Biorefinery roadmap based on catalytic production and upgrading 5-hydroxymethylfurfural

Abstract

Biorefineries, which utilize lignocellulosic biomass as renewable energy source and sustainable carbon feedstock, are a promising solution to alleviate the excessive dependence on the depleting fossil resources and address climate change and other environmental problems. Owing to the recalcitrance and over-functionalized nature of biomass, the conversion of biomass into desirable products requires a series of complex deconstruction, catalytic conversion, separation and purification processes. In the biorefinery roadmap, 5-hydroxymethylfurfural (HMF) stands out as a bridge connecting biomass raw materials to alternative fuels, chemicals and materials, which can displace petroleum-derived products. This review describes the recent advances in the design and development of catalytic systems for the conversion of biomass and their constituent carbohydrates to HMF via hydrolysis, isomerization and dehydration reactions, and the upgrading of HMF towards polymer monomers, fine chemicals, fuel precursors, fuel additives, liquid fuels, and other platform chemicals via hydrogenation, oxidation, esterification, etherification, amination and aldol condensation reactions, with emphasis on how the catalysts, solvents and reaction conditions determine the reaction pathway and product selectivity. We also attempt to provide a conceptual framework on how to evaluate the actual reaction efficiency, reusability, and economic and technical feasibility of different catalytic systems and highlight the key research challenges to be addressed.

---

Qidong Hou

Qidong Hou obtained his Bachelor's Degree and Master's Degree in 2013 and 2018, respectively, from the College of Environmental Science and Engineering, Nankai University, China. After he graduated, he worked as the Director of the Biorefinery Center in National & Local Joint Engineering Research Center of Biomass Resource Utilization, China. Dr Hou has wide experience in garbage classification and recycling, pollution control of solid waste, biomass pretreatment, catalytic conversion of biomass to important platform chemicals and their upgradation toward fuels, fine chemicals and degradable plastics through heterogeneous catalysis, advanced oxidation technology, selective oxidation and hydrogenation technology.

Xinhua Qi

Dr Xinhua Qi is a Professor at Nankai University, Tianjin, China. His research interests mainly focus on green processes for biomass conversion into value-added materials and chemicals. He has published more than 100 peer reviewed scientific papers, and these papers have been cited over 3200 times with an H index 30. He has also co-authored over 20 patents, 5 books and 3 book chapters on environmental engineering and biomass resources utilization. Prof. Qi has been supported by the National Key Research and Development Program of China, National Natural Science Foundation of China, the Natural Science Fund for Distinguished Young Scholars of Tianjin.

Meiting Ju

Meiting Ju is a Professor at the College of Environmental Science and Engineering, Nankai University, China, and Director of the National & Local Joint Engineering Research Center of Biomass Resource Utilization. He is a specialist in pollution control of organic solid waste, valorization of biomass toward fuels, chemicals and materials, industrial ecology, environmental impact assessment and environmental management. He has coauthored more than 100 scientific articles, 10 books and 30 authorized patents. His research focuses on efficient, economic and environmentally friendly biorefinery technologies to promote the establishment of a more sustainable industry.

---

1. Introduction

The modern society consumes a significant amount of fossil resources, including petroleum, coal and natural gas, to meet the major demand for energy, chemicals and materials.¹ Using the well-established petroleum refinery process, fossil carbon resources can be converted into transportation fuels, fertilizers, fine-chemicals, polymers, plastics, pharmaceuticals, detergents, food additives, electronics, sports equipment, clothes, dyes and agrochemicals.^2,3 However, the excessive dependence on unrenewable fossil fuel resources has resulted in a series of economic, social and environmental problems, including shortage of resources and energy crisis, massive emission of environmental pollutants, global climate change, and unsustainable development. Therefore, to reduce the dependence on fossil fuel resources, the exploitation of renewable resources, such as solar, wind, wave power, and biomass is of great importance.^4,5

Among these renewable resources, biomass is the only renewable carbon-based resource that can serve as both energy and feedstock, with a global production larger than 120 billion tons per annum (the dry basis weight is about 10 billion tons), containing 2.2 × 10²¹ J of energy.^6–10 Moreover, the utilization of biomass is considered to be approximately carbon neutral since the CO₂ emission associated with biomass-derived products can be offset by the CO₂ capture in photosynthesis.^11,12 In fact, biomass was continuously utilized as the predominant energy source and feedstock before the industrial age, and nowadays biomass still ranks the fourth used energy, which is about 10% of the global primary energy source.⁹ The renaissance of biomass utilization is based on the concept of “biorefinery”, which has been developed to displace the current energy and chemical industry based on petroleum refining, showing a great promise for the realization of sustainable development.

Biomass (Fig. 1) represents the biogenic organic matter originating from carbon dioxide (CO₂) and water (H₂O) via photosynthesis using energy from sunlight.⁴ Generally, biomass contains carbohydrates, lignin, fatty acids, lipids, proteins, and other components containing carbon (C), hydrogen (H), oxygen (O), nitrogen (N), phosphorus (P), chlorine (Cl), sulphur (S), and metal elements (K, Na, Ca, and Mg).^9,13,14 Lignocellulosic biomass, which is prevalent in the cell walls of plants, accounts for more than 90% of all plant biomass.¹⁵ Lignocellulosic biomass is the most abundant and available biomass source for biorefineries, without competing with food reserves.⁴ Lignocellulosic biomass is mainly composed of cellulose (35–50 wt%), hemicellulose (20–30 wt%) and lignin (20–30 wt%), in which aligned bundles of crystalline and amorphous cellulose microfibrils are embedded in a disordered matrix of hemicellulose and lignin.¹⁵ Cellulose is a linear polysaccharide composed of glucose units linked via β-1,4-glycosidic bonds, hemicelluloses are branched polysaccharides consisting primarily of xylose with a small quantity of mannose, galactose, rhamnose, and arabinose via multiple types of glycosidic bonds, and lignin is a three-dimensional amorphous polymer composed of three types of phenylpropanolic monomers (coniferyl alcohol, p-coumaryl alcohol and sinapyl alcohol) linked by carbon–carbon and ether bonds.^4,16,17 The natural recalcitrance of lignocellulosic biomass, which largely originates from the tight compaction and aggregation of its partially crystallized cellulose chains and the inherent inaccessibility of cellulose to enzymes and catalysts owing to the surrounding heterogeneous matrix composed of lignin and hemicellulose, is the major obstacle in the cost-effective deconstruction and valorization of biomass.¹⁸

Fig. 1 Structure of lignocellulosic biomass and its components. Compiled from ref. 2, 16 and 19.

For a long time, physical, biological, chemical and combined approaches have been investigated to transform lignocellulosic biomass into useful products, forming the roadmap of biorefinery technologies, which have potential to displace petroleum refining.^20–22 The direct combustion of lignocellulosic biomass can provide heat, which can be used for the generation of electricity. Alternatively, lignocellulosic biomass can also be converted into biomass briquetting fuel via physical compression. Traditional biological technologies, including aerobic and anaerobic fermentation, mainly produce organic fertilizers, feeds and biogas. In addition, the development of high-performance microbial cell factories can enable the production of more diverse chemicals of interest from biomass-derived carbon sources.²² The efficient biological conversion of lignocellulosic biomass to target products usually requires pretreatment to enhance the accessibility of biomass by altering its three-dimensional (3D) structure, interaction and composition.^2,6,16 Ethanol (EtOH), one of the most representative biomass biofuels, is produced industrially from starchy biomass, including corn starch and sugar cane, while its production from lignocellulosic biomass via pretreatment, enzymatic saccharification and fermentation is a more sustainable but challenging approach.^2,11 Pyrolysis (or gasification) of lignocellulosic biomass simultaneously produces biochar, bio-oil and gas products, which only consumes approximately 10% of the energy in the original biomass.^9,23 However, most biorefinery technologies either provide low-value products or generate a series of complex mixtures, which require tedious separation, purification and upgrading process to obtain the final products.⁹ Accordingly, the biorefinery strategy aims to convert lignocellulosic biomass to high-quality fine-chemicals, liquid transportation fuels and high-performance structural materials, which can partly or even completely displace the currently indispensable products derived from fossil resources and other unrenewable resources, and thus has attracted great attention.²⁴

Shifting from the ready-to-use petroleum refinery economy to biorefinery comes with considerable technical and financial challenges since petroleum is under-functionalized relative to the target products, while biomass is over-functionalized.²⁵ The conversion of biomass to final products requires a series of deconstruction, catalytic conversion, separation and purification processes. In these conversion processes, 5-hydroxymethylfurfural (HMF) stands out as a bridge (Fig. 2) connecting biomass raw materials to the biorefinery industry since it not only retains a reasonable proportion of the initial chemical complexity of biomass, but also can be converted into multiple target products, which are promising to displace the corresponding petrochemicals.^1,26 In traditional biorefinery processes aiming to obtain valuable products from the carbohydrate fractions, lignin is usually considered as a major obstacle in the efficient utilization of lignocellulosic biomass.²⁷ After pretreatment or fractionation, cellulose-rich material is collected as the predominant feedstock for the biorefinery, some of the hemicellulose is lost and lignin is converted into condensed residue due to its uncontrollable degradation under harsh reaction conditions.^28,29 In fact, lignin not only can be used for the generation of heat via combustion, but also to produce high value products as the most abundant resource of bio-aromatics.^28,30,31 Glucose can be obtained from cellulose via either enzymatic saccharification or acid-catalyzed hydrolysis. After removing three molecules of water from glucose, glucose can be converted to HMF, a pivotal platform chemical, which can be further converted to versatile products ranging from polymer monomers, fine chemicals, fuel precursors, fuel additives, and liquid fuels to other platform chemicals, covering a broad range of structural complexities and application demands.^1,2,26,32 Using a similar but more facile reaction route, hemicellulose can be transformed into furfural, a current commodity chemical with an annual production of about 300 kTon.^13,33 Furfural has already been extensively used in the chemical industry.^1,33,34 In comparison with furfural, HMF is more attractive owing to the abundance of cellulose feedstock for its synthesis and its versatility of HMF. Therefore, the development of environmentally friendly and cost-effective catalytic technology that can convert cellulose into HMF and its derivates offers great opportunity for the establishment of sustainable biorefineries.

Fig. 2 Basic framework of the biorefinery process based on HMF.

The last few decades have witnessed continuously growing interest in the synthesis of HMF and its catalytic upgrading toward biofuels, bio-based chemicals and materials, as evidenced by the number of publications on this topic (Fig. 3). The abundant original research on the transformation of biomass to HMF and its subsequent conversion to value-added chemicals has greatly promoted the development of this field. Although several recent reviews articles, chapters, and books on novel materials and biorefineries presented the synthesis and upgrading of HMF and furfural, in a wider context of materials or biomass valorization,^33,35–43 the chemistries, processes and sustainability issues for the synthesis of HMF and upgrading, and upstream and downstream processes have not been systematically summarized and critically evaluated in detail in these works. Also, many interesting results have been recently published on the development and applications of novel materials and solvents for the synthesis and upgrading of HMF.

Fig. 3 Number of publications on HMF per year (January 2000–July 2020). Source: Web of science (topic: 5-hydroxymethylfurfural, 5-hydroxymethyl-2-furfuraldehyde, 5-hydroxymethyl furfural, 5-HMF or HMF).

Herein, we aim to provide a roadmap for biorefineries based on HMF, covering the important literature since 2007 with focus on the recent development of the state-of-the-art catalytic systems and advanced reaction processes. This review includes two major sections. Section 2 summarizes the advances in the synthesis of HMF using various feedstock, covering the development of homogeneous catalysts, heterogenous catalysts, solvents, reaction pathways and process technology. Section 3 summarizes the upgrading of HMF toward value-added fuels and chemical commodities, including furan derivates, fuel additives, diols, dicarboxylic acids and hydrocarbons via selective oxidation, hydrogenation, etherification, coupling and condensation reaction. Considering the similar roles of furfural and HMF in the biorefinery, the synthesis and upgrading of furfural are also presented in this review, only emphasizing the cutting-edge catalytic technologies. For a more elaborate overview on the synthesis and upgrading of furfural, the reader is referred to the dedicated reviews on this topic.^33,34 Throughout, we attempt to reveal the reaction mechanism, highlight the key factors that influence the performance, and systematically evaluate the merits and drawbacks of disparate catalytic systems by comparing their intrinsic reaction efficiency, reusability, economic and technical feasibility and sustainability. The gaps in current research, the drawbacks in analytical and characterization methods, the key research problems and the future trends are also discussed. We hope that this review will serve as a useful reference for the academic research and industrial applications of biorefinery-based HMF, and consequently attract the attention of academic and industrial researchers toward crucial scientific and technical problems, the solution of which can help overcome the main bottlenecks in this field.

Assessing all the catalysts and all the possible HMF derivatives in the literature is beyond the scope of this manuscript since the number of catalysts and potential HMF derivatives are colossal. Therefore, we try to unravel all the types of catalysts, solvents and catalytic systems, and then examine the catalytic systems that fulfil one of the following criteria: (1) the typical catalytic systems that have been confirmed or improved by the following studies; (2) the novel materials and solvents that have been tested preliminarily in the synthesis and upgrading of HMF; and (3) new phenomena and new or general mechanisms observed or confirmed for the catalytic systems. Similarly, we examine the HMF derivatives according to the following criteria: (1) biofuels that can be derived either directly from HMF or from high volume HMF derivatives; (2) commercial chemicals that are produced directly from HMF; (3) non-commercial chemicals that are directly derived from HMF through well-established routes and that present high potential for commercialization as commodities; and (4) commercial products currently derived from petroleum that can be produced from biomass via HMF as the main intermediate.

2. Synthesis of HMF from biomass

Considering the chemical composition of HMF (C₆H₆O₃), it can be considered the dehydration product of a C₆ sugar after removing three molecules of water. In thermally processed foods containing a high concentration of sugars and amino acids, HMF is usually generated via the Maillard reaction as an impurity, which is a potential health hazard.⁴⁴ Structurally, HMF contains a furan ring bearing hydroxymethyl (CH₂–OH) and aldehyde (CHO) functional groups. Therefore, the dehydration of carbohydrates to HMF not only can decrease the oxygen content of carbohydrates, but also retain reasonable chemical complexity for subsequent conversion.²⁶ As a pivotal platform in biorefineries, the efficient production of HMF from biomass is prerequisite for the establishment of an HMF-based biorefinery industry. For the historical developments in the synthesis of HMF, the reader is referred to the earlier literature.^13,42,45,46 In this section, we aim to present an overview of the reactions, feedstocks, catalysts, solvents and process technologies for the synthesis of HMF, focusing on the development of efficient and selective catalytic systems to improve the intrinsic HMF productivity. For a simple extension of previous work, we only provide a simple description, but the catalytic performance and detailed reaction conditions are summarized in the tables for readers to find useful information readily.

2.1. Main feedstocks and reaction processes

Any C₆ sugar containing materials either from natural sources or synthetic approaches, including monosaccharides (fructose, glucose and mannose), disaccharides (sucrose, cellobiose and maltose), polysaccharides (inulin, starch and cellulose), food waste, industrial molasses, and lignocellulosic biomass and its hydrolysate can be used as a feedstock for the production of HMF.^47,48 The synthesis of HMF from biomass and its components involves a series of complex processes, including pretreatment (or fractionation) of lignocellulosic biomass, hydrolysis of cellulose to glucose, isomerization of glucose to fructose and dehydration of fructose. Meanwhile, these processes are accompanied by a series of side-reactions, including the decomposition of HMF to formic acid and levulinic acid and the condensation of sugars, reaction intermediates and HMF toward humins.

2.1.1. Conversion of fructose to HMF. Among the various feedstocks, fructose, which contains a five-ring structure similar to HMF, is the most amendable feedstock for the synthesis of HMF. Since fructose can be converted into HMF via a simple dehydration process, early studies usually employed fructose as the starting material. Many Brønsted acid-containing catalysts can catalyze the dehydration of fructose to HMF efficiently, and ionic liquids, organic solvents and metal salts (Fig. 4) may also promote the dehydration of fructose to HMF. The polysaccharide composed of fructose, inulin, can also be converted to HMF easily via hydrolysis to fructose, followed by the dehydration of fructose to HMF in a one-pot reactor using a Brønsted acid as the catalyst.

Fig. 4 Main reaction for the synthesis of HMF from biomass and its components. Adapted from ref. 49.

2.1.2. Conversion of glucose to HMF. Compared with fructose, the dehydration of glucose into HMF is more attractive but more challenging owing to its stable six-ring structure.⁵⁰ As the most abundant component in lignocellulosic biomass, glucose is 75% cheaper than fructose and is generally considered as the first platform chemical in a biorefinery.⁵¹ Usually, the conversion of glucose to HMF proceeds via a two-step reaction process involving the isomerization of glucose to fructose and subsequent dehydration of fructose to HMF with the cooperation of isomerization and dehydration catalysts in one-pot catalytic systems, which generally has a lower activation energy than the direct conversion of glucose to HMF (Table 1). The isomerization of glucose to fructose, which can be catalyzed by a Lewis acid, isomerase or Brønsted base, is a reversible reaction with a low yield and relatively low reaction efficiency due to the constraint of the reaction equilibrium.⁵² The production of high-fructose corn syrup (HFCS) via the isomerization of glucose to fructose by enzymes is an important industrial process in the food and chemical industry, but this process seems to be unsuitable for biorefineries due to its high cost. The isomerization of biomass-derived aldoses to the corresponding ketoses and epimerization of aldoses using base and Lewis acid catalysts have been extensively studied, and the reader is referred to these reviews.^53,54 Owing to the additional isomerization step, the direct conversion of glucose is more challenging than that of fructose, usually resulting in a lower HMF yield and selectivity. Besides, the conversion of glucose to HMF can also be achieved via a two-step process involving discrete isomerization and dehydration steps in two separate catalytic systems. Similarly, the conversion of xylose to furfural can be performed either through a one-step process over a single Brønsted acid catalyst, or two-step process via xylulose as the intermediate over a combined Lewis and Brønsted acid catalyst with lower activation energy.⁵⁵

Table 1 Activation energy for the main reaction steps for the synthesis of HMF in representative catalytic systems

Reaction process	Catalyst	Solvent	Test condition	Activation energy	Ref.
a Simulated activation energy by DFT.
Isomerization of xylose to xylulose	CrCl3	Water	105–145 °C, xylose loadings 1 wt%	64.9 kJ mol^−1	55
Dehydration of xylulose to furfural	HCl	Water	105–145 °C, xylulose loadings 1 wt%	96.7 kJ mol^−1	55
Dehydration of xylose to furfural	HCl	Water	105–145 °C, xylose loadings 1 wt%	133.9 kJ mol^−1	55
Isomerization of glucose to fructose	CrCl3	Water	130–150 °C, glucose loadings 10 wt%	64.4 kJ mol^−1	56
Isomerization of glucose to fructose	AlCl3	Water	100–120 °C, glucose loadings 0.25 M	110 kJ mol^−1	57
Isomerization of glucose to fructose	AlCl3	Water/THF	130–160 °C, glucose loadings 0.25 M	95 kJ mol^−1	58
Isomerization of fructose to glucose	AlCl3	Water/THF	130–160 °C, glucose loadings 0.25 M	66 kJ mol^−1	58
Isomerization of glucose to fructose	Sn-Beta	Water	80–120 °C, glucose loadings 0.52 M	93 kJ mol^−1	59
Isomerization of glucose to fructose	Triethylamine	Water	80–120 °C, glucose loadings 0.52 M	61 kJ mol^−1	59
Isomerization of glucose to fructose	Meglumine	Water	80–110 °C, glucose loadings 10 wt%	74 kJ mol^−1	52
Dehydration of fructose to HMF	5 mM–1 M H2SO4	Water	140–180 °C, fructose loadings 0.1–1 M	123 kJ mol^−1	13
Dehydration of fructose to HMF	50 mM H2SO4	Water/GVL (w/w = 1/3)	140–180 °C, fructose loadings 0.1–1 M	84 kJ mol^−1 ^a	60
Dehydration of fructose to HMF	50 mM H2SO4, KCl	Water/GVL (w/w = 1/3)	140–180 °C, fructose loadings 0.1–1 M	74 kJ mol^−1 ^a	60
Dehydration of fructose to HMF	50 mM H2SO4, KCl	Water/GVL (w/w = 1/9)	140–180 °C, fructose loadings 0.1–1 M	67 kJ mol^−1 ^a	60
Dehydration of fructose to HMF	EMIMBr	EMIMBr		57.3 kJ mol^−1 ^a	61
HMF degradation	5 mM–1 M H2SO4	Water	140–180 °C, HMF loadings 0.1–1 M	147 kJ mol^−1	13
Direct dehydration of glucose to HMF	5 mM–1 M H2SO4	Water	140–200 °C, glucose loadings 0.1–1 M	152 kJ mol^−1	62
Direct dehydration of glucose to HMF	HAP	EMIMBr	110–130 °C, glucose loadings 10 wt%	100.5 kJ mol^−1	49
Direct dehydration of glucose to HMF	Al-HAP	EMIMBr	110–130 °C, glucose loadings 10 wt%	80.7 kJ mol^−1	49
Direct dehydration of glucose to HMF	Sn-HAP	EMIMBr	110–130 °C, glucose loadings 10 wt%	79.2 kJ mol^−1	49
Direct dehydration of glucose to HMF	Sn/Al-HAP	EMIMBr	110–130 °C, glucose loadings 10 wt%	68.4 kJ mol^−1	49
Hydrolysis of cellulose to glucose	0.1 M–1 M H2SO4	Water	100–200 °C, cellulose loading 1.7 wt%	170 kJ mol^−1	63

---

2.1.3. Conversion of polysaccharides and biomass to HMF. The conversion of disaccharides and polysaccharides to HMF requires them to be depolymerized into monosaccharides via hydrolysis prior to the isomerization and dehydration steps. The hydrolysis step can be catalyzed either by an enzyme or Brønsted acid. Generally, the hydrolysis of disaccharides, inulin and starch is relatively facile, while the selective hydrolysis of cellulose to glucose is challenging owing to the recalcitrant nature imparted by its long molecular chain with plenty of intramolecular and intermolecular hydrogen bonds. Due to its complex composition and structure, the direct conversion of lignocellulosic biomass into HMF involves a series of complex processes, including pretreatment or fractionation of biomass to improve the availability of cellulose, depolymerization of cellulose to glucose, isomerization of glucose to fructose and dehydration of fructose to HMF. Therefore, the direct conversion of lignocellulosic biomass into HMF is more challenging than pure carbohydrates. Accordingly, various pretreatment methods, including physical, chemical, biological and combined methods have been investigated to reduce the crystallinity degree and molecular weight of cellulose to promote its subsequent conversion.⁶⁴ Besides, during the processing of cellulose, hemicelluloses or biomass, including pretreatment, fractionation, hydrothermal processing and carbonization, HMF is also omnipresent as a by-product with low yield.⁶⁵ Besides lignocellulosic biomass, chitin biomass (Fig. 5), including raw crustacean shells, purified chitin, and chitosan and its corresponding monomers, N-acetylglucosamine (GlcNAc) and glucosamine (GlcNH₂) can also be used as feedstock for the production of HMF.^66,67

Fig. 5 Production of HMF from chitin biomass. Adapted from ref. 66 and 67.

2.2. Reaction media for the production of HMF from biomass

Generally, the dissolution of a reactant and product in a solvent and the dissolution or dispersion of a catalyst in a solvent have predominant influence on reaction processes. In addition, the competing adsorption of solvents with reactants, intermediates, transition states and products on the catalyst surface can affect the reaction pathway.⁶⁸ The mass transfer between the catalyst surface and liquid phase has a predominant effect on the reaction rates. Besides, solvents can also function as homogeneous catalysts in some cases to alter the reaction pathway. Thus, it is necessary to search for cost-effective and environmentally friendly solvents to avoid the use of hazardous and harmful solvents. The use of ionic liquids, supercritical fluids, fluorous solvents, deep eutectic solvents, and biomass-derived solvents as green solvents in the conversion of lignocellulose has been reviewed in previous reports.^17,69,70 In this section, we analyze the influence of different solvents on the production of HMF and discuss their merits and disadvantages from the perspective of green chemistry, with emphasis on green solvents, biomass-derived solvents, and low boiling points solvents.

2.2.1. Water. Water is generally considered a perfect green solvent owing to its nontoxic nature, low-cost and incombustibility, but it suffers from many disadvantages in biorefineries, including energy-intensive distillation process, intractable wastewater, and difficulty in controlling the dissolution and reaction process.^17,69,71 For the conversion of monosaccharides to HMF, the presence of waster usually has obvious adverse impacts. On the one hand, the use of water as a solvent has a negative impact on the reaction equilibrium toward HMF since water is the main product of the dehydration process. On the other hand, water participates in the decomposition of HMF to formic acid and levulinic acid. Therefore, although water and supercritical water (scH₂O) have been widely investigated as reaction media for the conversion of sugars to HMF, the yield and selectivity for HMF are usually low in the medium of pure water. For the isomerization of glucose to fructose, water is the most widely used solvent using an enzyme, Lewis acid or Brønsted base as the catalyst. For the conversion of polysaccharides and biomass, water is an indispensable reactant for the hydrolysis step, but not a good reaction medium due to the low solubility of polysaccharides and biomass in pure water.^72–74

2.2.2. Organic solvents. Several organic solvents, in particular dimethyl sulfoxide (DMSO), are widely used as the reaction medium for the production of HMF. The efficient conversion of fructose to HMF in the medium of DMSO in the absence of additional catalyst has been widely reported,^75,76 but many contradictory results exist in current literature. Guo et al. reported that the HMF yield of 70.5% was obtained from pure DMSO at 140 °C for 4 h after purging the reaction mixture with N₂.⁷⁷ Nevertheless, many studies reported that DMSO is capable of converting fructose to HMF efficiently in air and an O₂ atmosphere, but only gives a low HMF yield in an N₂ atmosphere. For example, Whitaker et al. observed that the HMF yields from fructose in DMSO under vacuum (0) and nitrogen (32%) at 120 °C were remarkably lower than that (77%) obtained under an air atmosphere.⁷⁸ Besides the inconsistent catalytic performance, the actual active species for the dehydration of fructose have not been confirmed. Ren et al. considered that the catalytic activity of DMSO for the dehydration of fructose to HMF stems from the valence unsaturation of the S and O atoms and the unsaturated double bond of S=O.⁷⁹ Several studies attributed the superior catalytic performance of DMSO for the dehydration of fructose to HMF in the presence of O₂ to the acidic species, such as CH₃SO₃H and H₂SO₄, generated from the decomposition of DMSO at an evaluated temperature,^80–82 but the barium chloride precipitation test conducted by Whitaker showed that no sulfate ions were generated at 120 °C and 150 °C under an oxygen atmosphere.⁷⁸ Moreover, they demonstrated that the formed organic acid could not catalyze the dehydration reaction effectively. Therefore, they concluded that DMSO does not decompose to into a strong acid at a temperature lower than 150 °C and the dehydration of fructose in DMSO is dominated by the solvent effect. Considering these contradictory results, precise control experiments and detailed analysis of the composition of the reaction mixture under various conditions are necessary to eliminate the possible influence of O₂ and impurity in the reagents and illuminate the realistic role of DMSO in the conversion of fructose to HMF.

The mechanism of the dehydration of fructose and glucose to HMF in DMSO over different catalysts has also been widely investigated. For example, Zhang et al. studied the mechanism of fructose dehydration to HMF in DMSO over Amberlyst 70, PO₄³⁻/niobic acid, and sulfuric acid.⁸⁰¹³C, ¹H, and ¹⁷O NMR, and high-resolution electrospray ionization mass spectrometry (HR ESI-MS) confirmed the formation of intermediates 1 and 3 (Fig. 6), which is consistent with the low energy species calculated from the high level G₄MP₂ theoretical analysis. Intermediate 2 was also identified by HR ESI-MS and theoretical analysis, but the keto and the enol forms could not be distinguished experimentally. The same intermediates were observed over the three different catalysts, suggesting that there is a common mechanism for the dehydration of fructose to HMF in DMSO, which is independent of the source of protons. Though theoretical analysis, Ren et al. indicated that the H⁺ from the Brønsted acid prefers to interact with DMSO, leading to the formation [DMSOH]⁺.⁷⁹ [DMSOH]⁺, instead of the original Brønsted acid is the main catalytically active species for the dehydration of fructose to HMF, which can explain why the intermediates are independent of the source of protons. Jia et al. reported that the use of anhydrous DMSO as the reaction medium for the conversion of glucose over chromium trichloride hexahydrate (CrCl₃·6H₂O) catalyst not only promoted the formation of HMF, but also led to undesirable side-reactions, in particular the dehydration of glucose into cellobiose.⁸³ Yang et al. reported that the formation of a six-coordinated structure (CrCl₃–3DMSO) resulted in a low glucose conversion and HMF yield, while N,N-dimethylacetamide (DMA) resulted in a higher HMF yield and selectivity owing to the four-coordinated structures formed between CrCl₃ with DMA.⁸⁴

Fig. 6 Three intermediates identified in the acid-catalyzed dehydration of fructose to HMF in the medium of DMSO. Adapted from ref. 80.

The products from the conversion of carbohydrates in alcohols depend on the catalyst and structure of the alcohol since alcohols can react with sugars, HMF via etherification or acetylation, and levulinic acid via esterification (Fig. 7). When HCl was used as the catalyst, the use of isopropanol and tert-butanol as the solvent enabled the selective conversion of fructose to HMF.⁸⁵ In contrast, the use of an ethanol, n-propanol and n-butanol solvent gave a mixture of HMF and 5-alkoxymethylfurfural (AMF), while the use of methanol as the solvent led to a series of ethers with a small amount of HMF. An HMF yield of 68% was obtained from fructose in the medium of isopropanol using NH₄Cl as the catalyst.⁸⁶ Kuo et al investigated the conversion of fructose using TiO₂ nanoparticles as the catalyst in various solvents.⁸⁷ When the reaction was conducted in tetrahydrofuran (THF), CH₃CN, N,N-dimethylformamide (N,N-DMF), DMSO, or DMA, the main product was HMF with a yield ranging between 19–54%. The reaction conducted in ethanol, n-propanol and n-butanol primarily gave levulinic ester via either the rehydration of HMF-ether to form levulinic ester, or rehydration of HMF to levulinic acid followed by esterification to form levulinic ester, while the predominant products in the medium of 2-propanol, 2-butanol, and tert-butanol were HMF ether probably owing to the high barrier of the rehydration reaction owing to steric hindrance. The intrinsic reactivity of sugars also influences the reaction pathways and product distributions. For example, psicose and tagatose exhibited higher selectivity toward HMF and 5-(methoxymethyl)furfural (MMF) (combined yield of 55%) than other 2-ketohexoses, including fructose and sorbose, in an H₂SO₄-catalyzed reaction in methanol.⁸⁸

Fig. 7 Reaction pathway of carbohydrates in alcohols. Adapted from ref. 87.

2.2.3. Water–organic monophasic solvent systems: solvent effect. A mixture of soluble organic solvents, such as DMSO and THF (THF is miscible with water, but it forms a biphasic system with water in the presence of a suitable amount of NaCl), and water in an appropriate ratio forms a monophasic solvent system, which is one type of widely used solvent for the synthesis of HMF.⁸⁹ Regulating the solvent composition and the reaction environment for biomass conversion is an effective strategy to preferentially improve the rates of the desired reaction steps while inhibiting the undesirable ones, thus leading to an increase in the reaction efficiency and selectivity of the target product. For example, the combination of water with organic solvents, such as DMSO, THF and N,N-DMF, could increase the HMF selectivity by inhibiting the side-reactions toward condensation/polymerization products and humins.^83,90 Force-field-based molecular simulations suggested that the arrangement of co-solvents and water around glucose can facilitate the formation of HMF and levulinic acid and suppress the degradation of glucose to undesirable byproducts owing to the reduced mobility of glucose.⁹⁰

The application of biomass-derived γ-valerolactone (GVL) in the Brønsted acid-catalyzed dehydration of sugars and other biomass-derived oxygenates was systematically investigated by the research group of Professor James A. Dumesic, revealing the universal solvent effect in biomass conversion.^60,91–103 Mellmer et al. demonstrated that the use of a solvent system consisting of GVL and water in fructose dehydration could lead to a higher reaction rate and HMF selectivity.⁹¹ The combination of reaction kinetic studies and ab initio molecular dynamics simulations (AIMD) indicated that the solvation of biomass-derived oxygenates in the solvent system correlates well with the number of vicinal hydroxyl or oxygen-containing groups from these chemicals. Compared with the reaction performed in pure water, the use of the GVL/water solvent system led to a 450-fold increase in the fructose dehydration rate and only 10-fold increase in the HMF degradation rate owing to the better solvation of fructose than HMF. The following research by Mellmer et al. indicated that the addition of catalytic concentrations of inorganic salts, in particular chlorides, to the water/GVL solvent system can enable a further enhancement in the fructose to HMF conversion efficiency with a lower activation energy (Table 1).⁶⁰ The use of 5 mM chloride salts led to a 10-fold increase in reactivity with an HMF yield of more than 80%. The reaction kinetic results and AIMD analysis indicated that Cl⁻ plays an important role in the stabilization of the deprotonation transition state, thus leading to an enhancement in the fructose dehydration efficiency.

Mellmer et al. reported that the use of a solvent system consisting of water with polar aprotic cosolvents, including GVL, 1,4-dioxane and THF could increase the reaction rates and product selectivity for a series of Brønsted acid-catalyzed reactions, including xylose dehydration to furfural, 1,2-propanediol dehydration to propanal and cellobiose hydrolysis to glucose.^92–94 The use of GVL as the solvent promoted the solvation of protons from the catalyst and then reduced the activation energy, thus leading to an increase in the reaction rate. This universal solvent effect was observed for both homogeneous and heterogeneous catalysts, such as H₂SO₄ and H-BEA zeolite. The reaction kinetics were studied to quantify the effect of polar aprotic organic solvents on the acid-catalyzed conversion of xylose into furfural.⁹² Among the tested solvents, GVL led to the most significant increase in the reaction rate compared to water, with an improvement in the furfural selectivity.⁹⁵ The conversion of xylose in the water/GVL solvent system at high temperature (498 K) within around 2 min gave a high furfural yield (>90%), even at industrially relevant xylose concentrations (10 wt%).⁹⁶ The kinetic model indicated that the improved furfural yield and selectivity at the evaluated temperature are mainly attributed to the reduced bimolecular condensation between xylose and furfural.

Besides the promotion effect for the dehydration of fructose, the GVL/water solvent system also plays an important role in the direct conversion of glucose to HMF. Song et al. observed the formation of moderate yields of HMF, fructose and levoglucosan (LGA) for glucose conversion in a hot-compressed GVL/water (HCGW) solvent system, where the reaction pathway and product selectivity could be tuned by adjusting the GVL concentration.⁹⁷ Li et al. reported that the combined use of HCl as a catalyst and NaCl as a promoter in GVL/water (v/v = 4) system enabled a high HMF yield (62.45%) from glucose in the absence of additional catalysts responsible for glucose isomerization.¹⁰⁴ The water/GVL solvent system could also convert biomass into soluble carbohydrates directly in a packed-bed flow-through reactor owing to the enhanced solubilization of biomass in the mixed solvent.^98,99

Similar to the GVL/water system, Cao et al. found that the ratio of water and THF has an important influence on the reaction pathway and the product distribution for cellulose conversion.^100,101 When the reaction was performed in pure THF using H₂SO₄ as the catalyst in the absence of water under relatively harsh reaction conditions (170–230 °C; 5–20 mM H₂SO₄), cellulose predominantly decomposed to LGA and then LGA was dehydrated to levoglucosenone (LGO), obtaining a maximum LGO yield of 51% (Fig. 8). In contrast, an HMF yield of up to 44% was obtained when the reaction was carried out under mild reaction conditions (140–190 °C; 5 mM H₂SO₄) in THF in the presence of a small amount of water (<2–3 vol%).¹⁰² The dehydration of LGA over a solid Brønsted acid in pure THF gave LGO,¹⁰⁵ while the acid-catalyzed isomerization of LGO in the medium of water/THF solvent system gave HMF as the major product,¹⁰⁶ which is an alternative reaction pathway for the production of HMF.

Fig. 8 Reaction pathway for cellulose in the medium of THF. Adapted from ref. 100 and 101.

Although GVL can lead to a high reaction rate and improved selectivity, the recycling of GVL is challenging owing to its high boiling point (207–208 °C). As an excellent alternative to water/GVL, the inexpensive solvent system composed of acetone and water also enabled the selective dehydration of fructose to HMF using a common Brønsted acid catalyst, affording an HMF yield of up to 85% even at a fructose loading of 5 wt%.¹⁰³ Similarly, a mixture of hexafluoroisopropanol (HFIP), which has a low boiling point of 58 °C, and water could also serve as a monophasic reaction medium for the dehydration of fructose.¹⁰⁷

Understanding solvent effects in multicomponent systems at the molecular level is of great importance for the screening and establishment of highly effective solvent systems. Combining the advantage of experiments, classical molecular dynamics simulations and machine learning tools, Walker et al. suggested that the screening of mixed solvent systems for biomass conversion over acid catalysts can employ the following workflow: (1) elucidate the reaction network and select candidate solvent systems; (2) compute the reaction rate of different steps; (3) compute the thermodynamic selectivity for the desired product; (4) verify model-predicted solvent systems by experiment; and (5) evaluate the solvent recyclability, cost and avaibility.⁸⁹

2.2.4. Water–organic biphasic solvent systems. The combination of insoluble organic solvents with water can form a biphasic solvent system (Fig. 9), as an important type of reaction medium. Compared with sing-phase reaction systems, biphasic systems consist of a reaction phase and extraction phase, which can transfer the formed HMF from the reaction phase to the extraction phase instantaneously, inhibiting the degradation of HMF and then improving the HMF production efficiency.¹⁰⁸ Biphasic systems consisting of water and organic solvents have been widely used as the reaction medium for the production of HMF.¹⁰⁹ In this aspect, methyl isobutyl ketone (MIBK), butanol and THF are the most widely used organic solvents owing to their suitable partition coefficients, and salts, in particular NaCl are widely used to promote the transfer of HMF from the water phase to the organic phase via the salting-out effect.⁴⁵ As one of the most outstanding milestones in the production of HMF in a biphasic system, Román-Leshkov et al. reported that a biphasic system consisting of an aqueous phase, use of hydrochloric acid or an acidic ion-exchange resin the catalyst with DMSO or poly(1-vinyl-2-pyrrolidinone) as an additive to inhibit side reactions, and MIBK organic phase modified with 2-butanol to promote the transfer of HMF enabled the direct conversion of a high concentration fructose (10 to 50 wt%, with respect to the aqueous phase) to HMF with high selectivity (80% selectivity at 90% fructose conversion).¹⁰⁸ For the liquid–liquid extraction of HMF, a high distribution coefficient and high separation factor are both important to reduce the amount of co-extracted water. For example, Altway et al. analyzed the liquid–liquid equilibrium (LLE) data of HMF, water, and MIBK or 2-pentanol using NRTL and UNIQUAC models.¹¹⁰ They found that although the MIBK–HMF–water system has a lower distribution coefficient than the 2-pentanol–HMF–water system, the former exhibits better separation ability than the latter. Moreover, the introduction of a certain amount of salt can further improve the distribution coefficient of biphasic systems owing to the salting-out effect. The salting-out effect of different salts in the water–MIBK biphasic system increases in the following order: K₂SO₄ < KCl < Na₂SO₄ < NaCl. The simulation on cellulose conversion in a water/THF biphasic system using the multiphase lattice Boltzmann method indicated that a thinner liquid membrane thickness is beneficial for an improvement in the HMF yield.¹¹¹

Fig. 9 Conversion of carbohydrates to HMF in biphasic reaction systems.

Owing to the low toxicity, biodegradability, and high partition coefficient of 2-methyltetrahydrofuran (MeTHF), which can be synthesized from biomass-derived furfural or levulinic acid, it is considered as a promising alternative to the petroleum-derived THF.^17,45,69 Based on a comprehensive survey on the solvents used in biphasic systems for the synthesis of HMF, Esteban et al. screened 15 promising candidates using a conductor-like screening model for real solvents (COSMO-RS).¹¹² Among them, ethyl acetate (EtOAc), n-propyl acetate and isopropyl acetate were the preferred solvents after considering performance, environmental, health and safety (EHS) impacts integrally. As an organic solvent with a low boiling point, dimethyl carbonate (DMC) is also effective to improve the HMF production and separation efficiency.^113,114

2.2.5. Ionic liquids. As a type of important and versatile solvent, ionic liquids (ILs) have attracted remarkable attention for the catalytic transformation of lignocellulosic feedstocks to chemicals and fuel products.^17,42,71,115 As the most common carbohydrate-solubilizing ionic liquids, 1-ethyl-3-methylimidazolium chloride (EMIMCl) and 1-butyl-3-methylimidazolium chloride (BMIMCl) are capable of converting fructose to HMF directly (Table 2) in the absence of additional catalysts. The conversion of fructose in EMIMCl at 120 °C for 3 h gives an HMF yield of about 74%, but heating pure HMF in EMIMCl in the absence of an additional catalyst can only recover 28% of HMF, indicating that HMF is unstable in this ionic liquid.¹¹⁶ The addition of catalytic amounts of certain metal chlorides, including CrCl₂, CuCl₂, VCl₄, and H₂SO₄ (98%) to EMIMCl could stabilize HMF and then improve the HMF recovery to 85–98%. Compared with EMIMCl, HMF is more stable either in pure BMIMCl or in BMIMCl containing metal chlorides.¹¹⁷ The high-resolution NMR spectra suggested that the simple dissolution of fructose in the BMIMCl ionic liquid favors the formation of the open fructoketose form, which is beneficial for the subsequent dehydration reaction.¹¹⁸

Table 2 Conversion of carbohydrates to HMF in ionic liquids and deep eutectic solvents without the use of additional catalysts

Solvent	Reaction conditions	Substrate	Substrate loading^a	HMF yield	Ref.
a Relative to the solvent.
EMIMCl	120 °C, 3 h	Fructose	10 wt%	74%	116
BMIMBr	100 °C, 3 h	Fructose	40 wt%	95%	119
EMIMBr	100 °C, 3 h	Fructose	40 wt%	93%	119
[GLY(mim)3][OMs]3	100 °C, 3 h	Fructose	40 wt%	72%	120
[GLY(mim)3][Cl]3	100 °C, 3 h	Fructose	40 wt%	37%	120
[GLY(mim)3][Br]3	100 °C, 3 h	Fructose	40 wt%	26%	120
[GLY(mim)3][PF6]3	100 °C, 3 h	Fructose	40 wt%	19%	120
[GLY(mim)3][OMs]3	100 °C, 3 h	Glucose	40 wt%	16%	120
[GLY(mim)3][OMs]3	100 °C, 3 h	Sucrose	40 wt%	57%	120
ChCl/Citric acid	90 °C, 2 h	Sucrose	40 wt%	58.8%	121
ChCl/Malic acid	90 °C, 2 h	Sucrose	40 wt%	60%	121
[HMIM]HSO4	180 °C, 1 min	Fructose	3 wt%	37.4%	122
[HMIM]HSO4	180 °C, 5 min	Glucose	3 wt%	17.3%	122
[HMIM]HSO4	180 °C, 10 min	Cellulose	3 wt%	12.1–19.8%	122
[HMIM]HSO4	180 °C, 5 min	Glucose	3 wt%	17.3%	122
[HMIM]HSO4	180 °C, 30 min, in situ removal of HMF	Fructose	3 wt%	77.3%	122
[HMIM]HSO4	180 °C, 5 min, in situ removal of HMF	Glucose	3 wt%	76.1%	122
[HMIM]HSO4	180 °C, 10 min, in situ removal of HMF	Cellulose	3 wt%	55.9–68.3%	122

---

The use of a Brønsted acid catalyst in the BMIMCl ionic liquid can improve the fructose conversion efficiency considerably. For example, the use of the sulfonic ion-exchange resin Amberlyst 15 as the catalyst in BMIMCl gave an HMF yield of 82.2% with a fructose conversion of 100% at 120 °C within 1 min.¹²³ The addition of cosolvents including acetone, DMSO, ethanol, methanol, ethyl acetate, and scCO₂ to the BMIMCl/Amberlyst 15 catalytic system even enabled the efficient conversion of fructose to HMF at room temperature.¹²⁴

Br-Containing ionic liquids, including N-methyl-2-pyrrolidone bromide ([NMP]Br), 1-butyl-3-methylimidazolium bromide (BMIMBr) and 1-ethyl-3-methylimidazolium bromide (EMIMBr) are more effective in catalyzing the dehydration of fructose to HMF than the corresponding Cl-based ionic liquids, with a lower activation energy (Table 1).¹³ Under the optimized conditions, the HMF yield up to 95% and 93% at fructose conversion approaching 100% could be obtained with BMIMBr and EMIMBr, respectively, in the absence of an additional acidic catalyst.^119,125,126 Moreover, the reusability of BMIMBr was demonstrated for 6 runs of a recycling experiment. Employing DFT calculations, Li et al. suggested that the superior catalytic performance of BMIMBr for the dehydration of fructose to HMF is mainly attributed to the synergy of the cations and anions in BMIMBr, where the cations function as acid/base catalytic sites, and the anions can stabilize the intermediates and transition states by nucleophile covalent binding or H-bonding with the substrate, and the in situ-formed water serves as an H-shuttle.¹²⁷

SO₃H-based ionic liquids can be used to displace mineral acids to catalyze the dehydration of fructose to HMF.¹²⁸ The direct conversion of glucose and cellulose in the [HMIM]HSO₄ ionic liquid only gave a low HMF yield and selectivity.¹²² When the formed HMF was in situ removed from the catalytic system by on-line vacuum steam distillation, the conversion of glucose and cellulose gave high HMF yields of 77.3% and 55.9–68.3%, respectively, which is comparable to that obtained with fructose.

Alkaline ionic liquids have also been investigated as catalysts for the dehydration of fructose. For example, a catalytic amount of the alkaline ionic liquid 1-butyl-3-methylimidazolium hydroxide (BMIMOH) was active to catalyze the conversion of fructose to HMF using DMSO as the solvent under a nitrogen atmosphere, obtaining an HMF yield of 91.6% at 160 °C in 8 h, while the use of BMIMBr as the catalyst in DMSO gave a low HMF yield under the same conditions.¹²⁹ After 3 h reaction at 160 °C, BMIMOH afforded an HMF yield of 54.4% from sucrose in the medium of DMSO.¹³⁰

The introduction of functional groups in ionic liquids can also influence the conversion of sugars. For example, Siankevich et al. demonstrated that the use of appropriately functionalized IL mixtures could lead to an improved HMF production efficiency from glucose and cellulose over a CrCl₂ catalyst under mild conditions with limited side-reactions compared with the single IL.¹³¹ The IL mixture consisted of [C₂OHmim]Cl and modified IL, which has an additional CH₂CH₂Ph group on its imidazolium cation, affording an HMF yield of up to 92%. The strong hydrogen bond formed between the hydroxyl group in the [C₂OHmim]Cl ionic liquid with glucose weakens and activates the C–O bond, leading to a reduction in the reaction activation energy.¹³² The interaction between the sugar and CH₂CH₂Ph group-modified IL led to an increase in the reaction rate for fructose isomerization from the linear to furanosyl form. Consequently, the formation of humins was remarkably suppressed owing to the absence of a carbonyl group, which is required for aldol condensation in the fructouranosyl form. The combination of a sulfonic acid group-modified IL, [C₂OHmim]Cl and CH₂CH₂Ph-group modified IL was effective for the efficient transformation of cellulose into HMF, obtaining an HMF yield of up to 46%.

Electron microscopy revealed at the microscopic level that the nanostructured meshwork formed from the BMIMBF₄ ionic liquid and water is beneficial for the high conversion of fructose and HMF selectivity in the H₂SO₄-catalyzed fructose dehydration reaction.¹³³ In contrast, the use of BMIMBF₄ and EMIMBF₄ as the reaction medium exhibited an adverse influence on the conversion of carbohydrates to HMF over the SnCl₄ catalyst.¹²⁵ Qu et al. reported that a hydroxy-functionalized ionic liquid, 1-hydroxyethyl-3-methylimidazolium tetrafluoroborate ([AEMIM]BF₄), in the medium of DMSO showed higher catalytic activity than [EMIM]OH, [BMIM]OH and [MIMPS]₃PW₁₂O₄₀ for the conversion of sucrose to HMF, affording an optimized HMF yield of 68.71% at 160 °C for 6 h.¹³⁰ Under the same conditions, the conversion of cellobiose only yielded 24.73% of HMF. Under microwave irradiation, 1,1,3,3-tetramethylguanidine tetrafluoroborate ([TMG][BF₄]) in DMAc/LiCl exhibited a better catalytic performance than oil heating, obtaining an HMF yield of 28.63% under the optimal conditions.¹³⁴

The noncoordinating ionic liquid [BMIM][OTf] is not a suitable solvent for the conversion of fructose to HMF owing to the instability of HMF in [Bmim][OTf] under acidic conditions. Ghatta et al. found that adding an optimized amount of water to [Bmim][OTf] could inhibit the side reaction remarkably either using p-toluene sulfonic acid (PTSA) or HCl as the catalyst, thus leading to HMF yields higher than 80% even at a fructose loading of up to 14 wt%.¹³⁵ Alam et al. reported that the catalytic activity of ILs for the conversion of wood ear mushroom to HMF increases in the following order: [BBIM-SO₃][NTf₂] > [BBIM-SO₃][OTf] ≈ [DMA][CH₃SO₃] > [NMP][CH₃SO₃], corelating well with their proton donating ability.¹³⁶

The separation of HMF from ionic liquids is rather challenging owing to their hydrogen bond interaction, which can be formed either between anions and HMF or between cations and HMF.¹³⁷ Similar to water–organic biphasic systems, biphasic systems composed of an ionic liquid reactive phase and organic extraction phase (Fig. 9) have been developed to inhibit the degradation of HMF, and then improve the HMF production efficiency, but the extraction of HMF from ionic liquids is difficult due to the strong hydrogen bonds between HMF and ionic liquids. Zhou et al. reported that the addition of functional promoters such as methanol, ethanol and acetonitrile (CH₃CN) to the BMIMCl/MIBK biphasic system could disrupt the hydrogen bonding between HMF and BMIMCl, leading to an enhanced HMF extraction efficiency.^138,139 In addition, Chen et al. demonstrated that the bubbling effect is necessary for the rapid transfer of HMF from the reaction phase to the extraction phase.¹³⁹

2.2.6. Deep eutectic solvents. Deep eutectic solvents (DESs), which are liquid eutectic mixtures formed from two or three components via self-association or hydrogen-bonding interaction, are considered promising solvents for the green processing of lignocellulosic biomass and the conversion of its components.¹⁴⁰ The use of DESs consisting of choline chloride and different hydrogen bonding donors, including phenol, oxalic acid, ethylene glycol and urea, as the reaction medium has been investigated for the conversion of fructose to HMF.^141,142 In the presence of sulfonated amorphous carbon-silica as the catalyst, the DESs formed from choline chloride and phenol gave an HMF yield of 67% with selectivity of 100%, which is superior to that obtained with EMIMCl, DMSO, BMIMPF₆ and other DESs. DESs not only suppress the undesirable side reactions, but are also beneficial for the separation of HMF. HMF can be readily isolated from the DES phase using ethyl acetate/diethyl ether as the extracting agent. Hydrophobic deep eutectic solvents, including decanoic acid/n-tetraoctylammonium bromide, decanoic acid/thymol and thymol/lidocaine are promising solvents for the extraction of HMF from water since they have low water solubility (1.7–14.4 wt%) and high HMF solubility (75–85 wt%).¹⁴³

2.3. Homogenous catalysts for the production of HMF from biomass

A variety of homogeneous catalysts including metal chlorides, mineral acids, organic acids and bases (Table 3) have been investigated for the dehydration of carbohydrates in various reaction media. For the historical development of homogeneous catalytic systems for the synthesis of HMF, the reader is referred to the earlier reviews.^17,42,144 In this section, we review the recent advances in homogeneous catalytic systems with emphasis on the underlying mechanism and actual role of various homogeneous species.

Table 3 Homogenous catalysts for the conversion of carbohydrates to HMF and isomerization of glucose to fructose

Catalyst	Solvent	Reaction conditions	Substrate loading^b	Conversion	HMF yield^g	Ref.
a Relative to monosaccharide. b Relative to total solvent. c Relative to water. d Relative to glycolic acid. e pH = 7.5. f 1 mmol of natural deep eutectic solvents composed of equimolar betaine/HCl, carboxylic acid and H2O combined with 3 mL of ethyl acetate. g HMF yield if unspecified. — Not provided.
AlCl3 10 mol%^a	[EMIM]Cl	120 °C, 6 h	Glucose, 20 wt%	—	1.6%	145
MeAlCl2 10 mol%^a	[EMIM]Cl	120 °C, 6 h	Glucose, 20 wt%	—	7.6%	145
Et2AlCl 10 mol%^a	[EMIM]Cl	120 °C, 6 h	Glucose, 20 wt%	—	17%	145
AlEt3 10 mol%^a	[EMIM]Cl	120 °C, 6 h	Glucose, 20 wt%	—	51%	145
L^RAlMe2 5 mol%^a	[BMIM]Br	120 °C, 2 h	Glucose, 9.1 wt%	97%	58–60%	146
AlCl3 50 mol%^a	LiBr aqueous solution (0.97 g mL^−1) /ethyl acetate (v/v = 1/3)	120 °C, 2 h	Glucose, 0.54 wt%	∼82%	∼78%	147
AlCl3 50 mol%^a (3 runs)	LiBr aqueous solution (0.97 g mL^−1) /ethyl acetate (v/v = 1/3)	120 °C, 2 h	Glucose, 0.54 wt%	∼40%	∼20%	147
Al2(SO4)3 0.34 wt%^b	GVL/H2O (4/1), 40 g L^−1 NaCl	165 °C, 50 min	Cellulose, 0.97 wt%	—	43.5%	148
Al2(SO4)3 0.34 wt%^b	GVL/H2O (4/1), 40 g L^−1 NaCl	165 °C, 40 min	Cellulose, 4.32 wt%	—	36.1%	148
Al2(SO4)3 0.34 wt%^b	GVL/H2O (4/1), 40 g L^−1 NaCl	165 °C, 50 min	Cellulose, 8.64 wt%	—	19.6%	148
H2SO4 0.5% w/v^b	DMSO/H2O	150 °C, 2 h	Chlorella sorokiniana, 1.8 wt%	—	24%	149
H2SO4 0.5% w/v^b	DMSO/LiCl/H2O	150 °C, 2 h	Chlorella sorokiniana, 1.8 wt%	—	52%	149
H2SO4 0.5% w/v^b	DMSO/NaCl/H2O	150 °C, 2 h	Chlorella sorokiniana, 1.8 wt%	—	44.6%	149
H2SO4 0.5% w/v^b	DMSO/LiCl/H2O	150 °C, 2 h	Sugar cane bagasse, 1.8 wt%	—	1%	149
CrCl3 25 mol%^a	[BMIM]Cl	120 °C, 5 h	Microcrystalline cellulose, 5 wt%	—	58.4%	61
CrCl3 25 mol%, AlCl3 2.5 mol%^a	[BMIM]Cl	120 °C, 3 h	Microcrystalline cellulose, 5 wt%	—	58.3%	61
CrCl3 25 mol%, AlCl3 2.5 mol%^a	[BMIM]Cl	120 °C, 3 h	Cotton, 5 wt%	—	59.5%	61
CrCl3 25 mol%, AlCl3 2.5 mol%^a	[BMIM]Cl	120 °C, 3 h	Filter paper, 5 wt%	—	54.7%	61
CrCl3 6.8 mol%^a	[BMIM]Cl/toluene (5/22)	130 °C, 3 h	Microcrystalline cellulose, 1.9 wt%	—	55% (27% glucose)	61
ZrOCl2 5 wt%^b; [HO2CMMIm]Cl 1.7 wt%^b	Isopropanol	150 °C, 3 h	Glucose, 1.7 wt%	90%	43%	150
[DMA][CH3SO3] 0.5 wt%^b	DMA-LiCl	140 °C, 10 min, microwave	Wood ear mushroom, 5 wt%	—	44%	136
[BBIM-SO3][NTf2] 0.5 wt%^b	DMA-LiCl	140 °C, 10 min, microwave	Wood ear mushroom, 5 wt%	—	44%	136
SnCl4·5H2O 10 mol%^a	EMIMBr	100 °C, 3 h	Glucose, 10 wt%	100%	64.5%	125
SnCl4·5H2O 40 mol%^a	EMIMBr	100 °C, 1 h	Glucose, 40 wt%	100%	56%	125
SnCl4·5H2O 10 mol%^a	EMIMBr/GDE (w/w = 1/4)	100 °C, 3 h	Glucose, 10 wt%	100%	58.7%	125
SnCl4·5H2O 0.056 M^a	DMSO/H2O (v/v = 1)	Microwave, 140 °C, 40 min	Cooked rice, 5 wt%	—	22.7 wt%	151
Cu(NO3)2 0.3 wt%^b	H2O, pH = 5.5, 30 bar N2	110 °C, 1.5 h	Glucose, 1 wt%	18%	Fructose, 16%	152
HCl 0.075 M^b	H2O/THF (v/v = 1/4)	190 °C, 1 h	Corn stover	—	36%	153
H2SO4 0.0375 M^b	H2O/THF (v/v = 1/4)	190 °C, 1 h	Corn stover	—	15%	153
HNO3 0.075 M^b	H2O/THF (v/v = 1/4)	190 °C, 1 h	Corn stover	—	16%	153
Li2SO4 0.113 M^b, H2SO4 0.0375 M^b	H2O/THF (v/v = 1/4)	190 °C, 1 h	Corn stover	—	70%	153
Na2SO4 0.113 M^b, H2SO4 0.0375 M^b	H2O/THF (v/v = 1/4)	190 °C, 1 h	Corn stover	—	76%	153
K2SO4 0.113 M^b, H2SO4 0.0375 M^b	H2O/THF(v/v = 1/4)	190 °C, 1 h	Corn stover	—	66%	153
KCl 0.225 M^b, HCl 0.075 M^b	H2O/THF (v/v = 1/4)	190 °C, 1 h	Corn stover	—	44%	153
KNO3 0.225 M^b, HNO3 0.075 M^b	H2O/THF (v/v = 1/4)	190 °C, 1 h	Corn stover	—	0	153
LiCl 33.3 mol%^c	H2O	120 °C, 15 min	Glucose, 1 wt%	9.5%	Fructose, 7.7%	154
LiBr 33.3 mol%^c	H2O	120 °C, 15 min	Glucose, 1 wt%	51.8%	Fructose, 30.3% (mannose 1.4%)	154
LiI 33.3 mol%^c	H2O	120 °C, 15 min	Glucose, 1 wt%	90.2%	Fructose, 21.4% (mannose 3.8%)	154
CaBr2 16.7 mol%^c	H2O	120 °C, 15 min	Glucose, 1 wt%	94.4%	Fructose 22.4%, mannose 2.2%	154
ZnCl2 33.3 mol%^c	H2O	120 °C, 15 min	Glucose, 1 wt%	33.9%	Fructose, 0	154
[Hnmp]Zn2Cl5 500 mol%^a	BMIMCl/DMSO (w/w = 1/7)	130 °C, 8 h	Cellulose, 1.4 wt%	—	39.29%	155
NaCl 4.3 wt%^b	H2O/THF (v/v = 1/6)	200 °C, 8 h	Cellulose, 0.71 wt%	99.9%	58.9%	156
H3BO3 2.75 wt%^b	[MIM]Cl	120 °C, 3 h	Glucose, 10 wt%	95%	19%	157
H3BO3 2.75 wt%^b	[EMIM]Cl	120 °C, 3 h	Glucose, 10 wt%	95%	41%	157
H3BO3 2.75 wt%^b	[BMIM]Cl	120 °C, 3 h	Glucose, 10 wt%	47%	14%	157
H3BO3 2.75 wt%^b	[HMIM]Cl	120 °C, 3 h	Glucose, 10 wt%	68%	32%	157
H3BO3 2.75 wt%^b	[OMIM]Cl	120 °C, 3 h	Glucose, 10 wt%	63%	26%	157
H3BO3 100 g L^−1 ^b, NaCl 50 g L^−1 ^b	Water/MIBK (v/v = 0.25)	150 °C, 1.5 h	Fructose, 6 wt%	93%	60%	158
H3BO3 100 g L^−1 ^b, NaCl 50 g L^−1 ^b	Water/MIBK (v/v = 0.25)	150 °C, 6 h	Glucose, 6 wt%	41%	14%	158
H3BO3 80 mol%^d	Choline chloride/glycolic acid (molar ratio = 1)	140 °C, 2 h	Glucose 8 mol%^d	—	0	159
H3BO3 80 mol%^d	Dihydrogen citrate/glycolic acid (molar ratio = 1)	140 °C, 2 h	Glucose 8 mol%^d	—	42%	159
H3BO3 1 wt%^b	[BMIM]Cl	120 °C, 30 min	Glucose, 10 wt%	—	1.2%	160
H3BO3 1 wt%^b	[BMIM]Cl	140 °C, 40 min	Glucose, 10 wt%	—	0.76%	161
12-TPA 1 wt%^b	[BMIM]Cl	140 °C, 40 min	Glucose, 10 wt%	—	23.5%	161
12-TPA 1 wt%^b, H3BO3 0.5 wt%^b	[BMIM]Cl	140 °C, 40 min	Glucose, 10 wt%	—	51.9%	161
12-TPA 1 wt%^b, H3BO3 0.5 wt%^b	TEAC	140 °C, 40 min	Glucose, 10 wt%	—	47.6%	161
2-Methoxycarbonylphenylboronic acid	DMA	100 °C, 6 h	Glucose, 10 wt%	—	22%	162
MgCl2 200 mol%^a, 2-methoxycarbonylphenylboronic acid 100 mol%^a	DMA	120 °C, 4 h	Glucose, 10 wt%	—	57%	162
MgCl2 300 mol%^a, HCl 0.61%^b, 2-methoxycarbonylphenylboronic acid 120 mol%^a	[EMIM]Cl	105 °C, 2 h	Cellulose, 5 wt%	—	39%	162
ChCl 2 mM^b, H3BO3 4 mM^b	H2O (2.6 M NaCl)/MIBK (v/v = 1/5)	195 °C, 1 h	Starch, 2 wt%	—	35.9%	163
ChCl 2 mM^b, H3BO3 4 mM^b	H2O (2.6 M NaCl)/THF (v/v = 1/5)	195 °C, 1 h	Starch, 2 wt%	—	60.3%	163
ChCl 2 mM^b, H3BO3 4 mM^b (6 runs)	H2O (2.6 M NaCl)/MIBK (v/v = 1/5)	195 °C, 1 h	Starch, 2 wt%	—	19%	163
ChCl 2 mM^b, H3BO3 4 mM^b (10 runs)	H2O (2.6 M NaCl) /THF (v/v = 1/5)	195 °C, 1 h	Starch, 2 wt%	—	21.7%	163
Sulfamic acid 0.7 M^b	H2O	200 °C, 2 min	Chitosan, 3 wt%	—	21.48%	164
[PSMIM]HSO4 0.9 wt%^b, ZnSO4 0.06 M^b	THF/water (v/v = 20/2)	160 °C, 1 h	Cellulose, 2.3 wt%	—	58.8%	165
NaHSO4 4.9 g L^−1 ^b, ZnSO4 18.3 g L^−1 ^b	THF/water (v/v = 20/2)	160 °C, 1.5 h	Radiata pine hydrolysate	—	40% (furfural 58%)	166
H3PO4 1 wt%^c	Acetone/H2O (v/v = 2/1), NaCl 35 wt%^c	180 °C, 80 min	Radiata pine hydrolysate	—	36%	166
H3PO4 1 wt%^c	Acetone/H2O (v/v = 2/1), NaCl, NaCl 35 wt%^c	170 °C, 30 min	Radiata pine hydrolysate	—	Furfural 62%	166
Na2HPO4/NaH2PO4^e	H2O	110 °C	Glucose, 10 wt%	52%	Fructose 30%	167
KH2PO4, 2 wt%^b	H2O/MIBK (v/v = 1/4)	160 °C, 2 h	Sucrose, 5.4 wt%	100%	75.5%	168
KH2PO4, 2 wt%^b	H2O	160 °C, 1 h	Glucose, 1.8 wt%	49%	Fructose 13.5%	168
K2HPO4, 2 wt%^b	H2O	120 °C, 1 h	Glucose, 1.8 wt%	81%	Fructose 34.4%	168
Betaine/HCl, 1.8 wt%^b	H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Fructose, 2.7 wt%	—	66%	169
Betaine/H2SO4, 2.7 wt%^b	H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Fructose, 2.7 wt%	—	68%	169
Betaine/(C6H5)SO3H, 2.7 wt%^b	H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Fructose, 2.7 wt%	—	69%	169
Betaine/p-CH3(C6H4)SO3H, 3.6 wt%^b	H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Fructose, 2.7 wt%	—	66%	169
Betaine/HCl, 1.8 wt%^b	ChCl/H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Fructose, 2.7 wt%	—	87%	169
Betaine/H2SO4, 2.7 wt%^b	ChCl/H2O/MIBK	130 °C, 1 h	Fructose, 2.7 wt%	—	85%	169
Betaine/(C6H5)SO3H, 2.7 wt%^b	ChCl/H2O/MIBK	130 °C, 1 h	Fructose, 2.7 wt%	—	85%	169
Betaine/p-CH3(C6H4)SO3H, 3.6 wt%^b	ChCl/H2O/MIBK	130 °C, 1 h	Fructose, 2.7 wt%	—	88%	169
Choline-O-sulfate, 3.6 wt%^b	H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Fructose, 2.7 wt%	—	61%	169
Betaine/HCl, 1.8 wt%^b	H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Glucose, 2.7 wt%	—	3%	169
Betaine/HCl 2.7 wt%^b, CuCl2·2H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Glucose, 2.7 wt%	—	22%	169
Betaine/HCl 2.7 wt%^b, FeCl3·6H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Glucose, 2.7 wt%	—	29%	169
Betaine/HCl 2.7 wt%^b, MgCl2·6H2O 16.7 wt%^a	H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Glucose, 2.7 wt%	—	4%	169
Betaine/HCl 2.7 wt%^b, SnCl4·5H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK (v/v = 1/10)	130 °C, 1 h	Glucose, 2.7 wt%	—	26%	169
Betaine/HCl 2.7 wt%^b, AlCl3·6H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK	130 °C, 1 h	Glucose, 2.7 wt%	—	58%	169
Betaine/HCl 2.7 wt%^b, AlCl3·6H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK	130 °C, 1 h	Glucose, 2.7 wt%	—	64%	169
Betaine/HCl 2.7 wt%^b, NiCl2·6H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK	130 °C, 1 h	Glucose, 2.7 wt%	—	37%	169
Betaine/HCl 2.7 wt%^b, LaCl3·nH2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK	130 °C, 1 h	Glucose, 2.7 wt%	—	39%	169
Betaine/HCl 2.7 wt%^b, CaCl2·2H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK	130 °C, 1 h	Glucose, 2.7 wt%	—	14%	169
Betaine/HCl 2.7 wt%^b, MnCl2·4H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK	130 °C, 1 h	Glucose, 2.7 wt%	—	12%	169
Betaine/HCl 2.7 wt%^b, CoCl2·6H2O 16.7 wt%^a	ChCl 2.7 wt%^b, H2O/MIBK	130 °C, 1 h	Glucose, 2.7 wt%	—	18%	169
Betaine/HCl 100 mol%^c, malic acid 100 mol%^c	H2O/ethyl acetate^f	Microwave, 140 °C, 11 min	Fructose	—	94%	170
Betaine/HCl 100 mol%^c, malic acid (5 runs) 100 mol%^c	H2O/ethyl acetate^f	Microwave, 140 °C, 11 min	Fructose	—	35%	170
Betaine/HCl 100 mol%^c, malic acid 100 mol%^c	H2O/ethyl acetate^f	Microwave, 160 °C, 11 min	Sucrose	—	72%	170
Betaine/HCl 100 mol%^c, L-tartaric acid 100 mol%^c	H2O/ethyl acetate^f	Microwave, 160 °C, 11 min	Glucose	—	20%	170
Betaine/HCl 100 mol%^c, L-tartaric acid 100 mol%^c	H2O/ethyl acetate^f	Microwave, 160 °C, 11 min	Sucrose	—	20%	170
EDTA 50 mol%^a	H2O/n-butanol (v/v = 1/4)	160 °C, 2 h	Fructose, 1.8 wt%	—	61%	171
EDTA 50 mol%^a	H2O/MIBK (v/v = 1/4)	160 °C, 2 h	Fructose, 1.8 wt%	—	67%	171
EDTA 50 mol%^a	H2O/MeTHF (v/v = 1/4)	160 °C, 2 h	Fructose, 1.8 wt%	—	66%	171

---

2.3.1. Metal salts. Homogeneous metal salts, which possess a multifaceted nature and diverse properties, play important roles in various chemistries.¹⁴⁴ Certain metal chlorides function predominantly as Lewis acids to catalyze the isomerization of glucose to fructose, while some metal chlorides function as difunctional Lewis and Brønsted acid catalysts for the one-pot conversion of glucose and polysaccharides to HMF. Alkali chlorides serve as salting out agents to promote the transfer of HMF in biphasic systems. Some concentrated molten salt solutions that can dissolve polysaccharides are useful reaction media for the conversion of carbohydrates.

As one of the most outstanding milestones in the production of HMF, Zhang and co-workers first reported that CrCl₂ and CrCl₃ can convert glucose to HMF directly in the medium of EMIMCl ionic liquid, affording HMF yields of approximately 73% and 45%, respectively, while FeCl₃, CuCl, CuCl₂, VCl₃, MoCl₃, PdCl₂, PtCl₂, PtCl₄, RuCl₃, RhCl₃ and H₂SO₄ are not effective in catalyzing this conversion.¹¹⁶ They also observed that EMIMCl itself was capable of converting fructose to HMF, but HMF was unstable in pure EMIMCl under the reaction conditions. Fructose was identified as an the main intermediate for the dehydration of glucose to HMF over CrCl₂ and CrCl₃ in the [EMIM]Cl ionic liquid.¹⁷² Therefore, CrCl₂ and CrCl₃ are not only responsible for the glucose-to-fructose isomerization step, but also play an important role for in the stabilization of HMF in EMIMCl. Compared with EMIMCl and BMIMCl, the use of DBU-based (DBU = 1,8-diazabicyclo[5.4.0]undec-7-ene) ionic liquids as a solvent could further improve the HMF yield and selectivity for the conversion of glucose over CrCl₃.^173,174 In the medium of water, CrCl₃ and AlCl₃ are active for the isomerization of glucose to fructose, FeCl₃ and CuCl₂ are mediocre catalysts, while MgCl₂ is inactive.¹⁷⁵

The mechanism of CrCl₃-catalyzed glucose isomerization to fructose in the medium of the BMMCl ionic liquid was investigated by Jia and co-authors using in situ far-infrared analysis (FIR).¹¹⁷ It was found that the catalytic performances of metal chlorides correlate well with their different coordination structures formed with the oxygen atoms of the substrate. The superior activity of CrCl₃ for the conversion of glucose to HMF in comparison with VCl₃, FeCl₃, and PtCl₂ is mainly attributed to its preferential coordination with the glycolaldehyde group of glucose. Moreover, the relatively weak interactions of Cr(III) with the isolated hydroxy and carbonyl groups enable the good stability of HMF in the ionic liquid/CrCl₃ system. VCl₃ forms strong V–O coordination bonds with oxygens from both glucose and HMF, thus dominantly promoting the formation of humins. PtCl₂ cannot convert glucose efficiently owing to its weak coordination ability, while the poor catalytic performance of FeCl₃ is mainly ascribed to the strong and irreversible coordination bonds formed indiscriminately between FeCl₃ and any oxygen-containing compounds.

¹³C NMR and ¹H NMR were used to elucidate the mechanisms of glucose isomerization, epimerization, and other interconversions over CrCl₃ in aqueous solution.¹⁷⁶ It was found that the isomerization of glucose to fructose proceeds via a C₂–C₁ intramolecular hydride transfer process, despite the wide range of catalytic activity. For the epimerization of glucose to mannose, reverse C₂–C₁ hydride transfer and C₁–C₂ intramolecular carbon shift mechanisms (the Bilik reaction) occur simultaneously, where the former mechanism dominates the conversion process. Besides, it was observed that the metal chlorides, in particular chromium(III), are also active for the isomerization of glucose to sorbose though the C₅–C₁ hydride transfer process, which is similar to that reported for Ti-BEA. Jia et al. reported that in aqueous solution, CrCl₃·6H₂O mainly catalyzed the isomerization of glucose to fructose via the coordination of the chromium species with the end aldehyde group in the chain form of glucose.¹⁷⁷ Choudhary et al. identified that the [Cr(H₂O)₅OH]²⁺ species formed from CrCl₃ hydrolysis is active to catalyze the isomerization of glucose to fructose with an activation energy (64.4 kJ mol⁻¹) lower than many catalysts (Table 1).⁵⁶

In addition to CrCl₂ and CrCl₃, other Cr species have also been used for the conversion of glucose to HMF in ionic liquids. For example, a series of Cr(III)-NHC (NHC = N-heterocyclic carbene) complexes were demonstrated to be active for the direct dehydration of glucose to HMF in the BMIMCl ionic liquid and DMSO.^178,179 Zhang et al. reported that the small, uniform Cr(0)-nanoparticles formed from Cr(CO)₆ either under microwave irradiation (3.6 ± 0.7 nm) or generated in situ via thermolysis (2.3 ± 0.4) during the reaction are strong Lewis acids, which are responsible for the conversion of glucose to HMF.¹⁸⁰

The combination of CrCl₃ and HCl in water/THF/NaCl afforded a maximum HMF yield of 59% from glucose.⁵⁶ In the medium of EMIMCl, BMIMCl DMA-LiCl, or EMIMCl-DMA-LiCl ionic liquid, the combination of CrCl₂ (or CrCl₃) with HCl enabled the direct conversion of cellulose to HMF.^181,182 CrCl₂ and CrCl₃ are also active for the conversion of hemicellulose-derived mannose to HMF in [EMIM]Cl or N,N-dimethylacetamide (DMA), but the conversion efficiency of galactose, galactose, lactose and tagatose are relatively low.¹⁸³ Under microwave heating, the [BMIM]Cl/CrCl₃ catalytic system enabled the fast conversion of glucose, fructose, sucrose, cellobiose, and cellulose, affording HMF yields of 71% and 54% from glucose and cellulose in 30 s and 10 min, respectively.¹⁸⁴

Thus, it can be concluded that CrCl₃ is capable of converting various carbohydrates to HMF efficiently in aqueous solution, organic solvents, ionic liquids, and biphasic systems, and is compatible with other Lewis acid and Brønsted acid co-catalysts. Compared with CrCl₃, other metal chlorides, such as SnCl₄ and AlCl₃ only show good catalytic performances in certain reaction media. Thus, the selection of suitable solvents and co-catalysts is critical for the improvement in catalytic performance.

SnCl₄ can convert glucose, cellobiose, maltose and starch into HMF in the [EMIM]Br and [BMIM]Br ionic liquids, but is ineffective in [EMIM]Cl, [BMIM]Cl, [EMIM]BF₄, [BMIM]BF₄ and DMSO.¹²⁵ Moreover, a biphasic system consisting of glycol dimethyl ether or dimethyl carbonate as the extraction phase and EMIMBr/SnCl₄ as the reaction phase also enabled the efficient conversion of carbohydrate to HMF, obtaining HMF yields between 49.0% and 65.7%. However, the direct conversion of cellulose in the EMIMBr/SnCl₄ catalytic system gives low HMF yield, possible owing to the low solubility of cellulose in EMIMBr. The dissolution of SnCl₄ in water produces an acid solution with a pH of about 1.8 owing to the hydrolysis of SnCl₄ to the mononuclear species Sn(OH)_y(4 − y)⁺ and H⁺.¹⁸⁵ In aqueous solution, SnCl₄ is effective in converting sucrose, maltose, cellobiose, waste cooked rice and bread crusts to monosaccharide and HMF,¹⁸⁶ and converting glucose to HMF.¹⁸⁵ At evaluated temperature (150 °C), SnCl₄ could catalyze the one-pot conversion of inulin and cellulose to levulinic acid, mainly owing to synergistic effect of stannic oxide, H⁺ and Cl⁻ formed from the hydrolysis of SnCl₄.¹⁸⁷ In the medium of ZnCl₂ molten salt hydrate, the use of SnCl₄ also promoted the one-pot conversion of inulin to levulinic acid.¹⁸⁸ Besides, the conversion of cellulose in methanol over a tin(II) triflate catalyst yielded a mixture of α-hydroxy esters, including methyl lactate, methyl vinyl glycolate and methyl-4-methoxy-2-hydroxybutanoate, and methyl levulinate.¹⁸⁹

When AlCl₃ (Table 3) was used to catalyze the conversion of glucose in the medium of EMIMCl, only an HMF yield of 1.6% was obtained.¹⁴⁵ Compared with AlCl₃, aluminum alkyl or alkoxy compounds, AlEt₃ gives a much higher HMF yield. Density functional theory (DFT) calculations indicated that the metal–ligand functionality of the Al(OMe)₃ catalyst exhibit both Lewis acid and Lewis base properties, which are responsible for its good catalytic performance.¹⁹⁰ Saang'onyo et al. demonstrated that the air-stable dimethyl aluminum complexes L^RAlMe₂, which contain (aminomethyl)phenolate (L^R) ligands could give an HMF yield of 58% from glucose using BMIMBr as the solvent.¹⁴⁶

In the AlCl₃–H₂O(NaCl)/THF biphasic system, the Brønsted acid (H⁺) resulting from the hydrolysis of AlCl₃ is not only responsible for the dehydration of fructose to HMF, but also active for a series of undesirable side-reactions, including the direct degradation of glucose and fructose to formic acid, the polymerization of HMF to humins, and the rehydration of HMF to formic acid and levulinic acid.⁵⁷ The Lewis acidic [Al(OH)₂(aq)]⁺ species is the predominant catalytic center for the reversible isomerization of glucose to fructose, but its presence also induces the direct degradation of HMF to formic acid.⁵⁸ Moreover, both Brønsted and Lewis acids can catalyze the polymerization of glucose/fructose to humins and the dehydration of fructose to HMF. A low pH value in aqueous solution improved the rate of AlCl₃-catalyzed glucose dehydration in biphasic reaction systems composed of NaCl saturated aqueous solution and 2-sec-butyl phenol (SBP).¹⁹¹ The combination of maleic acid and AlCl₃ is more effective than that of HCl and AlCl₃ for the conversion of glucose toward HMF and levulinic acid in water.¹⁹² The degradation of HMF to humins using maleic acid and AlCl₃ as the catalyst was reduced by 50% compared with the catalytic system using HCl and AlCl₃ as the catalyst.¹⁹³ The catalytic complex Al-(MA)₂-(OH)₂ (aq) formed from maleic acid and AlCl₃ has a lower activation energy (95 kJ mol⁻¹) than that (149 kJ mol⁻¹) of HCl and AlCl₃ for the glucose-to-fructose isomerization. Simultaneously, the fructose conversion rate is 1.7 times faster than HCl and AlCl₃ with 3 times higher selectivity toward HMF.

Under microwave irradiation, AlCl₃ in water, water–MIBK biphasic solvent and DMSO gave HMF yields of 37.3%, 43.0%, and 52.4%, respectively, within just 5 min.¹⁹⁴ Under the same conditions, the AlCl₃/DMSO system could also convert starch and inulin, attaining HMF yields of 30.6% and 39.6%, respectively. In the medium of DMA, the addition of NaF inhibits the AlCl₃-catalyzed glucose conversion to HMF, while NaI and NaBr promote the reaction significantly.¹⁹⁵ When NaI was used as an additive, the HMF yield improved from 36% to 62% with the glucose conversion increasing from 71% to 86%. In the medium of H₂O/DMSO, AlCl₃ is more effective than other metal chlorides, such as CrCl₃ and SnCl₄ for the conversion of underused hexoses to HMF, obtaining HMF yields of 60% and 35% from hemicellulose-derived mannose and galactose, respectively.^196,197 In the medium of a DMSO/BMIMCl mixture, the direct conversion of cellulose to HMF could be achieved over AlCl₃, giving an HMF yield of 54.9% at 150 °C after 9 h.¹⁹⁸ The ball milling of cellulose and Al₂(SO₄)₃ combined with subsequent catalytic reaction in water/GVL/NaCl gave an HMF yield of 43.5%.¹⁴⁸ In this process, Al₂(SO₄)₃ is not only helpful for the disruption of the hydrogen bonds in the cellulose molecules during mechanochemical pretreatment, but also serves as a Lewis–Brønsted acid bifunctional catalyst in the conversion of cellulose to HMF.

Cai et al. found that although FeCl₃ is not as effective as CrCl₃ and AlCl₃ for the dehydration of glucose to HMF and the dehydration of xylose to furfural, the use of FeCl₃ as a catalyst in water/THF (4 : 1) mixture achieved a furfural yield of 95% with HMF yield of 51% directly from biomass, which is remarkably higher than that obtained with CrCl₃ and AlCl₃.¹⁹⁹ Yan et al. reported that in the medium of water, transition metal chlorides, including FeCl₃, RuCl₃, VCl₃, TiCl₃, MoCl₃, and CrCl₃ are active to convert cellulose to levulinic acid at the evaluated temperature (220 °C), while in the medium of the water/butanol/NaCl biphasic system, these catalysts, in particular RuCl₃, are active for the conversion of cellulose to HMF with a yield as high as 83.3%.²⁰⁰ They proposed that RuCl₃ could promote the disruption of hydrogen bonds and C–O–C bonds in cellulose and the subsequent glucose dehydration to HMF, and the water/butanol/NaCl biphasic system could reduce the undesirable degradation of HMF.

Germanium(IV) chloride (GeCl₄) is effective to catalyze the conversion of fructose, glucose, sucrose, maltose and cellulose to HMF in the [BMIM]Cl ionic liquid.²⁰¹ An HMF yield of 92% was obtained from fructose within 5 min at 100 °C. [BMIM]Cl/GeCl₄ gave HMF yields of 38.4% and 35% from glucose and cellulose, respectively. In addition, the removal of water from the reaction system with 5 Å molecular sieves further improved the HMF yield from 38.4% to 48.4%. The conversion of glucose, cellobiose and cellulose to HMF was also conducted under mild conditions in the BMIMCl/HfCl₄ system (100 °C, 2 h), obtaining HMF yields of 34.5%, 31.7% and 33.4%, respectively.²⁰² It was deduced that the Lewis acidic sites from HfCl₄ could disrupt the glycosidic bonds via binding with a glycosidic oxygen atom in a similar manner as protonic acid, resulting in the hydrolysis of polysaccharides to glucose.

Lanthanide-based catalysts, including CeCl₃, PrCl₃, NdCl₃, DyCl₃, YbCl₃, and Yb(OTf)₃ have been investigated for the dehydration of glucose to HMF in the medium of [EMIM]Cl, [BMIM]Cl, [HMIm]Cl and [OMIm]Cl.²⁰³ Among the tested catalytic systems, [BMIM]Cl/YbCl₃, [HMIm]Cl/YbCl₃, [OMIm]Cl/YbCl₃ and [BMIM]Cl/Yb(OTf)₃ gave HMF yields of around 20%.

Chan et al. reported that WCl₆ is more active than CrCl₂, CrCl₃ and AlCl₃ for the dehydration of fructose to HMF in the [BMIM]Cl ionic liquid at a low reaction temperature (22 °C), affording an HMF yield of more than 60%.²⁰⁴

Jerome reported that the use of DMSO and LiCl saturated aqueous solution as a solvent enabled the direct conversion of Chlorella sorokiniana biomass to HMF over H₂SO₄ catalyst, obtaining an HMF yield (52%) almost 40-fold higher than that obtained with sugar cane bagasse biomass.¹⁴⁹ They also found that the use of LiCl led to an HMF selectivity of up to 98.7% with an HMF yield 2-fold higher than that obtained without the use of metal salts.

Mensah et al. identified Cu(OH)⁺ as an active Lewis-acidic species for glucose isomerization over the Cu(NO₃)₂ catalyst, which follows the intramolecular 1,2-hydride shift mechanism, attaining a maximum fructose yield of 16% in aqueous solution at a pH of 5.3.¹⁵² Zhang et al. found that trace CrCl₂ and CuCl₂ in the EMIMCl ionic liquid play an important role in the conversion of carbohydrates.²⁰⁵ CrCl₂ is responsible for the selective formation and stabilization of HMF, while CuCl₂ is active for depolymerization of cellulose and the degradation of formed HMF.²⁰⁵ Similarly, the combined use of CrCl₃ with slight AlCl₃, which plays an important role in the decomposition of cellulose, achieved an HMF yield (58.3%) comparable to single CrCl₃ (58.4%) with a decrease in the reaction time by nearly 1/3.⁶¹

Under relatively harsh reaction conditions (200 °C), the combination of concentrated NaCl aqueous solution with THF enabled the direct conversion of cellulose to HMF in high yield (58.9%), and LiCl, KCl, NH₄Cl, LiBr, NaBr and KBr also gave HMF yields ranging from 40.7% to 58.9%, while MgCl₂ and CaCl₂ gave similar HMF yields with a faster rate.¹⁵⁶ These results indicate that Cl⁻ itself can promote the hydrolysis of cellulose, isomerization of glucose via the 1,2-hydride shift path, and accelerate the dehydration of fructose. Seawater, which contains Na⁺, K⁺, Mg²⁺, Cg²⁺, Cl⁻ and SO₄²⁻, was more effective for the direct conversion of glucose to HMF than NaCl solution under hydrothermal conditions, giving an HMF yield of 30% with a fructose yield of 4% at 211 °C after 15 min.²⁰⁶ The improved catalytic performance was attributed to the enhanced isomerization of glucose to fructose promoted by seawater. A biphasic reaction system consisting of concentrated seawater (ca. 30 wt% salts) with THF was directly used to produce HMF and furfural from cellulose and hemicellulose fractions, which were obtained by depolymerizing lignin in birch wood over Ru/C, affording an HMF yield of 43.3% with a furfural yield of 40.8%.²⁰⁷ The acidic seawater-catalyzed hydrothermal liquefaction (MHTL) of sugarcane bagasse under microwave irradiation gave the highest HMF yield of 8.1%.²⁰⁸

Cl- and Br-containing salts and ionic liquids play important roles in biorefineries based on HMF. As discussed above, NaCl not only can promote the transfer of HMF from the water phase to organic phase via the salting-out effect,⁶⁰ but also serve as a co-catalyst to improve the fructose dehydration rate (Fig. 10a).⁶⁰ HCl is a widely used acid catalyst for the dehydration of fructose to HMF,¹⁰³ hydrolysis of cellulose to glucose,⁷² and the direct conversion of cellulose to 5-(chloromethyl)furfural (CMF).^209–211 The catalytic effect of Cl⁻, regardless of the cations, is demonstrated by the direct conversion of cellulose to HMF in NaCl, MgCl₂ aqueous solution and seawater.^156,206 Similarly, Br⁻ also plays multiple roles in the dissolution and hydrolysis of polysaccharides, isomerization of glucose to fructose, dehydration of fructose and even oxidation of HMF (Fig. 10b). Liu et al. reported that a concentrated LiBr (65 wt%) solution enabled the efficient depolymerization of cellulose to short-chain oligomers and glucose in the absence of any additional acid catalyst, attaining a short-chain oligomer yield of 90.4% under mild conditions (130 °C) even at an initial cellulose concentration of more than 10 wt%.²¹² Yang et al. reported that [NMP]Br was unable to catalyze the isomerization of glucose to fructose.⁸⁴ In contrast, Yoo et al. found that a concentrated aqueous solution of LiBr (∼62 wt%) afforded a fructose yield of 30% from glucose under mild conditions (120 °C), which is much higher than that with other salts, demonstrating that Br⁻ is active for the isomerization of glucose to fructose in water.¹⁵⁴ The addition of [NMP]Br and [NMP]Cl to DMSO could improve the HMF formation rate significantly from glucose over CrCl₃ owing to the improved fructose dehydration to HMF promoted by [NMP]Br and [NMP]Cl.⁸⁴ For H₂SO₄-catalyzed fructose dehydration in aqueous solution, the combination of sodium with most anions, including chloride, bromide, iodide, mesylate, tosylate, perchlorate monohydrate, and sulfate led to a higher HMF formation rate compared with the salt-free system, while sodium nitrate led to a decrease in the HMF formation rate.²¹³ As co-catalysts, chlorine and bromide salts work differently in different catalytic systems. For example, the use of bromide salts improves the CrCl₃-catalyzed glucose dehydration toward HMF more remarkably in aqueous media than chlorine salt,²¹⁴ which is mainly attributed to the improved fructose dehydration catalyzed by bromide anions. In contrast, chlorine salt can accelerate the acid-catalyzed fructose dehydration in the GVL/water solvent system, while bromide salt does not lead to an increase in the reaction rate in this catalytic system.⁶⁰

Fig. 10 Roles of (a) Cl- and (b) Br-containing salts and ionic liquids in biorefineries based on HMF.

2.3.2. Homogeneous acids and bases. Boric acid (H₃BO₃) was proven to be a metal-free promoter for the dehydration of glucose to HMF, obtaining HMF yields between 14–41% in the medium of imidazolium-based ionic liquids.¹⁵⁷ However, the reuse of H₃BO₃ as a catalyst for fructose dehydration led to a decrease in the HMF yield and selectivity, probably owing to the formation of strong fructose–borate chelate complexes. In a water/MIBK biphasic system, H₃BO₃ and NaCl worked synergistically to convert fructose to HMF, obtaining an HMF yield of 60% even at a fructose loading of 6 wt%.¹⁵⁸ However, the catalytic system is not suitable to catalyze the conversion of glucose, only attaining an HMF yield of 14%. The molten mixtures formed from choline salts and carboxylic acids can also be used as reaction media for the conversion of glucose to HMF over H₃BO₃.¹⁵⁹ The molten mixtures composed of choline chloride and different carboxylic acids gave a poor HMF yield, while the combination of dihydrogen citrate with suitable carboxylic acids, including succinic acid, glycolic acid, malic acid, mandelic acid, salicylic acid and oxalic acid, gave HMF yields between 7–42%. In these molten mixtures, CrCl₃ only afforded an HMF yield of less than 3%.

The combination of CrCl₃ and H₃BO₃ in [BMIM]Cl gave an HMF yield (69.1%) higher than the sum of the HMF yields (1.4% and 60.3%) obtained with CrCl₃ and H₃BO₃, separately, demonstrating the synergistic effect of CrCl₃ and H₃BO₃.¹⁶⁰ Similarly, the combination of tungstophosphoric acid and H₃BO₃ gave an HMF yield (51.9%) remarkably higher than the separate catalyst.¹⁶¹ The combined catalyst was also effective in tetraethyl ammonium chloride (TEAC), affording an HMF yield (47.6%) higher than that obtained using caprolactam (CPL), DMA, N,N-DMF and DMSO as the solvent. The combination of ortho-carboxyl-substituted phenylboronic acids with hydrated MgCl₂ or ionic liquid [BMIM]Cl is also effective to catalyze the direct conversion of glucose to HMF.^162,215 Moreover, the combination of MgCl₂, HCl, 2-methoxycarbonylphenylboronic acid enabled the one-pot conversion of cellulose to HMF under relatively mild conditions.¹⁶² However, further work is needed to confirm the catalytic role of different species in this catalytic system for the conversion of glucose and cellulose to HMF.

Körner et al. investigated the effect of different Brønsted acids, including acetic acid, HCl, H₂SO₄ and H₃PO₄, on the hydrothermal conversion of fructose in water.²¹⁶ They found that the maximum HMF yield is almost independent of the pH or acid type, while a high concentration of H₃PO₄ has a remarkable accelerating effect beyond donating protons. Similarly, Widsten et al. reported that the use of H₃PO₄ as a catalyst in an acetone/water solvent system also enabled moderate HMF and furfural yields from radiata pine hydrolysate.¹⁶⁶ Fang et al. investigated the influence of Brønsted acids (H₂SO₄, HCl and HNO₃) and alkali metal cations (Li⁺, Na⁺, and K⁺) on the conversion of corn stover in the medium of an H₂O/THF system.¹⁵³ Among the tested catalysts, H₂SO₄/Na₂SO₄ gave an HMF yield of 76% at 190 °C for 1 h. They proposed that the presence of alkali metal cations may help inhibit the degradation of HMF not just via the salting-out effect. Similarly, the use of NaHSO₄/ZnSO₄ as a catalyst in the THF/water biphasic system also gave moderate HMF and furfural yields from the hydrolysate obtained via the prehydrolysis of radiata pine wood chips.¹⁶⁶ The biphasic system composed of concentrated NaHSO₄–ZnSO₄ aqueous solution and THF could convert cellulose to HMF directly, affording a high HMF yield of 53% with a cellulose conversion of 96% at 160 °C in 60 min.²¹⁷ However, more work is needed to reveal the role of NaHSO₄ and ZnSO₄ in the conversion of carbohydrate to HMF and the HMF stabilization mechanism.

The HCl-catalyzed degradation of HMF to humins was studied by Tsilomelekis et al. using ATR-FTIR spectroscopy, scanning electron microscopy (SEM) and dynamic light scattering (DLS).²¹⁸ They proposed that in the medium of water humins is formed either though a ring opening mechanism and/or through nucleophilic attack of the carbonyl group at the α- or β-position of the furan ring. Initially, HMF reacts with HMF molecules and other intermediates via aldol condensation and/or etherification reactions, leading to the formation of soluble oligomers. Subsequently, the humin particles grow via nucleophilic attack. Meanwhile, the direct addition of HMF to the primary particles and the aggregation of the primary particles to large humin particles, which are insoluble in water, also occur in water. They also demonstrated that humins are spatially and chemically heterogeneous materials composed of both insoluble macromolecules and small soluble species, and the soluble fraction increases positively with an increase in the donor number of the solvent.²¹⁹ Therefore, the use of DMSO as the co-solvent could suppress the nucleophilic attack, thus leading to humins with a smaller size (∼100 nm) than that (3–4 μm) in neat water. Besides HMF, Maruani et al. also identified the water-soluble oligomers of D-glucose (WSO) as the main intermediates for the formation of humins during the hydrothermal conversion of glucose in H₂SO₄ solution.²²⁰ They suggested that WSO can be inserted into the structure of humins via aldol condensation, leading to the formation of humins.

Supercritical CO₂ was tested as an acid catalyst by Labauze et al. for the dehydration of fructose.²²¹ The two-phase supercritical CO₂–water system gave an HMF yield of 48 mol% after 4 h of reaction at 160 °C under 25 MPa of CO₂, which was slightly higher than that (40%) without the use of a catalyst. When ethylenediaminetetraacetic acid (EDTA) was used as the catalyst, an HMF yield of 89% was obtained from fructose in a water–MIBK biphasic system containing PVP and 2-butanol as phase modifiers.¹⁷¹ After five times reuse, EDTA still gave an HMF yield of 88% with 4.3% loss of catalyst. Levulinic acid was used to catalyze the conversion of lignocellulosic biomass such as pinewood and eucalyptus sawdust to HMF and furfural by Seemala et al. in both mono and biphasic solvent systems.²²² The maximum HMF yield of 7.4% was obtained in the toluene/water (1 : 1) mixture at 180 °C for 15 min.

Feng et al. reported that betaine-based catalysts (BX) prepared from the anhydrous betaine derived from the betaine sugar industry with Brønsted acids, such as HCl, H₂SO₄, (C₆H₅)SO₃H and p-CH₃(C₆H₄)SO₃H, as well as choline-O-sulfate (ChOS) derived from ChCl and concentrated sulfuric acid could be used as renewable and sustainable catalysts for the dehydration of fructose in the medium of H₂O/MIBK and ChCl/H₂O/MIBK biphasic systems, obtaining HMF yields between 61–68%.¹⁶⁹ However, these catalysts are not effective for the conversion of glucose to HMF. The combination of BX with AlCl₃ in the ChCl/H₂O/MIBK biphasic system gave an HMF yield of 64%, which is slightly higher than that obtained (58%) with the AlCl₃/ChCl/H₂O/MIBK biphasic system.

Homogeneous organic Brønsted bases (Table 4), including primary, secondary, and tertiary amines are more effective than inorganic Brønsted bases for the isomerization of glucose to fructose in water via the proton transfer mechanism (Fig. 11a).²²³ In particular, triethylamine gave a high fructose yield (32%) and selectivity (63%), which are comparable to Sn-Beta zeolite, with an activation energy (61 kJ mol⁻¹) lower than that with Sn-Beta zeolite (93 kJ mol⁻¹).⁵⁹ The formation of an enediol intermediate is the rate-limiting step for the isomerization of glucose to fructose. A short reaction time at high temperature and an appropriate pH environment are required to limit the side-reaction and then to afford a high fructose yield. The conversion did not occur at a pH lower than 9, while the fructose yield decreased at a pH higher than 11.5 owing to the degradation of ring fructose and acyclic forms of both sugars (Fig. 11b). Chen et al. conducted a systematic comparison of homogeneous inorganic bases, organic amines, and heterogeneous anion exchange resins for the isomerization of glucose under identical reaction conditions and identified meglumine as the superior homogeneous base, giving a fructose yield of 35% with approximately 80% selectivity, with a relatively low activation energy (74 kJ mol⁻¹).⁵²

Fig. 11 Glucose isomerization to fructose via (a) proton transfer mechanism. Adapted from ref. 59 and 224. (b) Side reactions during the isomerization of glucose over base catalysts. Adapted from ref. 225. (c) Glucose isomerization to fructose via intramolecular hydride shift mechanisms. Adapted from ref. 224.

Table 4 Homogeneous bases for the isomerization of glucose to fructose

Catalyst^a	Solvent	Reaction conditions	Glucose loading^b	Conversion	Fructose yield	Ref.
a Relative to monosaccharide. b Relative to solvent.
Morpholine 10 mol%	Water	100 °C, 30 min	10 wt%	43%	17%	223
Piperazine 10 mol%	Water	100 °C, 30 min	10 wt%	43%	28%	223
Ethylenediamine 10 mol%	Water	100 °C, 30 min	10 wt%	43%	25%	223
Piperidine 10 mol%	Water	100 °C, 30 min	10 wt%	43%	29%	223
Pyrrolidine 10 mol%	Water	100 °C, 30 min	10 wt%	43%	29%	223
Triethylamine 12 mol%	Water	100 °C, 7 min	9.4 wt%	50%	31%	59

---

2.4. Heterogeneous catalysts for the production of HMF from biomass

Recently, much attention has been paid to replacing traditional liquid acids with solid acid catalysts with the aim of meeting the requirement of green and sustainable chemistry.^226,227 Solid acid materials including acid-functionalized carbon-based materials, acid-functionalized polymers, metal oxides, metal phosphates, zeolites, metal–organic frameworks and heteropoly acids (HPAs) have widely investigated as heterogeneous catalysts for acid-catalyzed biomass conversion reactions. The catalytic activity of solid acid catalysts can be manipulated by tuning their structural and acidic properties. The acidic characteristics such as acid type (Brønsted vs. Lewis acidity), concentration, acid intensity and local environment of acid sites have a critical influence on the activity and selectivity for many acid-catalyzed reactions.²²⁸ The structural features including morphology, size, specific surface area, pore distribution, location (intra- vs. extra-crystalline) and interfacial effect not only affect the acidic characteristics, but also influence the host–guest interaction, shape selectivity and reaction pathway. Moreover, the spatial proximity of acid sites and pore confinement (environmental effect) also influence the actual catalytic process. In this section, we not only summarize heterogeneous catalysts from the perspective of reaction efficiency, but also emphasize their stability and reusability.

2.4.1. Metal oxides. Magnesium oxide (MgO) can be used as a heterogeneous catalyst (Table 5) for isomerization reactions.^229,230 The maximal fructose yield of 33.4% with glucose conversion of 44.1% was obtained at 90 °C for 45 min in water. The use of polar aprotic solvents, including DMSO, DMA and sulfolane led to a decrease in the fructose yield. Moreover, MgO can be readily separated and recycled at least four times without obvious loss in its catalytic activity.²³⁰ MgO supported on biochar (MgO-biochar), which was synthesized via the pyrolysis of wood waste with MgCl₂ salt, could also be used as a basic catalyst, giving a fructose yield of 30% at 100 °C for 30 min in water.²³¹ However, the catalytic activity of MgO-biochar decreased remarkably in the recycling experiment, mainly owing to the leaching of Mg.

Table 5 Metal oxide-catalyzed conversion of carbohydrates to HMF and isomerization of glucose to fructose

Catalyst	Catalyst loading^a	Solvent	Reaction conditions	Substrate loading^a	Conversion	Yield^b	Ref.
a Relative to solvent. b HMF yield if unspecified. — Not provided.
MgO	0.5 wt%	H2O	90 °C, 0.75 h	Glucose 4 wt%	44.1%	Fructose 33.4%	229,230
MgO-biochar	1 wt%	H2O	100 °C, 0.5 h	Glucose 10 wt%	37.5%	Fructose 30%	231
MgO-biochar (3 runs)	1 wt%	H2O	100 °C, 0.5 h	Glucose 10 wt%	31.9%	Fructose 15%	231
Bulk-WO3-uc	1 wt%	H2O/THF	120 °C, 4 h	Glucose 1 wt%	100%	15%	232
Meso-WO3-uc	1 wt%	H2O/THF	120 °C, 2 h	Glucose 1 wt%	100%	57%	232
Meso-WO3-300	1 wt%	H2O/THF	120 °C, 3 h	Glucose 1 wt%	100%	56.2%	232
Meso-WO3-600, HCl	1 wt%	H2O/THF	120 °C, 3 h	Glucose 1 wt%	100%	40.8%	232
Meso-WO3-600	1 wt%	H2O/THF	120 °C, 3 h	Glucose 1 wt%	100%	30%	232
Meso-NbW-uc	1 wt%	H2O/THF	120 °C, 4 h	Glucose 1 wt%	100%	55%	232
Meso-TiW-uc	1 wt%	H2O/THF	120 °C, 4 h	Glucose 1 wt%	100%	50%	232
Meso-ZrW-uc	1 wt%	H2O/THF	120 °C, 4 h	Glucose 1 wt%	100%	50%	232
Meso-VW-uc	1 wt%	H2O/THF	120 °C, 2 h	Glucose 1 wt%	100%	23%	232
Nb4W4	2 wt%	H2O	120 °C, 2 h	Glucose 4.5 wt%	36.1%	18.8%	233
Nb7W3	2.9 wt%	2-Butanol/H2O (v/v = 2.5)	140 °C, 2 h	Glucose 0.28 wt%	100%	52%	234
10CuZr	—	1-Butanol/H2O (w/w = 2.75)	200 °C, 5.5 h	Glucose 2.5 wt%	94.5%	33.2%	235
WO3	1 wt%	H2O/THF (v/v = 1/9)	120 °C, 4 h	Glucose 1 wt%	100%	15%	236
NbW5	1 wt%	H2O/THF (v/v = 1/9)	120 °C, 4 h	Glucose 1 wt%	100%	60%	236
TiW5	1 wt%	H2O/THF (v/v = 1/9)	120 °C, 4 h	Glucose 1 wt%	100%	55%	236
HTaWO6	4 wt%	DMSO	140 °C, 0.5 h	Fructose 5 wt%	99%	67%	237
HTaWO6	4 wt%	DMSO	140 °C, 0.5 h	Glucose 5 wt%	7%	2%	237
Nb2O5	1 wt%	H2O/THF (v/v = 1/9)	120 °C, 0.5 h	Glucose, 1 wt%	100%	22%	236
Nb–P–Si oxides	0.5 g in total recirculation reaction line	H2O/isopropanol (v/v = 5), 3 mL min^−1	130 °C, 5 h	Fructose 1.8 wt%	50%	25%	238
TiO2	1 wt%	H2O/THF (v/v = 1/9)	120 °C, 0.5 h	Glucose 1 wt%	100%	0.6%	236
Hybrid-TiO2	2 wt%	H2O	130 °C, 7 h	Glucose 2 wt%	75%	45%	239
Inorganic TiO2	2 wt%	H2O	130 °C, 7 h	Glucose 2 wt%	50%	8%	239
NH4AlOHCO3	15 wt%	DMSO-BMIMCl	120 °C, 4 h	Glucose 2 wt%	—	52.2%	240
γ-Fe2O3	17.2 wt%	DMSO-BMIMCl	120 °C, 4 h	Glucose 2 wt%	—	2.5%	240
γ-Al2O3	11.0 wt%	DMSO-BMIMCl	120 °C, 4 h	Glucose 2 wt%	—	5.1%	240
Natural Al2O3	10 wt%	EMIMBr	140 °C, 3 h	Glucose 10 wt%	100%	24.0%	241
Al2O3-b-0.05	10 wt%	EMIMBr	140 °C, 3 h	Glucose 10 wt%	100%	49.7%	241
Al2O3, DIAION® RCP160M	2 wt%, 1 wt%	H2O–NMP–NaCl/MIBK	120 °C, 8 h	Glucose 1 wt%	90.48%	84.92%	242
CO2, TiO2, ZrO2	5 MPa, 0.7 wt%, 2 wt%	0.34 M NaCl/THF (v/v = 0.25)	200 °C, 3 h	Cellulose 2 wt%	—	48.4%	243
TiO2, ZrO2	0.7 wt%, 2 wt%	0.34 M NaCl/THF (v/v = 0.25)	200 °C, 3 h	Cellulose 2 wt%	—	41.4%	243
Al2O3–TiO2–W	—	NaCl, H2O/THF (v/v = 0.33)	170 °C, 2 h	Glucose 2 wt%	—	70%	244
Ta7W3	2 wt%	H2O/butanol (v/v = 1)	160 °C, 5 h	Glucose 2 wt%	100%	54%	245
ZrO2	0.67 wt%	DMSO	150 °C, 4 h	Glucose 1.67 wt%	76.6%	12.3%	246
B2O3(20 wt%)/ZrO2	0.67 wt%	DMSO	150 °C, 4 h	Glucose 1.67 wt%	83.6%	21.1%	246
B2O3(20 wt%)/Al2O3	0.67 wt%	DMSO	150 °C, 4 h	Glucose 1.67 wt%	89.1%	31.8%	246
B2O3(20 wt%)/ZrO2–Al2O3	0.67 wt%	DMSO	150 °C, 4 h	Glucose 1.67 wt%	90.8%	41.2%	246
Sn20/γ-Al2O3	4 wt%	DMSO/H2O (v/v = 4)	150 °C, 1 h	Glucose 4 wt%	∼99%	27.5% (lactic acid, 16.5%)	150
1.5 wt% SnO2/Al2O3-HWT	2 wt%	DMSO/H2O (v/v = 4)	110 °C, 4 h	Glucose 4.5 wt%	59.7%	Fructose + methylfructoside 30.4%, Mannose 11.0%	247
SA/50 (Si/Al = 50/50)	1 wt%	DMSO/H2O (v/v = 7/3)	160 °C, 2 h	Glucose 3 wt%	70%	38%	248
SA/50 (Si/Al = 50/50)	1 wt%	DMSO/H2O (v/v = 7/3)	160 °C, 2 h	Glucose 3 wt%	68%	37%	248
ASA (Si/Al = 90/10)	1 wt%	DMSO/H2O (v/v = 7/3)	160 °C, 2 h	Glucose 3 wt%	45%	18.8%	248
Cr(III)/CP, H2SO4	6.3 wt%, 1 wt%	DMSO/H2O (v/v = 4)	180 °C	Glucose 3.1 wt%	92.5%	64.7%	249
Cr(III)/CP (5 runs), H2SO4	6.3 wt%, 1 wt%	DMSO/H2O (v/v = 4)	180 °C, 3 h	Glucose 3.1 wt%	82.3%	58.2%	249
Cr(VI)/CP, H2SO4	6.3 wt%, 1 wt%	DMSO/H2O (v/v = 4)	180 °C, 3 h	Glucose 3.1 wt%	88.5%	60.7%	249
TiOSO4	1 wt%	BMIMCl	130 °C, 3 h	Fructose 10 wt%	—	92%	250
TiOSO4	1 wt%	BMIMCl	130 °C, 3 h	Glucose 10 wt%	—	45%	250
TiOSO4	1 wt%	BMIMCl	130 °C, 3 h	Cellobiose 10 wt%	—	52%	250
TiOSO4	1 wt%	BMIMCl	130 °C, 3 h	Cellulose 10 wt%	—	38%	250
TiOSO4 (8 runs)	1 wt%	BMIMCl	130 °C, 3 h	Cellulose 10 wt%	—	20%	250
PAL-SO3H	1 wt%	GVL/H2O (v/v = 95/5)	180 °C, 1 h	Xylose 2 wt%	90.6%	Furfural 87%	251

---

In the medium of pure water, commercially available γ-Al₂O₃ was unable to catalyze the conversion of glucose to fructose, while NaAlO₂ afforded a fructose yield as high as 52.1% with a glucose conversion of 61.4% under mild conditions (60 °C, 45 min).²²⁹ The combination of Al₂O₃ with metal salts is an effective strategy to improve the HMF production efficiency from glucose. García-Sancho et al. compared the catalytic performance of commercial acidic, neutral and basic Al₂O₃ for the dehydration of glucose to HMF in the medium of H₂O/MIBK.²⁵² Among the tested Al₂O₃ samples, the acidic Al₂O₃ exhibited the highest total acidity and the highest catalytic activity. However, the HMF yield under the optimized condition (175 °C, 120 min) was still lower than 20% owing to side reactions. Although CaCl₂ could not catalyze the isomerization of glucose to fructose alone, the introduction of CaCl₂ to the reaction system promoted the formation of α-D-glucopyranose and then shifted the equilibrium toward HMF with an increase in the reaction rate. Owing to the synergistic effect between CaCl₂ and γ-Al₂O₃, an HMF yield of up to 52% with a glucose conversion of 96% was obtained at 175 °C within 15 min. Similarly, Sampath et al. showed the synergy of Al₂O₃ with metal salts, including CuCl₂, CrCl₃ and FeCl₃, for the conversion of glucose to HMF in DMSO.²⁵³ Even at a glucose loading of up to 30 wt%, the HMF yield of 56% could be obtained by combining CuCl₂ and Al₂O₃. Furthermore, although the activity of Al₂O₃ decreased rapidly in the recycling experiment, it could be restored after calcination. Pumrod et al. reported that the combination of Al₂O₃, an ion exchange resin (DIAION® RCP160M) catalyst, and biphasic solvent consisting of water, NMP and MIBK could greatly improve the HMF yield and selectivity compared with the water/MIBK biphasic system.²⁴²

The reaction efficiency of glucose conversion to HMF was considerably improved via the combination of the EMIMBr ionic liquid and an Al₂O₃-based material with low Brønsted acidity and high Lewis acidity (Fig. 12).²³⁶ Different from the aqueous solution-based reaction system, the presence of Brønsted acid sites in EMIMBr accelerated the undesirable side-reaction remarkably, leading to a low HMF yield and selectivity. The simple alkaline treatment could block most of the Brønsted acid sites and weak Lewis acid sites, obtaining Al₂O₃-b-0.05 with a relatively high concentration of Lewis acid sites and low concentration of Brønsted acid sites. The EMIMBr/Al₂O₃-b-0.05 catalytic system could convert glucose to HMF efficiently, attaining an HMF yield of up to 49.7% even at high glucose concentration (10 wt%), which is close to the most efficient homogeneous catalytic systems, such as EMIMCl/CrCl₂ and BMIMCl/CrCl₃. The commercially available TiO₂ products, P25 and A-TiO₂ (anatase), gave HMF yields of 13.8% and 10.9%, respectively, while R-TiO₂ (rutile) gave a lower HMF yield.²⁵⁴ In the medium of a BMIMCl/DMSO mixture, γ-Al₂O₃ gave an HMF yield of only 5.1% from glucose, while ammonium aluminum carbonate hydroxide (NH₄AlOHCO₃) afforded a much higher HMF yield (52.2%).²⁴⁰

Fig. 12 Cooperation effect of a heterogeneous catalyst with ionic liquid EMIMBr for the conversion of glucose to HMF. Reproduced from ref. 236 with permission from Elsevier, Copyright 2018.

Mesoporous Al₂O₃–B₂O₃ with an Al/B molar ratio of 1 : 1, which was prepared from aluminum isopropoxide and boric acid via an evaporation-induced self-assembly method, exhibited abundant Lewis acid sites and undetectable Brønsted acid sites.²⁵⁵ In the medium of DMSO, the mesoporous Al₂O₃–B₂O₃ with an Al/B molar ratio of 1 : 1 gave an optimized HMF yield of 41.4% after 120 min reaction at 140 °C. Han et al. reported that B₂O₃ supported on ZrO₂–Al₂O₃ (B₂O₃(20 wt%)/ZrO₂–Al₂O₃) showed an HMF yield of 41.2% from glucose, which is obviously higher than that obtained with B₂O₃(20 wt%)/ZrO₂ and B₂O₃(20 wt%)/Al₂O₃.²⁴⁶ Marianou et al. reported that an SnO₂ supported on γ-Al₂O₃ (Sn₂O/γ-Al₂O₃) catalyst gave an HMF of 27.5% from glucose in the medium of DMSO/water.¹⁵⁰ They concluded that the introduction of Sn species, either in the oxide or ionic form, on the Al₂O₃ support, could not only catalyze the conversion of glucose to HMF, but also promote the co-synthesis of lactic acid. Yatoo et al. reported that SnO₂ supported on hot water-treated alumina (0.5 wt% Sn/Al₂O₃-HWT) was effective in catalyzing the isomerization of glucose to fructose and methylfructoside (total yield of 30.4%) in methanol.²⁴⁷ Amorphous silica–alumina materials are typical solid acids widely used for the conversion of biomass, but their catalytic performance is limited due to their lower Brønsted acid strength than corresponding crystalline zeolites. Wang et al. showed that flame-made amorphous silica–alumina (labeled as SA/50, Si/Al = 50/50) with a high Al content exhibited a much higher Brønsted acid strength than the conventional amorphous silica–alumina (ASA, Si/Al = 90/10) prepared via the co-precipitation method due to the synergy of penta- and tetra-coordinated aluminum species, leading to enhanced activity for the conversion of glucose to HMF.²⁴⁸

Niobium oxide (Nb₂O₅) and niobium phosphate (NbPO) have been extensively investigated for the conversion of various sugars to HMF. Nakajima et al. reported that the commercial niobic acid (Nb₂O₅·nH₂O) with water-tolerant Lewis acid sites can function as a heterogeneous catalyst for the conversion of glucose, attaining an HMF yield of 12.1% at 120 °C in pure water.²⁵⁶ Na⁺/Nb₂O₅·nH₂O obtained by blocking the Brønsted acid sites on Nb₂O₅·nH₂O with Na⁺ gave a comparable HMF yield with that of Nb₂O₅·nH₂O, suggesting the formation of HMF from glucose predominantly proceeds on the water-tolerant Lewis acid sites. After treating Nb₂O₅·nH₂O with H₃PO₄, ca. 70% of the Brønsted acid sites on H₃PO₄/Nb₂O₅·nH₂O was blocked with PO₄³⁻, thus remarkably suppressing undesirable side reactions and then improving the HMF yield to 52.1%. do Prado et al. reported that the dehydration of fructose to HMF over Nb₂O₅·nH₂O in water and DMSO gave optimized HMF yields of 22% and 47%, respectively.²⁵⁷

Niobium oxides with different structures and morphologies have been investigated to establish the relation among structure, acidity and catalytic performance.^258,259 It was found that the structure of niobium oxides could control their acidity, and both acid type and strength are closely related to the catalytic activity. Few-layer to monolayer or mesoporous Nb₂O₅·nH₂O exhibited abundant Lewis acid sites, which correspond to the low coordinated NbO₅ and in some cases NbO₄ sites derived from the oxygen vacancies in thin and flexible NbO₆ systems.²⁵⁹ Owing to its low surface area and high structural rigidity, the bulk Nb₂O₅·nH₂O only exhibited a few acid sites. HNb₃O₈ possessed a small amount of Brønsted acid sites due to its protonic structure, whereas the anhydrous crystalline H-Nb₂O₅ did not exhibited measurable acid sites. The conversion of fructose to HMF is mainly catalyzed by Brønsted acid, with weaker Brønsted acid sites favoring the selective formation of HMF. Among the Nb-based materials, including bulk HNb₃O₈, few-layer niobium oxide (hy-Nb), monolayer niobium oxide (hy-Nb-TEOA) and mesoporous niobium oxide, mesoporous niobium oxide gave the highest HMF yield from sucrose owing to the balanced Lewis and Brønsted acid concentration with appropriate acid strength.

Nb₂O₅ nanosheets synthesized via the bottom-up hydrothermal method in the presence of NH₄⁺ possess predominantly water-tolerant Brønsted acid sites and exhibit high catalytic activity for the hydrolysis of inulin to fructose in water.²⁶⁰ Similarly, the HNb₃O₈ nanosheets obtained by exfoliating layered HNb₃O₈ also function as a strong Brønsted acid.²⁶¹ In situ exfoliation of layered HNb₃O₈ under microwave irradiation enabled the efficient catalytic conversion of high concentration fructose (10 wt%) to HMF in water, attaining an HMF yield of 55.9% within 18 min.²⁶²

Nb₂O₅·nH₂O and NbPO have been used as catalysts to produce HMF from fructose in a continuous reactor at a reaction temperature in the range of 90 °C to 110 °C by Carniti et al., but the HMF selectivity was lower than 40%.²⁶³ Interestingly, the HMF selectivity increased with an increase in fructose conversion under the continuous flow condition, which is opposite to the tread usually observed in batch mode. They inferred that the easy deposition of secondary products on the catalyst surface led to the relatively low product selectivity at a low fructose conversion. The possibility for the conversion of glucose to HMF over Nb₂O₅ in a flow-through reactor under mild conditions (120 °C) was also attempted by Kawamura et al., but the HMF yield was only 0.47%.²⁶⁴

Nb₂O₅ also plays an important role in the acid-catalyzed depolymerization of disaccharides and the dehydration of other biomass-derived chemicals, including xylose, triose and glycerol. For example, amorphous Nb₂O₅ and mesoporous Nb₂O₅ are active for the dehydration of xylose to furfural.^265,266 Na⁺ exchange treatment did not reduce the original catalytic activity, indicating that the conversion proceeds predominantly on the Lewis acid sites. Mesoporous Nb₂O₅·nH₂O prepared using P123 as the structure-directing agent exhibited similar Lewis and Brønsted acid sites with bulk Nb₂O₅·nH₂O, but the former showed much higher catalytic activity for the hydrolysis of cellobiose than the latter, suggesting that the hydrophilic mesopores are crucial for the hydrophilic reaction.²⁶⁷

Similar to Nb₂O₅, TiO₂ is a typical water-tolerant solid acid catalyst, which has been widely investigated for the conversion of biomass.²⁶⁸ The pristine TiO₂ mainly contains Lewis acid sites, while partially phosphated TiO₂ exhibits higher acidity consisting of both Lewis and Brønsted acid sites. After 2 h reaction over bare TiO₂ at 120 °C, 91.5% of glucose was converted to complex by-products, including polymerized species, with the HMF selectivity of just 8.5%.²⁶⁹ The uncalcined hybrid organic–inorganic anatase (hybrid-TiO₂), which was synthesized via the hydrothermal method in the presence of citric acid, exhibited an HMF yield as high as 45% with the glucose conversion of 75% in monophasic water, which is much higher than that (8%) obtained with inorganic TiO₂ after calcination. This is because the large amount of five-fold coordinatively unsaturated Ti⁴⁺ sites in hybrid-TiO₂ serve as a Lewis acid to catalyze the isomerization of glucose to fructose.²³⁹ Protonated titanate nanotubes, which were synthesized via the hydrothermal treatment of TiO₂ in NaOH aqueous solution followed by H⁺ exchange, serve as a highly active heterogeneous catalyst owing to the presence of both Brønsted and Lewis acid sites.²⁷⁰ The protonated titanate nanotubes gave a much higher HMF yield (14%) from glucose than TiO₂ (2%) in water under mild reaction conditions (120 °C, 0.5 h).

DFT study indicated that the conversion of glucose to HMF over TiO₂ proceeds via a direct stepwise dehydration mechanism instead of the sequential isomerization–dehydration mechanism owing to the cooperative effect between the Lewis acid derived from tetrahedral Ti⁴⁺ and Lewis base of the OH group on the surface of anatase TiO₂.²⁷¹ Isotopic labelling and ¹³C NMR analysis confirmed this stepwise dehydration mechanism (Fig. 13) for glucose conversion over TiO₂ and phosphated TiO₂, and identified 3-deoxyglucosone as the main intermediate.²⁷² Firstly, the dehydration of ring-opening glucose at the C₃ carbon leads to the formation of a highly reactive enol intermediate and the enol intermediate is rapidly converted to 3-deoxyglucosone via keto-enol tautomerization. Secondly, the intramolecular acetalization of 3-deoxyglucosone and subsequent dehydration result in the formation of HMF.

Fig. 13 Stepwise dehydration of glucose to HMF over TiO₂ and phosphated TiO₂. Adapted from ref. 272.

Although mesoporous tungsten oxide without calcination (meso-WO₃-uc) gave a higher HMF yield than bulk tungsten oxide, its catalytic performance decreased remarkably after calcination at 600 °C owing to the loss of Brønsted acid sites.²³² The effectiveness of WO₃ for the conversion of glucose to HMF in a flow-through reactor under mild conditions (120 °C) was investigated by Kawamura et al., but the HMF yield was only 0.32%.²⁶⁴ Periodic DFT calculations suggested that the isomerization of glucose to fructose over tungstite (WO₃·H₂O) is attributed to the synergistic effect of the Lewis acid sites and neighboring proton donors.²⁷³ The Lewis acid site is responsible for the C₂–C₁ H-shift rate-determining step, while the hydrogen-bond network formed on the catalyst surface promotes the concomitant proton-transfer process.^232,236

Doping and recombination of metal oxides is an effective approach to regulate their acidity and alkalinity and then improve their catalytic performance. Periodic DFT calculations suggested that doping of tungstite (WO₃·H₂O) with Nb⁵⁺ and Ti⁴⁺ ions can reduce the overall barrier for the isomerization of glucose to fructose.²⁷³ Mesoporous Nb–W oxides have both Lewis acid sites and Brønsted acid sites, of which the strong Brønsted acid sites is formed via the isomorphous replacement of Nb⁵⁺ ions with higher-valence W⁶⁺ ions.²⁷⁴ Layered-type W–Ti–O possesses Brønsted acid sites and strong Lewis acid sites, which transform to Brønsted acid sites in the presence of water, and weak Lewis acid sites, which are water-tolerable.²⁷⁵ Guo et al. investigated the use of ordered mesoporous Nb_xW_(8−x) oxides, which were prepared via an evaporation-induced self-assembly method followed by calcination at 600 °C, for the conversion of glucose to fructose, mannose and HMF.²³³ The concentration of the Brønsted and Lewis acid distributions in the Nb_xW_(8−x) oxides could be tuned by regulating the Nb/W ratio. Owing to the high concentration of Brønsted and Lewis acid sites, the mesoporous Nb₄W₄ exhibited a relatively low activation energy (90.2 kJ mol⁻¹) for the conversion of glucose, affording an HMF yield of 18.8% with a mannose yield of 5.3% at just 120 °C. The Lewis acid sites not only catalyze the isomerization of glucose to fructose, but also promote the epimerization of glucose to mannose. Wiesfeld et al. reported that mesoporous tungsten oxide without calcination (meso-WO₃-uc) was more active than Nb, Ti, V and Zr-doped WO₃ without calcination for the dehydration of glucose.²³² Córdova-Pérez et al. found that the Al₂O₃–TiO₂–W material with a W concentration of 5 wt% exhibited an acid–basic site molar ratio of 2.35, which is lower than other Al₂O₃–TiO₂–W materials, and the highest HMF yield from glucose, suggesting that the basic sites also play an important role in the conversion of glucose to HMF.²⁴⁴ Similarly, one-dimensional γ-Al₂O₃ modified with 0.5–1 wt% of Nb₂O₅ exhibited strong Lewis acid sites and concentrated Brønsted acid sites, leading to HMF yields between 55.9–59.0% from glucose using DMSO as the solvent.²⁷⁶

In addition to doping and recombination, physical mixing is also an effective approach to regulate the catalytic performance. For example, Jing et al. reported that the combined use of commercial ZrO₂ and TiO₂ catalyst resulted in an HMF yield of 41.4% from cellulose at 200 °C for 3 h, and the introduction of high-pressure CO₂ further improved the HMF yield to 48.4%.²⁴³ Cui et al. designed a chromium-based ceramic material via the adsorption of Cr(III) and Cr(VI) by chitosan nanoparticles followed by moulding with SiO₂, Al₂O₃, Na₂SiO₃, MgO, and CaO (mass ratio of 10 : 5 : 5 : 5 : 2).²⁴⁹ They found that the combined use of chromium-based ceramic powder as the catalyst and H₂SO₄ as a co-catalyst enabled the efficient dehydration of glucose to HMF in a water/DMSO system, attaining HMF yields of 64.7% and 60.7% with Cr(III)/CP and Cr(VI)/CP, respectively.

The sulfonation of metal oxides is an effective approach to improve their catalytic performance for the dehydration of fructose to HMF. SnO₂–ZrO₂ prepared via the sol–gel method gave an HMF yield of around 54% from fructose, while the sulfated SnO₂–ZrO₂ obtained by impregnating SnO₂–ZrO₂ with H₂SO₄ solution gave an HMF yield of up to 75%.²⁷⁷ The catalytic activity of sulfated SnO₂–ZrO₂ remained almost unchanged in the recycling experiment for five times. The anchoring of MoO₃ and SO₄²⁻ on TiO₂ improved their acidity remarkably, especially the quantity of weak to moderate acid sites on TiO₂/MoO₃-30, thus leading to the improved conversion of cellulose to a series of products, including cellobiose, glucose, fructose, HMF, formic acid and levulinic acid.²⁷⁸ Compared with commercial Nb₂O₅, the sulfated mesoporous Nb₂O₅ exhibited much higher Brønsted acidity without an appreciable amount of Lewis acid sites, leading to an HMF yield of up to 88% from fructose in DMSO.²⁷⁹

2.4.2. Metal hydroxides. As a typical solid base, Mg–Al hydrotalcite (Table 6) is effective in catalyzing the isomerization of glucose to fructose via the proton transfer mechanism (Fig. 11a). Weak base sites and relatively low reaction temperatures favor the isomerization of glucose to fructose, while strong base sites and relatively low reaction temperature lead to the degradation of sugars to byproducts, including lactic acid, glyceric acid, glycolic acid and formic acid.^280,281 Therefore, rehydrated Mg–Al hydrotalcite with abundant weak base sites formed from exfoliation and vertical breaking of layers in the hydrotalcite structure exhibited higher catalytic activity than the as-synthesized Mg–Al hydrotalcite and calcined Mg–Al hydrotalcite. The catalytic performance of Mg–Al hydrotalcite could be further improved by increasing its surface weak base sites via the sonication-assisted rehydration of hydrotalcite.²⁸² Although commercial hydrophobic hydrotalcite gave a lower fructose yield and glucose conversion than hydrophilic hydrotalcites in carbonate or hydroxy form, hydrophobic hydrotalcite afforded a superior fructose selectivity of 92%.²⁸³ The catalyst deactivation was mainly owing to the adsorption of byproducts, which were formed by the retro-aldolization of glucose and fructose, especially the latter. In addition, the leaching of Mg²⁺ ions in hydrotalcites was caused by the acidic degradation products such as lactic acid. Yabushita et al. reported that the use of ethanol as a solvent could not only improve the fructose yield to 56% for the isomerization of glucose over hydrotalcite (Mg/Al ratio = 3) owing to the shift of the reaction equilibrium, but also enabled the reuse of the catalyst with a high catalytic performance at least three times.²²⁵ Upare et al. reported that the combination of hydrotalcite (Mg/Al ratio = 2) with 1-butanol could give a fructose yield higher than 50% with selectivity of 80% even at a glucose concentration of up to 10 wt%.²⁸⁴ In this catalytic system, the Mg²⁺ leaching from the hydrotalcite was negligible and the high purity glucose (91%) and fructose crystals (95%) could be readily separated from the solvent by subsequent cooling and filtration by virtue of the different solubility of glucose and fructose, which is more convenient than the conventional chromatographic separation method.

Table 6 Isomerization of glucose to fructose and conversion of glucose to HMF over metal hydroxides

Catalyst	Catalyst loading^a	Solvent	Reaction condition	Substrate loading^a	Glucose conversion	Yield^b	Ref.
a Relative to solvent. b Fructose yield if unspecified. — Not provided.
Commercial hydrotalcite	0.71 wt%	Water	90 °C, 24 h	Glucose 10 wt%	29%	27%	283
Hydrophilic hydrotalcites in carbonate form	0.71 wt%	Water	90 °C, 24 h	Glucose 10 wt%	41%	31%	283
Hydrophilic hydrotalcites in hydroxy form	0.71 wt%	Water	90 °C, 24 h	Glucose 10 wt%	71%	29%	283
Hydrotalcites (Mg/Al ratio = 3)	0.71 wt%	Ethanol	120 °C, 6 h	Glucose 0.71 wt%	70%	56%	225
Hydrotalcites (Mg/Al ratio = 3)	0.71 wt%	H2O	120 °C, 2 h	Glucose 0.71 wt%	28%	16%	225
Hydrotalcites (Mg/Al ratio = 3)	0.71 wt%	Methanol	120 °C, 2 h	Glucose 0.71 wt%	55%	43%	225
Hydrotalcites (Mg/Al ratio = 3)	0.71 wt%	1-Propanol	120 °C, 2 h	Glucose 0.71 wt%	56%	48%	225
Hydrotalcites (Mg/Al ratio = 3)	0.71 wt%	DMSO	120 °C, 2 h	Glucose 0.71 wt%	24%	16%	225
Hydrotalcites (Mg/Al ratio = 3)	0.71 wt%	N,N-DMF	120 °C, 2 h	Glucose 0.71 wt%	48%	35%	225
Hydrotalcites (Mg/Al ratio = 2)	10 wt%	1-Butanol	120 °C, 5 h	Glucose 10 wt%	62%	51%	284
Hydrotalcites (Mg/Al ratio = 2)	10 wt%	1-Butanol	100 °C, 5 h	Glucose 10 wt%	35%	32%	284
Hydrotalcites (Mg/Al ratio = 2)	10 wt%	H2O	100 °C, 5 h	Glucose 10 wt%	54%	30%	284
Hydrotalcites (Mg/Al ratio = 2)	10 wt%	Ethanol	90 °C, 5 h	Glucose 10 wt%	47%	32%	284
Hydrotalcites (Mg/Al ratio = 2)	10 wt%	DMF	100 °C, 5 h	Glucose 10 wt%	43%	28%	284
Hydrotalcites (Mg/Al ratio = 2)	10 wt%	GVL	100 °C, 5 h	Glucose 10 wt%	40%	26%	284
Hydrotalcites (Mg/Al ratio = 2)	10 wt%	Toluene	100 °C, 5 h	Glucose 10 wt%	50%	15%	284
Hydrotalcite, Amberlyst-15	3.3 wt%, 3.3 wt%	N,N-DMF	100 °C, 3 h	Glucose 3.3 wt%	73%	HMF 42%	285
Hydrotalcite, Amberlyst-15	3.3 wt%, 3.3 wt%	N,N-DMF	100 °C, 3 h	Cellobiose 3.3 wt%	52%	HMF 30%	285

---

The combination of the solid base Mg–Al hydrotalcite and solid acid Amberlyst-15 in N,N-DMF enabled the direct conversion of glucose and cellobiose to HMF via orthogonal tandem catalysis, affording HMF yields of 42% and 30% with glucose conversions of 73% and 53%, respectively, under the optimized conditions.²⁸⁵ Although humins were formed on the surface of the hydrotalcites, these catalysts could be reused three times without an obvious decrease in their catalytic performance after washing with N,N-DMF followed by drying in vacuo.

2.4.3. Metal phosphates. Various metal phosphates have been investigated for the conversion of carbohydrates to HMF (Table 7). The conversion of carbohydrates to HMF over CrPO₄, MnPO₄ and FePO₄ follows a similar mechanism (Fig. 14).^286–288 Xu and co-authors reported that the CrPO₄ catalyst is more effective than CrCl₃, H₃PO₄, NaHPO₄ and the simultaneous use of CrPO₄ and NaHPO₄ in the medium of H₂O/THF(NaCl), attaining HMF yields of 83%, 63% and 37% from fructose, glucose and cellulose, respectively.²⁸⁶ The pH of the aqueous phase was measured to be 2.48, suggesting the hydrolysis of CrPO₄ in the reaction system. Therefore, the authors inferred that the partially hydrolysis of CrPO₄ in the reaction system produces H⁺ and [Cr(H₂O)₅OH]²⁺. The depolymerization of cellulose to glucose and dehydration of fructose to HMF are mainly catalyzed by H⁺, while the isomerization of glucose to fructose is mainly catalyzed by the [Cr(H₂O)₅OH]²⁺ Lewis acid sites. In their following studies, Xu et al. showed that the CrPO₄ catalyst could also catalyze the efficient production of furfural and HMF from wheat straw, obtaining a furfural yield of 67% and HMF yield of 32% at 180 °C for 90 min.²⁸⁷ The reusability of the CrPO₄ catalyst was tested using the xylose dehydration reaction as an example. After four consecutive runs, the furfural yield decreased from 88% to 47%, possibly owing to the partial dissolution of the catalyst at the evaluated reaction temperature.

Fig. 14 Reaction pathway of biomass to HMF over FePO₄, MnPO₄ and CrPO₄. Adapted from ref. 286–288.

Table 7 Conversion of carbohydrates to HMF over metal phosphates

Catalyst	Catalyst loading^a	Solvent	Condition	Substrate loading^a	Conversion	Yield^b	Ref.
a Relative to solvent. b HMF yield if unspecified. — Not provided.
NbPO	10 wt%	H2O	130 °C, 6 h	Fructose, 1 wt%	87%	70%	289
NbPO	10 wt%	H2O/DMSO (w/w = 2/3)	130 °C, 6 h	Fructose, 1 wt%	95%	90%	289
NbPO	10 wt%	H2O	130 °C, 6 h	Fructose, 10 wt%	86%	54%	289
Mesoporous ZrPO	0.3 wt%	H2O	155 °C, 6 h	Glucose, 0.5 wt%	83.8%	46.6%	290
α-ZrPO	1.7 wt%	DMSO	120 °C, 3 h	Fructose, 10 wt%	97.2%	54.3%	291
Mesoporous ZrPO	1.7 wt%	DMSO	120 °C, 3 h	Fructose, 10 wt%	95.6%	52.8%	291
Amorphous ZrPO	1.7 wt%	DMSO	120 °C, 3 h	Fructose, 10 wt%	95.3%	54.7%	291
Amorphous ZrPO-SO4^2−	1.7 wt%	DMSO	120 °C, 3 h	Fructose, 10 wt%	100%	73.4%	291
ZrPO	—	DMSO/H2O (v/v = 9)	120 °C, 3 h, LHSV: 1 h^−1	Fructose, 5 wt%	94.5%	56.3%	292
ZrPO	—	DMSO/H2O (v/v = 9)	160 °C, 3 h, LHSV: 1 h^−1	Glucose, 5 wt%	72%	32%	292
HfO(PO4)x	0.6 wt%	NaCl–H2O/THF	175 °C, 2.5 h	Glucose, 2 wt%	99.7%	90.5%	293
HfO(PO4)x	0.6 wt%	NaCl–H2O/THF	190 °C, 4 h	Cellulose, 2 wt%	100%	69.8%	293
MnPO	0.5 wt%	H2O (5.4 M NaCl)/THF (v/v = 1 : 3)	160 °C, 1.5 h	Glucose, 1.3 wt%	96%	59%	288
MnPO	0.5 wt%	H2O (5.4 M NaCl)/THF (v/v = 1 : 3)	170 °C, 2 h	Cellulose, 10 wt%	—	44%	288
MnPO (3 runs)	0.5 wt%	H2O (5.4 M NaCl)/THF (v/v = 1 : 3)	160 °C, 1.5 h	Glucose, 1.3 wt%	82%	30%	288
FePO4	1.3 wt%	H2O (6 M NaCl)/THF (v/v = 1 : 3)	140 °C, 0.25 h	Glucose, 2.5 wt%	97.8%	23.1%	294
FePO4	0.05 wt%	H2O (6 M NaCl)/THF (v/v = 1 : 3)	140 °C, 0.25 h	Fructose, 2.5 wt%	99.9%	71.5%	294
FePO4	1.3 wt%	H2O (6 M NaCl)/THF (v/v = 1 : 3)	160 °C, 1 h	Cellulose, 2.5 wt%	87.4%	48.0%	294
FePO4	1.3 wt%	H2O (6 M NaCl)/THF (v/v = 1 : 3)	140 °C, 0.25 h	Camellia oleifera shell, 2.5 wt%	—	7.8%	294
Branch-like FePO4	0.1 wt%	H2O (1.1 M NaCl)/THF (v/v = 1 : 3)	160 °C, 1 h	Methyl cellulose, 0.25 wt%	—	∼48%	295
Flower-like FePO4	0.1 wt%	H2O (1.1 M NaCl)/THF (v/v = 1 : 3)	160 °C, 1 h	Methyl cellulose, 0.25 wt%	—	∼44%	295
Sphere FePO4	0.1 wt%	H2O (1.1 M NaCl)/THF (v/v = 1 : 3)	160 °C, 1 h	Methyl cellulose, 0.25 wt%	—	∼37%	295
Amorphous FePO4	0.1 wt%	H2O (1.1 M NaCl)/THF (v/v = 1 : 3)	160 °C, 1 h	Methyl cellulose, 0.25 wt%	—	∼33%	295
SnPO	0.33 wt%	H2O (NaCl) /MIBK	Microwave, 150 °C, 0.33 h	Glucose, 0.33 wt%	80%	50%	296
SnPO	0.5 wt%	H2O(NaCl)/THF	175 °C, 1 h	Glucose, 1.25 wt%	98%	61%	297
SnPO	5 wt%	EMIMBr	120 °C, 3 h	Glucose, 20 wt%	94.6	52.3%	254
HAP	10 wt%	EMIMBr	130 °C, 4 h	Glucose, 10 wt%	86.5%	31.4%	49
Sn-HAP	10 wt%	EMIMBr	130 °C, 6 h	Glucose, 10 wt%	97.5%	52.6%	49
Al-HAP	10 wt%	EMIMBr	130 °C, 2 h	Glucose, 10 wt%	81.6%	32.5%	49
Sn/Al-HAP	10 wt%	EMIMBr	130 °C, 3 h	Glucose, 10 wt%	96.0%	65.6%	49
Sn/Al-HAP	20 wt%	EMIMBr	130 °C, 3 h	Glucose, 10 wt%	—	46.0%	49
Sn/Al-HAP	10 wt%	EMIMBr	130 °C, 3 h	Starch, 10 wt%	—	46.5%	49
5% P–SnO2	0.95 wt%	GVL/H2O (v/v = 20)	180 °C, 1.5 h	Glucose, 0.95 wt%	—	46.4% (furfural 18.9%)	298
SnO2	0.95 wt%	GVL/H2O (v/v = 20)	180 °C, 1.5 h	Glucose, 0.95 wt%	85.1%	19.3% (furfural 7.2%)	298
SnPCP@MnO2-PDA	1 wt%	DMSO	150 °C, 5 h	Glucose, 4 wt%	92.2%	55.8%	299
SnPCP@MnO2-PDA	1 wt%	H2O/THF	150 °C, 5 h	Glucose, 4 wt%	73.2%	41.2%	299
SiO2	4.2 wt%	H2O/acetone (v/v = 5)	160 °C, 1.67 h	Glucose, 3.3 wt%	27.6%	6.0%	300
H3PO4–SiO2	4.2 wt%	H2O/acetone (v/v = 5)	160 °C, 1.67 h	Glucose, 3.3 wt%	63.1%	39.5%	300
Unsupported FePO4	4.2 wt%	H2O/acetone (v/v = 5)	160 °C, 1.67 h	Glucose, 3.3 wt%	65.4%	35.3%	300
H3PO4–SiO2–FePO4 (0.15)	4.2 wt%	H2O/acetone (v/v = 5)	160 °C, 1.67 h	Glucose, 3.3 wt%	99.9%	76.3%	300
H3PO4–SiO2–FePO4	4.2 wt%	H2O/acetone (v/v = 5)	160 °C, 1.67 h	Glucose, 3.3 wt%	65.8%	38.8%	300

---

Similarly, the inexpensive manganese phosphate catalyst (MnPO₄) afforded HMF yields of 59% and 44% from glucose and cellulose, respectively.²⁸⁸ The use of FePO₄ as a catalyst for the production of HMF was investigated the Zuo's group.^301,302 They found that the FePO₄ catalyst was active for the conversion of fructose, glucose, cellulose, and Camellia oleifera shell to HMF with moderate to high yields. They proposed that is FePO₄ hydrolyzed at the evaluated temperature to H₃PO₄, which is responsible for the hydrolysis of cellulose and dehydration of fructose and Fe(OH)₃, which are responsible for the isomerization of glucose to fructose, and the catalyst could be recovered owing to the reaction of H₃PO₄ with Fe(OH)₃ upon cooling. They observed that although the amorphous FePO₄ transformed to the crystalline form after the reaction, the catalyst could be reused in five reaction cycles without loss in its catalytic performance.³⁰² Huang et al. reported that the silica-supported phosphate and iron phosphate heterogeneous catalyst (H₃PO₄–SiO₂–FePO₄(0.15)) exhibited a higher HMF yield (76.3%) from glucose than the unsupported FePO₄.³⁰⁰

Aluminium phosphate-based materials are effective heterogenous catalysts to catalyze the dehydration reaction. For example, Zhang et al. reported that silicoalumino phosphate (SAPO-34) afforded an HMF yield as high as 93.6% from glucose in the medium of water/GVL at 170 °C for 40 min.³⁰³ In addition, they found that the presence of an appropriate amount of water (5–15 wt%) is beneficial for the selective formation of HMF. Romo et al. reported that a SAPO-34/5A zeolite bead catalyst, which was synthesized by growing SAPO-34 zeolite crystals on zeolite 5A beads, exhibited furfural and HMF yields of 45% and 20% from xylose and glucose in the medium of water, respectively.³⁰⁴

Similar to Nb₂O₅·nH₂O, porous niobium phosphate (NbPO), which was prepared via the hydrothermal method using cetyltrimethyl ammonium bromide (CTAB) as the template, could also be used as a water-tolerant solid acid.³⁰⁵ The concentration and ratio of Brønsted and Lewis acid sites could be tuned by regulating the pH value in the synthetic process. Among the NbPO synthesized at different pH, the porous NbPO synthesized at pH = 2 with the highest amount of Brønsted acid sites was the most active for the dehydration of fructose to HMF, obtaining the highest HMF yield of 45% in water.³⁰⁶ In contrast, the porous NbPO synthesized at pH = 7 exhibited the highest amount of Lewis acid sites with a certain amount of Brønsted acid sites, leading to the highest HMF yield of 33.6% from glucose in pure water. The NbPO modified by 0.1 and 10 M HCl solutions gave HMF yields of around 8% after 24 h reaction at 120 °C.³⁰⁷

Hierarchically porous titanium phosphate nanoparticles, which were synthesized from titanium isopropoxide and orthophosphoric acid using P123 as a structure directing agent, exhibited HMF yields of 17%, 22%, and 44% from cellulose, glucose and fructose, respectively, using DMA/LiCl as the solvent under microwave irradiation.³⁰⁸ The use of partially phosphated TiO₂ for the selective production of HMF from glucose was investigated by several groups. Atanda et al. reported that nanosized phosphated TiO₂ with a bulk Ti/P molar ratio of 10.1 in the medium of water/n-butanol mixture gave the highest HMF yield of 81% with the glucose conversion of 97% at 175 °C in 3 h.³⁰⁹ The same catalyst in a water/THF biphasic system gave an HMF yield of 83%, which is comparable to that in the water/n-butanol mixture.³¹⁰ The addition of N-methyl-2-pyrrolidone (NMP) to the water/THF biphasic system could further improve the conversion efficiency, affording high HMF yields of 90%, 94%, and 98% from glucose, cellobiose and sucrose, respectively. When mechanocatalytic depolymerized cellulose was used as the feedstock, an HMF yield of up to 74.7% was obtained in this catalytic system.³¹¹ Rao et al. reported that the phosphated TiO₂ catalyst, which was synthesized via a one-pot sol–gel method only gave an HMF yield of 53% with a glucose conversion of 98% in the water/THF system. The above studies all speculated that the conversion of glucose to HMF proceeds via the widely considered reaction pathway involving the isomerization of glucose to fructose and fructose dehydration steps, but the reaction pathway has not been confirmed by direct evidence to date. In contrast, Noma et al. reported that both TiO₂ and phosphoric acid-treated TiO₂ gave a low HMF yield (≤34%) with a selectivity lower than 70% under mild conditions (120 °C, 4 h) in the medium of water, water/butanol, water/MIBK or water/2-sec-butylphenol.²⁷² Also, these biphasic systems have been widely demonstrated to be effective to improve the HMF yield over other catalysts by inhibiting side reactions. Moreover, their following work demonstrated that the phosphorylation of TiO₂via a simple impregnation method could only anchor two phosphates per nm², which limits the selective formation of HMF, while photo-assisted phosphorylation could introduce about three phosphates per nm², thus affording an HMF selectivity of 81% with the glucose conversion of 84% at 135 °C for 4 h in the medium of water/2-sec-butylphenol.³¹² Isotopic labelling and ¹³C NMR analysis demonstrated that the conversion of glucose over phosphated TiO₂ proceeds via the stepwise dehydration mechanism with 3-deoxyglucosone as the main intermediate (Fig. 13), which is similar to that over TiO₂.^272,312 They proposed that the low HMF selectivity over the bare TiO₂ is attributed to the intense intermolecular reactions between the adsorbed glucose and/or 3-deoxyglucosone molecules due to the strong adsorption of glucose onto the surface of TiO₂ (Fig. 15), while the phosphorylation of TiO₂ can inhibit these intermolecular side reactions by reducing the adsorption of glucose, and then improve the HMF selectivity.³¹² In summary, the catalytic performance of TiO₂ (section 2.4.1) and titanium phosphate are relatively poor, while partially phosphated TiO₂ ³¹² and citric acid-modified TiO₂ ²³⁹ show much better catalytic performances. Therefore, the precise control of the surface composition of TiO₂ and the use of appropriate solvents are crucial for the improvement of the HMF production efficiency.

Fig. 15 Reaction pathway of glucose over TiO₂ with and without surface phosphates. Adapted from ref. 312.

Tin phosphate exhibited excellent catalytic activity for the conversion of glucose to HMF in water-containing solvents, including a water/DMSO mixture, water(NaCl)/MIBK, and water(NaCl)/THF biphasic system.^296,297,313 Zhang et al. reported that the tin phosphate generated in situ from SnCl₄ and (NH₄)₂HPO₄ afforded HMF yields of 71% and 33% from fructose and glucose, respectively.³¹³ Under microwave-assisted heating, large-pore mesoporous tin phosphate (SnPO) exhibited excellent catalytic activity for the direct conversion of fructose, glucose, sucrose, cellobiose, and cellulose in water(NaCl)/MIBK, affording HMF yields of 32–77%.²⁹⁶ The conversion of glucose over SnO₂ in a GVL/water mixture gave a low yield of HMF and furfural owing to the concomitant retro-aldol process, while phosphated SnO₂ gave an HMF yield of 46.4% with a furfural yield of 18.9%.²⁹⁸

A high concentration glucose (10 wt%) could be converted into HMF effectively in the medium of EMIMBr ionic liquid using SnPO as a heterogeneous catalyst, obtaining an HMF yield if up to 58.3%.²⁵⁴ Compared with bulk SnO₂ and small pore tin phosphate (sSnPO), the SnPO catalyst exhibited a lower Brønsted acid concentration and higher Lewis acid concentration, resulting from the tetra-coordinated Sn⁴⁺ sites, which are the main active species for the isomerization of glucose to fructose. The excellent conversion efficiency was mainly attributed to the synergistic effect of the SnPO catalyst and EMIMBr ionic liquid. Under the same reaction conditions, CrPO only gave an HMF yield of 14.1%. Similarly, tin(IV) phosphonate (SnBPMA) and zirconium phosphonate (ZrBPMA), which were prepared from SnCl₄·5H₂O or ZrOCl₂·8H₂O with N,N-bis(phosphonomethyl)aminoacetic acid, were also investigated for the conversion of fructose, sucrose and inulin to HMF in the EMIMBr ionic liquid by Ning et al.³¹⁴ An HMF yield of up to 86.5% was obtained from fructose in the EMIMBr/SnBPMA system within 1.5 h at 100 °C, with a reaction rate higher than that obtained with EMIMBr alone. SnBPMA and ZrBPMA gave HMF yields of 40.1% and 48.7% from sucrose and inulin, respectively. However, their catalytic performance for glucose conversion was not investigated. Similarly, the highly efficient transformation of glucose was achieved over an Al and Sn co-modified hydroxyapatite catalyst (Al/Sn-HAP) in the medium of EMIMBr, affording HMF yields of 70.5% and 46.6% at a glucose loading of 10 and 40 wt%, respectively.⁴⁹ The Al/Sn-HAP catalyst exhibited a remarkably lower activation energy (68.4 kJ mol⁻¹) than HAP (110.5 kJ mol⁻¹), Al-HAP (80.7 kJ mol⁻¹) and Sn-HAP (79.2 kJ mol⁻¹) for the conversion of glucose (Table 1). Moreover, the combination of the Al/Sn-HAP catalyst with EMIMBr enabled the efficient transformation of starch to HMF via tandem catalysis (Fig. 16), obtaining an HMF yield of 46.5% at a relatively high substrate loading.

Fig. 16 Conversion of starch to HMF via tandem catalysis. Adapted from ref. 49.

Alkaline-earth metal phosphates, such as calcined calcium and strontium phosphate, only afforded HMF yields of around 20% from glucose in hot compressed water at 220 °C for 5 min.³¹⁵ Zirconium phosphates (ZrPO) showed moderate catalytic activity for the conversion of fructose and glucose to HMF in the medium of DMSO or water/DMSO.^291,292

Mesoporous tantalum oxide and tantalum phosphate were tested as heterogeneous catalysts for the conversion of glucose to HMF using a water/MIBK biphasic system as the reaction medium by Jiménez-Morales et al.^316,317 Mesoporous tantalum oxide was prepared via the sol–gel method from the Ta(OC₂H₅)₅ precursor using the triblock co-polymer Pluronic L-121 as the structure-directing agent, while the mesoporous tantalum phosphate was prepared by the reaction of tantalum penta-ethoxide with phosphoric acid using hexadecyltrimethylammonium bromide as a surfactant in absolute ethanol. Pyridine-FTIR analysis showed that the concentration of Lewis acid sites in the mesoporous tantalum oxide was 98.1 μmol g⁻¹, while the Brønsted acid sites disappeared after heating at 125 °C under vacuum owing to the removal of coordinated water molecules. Compared with the mesoporous tantalum oxide, the mesoporous tantalum phosphate exhibited much higher acidity (1.48 mmol NH₃ g⁻¹) with a Brønsted acid concentration of 309 μmol g⁻¹ and Lewis acid concentration of 97 μmol g⁻¹. The mesoporous tantalum oxide gave an HMF yield of 23% at a glucose conversion of 69% under the optimized conditions (175 °C, 1.5 h). In contrast, the mesoporous tantalum phosphate gave an HMF yield of up to 32.8% at the glucose conversion of 56.3% under the optimized conditions (170 °C, 1 h), and fructose was not observed over this catalyst owing to the rapid dehydration of fructose to HMF over the high concentration of Brønsted acid sites. The leaching of phosphorus or tantalum species was not detected during the reaction process and the catalytic activity could be maintained for three catalytic runs, demonstrating the high stability of mesoporous tantalum oxide and tantalum phosphate. Yang et al. reported that commercial tantalum hydroxide (Ta₂O₅·nH₂O, TA sample) and TA-p (TA treated with 1 M phosphoric acid solution at room temperature for 52 h and then calcined at 300 °C) gave HMF yields of 62% and 90% from fructose in the medium of water/2-butanol mixture, respectively.³¹⁸ Moreover, an HMF yield as high as 58% with the glucose conversion of 70% was obtained over TA-p at 160 °C in 140 min.

Cao et al. prepared a series of hafnyl phosphates, HfO(PO₄)_x (x = 1.0, 1.5 and 2.0), as heterogeneous catalysts for the conversion of cellulose and other sugars in the medium of the NaCl–H₂O/THF biphasic system.²⁹³ An HMF yield as high as 69.8% was achieved from cellulose over HfO(PO₄)₂ after 240 min reaction at 190 °C. The conversion of fructose, glucose, cellobiose, sucrose, starch and inulin in this reaction system gave high HMF yields of 94.8%, 90.5%, 79.3%, 86.6%, 75.3% and 80.4%, respectively, whereas the direct conversion of wheat straw only gave an HMF yield of 18.6%. They concluded that the introduction of the phosphate group could deactivate the unselective Lewis acid sites in the catalysts and then suppress the HMF decomposition to levulinic acid and formic acid. Moreover, the deposition of humins on the catalyst surface was also remarkably inhibited owing to the poorer HMF adsorption capability (9.3 mg g⁻¹) of HfO(PO₄)₂. Therefore, the catalyst could maintain its catalytic activity after five catalytic cycles.

In the batch reactor, cerium phosphate afforded a best HMF yield of 52% with the selectivity of 93% from fructose in the medium of water, while in the flow reactor the catalyst only gave an HMF yield of 24% with a selectivity higher than 95%.³¹⁹ The use of biphasic systems consisting of dimethyl carbonate (DMC) and water as the reaction medium enabled an HMF yield of up to 70% with the selectivity of 93.3% from fructose over cerium phosphate.¹¹⁴ Moreover, the reusability of the cerium phosphate catalyst in water/DMC was better than that in pure water.

Jia et al. reported that the combined use of phosphorus pentoxide (P₂O₅) and metal chlorides, such as NiCl₂ and NaCl, could improve the HMF yield and selectivity remarkably, attaining an HMF yield of 75% with 85% selectivity from fructose after 30 min reaction in DMSO at 80 °C. Under the same conditions, P₂O₅, H₃PO₄ and NiCl₂ gave low HMF yields and fructose conversion, demonstrating the synergy effect between P₂O₅ and metal chlorides for the dehydration of fructose to HMF.³²⁰

A series of studies on homogeneous metal phosphates indicated that HPO₄²⁻ plays an important role in the isomerization of glucose. In the aqueous solution of NaH₂PO₄ and Na₂HPO₄ at pH = 7.5, glucose can be isomerized to fructose with a yield of 30%.¹⁶⁷ The isomerization proceeds via an enediol anion intermediate, similar to the base-catalyzed process. In addition, the aqueous solution of NaH₂PO₄ and Na₂HPO₄ is also effective for the isomerization of ribose and lyxose to rare keto-pentoses, ribulose and xylulose, which are important feedstock for the synthesis of commodities and fine chemicals.³²¹ Ma et al. reported that KH₂PO₄ is active to convert sucrose to HMF in the MIBK/H₂O biphasic system, attaining an HMF yield of up to 70% under the optimum conditions.¹⁶⁸ Even at a sucrose loading of up to 17.2 wt%, this catalytic system still afforded an HMF yield of 62.5% (higher than 50%), also indicating that HPO₄²⁻ could catalyze the isomerization of glucose into fructose. The incorporation of a suitable organic moiety in metal phosphates not only can make the catalyst more flexible and robust, but also impart more hydrophobicity on the catalyst surface.³²² Therefore, the development of organic–inorganic hybrid metal phosphonates may further broaden the portfolio of metal phosphate catalysts.

2.4.4. Zeolites. Zeolite-based catalysts with different types (Brønsted, Lewis, or both), amount, strength and location of acid sites have been widely used for biomass valorization.³²³ As an important milestone in the isomerization of glucose to fructose, Moliner et al. reported that Sn-Beta zeolite can be used as a water-tolerating heterogeneous Lewis acid to catalyze the isomerization of glucose to fructose, affording a fructose yield of 31% at the glucose loading of 10 wt% under mild conditions.³²⁴ Different from the proton transfer mechanism over homogeneous and heterogeneous Brønsted base catalysts, the isomerization of glucose to fructose over Sn-Beta zeolite proceeds though the intramolecular hydride shift mechanism, which is analogous to metalloenzymes (Fig. 11c).^224,324 As shown in Fig. 17, four types of Sn species may be formed in the Sn-Beta zeolite. The isolated framework Sn site, which is mononuclear Sn⁴⁺ incorporated in the framework of zeolite, and extra-framework tin oxide (SnO₂) can be distinguished by Sn-NMR, UV-vis absorption and FTIR spectroscopy using pyridine as a probe.^325,326 Generally, framework Sn sites can be formed at a relatively low Sn loading, while a high Sn loading leads to the formation of extra-framework SnO₂ and even partial blockage of the zeolite micropores.³²⁵

Fig. 17 Schematic representation of the structure of Sn-Beta zeolite. Adapted from ref. 325 and 326.

The framework-isolated Sn sites have been proven to be strong Lewis acid sites responsible for most Lewis acid-catalyzed reactions. The open and closed tetrahedral Sn sites in the framework of zeolite can be quantified by FTIR spectroscopy using deuterated acetonitrile as the probe.³²⁶ The open Sn sites with proximal silanol groups are responsible for the isomerization of glucose to fructose via an intramolecular hydride shift process.^327,328 Tin silsesquioxane with a neighboring SiOH group favors the selective isomerization of glucose to fructose via the intramolecular H-shift mechanism. In contrast, the methyl-ligated tin silsesquioxane without a neighboring SiOH group, a homogeneous model of Sn-Beta, which does not contain “open” sites, prefers the conversion of glucose to mannose via the intramolecular C-shift mechanism, with a much lower reaction rate.^328,329 These results also confirm the critical role of the open Sn sites in the formation of fructose. In the medium of methanol, the framework Sn sites mainly dominant the epimerization of glucose to mannose by a Lewis acid-mediated intramolecular carbon shift process.³³⁰

SnO₂ can be located either at the external surface of the zeolite crystal or within the hydrophobic channel of the zeolite, with the catalytic performance depending on the reaction type and chemical environment.³³⁰ The isomerization of glucose to fructose can proceed over the SnO₂ cluster located within the hydrophobic channel of the zeolite via a base-catalyzed proton-transfer process, in either water or methanol. In water, the SnO₂ particles supported on the external surface of the zeolite crystal or amorphous silica cannot catalyze the isomerization, but in methanol the isomerization can proceed via the proton-transfer process. These results indicate that the contact of Sn sites with bulk water may suppress the isomerization process.

The synthetic process and conditions have an important influence on the Sn sites in Sn Beta zeolite. Compared with hydrophilic zeolite (Sn-Beta–OH), hydrophobic the Sn-Beta zeolite synthesized in fluoride medium (Sn-Beta–F) exhibited a 50 times higher first-order rate constant for glucose isomerization to fructose in water, mainly owing to the reduced kinetic inhibition of the open tetrahedral Sn sites with a few coordinated water molecules.³²⁶ To avoid the use of HF, a two-step synthesis process involving the dealumination of commercial Beta zeolite in acid followed by grafting of the Sn precursor in isopropanol under reflux conditions was developed to synthesize an Sn Beta zeolite.^325,331 Compared with the conventional synthetic method, this method enabled the fast preparation of highly active Sn Beta zeolite with an Sn loading of up to 2 wt% using a cheap Sn-precursor and industrially available Beta zeolite as the feedstock.

Since Sn-Beta zeolite lacks Brønsted acid sites for dehydration, the combination of Sn-Beta zeolite with a Brønsted acid is usually required for the direct conversion of glucose to HMF (Table 8) in the medium of water or organic solvents.³³² The combination of Sn-Beta zeolite (or Ti-Beta) with HCl in a water/THF/NaCl biphasic system gave an HMF selectivity of over 70%, suggesting that Sn-Beta can catalyze the isomerization of glucose in an acidic environment.³³³ When GVL, GHL, THF : MeTHF (1 : 1) and THF were used as the solvent, the combined use of Amberlyst-70 and Sn-Beta as the solid acid catalyst in a salt-free reaction system resulted in HMF yields of 59%, 55%, 60% and 63%, respectively, which are comparable to or higher than that obtained with a homogeneous Lewis acid (AlCl₃) and Brønsted acid (HCl).³³⁴ In addition to obtaining a high HMF yield, this catalytic system could also reduce the use of the homogeneous catalyst and salt. Moreover, the separation and purification of HMF can be readily achieved by distillation owing to the low boiling point of THF.

Table 8 Zeolites for the conversion of carbohydrates to HMF

Catalyst	Catalyst loading^a	Solvent	Reaction condition	Substrate loading^a	Conversion	Yield^b	Ref.
a Relative to solvent. b HMF yield if unspecified. — Not provided.
HY zeolite	5 wt%	[bmpyrr][Cl]0.5[NTf2]0.5	80 °C, 5 h	Fructose, 5 wt%	—	73%	335
HY zeolite	5 wt%	[bmpyrr][Cl]0.5[NTf2]0.5	120 °C, 4 h	Glucose, 5 wt%	—	38%	335
HY zeolite	5 wt%	[bmpyrr][Cl]0.5[NTf2]0.5	120 °C, 3 h	Sucrose, 5 wt%	—	62%	335
SBA-15-SO3H	6.6 wt%	DMSO	120 °C, 1 h	Fructose, 3.3 wt%	100%	96%	336
SBA-15/5-aminoisophthalic acid	0.83 wt%	DMSO	Microwave, 135 °C, 20 min	Fructose, 1.1 wt%	—	74%	337
SBA-15/5-aminoisophthalic acid	0.83 wt%	DMSO	Microwave, 135 °C, 20 min	Glucose, 1.1 wt%	—	64%	337
SBA-15/5-aminoisophthalic acid	0.83 wt%	DMSO	Microwave, 135 °C, 20 min	Cellulose, 1.1 wt%	—	42%	337
2SZ@SBA-15-SO3H-NH2	2 wt%	BMIMCl	Microwave, 120 °C, 3 h	Cellulose, 5 wt%	78.1%	42%	338
Mesoporous organosilica-SO3H	1 wt%	BMIMCl	100 °C, 5 h	Avicel, 5 wt%	—	10.5% (glucose 40.1%, cellobiose 22.4%)	339
ZSM-5	0.5 wt%	H2O	150 °C, 1 h	LGO, 1 wt%	65.3%	33.7%	340
ZSM-5	0.67 wt%	H2O/THF/NaCl	170 °C, 1 h	Remnant algal biomass, 4 wt%	65.3%	34.4%	341
Mordenite	0.5 wt%	H2O	150 °C, 1 h	LGO, 1 wt%	16.1%	4.2%	340
Nb(0.05)-Beta 18	0.6 wt%	H2O	180 °C, 24 h	Glucose, 3.6 wt%	41.7%	17.2%	342
Nb(0.05)-Beta 18	0.6 wt%	NaCl/H2O/MIBK	180 °C, 12 h	Glucose, 3.6 wt%	97.3%	82.1%	342
Al/MCM-41	0.8 wt%	ChCl/H2O/MIBK	195 °C, 1.5 h	Glucose, 2.5 wt%	—	57%	343
Al/MCM-41	0.8 wt%	ChCl/MIBK	195 °C, 1.5 h	Glucose, 2.5 wt%	—	40%	343
Al/MCM-41	0.8 wt%	H2O/MIBK	195 °C, 1.5 h	Glucose, 2.5 wt%	—	30%	343
Cr/MCM-41	0.8 wt%	ChCl/H2O/MIBK	195 °C, 1 h	Glucose, 2.5 wt%	—	44%	343
Zr/MCM-41	0.8 wt%	ChCl/H2O/MIBK	195 °C, 2 h	Glucose, 2.5 wt%	—	42%	343
Zr-salen-MCM-41	2.5 wt%	DMSO	140 °C, 4 h	Fructose, 5 wt%	—	92.0%	344
Zr-salen-MCM-41	2.5 wt%	DMSO	140 °C, 4 h	Glucose, 5 wt%	—	36.6%	344
H-β-zeolite	5–10 wt%	H2O (HOAc)	180 °C, 1 h	Chitosan	—	15.3–28.2%	345
2SZ@SBA-15	1.5 wt%	iPrOH/DMSO (v/v = 9)	120 °C, 10 h	Glucose, 3.6 wt%	89.3%	57.3%	346
5 wt% CeO2-2SZ@SBA-15	1.5 wt%	iPrOH/DMSO (v/v = 9)	120 °C, 10 h	Glucose, 3.6 wt%	93.9%	66.5%	346
5 wt% CeO2@SBA-15	1.5 wt%	iPrOH/DMSO (v/v = 9)	120 °C, 10 h	Glucose, 3.6 wt%	65.6%	14.1%	346
5 wt% CeO2-2SZ@SBA-15	1.5 wt%	iPrOH/DMSO (v/v = 9)	120 °C, 10 h	Cellobiose, 3.6 wt%	85.4%	53.8%	346
5 wt% CeO2-2SZ@SBA-15	1.5 wt%	iPrOH/DMSO (v/v = 9)	120 °C, 10 h	Starch, 3.6 wt%	75.5%	45.2%	346
5 wt% CeO2-2SZ@SBA-15	1.5 wt%	iPrOH/DMSO (v/v = 9)	120 °C, 10 h	Cellulose, 3.6 wt%	—	ND	346
Al0.33Nb0.67/SBA-15	0.33 wt%	H2O/MIBK (v/v = 1/2)	170 °C, 6 h	Cellulose, 1.6 wt%	93.0%	10.4%	347
Al0.33Nb0.67/SBA-15	0.33 wt%	H2O/MIBK (v/v = 1/2)	170 °C, 6 h	Cellulose, 1.6 wt%	93.4%	55.7%	347
SO4^2−/ZrO2@HZSM-5	1 wt%	DMSO	195 °C, 1.5 h	Glucose, 3 wt%	—	23.6%	348
SO4^2−/ZrO2@HZSM-5	1 wt%	ChCl/DMSO	195 °C, 1.5 h	Glucose, 3 wt%	—	41.3%	348
SO4^2−/ZrO2@HZSM-5	1 wt%	NaCl/H2O/DMSO	195 °C, 1.5 h	Glucose, 3 wt%	—	61.1%	348
SO4^2−/ZrO2@HZSM-5	1 wt%	NaCl/H2O/ethyl acetate	195 °C, 1.5 h	Glucose, 3 wt%	—	40.7%	348
SO4^2−/ZrO2@HZSM-5	1 wt%	NaCl/H2O/ethyl acetate	195 °C, 1.5 h	Glucose, 3 wt%	—	54.7%	348
Al-Mont	1.29 wt%	NaCl/H2O/THF	180 °C, 2.5 h	Glucose, 1.3 wt%	∼97%	80.4%	349
Al-Mont	1.29 wt%	NaCl/H2O/THF	180 °C, 2.5 h	Fructose, 1.3 wt%	—	98.5%	349
Al-Mont	1.29 wt%	NaCl/H2O/THF	180 °C, 2.5 h	Sucrose, 1.3 wt%	—	86.1%	349
Al-Mont	1.29 wt%	NaCl/H2O/THF	180 °C, 2.5 h	Inulin, 1.3 wt%	—	81.6%	349
Al-Mont	1.29 wt%	NaCl/H2O/THF	180 °C, 2.5 h	Starch, 1.3 wt%	—	60.1%	349
M-Mont (M = Fe, Zn, Zr, Ca)	1.29 wt%	NaCl/H2O/THF	180 °C, 2.5 h	Glucose, 1.3 wt%	—	18–24%	349
Nb-MMT-900	0.8 wt%	NaCl/H2O/MIBK	170 °C, 3 h	Glucose, 1.2 wt%	99%	70.52%	350
Nb-MMT-900	0.8 wt%	H2O	170 °C, 3 h	Glucose, 1.2 wt%	63.0%	18.4%	350
Nb-MMT-900	0.8 wt%	NaCl/H2O/MIBK	170 °C, 3 h	Cellulose, 1.2 wt%	54.68%	10.51%	350
HY-1 zeolite	0.7 wt%	H2O/GBL (v/v = 9/105)	150 °C, 50 min	Fructose, 5 wt%	99.8%	69.2% (furfural 7.7%)	351
HY-2 zeolite	0.7 wt%	H2O/GBL (v/v = 9/105)	150 °C, 50 min	Fructose, 5 wt%	98.8%	39.1% (furfural 28.6%)	351
HY-3 zeolite	0.7 wt%	H2O/GBL (v/v = 9/105)	150 °C, 50 min	Fructose, 5 wt%	98.5%	21.4% (furfural 37.8%)	351
Hβ zeolite	1 wt%	H2O/GBL (v/v = 5/95)	150 °C, 1 h	Fructose, 5 wt%	99.9%	14.0% (furfural 50.25%)	352
Hβ zeolite	1 wt%	H2O/NMP (v/v = 5/95)	150 °C, 1 h	Fructose, 5 wt%	97.4%	83.3% (furfural 6.7%)	352
β zeolite, HCl	0.27 wt%, 200 ppm	H2O	150 °C, 1 h	Glucose, 3.3 wt%	—	32.4%	332
SAPO-34/5A zeolite bead	1.2 wt%	H2O	190 °C, 3 h	Glucose, 2 wt%	—	20%	304
[BMIM]Br encapsulated H-MOR	1.1 wt%	H2O (10 wt% NaCl)	170 °C, 3 h	Glucose, 3.3 wt%	75%	39%	353
[BMIM]Br encapsulated H-MOR (2–5 runs)	1.1 wt%	H2O (10 wt% NaCl)	170 °C, 3 h	Glucose, 3.3 wt%	42–45%	Around 25–30%	353

---

Owing to the excellent activity of EMIMBr for the dehydration of fructose, an Sn-containing zeolite could also act as a heterogeneous catalyst for the efficient conversion of glucose into HMF in the medium of EMIMBr ionic liquid without the use of an additional Brønsted acid. Sn-MCM-41 gave an HMF yield as high as 70% at 110 °C within 4 h, while MCM-41, Ti-MCM-41, Cr-MCM-41, Zr-MCM-41 and Sn-Beta only gave HMF yields of 4%, 5%, 11%, 6% and 15%, respectively.³⁵⁴ Sn-MCM-41 could be readily recovered and reused for the dehydration of glucose with a slight decrease in its catalytic activity. However, the reason why Sn-MCM-41 exhibited an excellent catalytic performance but Sn-Beta showed a poor catalytic performance was not investigated in this study. Jia and co-authors compared the catalytic performance of Sn/MCM-41, Al/MCM-41 and Cr/MCM-41 for the conversion of mannose into HMF in DMSO.³⁵⁵ They found that Sn/MCM-41 exhibited better catalytic activity than the other two catalysts, resulting in an HMF yield of 45% with glucose conversion of 88% at 150 °C within 60 min. The as-synthesized Sn/MCM-41 catalyst gave HMF yields of 42% and 28% from glucose and galactose, respectively. The control experiment using nano-SnO₂ as the catalyst attained a low mannose conversion, suggesting that the Sn species in the framework of Sn/MCM-41, instead of the surface SnO₂ are the main catalytic center for sugar conversion.

The Sn montmorillonite (Sn-Mont) catalyst prepared from Ca-Mont and SnCl₄·5H₂O aqueous solution by ion-exchange contains both Lewis acid and Brønsted acid sites, resulting from Sn⁴⁺ and partially hydrolyzed Sn–OH groups, respectively.³⁵⁶ The Sn-Mont catalyst gave an HMF yield of up to 53.5% from glucose at 160 °C for 3 h using a monophasic THF/DMSO mixture as the solvent. Moreover, the one-pot conversion of cellulose to HMF in high yield (39.1%) was also achieved using the THF/H₂O-NaCl biphasic system as the reaction medium. Al-doped montmorillonite (Al-Mont) exhibited higher catalytic activity than Fe-, Zn-, Zr- or Ca-doped montmorillonite, obtaining HMF yields of 80.4%, 60.1% and 81.6% from glucose, starch and inulin, respectively.³⁴⁹ Similarly, a niobium-loaded montmorillonite (Nb-MMT) catalyst bearing both Lewis and Brønsted acid sites, which was readily prepared via a cation-exchange method, gave HMF yields of 70.52% and 10.51% from glucose and cellulose, respectively.³⁵⁰

Catalyst deactivation is a serious and universal issue limiting the application of zeolites in biomass conversion. The stability of zeolites depends on many factors, including their framework type (MOR, BEA, MFI or FAU), preparation process (hydrothermal synthesis or post-modification), hydrophobicity, hydrophilicity, solvent (water or alcohol) and reactant. Generally, the deactivation of tin-containing zeolites mainly results from the amorphization of the zeolite framework and leaching, restructuring or fouling of the active tin sites.³⁵⁷ A series of studies showed that the Sn-Beta zeolite deactivates rapidly both in pure water ^358,359 and alcohols.^360,361 In contrast, Guo et al. reported that the use of a dioxane/water mixture (containing 5 wt% water) as the reaction medium not only greatly improved the stability and reusability of the Sn-Beta zeolite, but also increased the fructose yield to 41.5%.³⁵⁸ Thermogravimetric analysis (TGA) and FTIR analysis showed that the hydrophilization of Sn-Beta is inhibited in the dioxane/water mixture, which can explain the good stability of Sn-beta in this medium.

Similar to Sn-Beta, Ti-substituted zeolite is also a typical Lewis acid, which is active for the isomerization of glucose in water, and the catalytic activity of Ti beta zeolite is lower than that of Sn-Beta owing to its higher activation energy.^324,362 Ti-Beta catalyzes the isomerization of glucose to sorbose via the intramolecular C₅–C₁ hydride shift mechanism and the isomerization of glucose to fructose via the intramolecular C₂–C₁ hydride shift mechanism simultaneously in both water and methanol, with water favoring the generation of fructose and methanol favoring the generation of sorbose.³⁶³ Cordon et al. reported that confining Ti sites within the twelve-membered ring (12-MR) micropores of Ti-Beta could greatly improve the reaction rate of D-glucose isomerization to D-fructose and L-sorbose simultaneously, and a further decrease of micropore size (10-MR pores in Ti-MFI and Ti-CON) resulted in a decrease in the glucose isomerization rate, possible owing to the intrapore reactant diffusion restrictions.³⁶⁴ Moreover, tighter confinement in Ti zeolites led to a remarkable increase in the selectivity toward L-sorbose over D-fructose, attaining an L-sorbose/D-fructose ratio of more than 10 on Ti-MFI.

Xu et al. reported that Cr/β-zeolite is highly effective for the conversion of a series carbohydrates, including fructose, glucose, sucrose, cellobiose and starch to HMF in the H₂O/THF/NaCl biphasic system.³⁶⁵ The Lewis and Brønsted acid sites on the Cr/β-zeolite surface work synergistically to catalyze the isomerization of glucose to fructose and dehydration of fructose, obtaining a maximum HMF yield of 72% after 1.5 h reaction at 150 °C. Nb-Modified Beta-zeolite (Nb(0.05)-Beta 18) in the NaCl/H₂O/MIBK biphasic system gave an HMF yield of 79.8% at the glucose conversion of 97.4% at 180 °C for 12 h.³⁴²

Different from Sn-containing zeolite materials, zeolites with a high Al content usually catalyze the direct conversion of fructose and glucose to HMF owing to the coexistence of sufficient Brønsted acid sites and Lewis acid sites.³⁶⁶ Jia et al. investigated the catalytic performance of MFI, BEA, and Y zeolite for the dehydration of fructose to HMF in different solvents including DMSO, acetone, GVL, and propylene carbonate (PC)/water (1 : 1 v/v) biphasic system under microwave irradiation.³⁶⁷ They found that in the medium of water, the HMF yield was independent of the particle size of MFI zeolite, and the highest HMF yield of 49.2% with the fructose conversion of 72.4% was obtained over the hydrophobic Y zeolite in DMSO. Jiménez-Morales et al. reported that the mesoporous 10 wt% aluminium-doped MCM-41 silica (10Al-MCM-41) in a water(NaCl)/MIBK biphasic system showed a glucose conversion of 98% and HMF yield of 63% at 195 °C in just 30 min.³⁶⁸ Using the same reaction conditions, the protonated zeolites H-Beta, H-ZSM-5 and H-Y attained HMF yields of 56%, 42% and 42%, respectively.^369,370 Jiménez-Morales et al. also investigated the use of Zr-doped MCM-41 silica (Zr-MCM-41) for the conversion of glucose to HMF using similar reaction conditions, obtaining an HMF yield of 23%.³⁷¹ Feng et al. investigated a series of MCM-41-supported metal catalysts, including Al, Cr, Fe, Cu, Zn and Zr, for the catalytic conversion of glucose to HMF, with the Al/MCM-41 catalyst in the medium of ChCl/H₂O/MIBK attaining the highest HMF yield of 57% after 1.5 h reaction at 195 °C.³⁴³

An Al-containing zeolite was also effective in catalyzing the conversion of glucose to HMF in the medium of [BMIM]Cl ionic liquid. Hβ-zeolite with a BEA structure and an appropriate Si/Al ratio of 25 showed higher catalytic activity than HY-zeolite, H-mordenite, Hβ-zeolite and HZSM-5, probably owing to the balanced density and strength of Lewis acid sites and Brønsted acid sites.³⁷² An HMF yield of up to 50.3% with 80.6% glucose conversion was obtained over the Hβ-zeolite at 150 °C for 50 min. The catalytic system was also effective for the conversion of other carbohydrates, including sucrose, maltose, cellobiose, starch and cellulose, obtaining HMF yields of 67.6%, 47.8%, 49.3%, 45.4% and 46.5%, respectively.

Velaga et al. reported that H-mordenite zeolite with mesoporosity, which was synthesized using a seed-assisted and template-free method, exhibited a maximum HMF yield of 66% with the glucose conversion of 76% at 180 °C for 1 h in a water(NaCl)/MIBK biphasic system.³⁷³ Mordenites modified with NH₄Ac, NH₄Cl or NH₄F were investigated as heterogeneous catalysts for the direct conversion of glucose into HMF using the BMIMBr ionic liquid or water–acetone/ethyl acetate biphasic system as the reaction medium.³⁷⁴ The NH₄Cl-modified mordenite afforded 64% HMF yield with 97% glucose conversion in BMIMBr ionic liquid. In contrast, the highest HMF yield (50%) from the biphasic system was obtained with the mordenite treated with 1 M NH₄Cl and 2.4 M NH₄F, mainly owing to the presence of strong Lewis acid sites.

Alkali metal chlorides, including LiCl, NaCl and KCl, have been demonstrated to be effective to improve the catalytic performance of ionic liquid/zeolite catalytic systems for the conversion of cellulose to HMF.³⁷⁵ An H-form zeolite in [EMIM]Cl ionic liquid only gave a maximum HMF yield of 18.5% after 150 min reaction at 160 °C. The combination of zeolite with LiCl gave the highest HMF yield of 70.3% at 160 °C for 30 min, while zeolite/KCl and zeolite/LiCl afforded the highest total furans yields of 82.0% and 81.2%, respectively, at 140 °C for 75 min. At a relatively mild reaction temperature, zeolite/NaCl-KCl was more effective than zeolite/NaCl and zeolite/KCl, attaining the highest HMF yield of 58.2% with total furan yield reaching 66.2% at 120 °C.

Xia et al. prepared a series of Fe/β-zeolite catalysts with different Fe/Al ratios via a liquid ion-exchange method to catalyze the conversion of glucose to HMF.^376,377 Although the introduction of Fe species to the zeolite did not increase the concentration of Lewis and Brønsted acid sites obviously, the Fe/β-zeolite catalyst attained a higher HMF yield than the β-zeolite. Both the HMF yield and Lewis/Brønsted acid ratio vs. Fe/Al ratio exhibited a “volcano relationship”. The control experiment suggested that the extra-framework isolated Fe species bound to the framework Al sites are the main active sites to catalyze the isomerization of glucose to fructose.

In addition to their direct use as catalysts, zeolites are also excellent supports for active components to improve their catalytic performance and stability. Osatiashtiani et al. prepared a ZrO₂/SBA-15 catalyst by grafting a sulfated zirconia monolayer on SBA-15.³⁷⁸ The Brønsted acid, Lewis acid and base sites mainly depended on the film thickness and sulfate loading. The presence of base and Lewis acid sites in bilayer ZrO₂/SBA-15 contributed to the isomerization of glucose to fructose. Compared with nonporous sulfated zirconia, the bilayer ZrO₂/SBA-15 afforded a 3-fold enhancement in the HMF production rate and better hydrothermal stability. Similarly, Zhang et al. synthesized a 5 wt% CeO₂-2SZ@SBA-15 catalyst, bearing basic sites, Lewis and Brønsted acid sites simultaneously, by incorporating ceria and sulfated zirconia (SZ) into SBA-15 via the layer-by-layer grafting method followed by the wet impregnation method.³⁴⁶ Consequently, 5 wt% CeO₂-2SZ@SBA-15 gave a much higher HMF yield (66.5%) from glucose than 2SZ@SBA-15 and 5 wt% CeO₂@SBA-15. Besides, the grafting of basic sites, such as NH₂, and Brønsted sites, such as SO₃H, was also widely attempted to improve the conversion of carbohydrates to HMF.³³⁸ Feng et al. reported that SO₄²⁻/ZrO₂ supported on HZSM-5 zeolite (SO₄²⁻/ZrO₂@HZSM-5) was active for the catalytic dehydration of glucose into HMF, obtaining an HMF yield of 61.1%.³⁴⁸

The octahedrally coordinated extra-framework Al sites in H-BEA-25 zeolite could catalyze the isomerization of glucose to fructose effectively with an activation energy ranging between that reported for Ti-BEA and Sn-Beta.³⁷⁹ Although Al-based zeolites are not as effective as Sn-Beta zeolites to isomerize glucose to fructose in water, Al-based zeolites can catalyze the isomerization of glucose to fructose via a two-step process (Fig. 18).³⁸⁰ In the first step, glucose is isomerized to fructose in the medium of alkyl alcohol followed by instantaneous reaction with alkyl alcohol via etherification, leading to the formation of alkyl fructoside. In the second step, fructose is obtained by the hydrolysis of alkyl fructoside in water. Zeolite H-USY (Si/Al = 6) showed higher catalytic activity than the beta, ZSM-5, mordenite and H-USY zeolites with a higher Si/Al ratio, attaining the highest fructose yield of 55% at 120 °C for 1 h. A single-unit-cell Sn-MFI nanosheet with exclusively framework Sn sites was also used for the two-step glucose isomerization process, resulting in a fructose yield (65%) remarkably higher than that obtained with Sn-MCM-41 and Sn-Beta.³⁸¹

Fig. 18 Isomerization of glucose to fructose over zeolite via a two-step process. Adapted from ref. 380.

The conversion pathway of fructose over zeolite catalysts can be controlled by manipulating the solvent effect or tuning the channel of zeolite. For example, the use of Hβ zeolite in the medium of γ-butyrolactone (GBL)/water mixture led to the reversible tetrahedral-octahedral framework aluminum transformation and promoted the selective formation of furfural (50.25% yield) from fructose (Fig. 19).³⁵² The synergistic effect of GBL with HY-3 zeolite with apertures of 7.4 Å favored the generation of acyclic fructose from cyclic fructose (8.6 Å), while the Brønsted acid sites in the channels of HY-zeolite catalyzed the decomposition of acyclic fructose to xylose and formaldehyde via the selective cleavage of the C–C bond, and the following xylose dehydration, resulting in the formation of furfural.³⁵¹

Fig. 19 Conversion of fructose to furfural over zeolite. Adapted from ref. 351 and 352.

2.4.5. Heteropolyacids. As one type of promising alternative to mineral acids, heteropolyacids, featuring strong Brønsted acidity and redox property, have been widely used for the hydrolysis of polysaccharides and dehydration of fructose.³⁸² For example, Tang et al. found that mesoporous TiO₂–Al₂O₃ nanofiber-supported Keggin-type phosphotungstic acid (fibrous HPW/TiO₂–Al₂O₃) exhibited higher catalytic activity and stability than powdery HPW/TiO₂–Al₂O₃, obtaining a maximum HMF yield of 80.3% (Table 9) from fructose.³⁸³ Similarly, phosphomolybdic acid (H₅PMo₁₂O₄₁) supported on mesoporous-ZrO₂ also exhibited high activity and stability in the dehydration of fructose to HMF.³⁸⁴ Huang et al. reported that silica-supported phosphotungstic acid (SiO₂-ATS-PTA) exhibited a much higher HMF yield than unsupported phosphotungstic acid in an acetone/water solvent system, suggesting that increasing the specific surface area is an effective strategy to improve their catalytic performance.³⁸⁵

Table 9 Conversion of carbohydrates to HMF over heteropolyacids

Catalyst	Catalyst loading^a	Solvent	Reaction condition	Substrate loading^a	Conversion	Yield^b	Ref.
a Relative to solvent. b HMF yield if unspecified. — Not available.
Fibrous HPW/TiO2–Al2O3	7 wt%	DMSO	120 °C, 3 h	Fructose, 12 wt%	89.9%	80.3%	383
Powdery HPW/TiO2–Al2O3	7 wt%	DMSO	120 °C, 3 h	Fructose, 12 wt%	86.7%	61.3%	383
ZMPA (30)	1 wt%	DMSO	120 °C, 0.5 h	Fructose, 7.2 wt%	100%	80.3%	384
mesoporous-ZrO2	1 wt%	DMSO	120 °C, 0.5 h	Fructose, 7.2 wt%	90%	29%	384
SnCl2-PTA/H β zeolite	1.25 wt%	NaCl/H2O/THF	180 °C, 2 h	Wheat straw, 2.5 wt%	—	30% (furfural 71%)	386
SnCl2-PTA/H β zeolite (4 runs)	1.25 wt%	NaCl/H2O/THF	180 °C, 2 h	Wheat straw, 2.5 wt%	—	19% (furfural 43%)	386
[PzS]H2PW	5.4 mM	3.1 wt% NaCl, H2O/THF (v/v = 0.2)	180 °C, 2 h	Glucose, 0.83 wt%	—	58.2% (LGA, 12.8%)	387
SiO2-ATS-PTA	3.3 wt%	Acetone/H2O (v/v = 5)	160 °C, 140 min	Glucose, 4 wt%	99.87%	78.31%	385
SiO2-ATS-PTA	3.3 wt%	Acetone/H2O (v/v = 5)	160 °C, 140 min	Glucose, 4 wt%	93.36%	68.35%	385
SiO2-ATS-PTA	3.3 wt%	H2O (v/v = 5)	160 °C, 140 min	Glucose, 4 wt%	99.77%	52.21%	385
PTA	1.4 wt%	Acetone/H2O (v/v = 5)	160 °C, 140 min	Glucose, 4 wt%	75.38%	52.27%	385
SiO2-ATS	1.9 wt%	Acetone/H2O (v/v = 5)	160 °C, 140 min	Glucose, 4 wt%	75.38%	52.27%	385
HReO4	0.5 wt%	DMSO	140 °C, 1 h	Fructose, 3.6 wt%	100%	100%	388
HReO4	0.5 wt%	N,N-DMF	140 °C, 1 h	Fructose, 3.6 wt%	64%	50%	388
HReO4	0.5 wt%	MeCN	140 °C, 1 h	Fructose, 3.6 wt%	35%	25%	388
HReO4	0.5 wt%	THF	140 °C, 1 h	Fructose, 3.6 wt%	66%	30% (levulinic acid 28%)	388
HReO4	0.5 wt%	1,4-Dioxane	140 °C, 1 h	Fructose, 3.6 wt%	100%	0% (levulinic acid 100%)	388
HReO4	5 mol%	1,4-Dioxane	140 °C, 2 h	Xylose, 1.5 wt%	—	Furfural 80%	389
[MimAM]H2PW12O40	2.5 mM	NaCl (0.5 M)/H2O/THF	160 °C, 7.5 h	Glucose, 2.5 wt%	99.8%	53.9%	390
[MimAM]H2PW12O40	2.5 mM	DMSO	160 °C, 7.5 h	Glucose, 2.5 wt%	99.8%	83.2%	390
ChH2PW12O40	0.02 M	H2O/MIBK	140 °C, 8 h	Cellulose, 1.8 wt%	87.0%	75.0%	391
ChH4CeW12O40	5 mM	H2O/DMSO/MIBK (v/v = 1/2/7)	140 °C, 6 h	Glucose, 1.4 wt%	—	67.5%	392

---

The introduction of Lewis acid sites in heteropolyacids is necessary to improve their catalytic performance for the conversion of glucose to HMF. Wang et al. reported that the introduction of aluminum to phosphotungstic acid could form Lewis acid sites owing to the electron-withdrawing effect of terminal WO on hydrated aluminum.³⁹³ Al-Doped phosphotungstic acid in H₂O/DMSO gave a much higher HMF yield (61.7%) from glucose than that of phosphotungstic acid (around 25%) after 4 h reaction at 170 °C owing to the suitable ratio of Brønsted and Lewis acid sites. The Brønsted–Lewis-surfactant-combined heteropolyacid Cr[(DS)H₂PW₁₂O₄₀]₃ (DS represents OSO₃C₁₂H₂₅ dodecyl sulfate) was used as a heterogeneous catalyst for the one-pot conversion of cellulose to HMF in water, obtaining an HMF yield of 52.7% with the glucose conversion of 77.1% at 150 °C within 2 h, which is much higher than that obtained with CrCl₃, Cr(DS)₃, H₃PW₁₂O₄₀ and Cr[H₂PW₁₂O₄₀]₃.³⁹⁴ Xu et al. reported that the SnCl₂-PTA/H β zeolite, which was prepared by introducing SnCl₂ and phosphotungstic acid (PTA) onto H β zeolite, gave an HMF yield of 30% and furfural yield of 71% from wheat straw.³⁸⁶ However, the catalytic activity of the SnCl₂-PTA/H β zeolite catalyst decreased remarkably after four cycles owing to the leaching of Sn species.

The design of hybrid catalysts composed of heteropolyacids with ionic liquids or deep-eutectic solvents can regulate the active sites and then improve their catalytic performance. For example, the acid–base bifunctional ionic hybrid catalyst [MimAM]H₂PW₁₂O₄₀, which was prepared using an amino-functionalized imidazolium ionic liquid and H₃PW₁₂O₄₀via ion exchange, afforded HMF yields of 53.9% and 83.2% from glucose using THF/H₂O–NaCl and DMSO as the reaction medium, respectively.³⁹⁰ Similarly, (HOCH₂CH₂N(CH₃)₃)H₂PW₁₂O₄₀ (abbreviated as ChH₂PW₁₂O₄₀) synthesized from choline chloride and H₃PW₁₂O₄₀ was more active than H₃PW₁₂O₄₀ for the one-pot conversion of cellulose to HMF in water/MIBK, affording an HMF yield up to 75% within 8 h at 140 °C.³⁹¹ Moreover, the heteropolyacid catalyst could be readily recycled after cooling to room temperature owing to its thermoregulate property.

2.4.6. Functionalized carbon-based solid acids. Carbon-based solid acids, mainly referring to carbon materials covalently functionalized by PhSO₃H or SO₃H groups, are a novel category of metal-free solid acids, which feature a unique carbon structure and Brønsted acidity (−H₀ = 8–11).³⁹⁵ These carbon-based solid acids can be used as versatile water-tolerant solid acids for the hydrolysis of polysaccharides, dehydration of fructose and etherification of HMF, as promising alternatives to conventional mineral acids and solid acids.

Numerous carbon-based materials including activated carbon, biochar, carbon nanotubes (CNTs), hydrothermal carbon, graphene, graphene oxide, carbon quantum dots, graphene quantum dots, ordered mesoporous carbon, nanostructured carbon, N-doped carbons, carbon nitride and carbon-based composites have been used as carbon supports or precursors for the preparation of SO₃H-containing functional carbon materials via sulfonation. Sulfonated carbon can be prepared either though simultaneous sulfonation and carbonization or post-grafting functionalization approach. For example, the direct hydrothermal treatment of glucose and p-toluenesulfonic acid (TsOH) gave a carbonaceous material with a high density of acid sites (2.0 mmol g⁻¹).³⁹⁶ Both biomass and its derivates, including fructose, glucose,^397,398 sucrose, sucralose,³⁹⁹ cellulose, lignin,^400,401 yeast cells,⁴⁰² and cow dung,³⁵⁵ have been used as feedstock for the preparation of SO₃H-functionalized carbonaceous solid acids via carbonization followed by sulfonation.

Sulfonated carbon has been widely investigated for the hydrolysis of cellulose to glucose.^397,400,403 To improve the reaction efficiency of cellulose hydrolysis, cellulase-mimetic solid acids (Fig. 20) bearing both cellulose-binding sites (such as –OH and –Cl) and catalytic sites (SO₃H and COOH) were designed by imitating the catalytic mechanism of cellulase.^{400,404–406} The binding and catalytic sites work energetically to promote the depolymerization of cellulose, where cellulose firstly attaches onto the catalyst surface via hydrogen bonds and then the β-1,4-glycosidic bond is disrupted by the acidic sites. Besides, the design of a mesoporous structure in carbon materials could promote the adsorption of 1,4-β-D-glucans, thus enabling hydrolysis over the weak acid sites (–COOH).^407–409 However, to date, cellulose hydrolysis over these solid acids still require a much higher reaction temperature than the optimum temperature for cellulase, suggesting that the activation energy of cellulose hydrolysis over cellulase-mimetic solid acid is considerably higher than that over cellulase. Thus, it is necessary to deepen our understanding on the catalytic mechanism of cellulase to design real cellulase-mimetic solid acids.

Fig. 20 (a) Cellulase-mimetic solid acid catalyst and (b) cellulose hydrolysis process. Adapted from ref. 404.

Numerous SO₃H-functionalized carbon materials have been demonstrated to be effective for the dehydration of fructose to HMF. For example, N-rich porous carbonaceous solid acids can be prepared by grafting strong acidic groups on porous carbon obtained by carbonization of polypyrrole in the presence of KOH, leading to higher catalytic activity than Amberlyst 15, H-ZSM-5, H-USY and sulfonic group-functionalized ordered mesoporous silicas for the dehydration of fructose to HMF (Table 10).⁴¹⁰ Sulfonic acid-functionalized lignin-derived mesoporous carbon (LDMC-SO₃H) with well-ordered two-dimensional hexagonal mesoporous characteristics was synthesized via phenolation and evaporation-induced self-assembly reaction.⁴¹¹ The LDMC-SO₃H catalyst gave an HMF yield as high as 98.0% at 140 °C for 2 h in the medium of DMSO. Adsorption kinetic studies suggested that the low adsorption capacity of HMF and high adsorption capacity of fructose on LDMC-SO₃H contribute greatly to the high selectivity toward HMF. Li et al. reported that sulfonated graphene quantum dots (SGQDs) could combine the advantages of homogenous and heterogeneous catalysis, affording an HMF yield (51.7%) comparable to homogenous H₂SO₄ (44.2%) and HCl (60.1%) at a relatively high fructose concentration (6.7 wt% with respect to the reaction medium).⁴¹² Besides, sulfonated graphene (SG) also exhibited high catalytic activity for the conversion of corn stalk and xylose into furfural.⁴¹³

Table 10 Conversion of carbohydrates to HMF and isomerization of glucose to fructose over functionalized carbon materials

Catalyst	Catalyst loading^a	Solvent	Condition	Substrate loading^a	Conversion	Yield^b	Ref.
a Relative to solvent. b HMF yield if unspecified. — Not provided.
10%TiO2C_S	1.7 wt%	DMSO	120 °C, 1 h	Fructose, 8.3 wt%	—	95%	414
PC-2-300	4 wt%	DMSO	160 °C, 3 h	Fructose, 5 wt%	—	93.7%	415
PC-2-300, 4 wt%	4 wt%	H2O/2-butanol (v/v = 2/3)	160 °C, 3 h	Fructose, 5 wt%	—	80.9%	415
BC-SO3H-PA	6.7 wt%	H2O	Microwave (350 W), 90 °C, 1 h	Cellulose, 13.3 wt%	—	23.1%	416
BC-SO3H	6.7 wt%	H2O	Microwave (350 W), 90 °C, 1 h	Cellulose, 13.3 wt%	—	<0.1%	416
PS-SO3H	6.7 wt%	H2O	Microwave (350 W), 90 °C, 1 h	Cellulose, 13.3 wt%	—	0	416
PS-NH2-PA	6.7 wt%	H2O	Microwave (350 W), 90 °C, 1 h	Cellulose, 13.3 wt%	—	2.5%	416
Ctobacco stem-SO3H	1 wt%	GVL/H2O (v/v = 4.7/0.3)	130 °C, 30 min	Fructose, 2 wt%	∼100%	93.7%	417
Ctobacco stem-SO3H	1 wt%	GVL/H2O (v/v = 4.7/0.3)	180 °C, 30 min	Glucose, 2 wt%	∼96%	43.8%	417
Cpolyurethane-SO3H	0.3 wt%	1,4-Dioxane	140 °C, 2 h	Fructose, 0.9 wt%	100%	70.1%	418
SC-CCA	1.2 wt%	GVL/H2O (v/v = 10)	130 °C, 20 min	Fructose, 2.4 wt%	100%	78.1%	419
SC-CCA	1.2 wt%	GVL/H2O (v/v = 10)	180 °C, 10 min	Glucose, 2.4 wt%	∼94%	33.2%	419
SC-CCA	1.2 wt%	GVL/H2O (v/v = 10)	180 °C, 50 min	Cellulose, 2.4 wt%	—	22.5%	419
BCSA-IL-X (X = CF3SO3H, HBF4, HPF6	0.67 wt%	H2O	350 W microwave, 80 °C, 3 h	Cellulose, 1.33 wt%	—	12.70–27.94%	420
BCSA-IL-Cl	0.67 wt%	H2O	350 W microwave, 80 °C, 3 h	Cellulose, 1.33 wt%	—	<0.1%	420
Bone char	0.5 wt%	H2O	90 °C, 3 h	Glucose, 1 wt%	27.2%	15%	421
Bone char; BAIL	0.25 wt%; 0.25 wt%	H2O	170 °C, 12 h	Glucose, 2 wt%	72%	39%	421
CS-CaO-800	0.6 wt%	H2O	80 °C, 40 min	Glucose, 5 wt%	40.9%	29.2%	422
CS-CaO-800	0.6 wt%	H2O	80 °C, 40 min	Glucose, 20 wt%	34.7%	25.0%	422
Sibunit-4	1 wt%	H2O	180 °C, 5 h, 1 MPa Ar	Cellulose, 1 wt%	—	6.6–9.4% (glucose 21.1–25.1%)	423
Al-Biochar	2.5 wt%	Acetone/H2O (v/v = 1)	Microwave, 140 °C, 5 min	Glucose, 5 wt%	29.1%	Fructose 21.5%	424
Al-Biochar (3 runs)	2.5 wt%	Acetone/H2O (v/v = 1)	Microwave, 140 °C, 5 min	Glucose, 5 wt%	19.5%	Fructose 7.8%	424
Sn-Biochar	2.5 wt%	H2O	Microwave, 150 °C, 20 min	Glucose, 5 wt%	∼74%	Fructose 12.1%	425
SO3H-OAC	1.25 wt%	THF/H2O (v/v = 3)-saturated NaCl	160 °C, 3 h	Glucose, 1.25 wt%	100%	93%	426
SO3H-OAC	1.25 wt%	THF/H2O (v/v = 3)	160 °C, 3 h	Glucose, 1.25 wt%	59.5%	30.6%	426
SO3H-OAC (5 runs)	1.25 wt%	THF/H2O (v/v = 3)-saturated NaCl	160 °C, 3 h	Glucose, 1.25 wt%	—	74%	426
PAB2-600	2 wt%	DMSO/H2O (v/v = 3)	180 °C, 20 min	Food waste, 5 wt%	86.5%	30.2%	427
SBC	5 wt%	DMSO/H2O (v/v = 3)	180 °C, 30 min	Bread waste, 5 wt%	86.5%	30.2%	428
CNTs-SO3H-NH2-Cr(III)	2.5 wt%	BMIMCl	130 °C, 3 h	Glucose, 5 wt%	86.5%	58.6%	429
CNTs-SO3H-NH2-Cr(III)	2.5 wt%	BMIMCl	130 °C, 3 h	Cellulose, 5 wt%	—	40.2%	429
CNTs-SO3H-NH2-Cr(III) (5 runs)	2.5 wt%	BMIMCl	130 °C, 3 h	Cellulose, 5 wt%	—	36%	429
CNTs-SO3H-NH2	2.5 wt%	BMIMCl	130 °C, 3 h	Cellulose, 5 wt%	—	31.1%	429
N-Doped biochar	1 wt%	H2O	120 °C, 20 min, microwave	Glucose, 5 wt%	12%	Fructose, 10.1%	430
N-Doped biochar	1 wt%	H2O	160 °C, 5 min, microwave	Glucose, 5 wt%	14.7%	Fructose, 14%	430
N-Doped biochar	1 wt%	H2O/acetone (v/v = 1)	160 °C, 5 min, microwave	Glucose, 5 wt%	22%	Fructose, 18%	430
SGQDs	0.8 wt%	DMSO/water/MIBK/butanol (v/v = 7/3/14/6)	170 °C, 2 h	Fructose, 6.7 wt%	91.8%	51.7%	412
GQDs	0.8 wt%	DMSO/water/MIBK/butanol (v/v = 7/3/14/6)	170 °C, 2 h	Fructose, 6.7 wt%	91.8%	31.2%	412
SGQDs	0.8 wt%	DMSO/water/MIBK/butanol (v/v = 7/3/14/6)	170 °C, 2 h	Glucose, 3.3 wt%	45.1%	19.5%	412
SGQDs	0.8 wt%	DMSO/water/MIBK/butanol (v/v = 7/3/14/6)	170 °C, 2 h	Cellobiose, 3.3 wt%	91.2%	13.2%	412
SGQDs	2 wt%	DMSO/water/MIBK/butanol (v/v = 7/3/14/6)	170 °C, 2 h	Ball-milled cellulose, 0.5 wt%	78.9%	22.2%	412

---

Besides SO₃H-functionalized carbon materials, carbon materials only containing weak acidic sites, such as COOH groups^398,431 and P–O groups^432,433 are also effective in catalyzing the hydrolysis of polysaccharides and the dehydration of carbohydrates to HMF. For example, the carbonaceous microsphere material obtained via the hydrothermal carbonization of glucose was directly used for the dehydration of fructose in BMIMCl, attaining an HMF yield of 88.1% at 100 °C in 90 min.³⁹⁸ A phosphorylated carbonaceous material was also used for the dehydration of fructose to HMF.^432,433 It was found that tuning the concentration of P–O groups on the catalyst is an effective approach to improve the catalytic activity and selectivity. Besides, phosphoric acid-activated wood biochar gave an HMF yield of 30.2% from starch-rich food waste.⁴²⁷

Due to the lack of glucose-to-fructose isomerization ability, SO₃H-functionalized carbon materials are unable to catalyze the efficient conversion of glucose to HMF. The introduction of basic and Lewis acidic species in carbon materials (Fig. 21) is an effective approach to improve their catalytic activity for glucose isomerization. Bone char, which was prepared via the calcination of bone cattle powder, exhibited obvious catalytic activity for the isomerization of glucose to fructose (19%) owing to its basic sites resulting from Ca (23 wt%), Mg (1.4 wt%) and Na (1.4 wt%) elements.⁴²¹ The combination of basic bone char and acidic IL 1-methyl-3-(3-sulfopropyl)-imidazolium hydrogen sulfate as the catalyst gave an HMF yield of 39% with the glucose conversion of 72% in the medium of water. Similarly, the carbon/CaO composite (CS-CaO-800) synthesized via the pyrolysis of crab shells was effective in catalyzing the isomerization of glucose to fructose, affording a fructose yield of 25.0% at an ultrahigh glucose loading (20 wt%).⁴²² Yu et al. reported that an aluminium-biochar composite (Al-biochar), which was synthesized via the pyrolysis of AlCl₃-treated saw dust, gave a fructose yield of 21.5% in the medium of acetone/H₂O at 160 °C for 5 min under microwave heating.⁴²⁴ They considered that the active Al sites contribute to approximately 70% of the isomerization activity and the possible active species are the Al₂O₃, Al(OH)₃, AlO(OH), and Al–O–C moieties in the amorphous phase. However, the fructose yield decreased to 7.8 mol% in the third recycling run. Zhang et al. introduced Al₂O₃ and TiO₂ in sulfonated carbon with hierarchically ordered pores (SCHOP) to strengthen the Lewis acid sites.⁴³⁴ The obtained Al–Ti@SCHOP composite catalyst exhibited higher catalytic activity in DMSO than the mixture of SCHOP, Al₂O₃ and TiO₂, giving a maximum HMF yield of 57.36% from glucose at 130 °C within 5.0 h. After five recycles, the HMF yield only decreased by 6.2%.

Fig. 21 Conversion of glucose to HMF over acid-base bifunctional catalyst. Adapted from ref. 416.

The introduction of N-containing groups is also effective to impart glucose isomerization ability and then promote the conversion of glucose to fructose and HMF (Fig. 21). N-Doped biochar bearing strong basic pyridinic nitrogen, which was obtained via the co-pyrolysis of spent coffee grounds and melamine, was used as a solid base catalyst for the isomerization of glucose, affording the maximum fructose yield of 18% in the medium of water/acetone mixture.⁴³⁰ Chen et al. prepared bamboo-derived biochar bearing both SO₃H and polyamide (BC-PA-SO₃H) for the direct conversion of cellulose, attaining an HMF yield of 23.10% in water under microwave-assisted conditions.⁴¹⁶ In contrast, the PA-free BCSA, PS-SO₃H, and microporous amino sulfonic resin (PS-NH₂-SO₃H) were unable to convert cellulose to HMF. Besides, BCSA-PA and PS-NH₂-SO₃H gave remarkably higher HMF yields from glucose than that from fructose, which is different from traditional solid acids. They proposed that the superior catalytic performance of BC-PA-SO₃H was attributed to the synergistic effect of the acidic and basic sites and the efficient mass transfer between the substrate and catalyst. An acid–base and Cr(III) species multi-functionalized multi-walled carbon nanotube (CNT) catalyst exhibited HMF yields of 58.6% and 40.2% from glucose and cellulose, respectively.⁴²⁹ Nahavandi et al. claimed that the oxidation of activated carbon (OAC) followed by sulfonation is an effective strategy to impart –SO₃H, –OH, and –COOH functional groups as well as strong Brønsted base sites, thus leading to excellent catalytic performance for the dehydration of glucose to HMF (yield: 93%).⁴²⁶ However, the presence of Brønsted base sites was only implied by carbon dioxide temperature-programmed desorption (CO₂-TPD), and the origin of the Brønsted base sites and their catalytic role were not provided. Since both the catalytic performance and stability of acid–base bifunctional carbon materials have remarkable differences in current reports, more strict control experiments are required to obtain credible experimental results for these bifunctional catalysts.

The use of a GVL/water mixture seems to be effective to promote the direct conversion of glucose and cellulose to HMF over sulfate and sulfonated carbon, without the use of additional active sites responsible for glucose isomerization.^417,419 For example, Huang et al. showed that in the medium of water/GVL, biomass-derived mesoporous carbon functionalized with benzenesulfonic acid gave moderate HMF yields of 33.2%, 22.5% and 32.3% from glucose, cellulose and biomass, respectively.⁴¹⁹ A similar synergistic effect was also observed for the conversion of glucose to HMF over sulfonated carbon and NaHSO₄. For example, Thanh et al. reported that a carbonaceous solid acid catalyst prepared via the incomplete carbonization of Acacia mangium wood sawdust followed by H₃PO₄ treatment and sulfonation only gave an HMF yield of 26% from the hydrolysate solution of Acacia mangium wood sawdust, which contained 0.05 M citric acid and 0.5 M glucose, while the combined use of the carbonaceous solid acid and NaHSO₄ gave a high HMF yield (64%), which is consistent with the phenomenon observed in the NaHSO₄/ZnSO₄ system.⁴³⁵ However, more work is needed to reveal the synergistic mechanism of GVL with sulfate or sulfonated carbon catalysts.

Since carbon-based materials are generally prepared from organic waste with complex compositions, they may contain various impurities, in particular metal species, and complex functional groups. As discussed in section 2.3.1, trace metal species may play an important role in the conversion of carbohydrates. Therefore, the comprehensive characterization of carbon-based materials, especially the type and content of metal species and functional groups, is of great importance for understanding their actual catalytic species and reaction mechanism.

2.4.7. Functionalized MOFs and COFs. The use of metal–organic framework (MOFs), covalent organic frameworks (COFs) and their derivates as heterogeneous catalysts for the conversion of biomass has attracted considerable attention owing to their high specific surface area, adjustable composition and functional groups.⁴³⁶ Sulfonated MOF and COF are highly effective for the dehydration of fructose to HMF (Table 11). Hu et al. reported that the use of sulfonated MOF NUS-6(Hf), which was prepared via a modulated hydrothermal synthetic method, afforded an HMF yield of 98% at the fructose conversion of 99% in DMSO.⁴³⁷ Compared with NUS-6(Zr), NUS-6(Hf) exhibited stronger Brønsted acidity owing to the presence of Hf–μ₃-OH groups and higher stability owing to the high dissociation energy (802 kJ mol⁻¹) of the Hf–O bonds. Moreover, the smaller pore size of NUS-6(Hf) could inhibit the unwanted side reactions to some extent. Peng et al. reported that TFP-DABA, a sulfonated two-dimensional COF prepared from 1,3,5-triformylphloroglucinol and 2,5-diaminobenzenesulfonic acid via a Schiff base condensation reaction followed by irreversible enol-to-keto tautomerization, was active for the dehydration of fructose, obtaining an HMF yield of 97% at 100 °C for 1 h in DMSO.⁴³⁸ These works suggest that the elegant regulation of the pore structure of SO₃H-functionalized materials is an effective approach to inhibit the undesirable degradation of HMF.

Table 11 Conversion of carbohydrates to HMF and isomerization of glucose to fructose over functionalized MOFs and COFs

Catalyst	Catalyst loading	Solvent	Reaction condition	Substrate loading^a	Conversion	Yield^b	Ref.
a Relative to solvent. b HMF yield if unspecified. — Not provided.
NUS-6(Hf)	5 wt%	DMSO	100 °C, 1 h	Fructose, 5 wt%	99%	98%	437
TFP-DABA	0.8 wt%	DMSO	100 °C, 1 h	Fructose, 5 wt%	99%	97%	438
UiO-66(Zr)-MSBDC(20)	0.33 wt%	H2O	140 °C, 3 h	Glucose, 10 wt%	31%	6% (fructose 22%)	439
UiO-66(Zr)-MSBDC(20) (3 runs)	0.33 wt%	H2O	140 °C, 3 h	Glucose, 10 wt%	20.6%	3.4% (fructose 6.9%)	439
UiO-66-Zr MOF	2.6 wt%	DMSO	140 °C, 12 h	Glucose, 2 wt%	—	49.7%	440
UiO-66(Zr)-NH2-SO3H	2.2 wt%	DMSO	130 °C, 8 h	Glucose, 2 wt%	—	48.2%	441
UiO-66(Zr)-NH2-SO3H (5 runs), 2.2 wt%	2.2 wt%	DMSO	130 °C, 8 h	Glucose, 2 wt%	—	38.7%	441
UiO-66-SO3H/PDA@PU	1 wt%	DMSO	120 °C, 2 h	Glucose, 2 wt%	—	56.6%	442
UiO-66-SO3H-NH2/PDA@PU	1 wt%	DMSO	120 °C, 2 h	Glucose, 2 wt%	—	70.3%	442
UiO-66-SO3H-NH2/PDA@PU (5 runs)	1 wt%	DMSO	120 °C, 2 h	Glucose, 2 wt%	—	∼62%	442
PHs-SO3H@UiO-66(Hf)-NH2	2 wt%	BMIMCl	120 °C, 1 h	Cellulose, 5 wt%	—	49.6%	443
PHs-SO3H@UiO-66(Hf)-NH2 (4 run)	2 wt%	BMIMCl	120 °C, 1 h	Cellulose, 5 wt%	—	42.7%	443

---

Recently, numerous MOFs with Lewis acidity have been synthesized and used as promising adsorbents and catalytic materials owing to their high porosity, tunable functionality and abundant metal sites.⁴⁴⁴ More importantly, the abundant unsaturated metal centers and electron-deficient groups impart unique Lewis acid sites to MOFs. The development of MOFs with tuned Lewis acidity and Brønsted acidity may improve their catalytic performance for sugar isomerization and dehydration reaction. Su et al. reported the use of a sulfonic acid-functionalized MOF, MIL-101(Cr)-SO₃H, as a bifunctional heterogenous catalyst for the conversion of glucose to HMF.⁵⁰ In this reaction system, the Cr³⁺ centers in MIL-101(Cr)-SO₃H function as Lewis acid sites, which are active for the isomerization of glucose to fructose, while –SO₃H serves as Brønsted acid sites responsible for the dehydration of fructose to HMF. The batch reaction in GVL/water (10 wt% water) gave a maximum HMF yield of 44.9% with the selectivity of 45.8%. In addition, the continuous reaction conducted in a fixed-bed reactor produced HMF with a steady yield, demonstrating that the catalyst was highly stable under the tested conditions. Zi et al. showed that the unfunctionalized MOF MIL-53(Al) was effective for the conversion of carboxymethyl cellulose (CMC) to HMF using water as the solvent, attaining an HMF yield of 40.3% with a total reducing sugar (TRS) yield of 54.2% at 200 °C for 4 h.⁴⁴⁵ They proposed that the Al species in MIL-53(Al) could promote the isomerization of glucose to fructose via the enediol structure formed from the coordination of Al species with the hydroxyl groups of glucose, while the hydrolysis of CMC to glucose and the dehydration of fructose are predominantly catalyzed by the Brønsted acid sites.

The introduction of chromium hydroxide particles on and within MIL(Cr)-101 enabled the efficient two-step isomerization of glucose to fructose (Fig. 18) in consecutive reactions in ethanol and aqueous media, attaining a fructose yield of up to 59.3%.⁴⁴⁶ The isomerization of glucose to fructose proceeded predominantly over chromium hydroxide via the proton-transfer mechanism, which is same with the base-catalyzed isomerization process, while the ketalization process is mainly catalyzed by Lewis acidic MIL(Cr)-101.

The acidic and basic sites in acid–base bifunctional MOFs can work synergistically to achieve the conversion of glucose to HMF. UiO-66-Zr, UiO-66(Zr)-NH₂-SO₃H and UiO-66-Zr MOF supported on polyurethane foam (UiO-66-SO₃H/PDA@PU) with appropriate acid–base bi-functional sites, which could be readily synthesized by tuning the ratio of organic ligands, afforded HMF yields of 49.7–70.3% from glucose in DMSO.^440–442 Zhao et al. reported that the PHs-SO₃H@UiO-66(Hf)-NH₂ catalyst bearing both acidic and basic sites afforded an HMF yield of 49.6% from cellulose under relatively mild conditions (120 °C, 1 h).⁴⁴³ Burnett et al. reported that the hydrothermally stable ytterbium MOF (Yb₆(BDC)₇(OH)₄(H₂O)₄) bearing bridging hydroxyl and metal-coordinated water exhibited both Brønsted and Lewis acidity and notable catalytic performance for the direct conversion of glucose to HMF (selectivity of 65% with glucose conversion of 38%) in pure water.⁴⁴⁷

Besides, MOFs can be used as precursors for the design and synthesis of novel solid acid catalysts. For example, silica-supported Lewis-acidic oxozirconium clusters (Zr₆@SiO₂) were synthesized via the nanocasting of NU-1000, an MOF with 3 nm channels and oxozirconium clusters, with silica followed by removing the organic linkers at high temperature, to improve the thermal stability of the MOF-derived single-site catalytic clusters.⁴⁴⁸ NU-1000 after dehydration at 300 °C under vacuum exhibited higher Lewis acidity and catalytic activity than Zr₆@SiO₂, but its activity vanished after thermal treatment at 500 °C. In contrast, the oxozirconium clusters in Zr₆@SiO₂ did not aggregate, even upon exposure to air at 600 °C, and the catalytic activity of Zr₆@SiO₂ for glucose isomerization to fructose (fructose yield: ∼23%) was maintained even after thermal treatment at 500 °C.

2.4.8. Functionalized polymers. SO₃H-functionalized polymers have been widely investigated for the dehydration of fructose (Table 12). The introduction of porosity and acidic groups in polymers is crucial for their catalytic activity. Du et al. designed a sulfonic-acid-functionalized porous organic polymer (POP) catalyst for the dehydration of fructose to HMF in a water/dioxane system.⁴⁴⁹ Owing to the hierarchical porosity and abundant accessible acid sites, HO₃S-POPs exhibited catalytic activity comparable to homogeneous sulfonic acids and higher than that of the commercially available Amberlyst-15. Cho et al. reported that manipulating the size of microporous organic networks (MONs) bearing aliphatic sulfonic acids could improve their catalytic performance for the conversion of fructose to HMF.⁴⁵⁰ Nano-sized MON-based solid sulfonic acid with a diameter in the range of 90–100 nm, which was prepared via the Sonogashira coupling of tetra(4-ethynylphenyl)adamantane and 1,4-dibromo-2,5-di(trimethylsilylethynyl)benzene in the presence of poly(vinylpyrrolidone) followed by the incorporation of SO₃H, afforded the maximum HMF yield of up to 98% in the medium of DMSO. The better catalytic performance than the micron-sized MON-AS was attributed to its reduced size and improved specific surface area (503 m² g⁻¹). They also observed that the stability of the aliphatic sulfonic acid in the polymer was remarkably higher than the aromatic sulfonic acid during the dehydration of fructose.⁴⁵¹

Table 12 Functionalized polymers for the conversion of carbohydrates to HMF

Catalyst	Catalyst loading^a	Solvent	Reaction condition	Substrate loading^a	Conversion	Yield^b	Ref.
a Relative to solvent. b HMF yield if unspecified. — Not provided.
HO3S-POPs	∼0.18 wt%	H2O/dioxane (v/v = 1/4)	Microwave, 140 °C, 15 min	Fructose, 2.5 wt%	100%	∼70%	449
SO3H-PANI-FeVO4	2.2 wt%	DMSO	120 °C, 6 h	Sucrose, 2.5 wt%	—	87%	452
MSPFR	1.2 wt%	GVL/H2O (v/v = 10)	140 °C, 60 min	Fructose, 2.4 wt%	99.3%	82.6%	453
MSPFR	1.2 wt%	GVL/H2O (v/v = 10)	190 °C, 60 min	Glucose, 2.4 wt%	97.6%	33.0%	453
MSPFR	1.2 wt%	GVL	190 °C, 100 min	Corn stover, 2.4 wt%	—	30.7% (furfural 43.4%)	454
MSPFR	1.2 wt%	GVL	190 °C, 100 min	Corn stover, 6.1 wt%	—	16.7% (furfural 1.5%)	454
MSPFR	1.2 wt%	H2O	190 °C, 100 min	Corn stover, 6.1 wt%	—	<5% (furfural 19.9%)	454
Polytriphenylamine–SO3H	0.3 wt%	GVL	175 °C, 0.5 h	Corncob, 1.25 wt%	—	32.3% (furfural 73.9%)	207
H-control-SO3H	1 wt%	DMSO	100 °C, 5 h	Fructose, 6.1 wt%	—	84%	451
H-control-SO3H (5 runs)	1 wt%	DMSO	100 °C, 5 h	Fructose, 6.1 wt%	—	40%	451
H-TA-CMP-ASO3H	1 wt%	DMSO	100 °C, 5 h	Fructose, 6.1 wt%	—	85%	451
H-TA-CMP-ASO3H (5 runs)	1 wt%	DMSO	100 °C, 5 h	Fructose, 6.1 wt%	—	82%	451
Lignosulfonate-based acidic resin	1 wt%	DMSO	120 °C, 1.5 h	Fructose, 1.4 wt%	100%	90%	455
Lignosulfonate-based acidic resin	1 wt%	DMSO	120 °C, 2.5 h	Inulin, 1.4 wt%	∼93%	73%	455
Phosphate-functionalized porous organic polymers (B-POP)	0.75 wt%	DMSO/1,4-dioxane	130 °C, 0.5 h	Fructose, 5 wt%	100%	85%	456
SPPS	1.8 wt%	EMIMBr	140 °C, 4 h	Glucose, 18 wt%	—	87.2%	457
SPPS	1.8 wt%	EMIMBr	140 °C, 4 h	Glucose, 18 wt%	—	87.2%	457
SPPS	1.8 wt%	EMIMBr	180 °C, 4 h	Cellulose, 18 wt%	—	87.2%	457
SPPS	1.8 wt%	EMIMBr	100 °C, 4 h	Glucose, 18 wt%	—	45.2%	457
SPPS (5 runs)	1.8 wt%	EMIMBr	140 °C, 4 h	Glucose, 18 wt%	—	∼87%	457
SPPS	—	EMIMBr/H2O (v/w = 5/0.27)	80 °C, 72 h	HMF, —	32%	68%	457
Sg-CN	1.25 wt%	H2O	100 °C, 0.5 h	Fructose, 12 wt%	—	96%	458
Sg-CN	1.25 wt%	H2O	150 °C, 8 h	Glucose, 12 wt%	—	Levulinic acid, 41%	458
SGCN	0.1 wt%	H2O	200 °C, 5 h	Glucose, 1 wt%	100%	94%	459
SGCN	0.1 wt%	H2O	200 °C, 5 h	Glucose, 1 wt%	80%	30%	459
P-BnNH3Cl	7 wt%	H2O/DMSO (v/v = 3/22)	120 °C, 10 h	Glucose, 9 wt%	89%	66.4%	460
P-BnNH3Cl	7 wt%	H2O/DMSO (v/v = 3/7)	140 °C, 12 h	Starch, 8.1 wt%	96%	41%	460
P-BnNH3Cl; HCl	7 wt%; 0.096 M	H2O/DMSO (v/v = 3/7)	120 °C, 10 h	Cellulose, 8.1 wt%	100%	40%	460
Amberlyst 70	1 wt%	H2O	150 °C, 1 h	LGO, 1 wt%	46.2%	29.6%	340
N-MON-AS (2 wt% SO3H)	0.92 wt%	DMSO	140 °C, 20 h	Fructose, 5 wt%	—	98%	450
N-MON-AS (2 wt% SO3H)	0.92 wt%	DMSO	100 °C, 20 h	Fructose, 5 wt%	—	91%	450
N-MON-AS (2 wt% SO3H)	0.09 wt%	DMSO	100 °C, 20 h	Fructose, 5 wt%	—	73%	450
Micron-sized MON-AS (2 wt% SO3H)	2.4 wt%	DMSO	140 °C, 20 h	Fructose, 5 wt%	—	76%	450
Micron-sized MON-AS (2 wt% SO3H)	0.24 wt%	DMSO	140 °C, 20 h	Fructose, 5 wt%	—	64%	450
DICAT-2	11.3 wt%	DMSO	MW, 130 °C, 2 min	Fructose, 6.3 wt%	98%	90%	461
DICAT-2	11.3 wt%	Isopropanol	MW, 130 °C, 2 min	Fructose, 6.3 wt%	94%	85%	461
DICAT-1	6.7 wt%	Isopropanol/DMSO (v/v = 7/3)	960 °C, 2 h	Sucrose, 9 wt%	—	77%	462
Cr-Tanned leather	10.25 wt%	DMSO	165 °C, 24 h	Glucose, 5 wt%	—	28%	463
COP-SO3H/SB-15	2.75 wt%	DMSO	120 °C, 1 h	Fructose, 5 wt%	99.5%	78%	464
COP-SO3H/SB-15	2.75 wt%	DMSO	120 °C, 1 h	Glucose, 5 wt%	47.8%	5%	464
COP-SO3H/SB-15	2.75 wt%	DMSO	120 °C, 1 h	Glucose, 5 wt%	70%	40%	464
Amberlyst-15	0.5 wt%	NaCl/H2O/THF	160 °C, 1 h	Glucose, 1.25 wt%	96%	48%	465
Fe(III)/Amberlyst-15	0.5 wt%	NaCl/H2O/THF	160 °C, 1 h	Glucose, 1.25 wt%	97%	68%	465
Al2O3/Amberlyst-15	1.5 wt%	DMSO	300 W ultrasound, 25 °C, 1 h	Fructose, 1.5 wt%	∼63%	47%	466
Yb(OTf)2/PhSO3H-MPR	3.5 wt%	H2O/SBP	170 °C, 4 h	Glucose, 0.9 wt%	75%	52%	467
Yb(OTf)2/PhSO3H-MPR	3.5 wt%	H2O/THF	170 °C, 4 h	Glucose, 0.9 wt%	35%	11%	467
Yb(OTf)2/PhSO3H-MPR	3.5 wt%	H2O/GVL	170 °C, 4 h	Glucose, 0.9 wt%	25%	13%	467
SPA-Imd-TinPCP	1 wt%	DMSO	160 °C, 5 h	Glucose, 4 wt%	89.5%	59.5%	468
SPA-Imd-TinPCP	1 wt%	H2O/THF	160 °C, 5 h	Glucose, 4 wt%	86.2%	49.8%	468
Sn-PIL	0.5 wt%	H2O/THF	130 °C, 1 h	Glucose, 1 wt%	99%	51.1%	469
PEG-CF3COO^−/SO3H based IL	0.15 M	CH3CN	75 °C, 3.5 h	Fructose, 2.5 wt%	100%	74.2%	470
PEG-CCl3COO^−/SO3H based IL	0.15 M	CH3CN	75 °C, 3.5 h	Fructose, 2.5 wt%	100%	73.5%	470
PEG-CH3COO^−/SO3H based IL	0.15 M	CH3CN	75 °C, 3.5 h	Fructose, 2.5 wt%	96.5%	51.6%	470
PEG-HSO4^−/SO3H based IL	0.15 M	CH3CN	75 °C, 3.5 h	Fructose, 2.5 wt%	100%	72.6%	470
PEG-CF3COO^−/SO3H based IL (3 run)	0.15 M	CH3CN	75 °C, 3.5 h	Fructose, 2.5 wt%	80%	51.6%	470

---

Besides artificial polymers, cellulose and lignin can also be used as precursors for the synthesis of sulfonated polymers. Pawar et al. reported that sulfonic acid-anchored cellulose (DIC_AT-2), which was prepared via the controlled sulfonation of cellulose by chlorosulfonic acid, functioned as an effective solid acid catalyst for the dehydration of fructose to HMF under microwave irradiation in the medium of isopropanol.⁴⁶¹ Lignosulfonate-based acid resin, which was obtained from lignosulfonate via ion-exchange with H₂SO₄ solution, could also be used as a renewable solid acid for the efficient dehydration of fructose and inulin to HMF in the medium of DMSO.⁴⁵⁵

The introduction of certain functional groups in SO₃H-functionalized polymers can regulate the intensity of acid sites or influence the interaction between the catalyst and substrate and then improve their catalytic activity and selectivity. For example, sulfonated polyaniline (SPAN) gave an HMF yield of up to 71% from fructose, with complete restriction of HMF rehydration (Fig. 22) to levulinic acid and formic acid.⁴⁷¹ The SO₃H in SPAN preferred to form a stable six-member ring structure with the benzenoid amine (–NH–) and quinoid imine (–N=) nitrogen via a hydrogen bond, which limits the Brønsted acidity of the catalyst. SO₃H is active for the dehydration of fructose to HMF, while the rehydration of HMF is completely suppressed owing to the decreased Brønsted acidity. Moreover, the stability and recyclability of the SPAN catalyst were improved owing to the formation of hydrogen bonds. Tang et al. reported that acidic resin bearing adjacent SO₃H and OH, which was synthesized via the one-pot condensation/sulfation of sodium sulfite, cyclopentanone and formaldehyde followed by ion-exchange with H⁺ (Fig. 23), gave an optimized HMF yield of 97.1% in the medium of DMSO under mild conditions (120 °C, 1.5 h).⁴⁷² The improved catalytic performance was attributed to the enhanced adsorption of fructose on the catalyst owing to the hydrogen bonding between OH and fructose. The introduction of SO₃H-functionalized ionic liquids with CF₃COO⁻, CCl₃COO⁻, AcO⁻ and HSO⁻₄ anions onto PEG-600 formed acidic catalysts, which were active to convert fructose to HMF in the medium of CH₃CN.⁴⁷⁰

Fig. 22 Dehydration of fructose to HMF over SPAN with inhibited HMF rehydration. Adapted from ref. 471.

Fig. 23 Preparation of acidic resin bearing adjacent SO₃H and OH. Adapted from ref. 472.

Johnson et al. investigated the stability of several commercially available sulfonic acid-containing solid acids, including Amberlyst 15, Amberlyst 45 and Nafion for the dehydration of fructose to HMF in a condensed phase flow reactor for a long reaction time of 48 h, revealing that in the medium of water, the catalyst deactivation rate resulting from carbon deposition (fouling) was dramatically faster than that resulting from sulfonic acid leaching.⁴⁷³ After comparing the influence of reactant, solvent, residence time, and feed concentration on the fouling rates, they found that the use of the polar aprotic solvent DMSO with a dilute (50 mM) reactant stream was the only successful approach to inhibit fouling, while operating at relatively mild conditions in aqueous systems did not improve the longevity of the catalyst obviously. The higher HMF yield from DMSO is mainly ascribed to the increased acidity, while the improvement of catalyst lifetime is attributed to the efficient desorption of products owing to the tuned surface polarity.

Currently, most studies on the dehydration of carbohydrate to HMF were carried out at a relatively low substrate loading. The use of a high substrate loading is of great importance for the reduction of solvent usage, energy consumption, reaction volume and operating cost. However, an increase in the substrate loading results in a remarkable decrease in the HMF yield and selectivity owing to the severe side reactions.⁴⁷⁴ Pyo et al. reported that the use of acidic ion exchange in DR-2030 as a catalyst in the medium of DMSO enabled the highly efficient production of HMF from fructose solution at a fructose concentration as high as 30 wt%, obtaining HMF yields of 85% and 82% at 110 °C in batch and continuous-flow reactions, respectively.⁴⁷⁵

Besides acid-functionalized polymers, base-functionalized polymers have also been developed for the dehydration of fructose to HMF. For example, Zhu et al. synthesized formyl-modified polyaniline (FS-PAN) as a solid organic-base catalyst (Fig. 24) for the dehydration of fructose to HMF.⁴⁷⁶ In the medium of DMSO, FS-PAN gave an HMF yield of up to 90.4 mol% at 140 °C for 4 h, which is much higher than that (44.9%) obtained with H₂SO₄-doped polyaniline (S-PAN). They proposed that the basicity of the polymer is mainly due to the nitrogen atoms incorporated between the phenyl rings in the backbone of the polyaniline chain, and the grafting of electron-withdrawing formyl groups further increased the basicity due to the greater localization of electrons at the amide nitrogen atom. The HMF yield increased linearly from 70% to 90% with an increase in the loading of formyl groups in FS-PAN, suggesting that the amide plays an important role in the synthesis of HMF. Different from S-PAN, the side reactions over FS-PAN, including the rehydration of HMF to levulinic acid and formic acid and the condensation of reaction intermediates to humins are completely inhibited. Therefore, FS-PAN can be reused for at least four runs without obvious loss of catalytic activity. However, since DMSO itself can convert fructose to HMF (yield: 70%) efficiently under the same conditions, it cannot be concluded that amide is the main active site responsible for the conversion of fructose to HMF. One possible explanation for the improvement of HMF yield is that the neutralization of the acidic species with FS-PAN can reduce the decomposition of HMF.

Fig. 24 Structure of formyl-modified polyaniline. Adapted from ref. 476.

As discussed in section 2.2.5, almost quantitative dehydration of fructose to HMF can be achieved in the medium of Br-based ionic liquids without the use of an additional catalyst, indicating the important role of Br⁻ in fructose dehydration. Ruby et al. demonstrated that poly(N-alkylvinylpyridinium bromides) could function as acid-free and metal-free heterogeneous catalysts for the dehydration of fructose into HMF, attaining a maximum HMF yield of 77% with the acetals of HMF as the main by-products in the medium of ethanol after 0.5 h at 180 °C.⁴⁷⁷ Under the same conditions, an I-based polymer gave a comparable HMF yield, while OH-, Cl⁻ and NTf₂-based polymers gave much lower fructose conversion and HMF yields than the Br-based polymer. Owing to the absence of Lewis and Brønsted acid sites, both the glucose conversion and HMF yield were low over these catalysts.

Verma et al. reported that the sulfonated carbon nitride (Sg-CN), which was synthesized via the calcination of urea at 500 °C followed by sulfonation using chlorosulfonic acid, exhibited a high HMF yield (96%) in the medium of water under mild conditions (100 °C, 0.5 h) even at a fructose loading as high as 12.4 wt%.⁴⁵⁸ Glucose could be converted under relatively harsh conditions (150 °C, 8 h) with levulinic acid as the main product. In contrast, another study on sulfonated carbon nitride (S-GCN), which was conducted by Chhabra et al., indicated that glucose, fructose, cellobiose, sucrose and starch can be converted to HMF selectively owing to the presence of Brønsted base and Brønsted acid sites in S-GCN, but the reaction was performed under very harsh conditions (200 °C, 5 h) and low substrate loading (1 wt%).⁴⁵⁹ Thus, more work is needed to reveal the reason for these conflicting results.

Similar to carbon-based solid acids, cellulase-mimetic sulfonated polymers have also been designed for the depolymerization of cellulose.⁴⁰⁴ For instance, Yu et al. designed a soluble sulfonated hyperbranched poly(arylene oxindole) with adjacent hydroxyl and sulfonic acid groups to mimic the separate acidic and alkaline active sites in cellulase.⁴⁷⁸ SHPAO bearing hydroxyl and sulfonic acid groups was more active than H₂SO₄ and other hyperbranched polymers for the depolymerization of cellulose to glucose, obtaining a glucose yield of 56% with 93% total selectivity toward useful hydrolytic and dehydration products. In contrast, when non-functionalized SHPAO with only sulfonic acid groups was used as a solid acid catalyst, cellulose was mainly converted to levulinic acid.⁴⁷⁹ The excellent catalytic activity of SHPAO bearing hydroxyl and sulfonic acid groups is ascribed to the presence of neighboring hydroxyl groups and sulfonic acid group. However, similar to the carbon-based solid acids, the stability of cellulase-mimetic sulfonated polymers is a major concern regarding their actual application. For example, Tyufekchiev et al. demonstrated that the good catalytic performance of the sulfonated chloromethyl polystyrene is attributed to the HCl generated from hydrolysis of C–Cl under cellobiose hydrolysis conditions, instead of the previously believed cooperation action of C–Cl moieties to bind cellulose via hydrogen bond and SO₃H to catalyze the disruption of glycosidic bond.⁴⁸⁰

As is consistent with the phenomenon observed with carbon-based solid acids, the use of the GVL/water mixture as the reaction medium can also promote the direct conversion of glucose and cellulose to HMF over sulfonated polymers without specific active sites responsible for glucose isomerization. For example, Li et al. reported that p-hydroxybenzenesulfonic acid-formaldehyde resin (MSPFR), which was prepared from formaldehyde and p-hydroxybenzenesulfonic acid via the hydrothermal method, exhibited HMF yields of 82.6% and 33.0% from fructose and glucose, respectively, in the medium of GVL/H₂O mixture.⁴⁵³ Zhang et al. reported that the MSPFR catalyst afforded a furfural yield of 43.4% with an HMF yield of 30.7% at 190 °C within 100 min from raw corn stover.⁴⁵⁴

The introduction of Lewis acidic metal species and N-containing basic groups in sulfonated polymers is an effective approach to facilitate the conversion of glucose to HMF. Bobbink et al. reported that chromium-tanned leather, a robust Cr-containing material produced via chemical treatment of animal hides by chromium salts in the leather industry, was effective for the dehydration of glucose to HMF, obtaining an HMF yield of 28% with levoglucosenone (LGO) as the main by-product.⁴⁶³ However, the HMF yield dropped to 16% during the second recycling of the catalyst. The Fe³⁺-modified Amberlyst-15 resin (Fe(III)/Amberlyst-15) exhibited much higher catalytic activity than Amberlyst-15 for the direct conversion of glucose to HMF owing to the synergistic effect of Fe³⁺ and SO₃H, which are active for the isomerization of glucose to fructose and the dehydration of fructose to HMF, respectively.⁴⁶⁵ Wang et al. prepared an ordered mesoporous phenolic resin (MPR) bearing both Brønsted acid and Lewis acid sites (Yb(OTf)₂/PhSO₃H-MPR) via sulfonation followed by post-grafting method.⁴⁶⁷ Lewis acid sites were formed in the pore channels of the ordered phenolic resin owing to the coordination of benzenesulfonic acid with Yb(OTf)₃. As expected, this catalyst enabled the one-pot cascade conversion of glucose to HMF with a yield of 52% in the medium of water/SBP biphasic system. The good catalytic performance was attributed to the synergistic effect of uniformly distributed Brønsted and Lewis acid sites. Besides, the catalytic activity could be maintained for five runs. Babaei et al. reported that the sulfonic acid-functionalized triazine covalent organic polymer supported on mesoporous SBA-15 (COP-SO₃H/SB-15) afforded HMF yields of 78% and 45% from fructose and glucose in DMSO, respectively, under the optimum reaction conditions.⁴⁶⁴ Cao et al. reported that a poly-benzyl ammonium chloride resin (PBnNH₃Cl) functioned as a bifunctional organocatalyst for the isomerization of glucose and dehydration of fructose (Fig. 25), thus affording good HMF yields from glucose and starch.⁴⁶⁰ However, additional HCl is required to achieve the effective conversion of cellulose to HMF.

Fig. 25 Direct conversion of glucose to HMF over PBnNH₃Cl. Adapted from ref. 460.

Li et al. claimed that the use of sulfonated poly(phenylene sulfide) (SPPS-SO₃H) containing just strong Brønsted acid sites as a heterogeneous catalyst in ionic liquid EMIMBr gave HMF yields as high as 87.2% and 68.2% from glucose and cellulose, respectively, in the medium of EMIMBr ionic liquid.⁴⁵⁷ Several new phenomena, which are contradictory to previously established knowledge, were observed in the SPPS-SO₃H/EMIMBr catalytic system. Firstly, SPPS-SO₃H containing just strong Brønsted acid sites could directly convert glucose and cellulose into HMF in high yields, but HMF is unstable in this catalytic system even at 80 °C. Secondly, both the HMF yield and selectivity from glucose and cellulose were much higher than that obtained from fructose. With the assistance of DFT calculation, they proposed that the SO₃H group in SPPS functions as both a proton donor (Brønsted acid) and proton acceptor (conjugate base) in the conversion of glucose, and the anions and cations of EMIMBr combined with SO₃H-SPPS help to stabilize the reaction intermediates and transition states, thus resulting in the facile conversion of glucose to HMF. Thirdly, high temperatures (140 °C and 180 °C) were required to achieve the maximum HMF yields from glucose and cellulose, respectively, while increasing the temperature from 100 °C to 140 °C led to a decrease in the HMF yield.

2.4.9. Other heterogenous catalysts. The construction of hybrid catalysts is an promising approach to improve the catalytic performance, including catalytic activity, selectivity, lifetime and recyclability.⁴⁸¹ Moreover, the rational design of hybrid catalysts can provide a special microenvironment, enabling new reaction pathways, in particular cascade catalytic reactions, which are difficult to achieve with a single catalyst.⁴⁸² For example, Huang et al. investigated the combined use of immobilized thermophilic glucose isomerase enzyme and SO₃H-functionalized mesoporous silica for the tandem conversion of glucose to HMF.⁴⁸³ The enzyme was immobilized on NH₂-functionalized mesoporous silica with high affinity. This immobilization not only enabled higher enzyme activity and stability in buffer, but also reduced their inactivation in organic solvents. The combined catalysts in the mixture of THF and H₂O (4 : 1 v/v) gave a fructose yield of 61% with an HMF yield of 30% at 363 K after 24 h reaction.

Various approaches have been attempted to facilitate the recovery and reuse of heterogenous catalysts. For example, the design of heterogenous catalysts with a large bulk size is an effective strategy to facilitate their recovery and reuse. For example, millimeter-size γ-Al₂O₃ beads bearing propyl sulfonic acid and alkyl groups were used for the dehydration of fructose to HMF.⁴⁸⁴ It was found that the C₁₆–SO₃H–γ-Al₂O₃ catalyst, which was obtained by calcination at 650 °C, exhibited the highest hydrophobicity and catalytic activity, obtaining an HMF yield of 84% in DMSO/H₂O at 110 °C for 4 h. Owing to their hydrophobicity and millimeter size, the reuse of the γ-Al₂O₃ beads could be readily achieved. The combination of magnetic Fe₃O₄ with SO₃H-functionalized solid acid allowed the magnetic separation of the heterogeneous catalyst from reaction system.⁴⁸⁵ For instance, the dehydration of fructose to HMF under solvent-free conditions (Fig. 26) was investigated by Shaikh et al. using dicarboxylic acid-modified ferrite as the catalyst.⁴⁸⁶ The successful surface modification prevented the agglomeration of the ferrite nanoparticles, including Fe₃O₄ and CoFe₂O₄. With the assistance of sonication, tartaric acid-modified Fe₃O₄ gave an HMF selectivity of 87% with the fructose conversion of 98% at 80 °C for 1 h under solvent-free conditions. After the reaction, the catalyst could be recovered via dilution with water followed by separation with an external magnet. Although the recovered catalyst had a larger average size (18 nm) than that (11 nm) of the fresh catalyst, its catalytic performance was maintained for at least five times. They also claimed that GO enabled the efficient dehydration of fructose to HMF under solvent-free conditions.⁴⁸⁷ Besides the development of heterogeneous catalysts, the design of adsorbents that can adsorb the homogeneous catalyst from the catalytic system is also an effective strategy to achieve the recycling of the catalyst.⁴⁸⁸

Fig. 26 Dehydration of fructose to HMF over tartaric acid-modified Fe₃O₄ under solvent-free conditions. Adapted from ref. 486.

The dehydration of fructose in low boiling point organic solvents is highly desirable considering the recycling of the solvent and separation and purification of the HMF product. Since carbohydrates are generally insoluble in low boiling point organic solvents, the direct conversion of carbohydrates in these solvents is difficult. As discussed in section 2.2, fructose can be readily transformed into HMF in the medium of DMSO and several ionic liquids, but the separation of HMF from these solvents is cumbersome and energy-consuming. Sun et al. attempted to create solvation environments similar to solvents (Fig. 27), such as DMSO-, NMP- and Br-containing imidazolium-type ionic liquids in heterogeneous catalysts by incorporating these solvent moieties into porous materials.⁴⁸⁹ The catalyst bearing both SO₃H and ionic liquid moieties enabled the highly efficient conversion of fructose using the readily separable THF as the reaction medium without additional water, obtaining an HMF yield as high as 98.8% at 120 °C within just 10 min. Guo et al. prepared a DMSO-like polymeric solid catalyst, Au@(polythiophene-polythiophene oxides), via the chemical over-oxidative polymerization of thiophene with the assistance of Au nanoparticles. The DMSO-like catalyst exhibited much better catalytic performance than its liquid analog DMSO for the dehydration of fructose to HMF in the medium of low-boiling solvents, such as 1,4-dioxane and water.⁷⁷ Yang et al. reported that the coating of AlCl₃/SiO₂ with choline chloride could promote the dissolution of carbohydrates, thus enabling the dehydration of carbohydrates to HMF in MeTHF and MIBK.⁴⁹⁰ However, the catalytic activity dropped remarkably during the second and third recycling. Similarly, the ionic liquid BMIMBr encapsulated in H-MOR zeolite, which was synthesized via a ship-in-a-bottle strategy, enabled the efficient conversion of glucose to HMF in the water/MIBK/NaCl biphasic system, giving an HMF yield of 42% at 170 °C for 3 h.³⁵³

Fig. 27 Dehydration of fructose to HMF in low-boiling points organic solvent based on the collaboration of the solvation environment and SO₃H incorporated in a heterogeneous catalyst. Adapted from ref. 489.

2.5. Process technology

To achieve large-scale industrial production and application, process technology should also be developed together with fundamental studies.⁴⁹¹ In addition to the development of catalysts and solvents, the scaling up of catalytic technologies from the laboratory to a pilot plant also involves the design and optimization of the reactor system, development of separation and purification process, selection of an energy source and feedstock and evaluation of economic and environmental benefits of an integrated HMF production process. In this section, we give an overview of how to improve the energy utilization efficiency by employing other heating methods, how to achieve the stabilization of HMF and how to achieve the separation and purification of HMF.

2.5.1. Pretreatment of feedstocks. Pretreatment of biomass feedstocks is a crucial step to overcome the recalcitrance of lignocellulosic biomass. Recently, the development of the lignin-first strategy, which enables the efficient and selective conversion of lignin to aromatics without compromising the subsequent conversion of carbohydrates, has provided an effective approach for the full exploitation of lignocellulosic biomass.^15,28 The efficiency of glucose production from biomass increases steadily with the development of pretreatment technology,^15,28 solid acid catalysts,⁴⁰⁹ solvent systems,^98,99 enzyme production⁴⁹² and mechanochemical methods, providing a cheaper feedstock for the production of HMF.⁴⁰¹ Meanwhile, ball milling of cellulose, natural wood, waste paper, and cotton clothing can also improve the HMF production efficiency markedly.^{148,493–496}

2.5.2. Heating manner for HMF synthesis: traditional heating vs. solar energy, microwave and ultrasound. Solar energy not only can be used for the photocatalytic valorization of lignocellulosic biomass and its derivates, including selective depolymerization of lignin in biomass,¹⁵ reforming of cellulose and its intermediates to H₂ ^497,498 and selective oxidation of platform chemicals,⁴⁹⁹ but also as an energy resource to assist the thermal catalytic dehydration process. Tsutsumi et al. reported the use of a semiconductor material with visible light irradiation to assist the Brønsted acid-catalyzed conversion of fructose HMF.⁵⁰⁰ Among the investigated semiconductor materials, the silanol group-coated silicon semiconductor (Si–OH) enabled almost quantitative conversion of fructose to HMF in the medium of DMSO/H₂O using 4.6 M H₃PO₄ as an acid catalyst, obtaining an HMF yield of up to 97% at 80 °C. Owing to the selective adsorption of fructose and desorption of HMF on Si–OH (Fig. 28), Si–OH under visible light irradiation could selectively transfer the heat from optical energy to fructose, but not to HMF. Therefore, the presence of Si–OH increased the fructose dehydration rate remarkably but did not increase the HMF degradation rate, thus leading to the selective production HMF from fructose. Similarly, the cellobiose hydrolysis efficiency over the Ir/HY catalyst could also be improved remarkably under visible light illumination owing to the photothermal effect.⁵⁰¹

Fig. 28 Visible light irradiation-assisted fructose dehydration to HMF over Brønsted acid. Adapted from ref. 500.

Microwave-assisted processes can improve the reaction efficiency by accelerating the mass and heat transfer. The conversion of sugars and biomass to HMF using homogeneous catalysts, solid acid catalysts and non-catalytic routes under microwave irradiation has been widely studied, demonstrating that microwave heating can improve the reaction efficiency and product selectivity in comparison with conventional heating.^502,503 For example, the direct conversion of lignocellulosic biomass (Phyllostachys aureosulcata) in a water/MIBK biphasic reaction system using HCl as the catalyst under microwave irradiation could afford relatively high HMF (42.44%) and furfural (48.90%) yields.⁵⁰⁴

The artificial introduction of microwave adsorption groups in solid catalysts not only can improve the energy utilization efficiency, but also help to enhance the product selectivity via the oriented transfer of energy to the desired reaction process. For example, Ji et al. reported the use of a CNT–TiO₂/SO₄²⁻ composite catalyst consisting of a carbon nanotube core and acidified TiO₂ shell as a microwave-responsive catalyst to promote the catalytic conversion of sugars, including fructose, glucose and sucrose to HMF.⁵⁰⁵ The inner CNT could generate heat under microwave radiation, and then the generated heat could transfer to the acidified TiO₂ shell, thus enabling a six-times higher energy utilization efficiency (4.2 mol kJ⁻¹ L⁻¹) compared with TiO₂. The maximal HMF yields of 67%, 52% and 65% were attained from fructose, glucose, and sucrose within 30 min, respectively. Ji et al. also reported that the core/shell structured CNT–polyaniline/SO₄²⁻ hybrid material could attain an HMF yield of 63% from fructose within 10 min with high energy efficiency (7.6 mol kJ⁻¹ L⁻¹) owing to the localized heating over the catalyst surface under microwave radiation.⁵⁰⁶ Compared with CNT–TiO₂/SO₄²⁻, the thinner PANI shell in CNT–polyaniline/SO₄²⁻ could further improve the overall microwave absorption, thus leading to a higher heat transfer efficiency and improved reaction efficiency. Similarly, the use of Nb-doped TiO₂/SO₄²⁻ (Nb–TiO₂/SO₄²⁻) as both a catalyst and microwave-responsive material (Fig. 29) could further improve the efficiency of fructose conversion to HMF under microwave heating. Nb doping not only increased the concentration of surface sulfonic groups, but also improved the acid strength of the sulfonic groups owing to the electron transfer from the bulk catalyst to surface sulfonic groups.⁵⁰⁷ Xiouras et al. demonstrated that although the reaction kinetics of xylose under microwave radiation is similar to that under the conventional heating method, the microwave-assisted method could reduce the energy consumption by 30% on the laboratory scale compared with the conventional heating method.⁵⁰⁸

Fig. 29 Use of Nb-doped TiO₂/SO₄²⁻ (Nb-TiO₂/SO₄²⁻) as both a catalyst and microwave-responsive material for the rapid dehydration of fructose to HMF. Adapted from ref. 507.

Under the power of ultrasonic irradiation (300 W), SBA-15 grafted with alumina (Al₂O₃/SBA-15) enabled the conversion of fructose to HMF at room temperature, affording an HMF yield of 47% in the medium of DMSO within 2.5 h.⁴⁶⁶ Similarly, sonochemical activation could also accelerate the enzymatic isomerization of glucose to fructose with a considerable reduction in the reaction time and temperature.⁵⁰⁹

Generally, the introduction of optical energy, microwave radiation and ultrasonic irradiation in a suitable manner can improve the reaction efficiency and product selectivity. However, since optical energy, microwave irradiation and ultrasonic irradiation are usually generated from electricity ultimately resulting from fossil fuel, whether these technologies can improve the actual energy utilization efficiency is still uncertain. Thus, the accurate evaluation of the overall energy utilization efficiency and thorough cost accounting are very important for practical application.

2.5.3. Reactor concepts: batch reaction vs. continuous reaction. Many studies indicate that the dehydration of fructose to HMF over solid acid catalysts under continuous conditions can afford a higher HMF yield than that obtained under batch conditions owing to the high loading of catalyst encapsulated in the packed bed reactor (PBR) and the inhibited side reactions due to the rapid separation of highly reactive target products with the catalyst.^283,510,511 The regulation of catalysts is an effective approach to further enhance the HMF production efficiency under continuous conditions. Morales-Leal et al. investigated the successive production of HMF from fructose in a continuous reactor packed with functionalized alumina bearing both sulfonic and thiol groups as the catalyst (Fig. 30).⁵¹² The surface acidity of alumina was enhanced by the grafting sulfonic and thiol groups, and a fructose conversion of 95% with HMF selectivity of 73% was obtained from fructose dissolved in a mixture of THF and water (4 : 1 w/w) during the continuous reaction at 90 °C. They inferred that the thiol groups could catalyze the tautomerization of fructose to its furanose form, thus enhancing the HMF production efficiency. Sonsiam et al. reported that the addition of NMP to H₂O/MIBK could improve the HMF yield under continuous conditions.⁵¹³ Weingart et al. reported that an HFIP/water mixture with a low boiling point, which enabled the ready separation of HMF, could also serve as a solvent for the dehydration of fructose in a continuous packed-bed reactor, affording an HMF yield of 76%.⁵¹⁴

Fig. 30 Dehydration of fructose to HMF in a continuous reactor. Adapted from ref. 512.

The development of solid acid catalysts with high stability and reusability is essential for the continuous reaction. Morales-Leal et al. observed that 15% of sulfur from the sulfonic and thiol group-functionalized alumina was loss rapidly at just 70 °C under the continuous reaction.⁵¹² Gallo et al. compared the activity and stability of SO₃H-functionalized mesoporous carbons (CMK-3 and CMK-5), propylsulfonic acid-functionalized SBA-15 and commercial Nafion SAC-13 for the continuous conversion of fructose to HMF in a fixed bed flow reactor.⁵¹⁵ Among the tested catalysts, phenylsulfonic acid-functionalized CMK-5 (CMK-5-PSA) attained the highest HMF production rate. Moreover, the deactivation rate coefficient of CMK-5-PSA (0.002 h⁻¹) was remarkably lower than that of pSO₃H-SBA-15 (0.120 h⁻¹) and NafionSAC-13 (0.008 h⁻¹).

The continuous production of HMF from glucose was conducted using Sn-exchanged Amberlyst-15 as the catalyst, which was prepared from Amberlyst-15 and SnCl₂ solution via ion-exchange, obtaining an HMF yield of 10.35%.⁵¹⁶ The use of a biphasic catalytic system consisting of phosphate buffer saline (PBS) as the reaction phase and 2-sec-butyl phenol as the extraction phase in a continuous microreactor could suppress the side reactions, thus leading to high HMF yields (80.9% and 75.7%) from fructose and glucose, respectively.⁵¹⁷ However, the recovery of phosphate buffer saline is difficult and the separation of HMF from 2-sec-butyl phenol is energy-intensive owing to its high boiling point. Thus, the efficient production of HMF from glucose in a continuous reactor will be an important task in the future.

2.5.4. Stabilization, separation and purification of HMF. The stabilization, separation and purification of HMF are highly important in its practical production and application. The separation of HMF from the catalytic system is usually conducted via extraction and distillation under vacuum. Aging of the HMF product (Fig. 31), which is usually isolated and stored as an oil-like product with a purity of 97–99%, occurs rapidly owing to the arrangement of the HMF molecules in solution, and the decrease in the HMF quality seriously limits its subsequent upgrading to value-added products.⁵¹⁸ For example, after two weeks of storage, large amounts of HMF degraded to dimers and larger oligomers via intermolecular etherification, and the further use of aged HMF as the starting material only gave 15% yield of the model drug Ranitidine. Similarly, when low-grade HMF, which was obtained from glucose dehydration, was used to displace high-grade HMF (purity, 99%) for hydrogenolysis, the yield of the target product decreased from 45.6% to 16.4%.²³⁵ Highly stable crystalline HMF with a purity of up to 99.9% could be obtained by extraction, purification and recrystallization of crude HMF products, which were synthesized from fructose and cellulose in the BMIMCl/H₂SO₄ and BMIMCl/CrCl₃ catalytic systems, respectively.⁵¹⁸ The crystalline HMF could be readily converted to the model drug Ranitidine with high yield (65%).

Fig. 31 (a) Comparison of oil-like HMF with low purity and crystalline HMF with high purity, and (b) introduction of protecting group at the O(6) position of glucose. Adapted from ref. 518.

The introduction of a protecting group at the O(6) position in glucose enabled the highly selective synthesis of 5-(TBDPS-oxymethyl)furfural (TBDPS = Ph₂tBuSi), an HMF derivate, with an isolated yield of up to 81%, which is much higher than the HMF yield (40–50%) obtained with glucose.⁵¹⁸ Compared with HMF, 5-(TBDPS-oxymethyl)furfural can be readily separated from the reaction mixture and stored at room temperature for a long time without obvious aging. Gomes et al. demonstrated that the addition of a small amount of sodium dithionite (Na₂S₂O₄) not only improved the stability of HMF, but also inhibited side reactions in a series of processes, thus enabling the high efficiency of HMF distillation, HMF synthesis in a biphasic system, Knoevenagel condensation of HMF with Meldrum's acid, Cannizaro reaction and condensation of HMF with primary diamines toward pyridinium salts.⁵¹⁹ Besides the stabilization of HMF during its synthesis and the use of high purity HMF as a feedstock, the stabilization of HMF during its upgrading is also important to avoid its undesirable degradation, and then improve the upgrading efficiency and the selectivity of target product. This aspect will be discussed in detail in section 3.5.

Similarly, the reversible stabilization of xylose, glucose and lignin monomers (Fig. 32) in the depolymerization of biomass by generating acetal with formaldehyde enables the selective depolymerization of cellulose, hemicellulose and lignin to simple sugars and phenolic monomers by inhibiting the degradation of target products, including the dehydration of sugars to furans and their subsequent degradation and lignin condensation.^30,520,521 The obtained diformylxylose and diformylglucose products can be utilized as feedstock for the production of fufural and HMF using the catalytic system designed for xylose and glucose dehydration with a similar reaction efficiency, while the hydrogenolysis of formaldehyde-stabilized lignin affords the production of guaiacyl and syringyl monomers at near theoretical yields, which is more efficient than conventional catalytic systems without formaldehyde stabilization.

Fig. 32 (a) Stabilization of xylose and glucose by generating acetal with formaldehyde during the depolymerization of hemicellulose and cellulose in lignocellulosic biomass. Adapted from ref. 30 and 520. (b) Stabilization of lignin by formaldehyde in extraction of lignin from lignocellulosic biomass. Adapted from ref. 521.

As discussed in sections 2.2.4 and 2.2.5, the construction of water–organic biphasic systems and ionic liquid–organic biphasic systems are not only responsible for the inhibition of HMF degradation, but also promotes the separation of HMF. Introducing a microfiltration membrane in the water/MIBK biphasic system (Fig. 33) could create a microreactor to further improve the liquid–liquid extraction of HMF from the reactive phase (water) to the organic phase (MIBK).^522,523 Consequently, the side-reaction was inhibited, resulting in an HMF yield of 93.0% from fructose. Sarwono et al. employed a nanofiltration membrane (NF) to separate IL and HMF from the BMIMCl/CrCl₃ catalytic system, as an alternative to the liquid–liquid extraction method (LLE).⁵²⁴ The HMF recovery rate reached 94.87% with an HMF flux rate of 0.2854 L m⁻² h⁻¹ in the first run, but the HMF recovery rate and flux rate decreased to 71.65% and 0.2397 L m⁻² h⁻¹ in the third run, respectively. Although the complete separation of HMF and IL was not achieved, as indicated by NMR analysis, the NF membrane exhibits great potential for the environmentally friendly separation and purification of HMF. Yan et al. reported that the simultaneous circulation of the reaction phase (water) and extraction phase (MIBK) via pumping, distillation and cooling (Fig. 34) enabled the highly efficient conversion of fructose and glucose to HMF (74% and 90%) in the water/MIBK/HCl and water/MIBK/AlCl₃ catalytic systems, respectively.⁵²⁵

Fig. 33 Introduction of microfiltration membrane to improve the liquid-liquid extraction of HMF from water to MIBK. Adapted from ref. 522.

Fig. 34 Conversion of fructose and glucose to HMF in water/MIBK biphasic system with the simultaneous circulation of the reaction phase and extraction phase. Adapted from ref. 525.

Based on the systemic investigation of the solubility, saturation and precipitation points, stability, phase separation and phase extraction of mixtures of DMSO with different chemical agents, Gajula et al. designed an integrated strategy to construct solvent systems for the efficient separation of furan derivates from DMSO (Fig. 35), which is challenging due to the high boiling point of DMSO and the instability of the target compounds.⁵²⁶ For instance, the unconverted sugars can be precipitated by adding an organic solvent such as MIBK, and the combination of organic solvents and sugars is beneficial for the extraction of the HMF product from DMSO into the organic solvent. Meanwhile, the combination of water and organic solvent can also promote the phase separation of the organic solvent and DMSO to separate HMF. Water can be used to precipitate 2,5-diformylfuran (DFF), while water or sugar can be used to precipitate 2,5-furandicarboxylic acid (FDCA).

Fig. 35 Separation of HMF and its derivates from DMSO by regulating the composition of the catalytic system. Adapted from ref. 526.

To achieve high-yield HMF production from glucose, Alipour et al. devised a novel pathway to decouple the glucose isomerization, fructose dehydration, solvent recycling and HMF separation process (Fig. 36) via a series of liquid–liquid extraction processes.⁵²⁷ In the first step, glucose was converted to fructose by immobilized glucose isomerase enzyme, and then the formed fructose was transferred continuously to the organic phase (octanol) by the lipophilic aryl boronic acid, a ketose-selective binding agent. The simultaneous isomerization and reactive-extraction in turn promoted the isomerization of glucose to fructose by shifting the reaction equilibrium. In the second step, the fructose in the organic phase was extracted to an acidic ionic liquid 1-ethyl-3-methylimidazolium hydrogen sulfate ([EMIM]HSO₄). After the second step, the fructose yield reached 89% and nearly a pure solution of fructose in [EMIM]HSO₄ was obtained. In the third step, fructose was dehydrated to HMF in the medium of [EMIM]HSO₄ using HCl as the catalyst under mild reaction conditions (50 °C, ambient pressure), and simultaneously the formed HMF was extracted to THF. This integrated method enables an HMF yield as high as 80% with prolonged reuse of the catalysts, solvents and reaction media. Moreover, this integrated method has also been proven to be effective for the conversion of corn stover hydrolysate produced via dilute-acid pretreatment. Similarly, Gimbernat et al. developed an integrated cascade process involving the continuous isomerization of glucose to fructose in the aqueous phase, complexing, extracting and transporting fructose to the organic phase (MIBK), and releasing fructose at the interface with the second aqueous phase, and then dehydrating fructose to HMF over acidic resin to avoid the use of costly ionic liquids.⁵²⁸ Nevertheless, the global HMF yield was just 4% with a glucose conversion of 70%.

Fig. 36 Three-step process for the production of HMF from glucose. Adapted from ref. 527.

Various porous materials, including MOFs,^529,530 polymers,^531,532 activated carbons,⁵³³ and zeolites ⁵³⁴ have been used for the selective adsorption of HMF to facilitate the separation of HMF from the catalytic system. For example, Zhang et al. designed a hollow-structured porous aromatic polymer (H-PAP), which had a cavity diameter between 228 ± 11 and 464 ± 15 nm, for the selective adsorption of HMF from aqueous solution, without the adsorption of fructose, levulinic acid and formic acid.⁵³⁵ The apparent adsorption amount of HMF mainly depended on the micropore surface area, micropore volume and cavity diameter, while the selectivity of HMF adsorption was attributed to the surface hydrophobicity. The HMF product with a purity of up to 94.4% could be recovered from the reaction mixture of acid-catalyzed fructose conversion. Yabushita showed that the hydrophobic cavities of p-tert-butylcalix[4]arene macrocycles grafted on amorphous silica (calix/SiO₂) were crucial for the selective adsorption of HMF from aqueous solution containing both HMF and glucose.⁵³⁶ They proposed that the selective adsorption of HMF is mainly attributed to the weak dispersive (van der Waals) interactions between the aromatic guests and tert-butyl upper-rim substituents in calixarene hosts.

2.5.5. Technoeconomic analysis. The high production cost of HMF and its derivates is the main obstacle it their large scale production and application of HMF. A small-scale HMF production plant has already been operating in Switzerland since 2014 with an annual production of 300 t, but the large-scale production of HMF has not been achieved to date, as indicated by the high price of the commercial HMF product (Sigma Aldrich, 3500 € per kg).⁴⁶ As a result of the high cost of HMF, the present cost of 2,5-furandicarboxylic acid (FDCA), one of the most important HMF-derived products, is almost 500-fold higher than that of fossil-derived terephthalic acid (TPA).⁵³⁷ de Carvalho et al. reported that when fructose was dehydrated in water or acetone/water solvent system (v/v = 1) using niobium phosphate as the catalyst, the minimum selling price (MSP) of HMF was estimated to be $3050 and 2210 per ton, respectively.⁵³⁸ As discussed in section 2.2.3, the efficient conversion of cellulose to HMF with high yield (44%) was achieved in the medium of THF/water (water content <2–3 vol%) solvent system.¹⁰² Techno-economic analysis indicates that the lowest production costs of LGO and HMF are lower than $3.00 per kg, making this reaction system promising for industrial production.¹⁰¹ Motagamwala et al. reported that the dehydration of fructose over Amberst-15 catalyst in acetone/water solvent system (v/v = 4) enabled the production of HMF (Fig. 37a) at an MSP of $1710 per ton.¹⁰³ In addition, the conversion of glucose to HMF via a two-step process involving the isomerization of glucose to fructose over Sn-Beta zeolite and dehydration of fructose to HMF (Fig. 37b) afforded the production of HMF at an MSP of $1460 per ton. Yan et al. reported that the conversion of fructose and glucose in the water/MIBK/HCl and water/MIBK/AlCl₃ catalytic systems with the simultaneous circulation of the reaction phase and extraction phase enabled the efficient production of HMF at an MSP of $1716 and $1215 per ton.⁵²⁵

Fig. 37 Process model for the production of HMF using fructose (a) and glucose (b) as feedstock. Adapted from ref. 103.

Kougioumtzis et al. attempted to scale up the production of HMF from hemicellulose-free biomass via the hydrolysis of biomass to sugars by H₂SO₄ followed by the dehydration of sugars to HMF by Sn/Al₂O₃ on an industrial scale (biomass through-put of 1500 kg h⁻¹) using the chemical process optimization software ASPEN PLUS™.⁵³⁹ They obtained an optimized HMF yield of 8% (with respect to cellulose) and by-product yield of 13% (with respect to biomass), with an HMF production rate of 54 kg h⁻¹. They estimated that the total electricity demand is 22.2 MW per t_HMF with the heat demand of 218 MW per t_HMF.

3. Upgrading of HMF

The upgrading of HMF to high-value fuels, fine chemicals and materials involves a series of complex reaction processes, including selective oxidation, hydrogenation, etherification, coupling, functionalization and condensation reaction and the combination of different reaction networks.^4,540,541 The selective tailoring of C=O, C–O and furan ring functional groups in HMF toward the desired products requires the development of active and selective catalysts with specific compositions, structures and active sites, the development of suitable reaction media and the regulation of reaction conditions.⁵⁴² The one-pot synthesis of high-value products from carbohydrates via HMF as an intermediate calls for the elegant manipulation of carbohydrate dehydration and the subsequent HMF upgrading step with good control of the accompanying side reactions.⁵⁴³ In this section, the upgrading of HMF is discussed mainly according to the reaction processes, but also considering the classification of products and the associated catalysts. We not only summarize the recent advances of HMF upgrading technologies, but also highlight the main bottlenecks of current catalytic systems and possible solutions.

3.1. Synthesis of 5-(chloromethyl)furfural and acetoxymethylfurfural

Due to the similarity of 5-(chloromethyl)furfural (CMF) with HMF, CMF is also considered a promising intermediate for the production of biofuels, polymers, fine chemicals, agrochemicals and pharmaceuticals.^209,544,545 However, different from HMF, a high yield of CMF can be readily achieved from glucose, sucrose, cellulose and raw biomass in the HCl/dichloroethane/LiCl or HCl/dichloroethane system under mild conditions (Fig. 38), but the synthesis of CMF requires the quantitative consumption of HCl.⁵⁴⁶ It was proposed that cellulose hydrolysis to glucose, glucose isomerization to fructose and fructose dehydration to HMF all proceed in the aqueous phase containing HCl, while the conversion of HMF to CMF promotes the transfer and sequestration of hydrophobic CMF, thus reducing the further degradation of CMF. Moreover, the separation and purification of CMF are much easier than HMF owing to the lipophilicity and stability of CMF under acidic conditions. Owing to its rich derivative chemistry, CMF is an important starting material for the production of other high-value HMF derivates.⁵⁴⁶ For example, CMF can be readily converted to levulinic acid in hot water with high yield (Fig. 39).⁵⁴⁷

Fig. 38 Direct conversion of carbohydrates and biomass to CMF under mild conditions. Adapted from ref. 209 and 544–546.

Fig. 39 Conversion of CMF to levulinic acid. Adapted from ref. 547.

Acetoxymethylfurfural, the acetylation product of HMF, is also considered a potential intermediate since the acetyl moiety imparts more hydrophobic property and less reactivity to acetoxymethylfurfural.⁵⁴⁸ Bicker et al. reported that fructose can be converted to acetoxymethylfurfural (Fig. 40) in subcritical acetic acid (scAcOH) with an acetoxymethylfurfural yield of 37% with the fructose conversion of 98%.⁵⁴⁹ Gavilà et al. reported that the acetolysis of cellulose acetate in the presence of 2 equivalents of acetic anhydride using sulfuric acid as a catalyst gave a maximum acetoxymethylfurfural yield of ca. 40%.⁵⁵⁰

Fig. 40 Conversion of fructose to acetoxymethylfurfural. Adapted from ref. 549.

3.2. Oxidation of HMF

The oxidation of HMF can give a series of high-value products, including 2,5-diformylfuran (DFF), 2,5-furandicarboxylic acid (FDCA), 5-formyl-2-furan carboxylic acid (FFCA), and 5-hydroxymethyl-2-furancarboxylic acid (HMFCA) (Table 13) via thermochemical, biocatalysis, photocatalysis and electrocatalysis technologies. The reaction pathway (Fig. 41) and product selectivity depend on many factors, including the catalyst, solvent, additive, pH, oxidant and reaction conditions. Generally, thermal aerobic oxidation over metal oxide catalysts in neutral aqueous solution and photocatalytic oxidation in trifluorotoluene (PhCF₃) favor the selective oxidation of the alcohol side chain, resulting in the conversion of HMF to DFF.^551,552 In contrast, the combination of noble metal catalysts and basic solvents (or basic supports) is prone to oxidize the aldehyde side chain first.^553,554 Ag-based catalysts can selectively oxidize HMF to HMFCA, while Au, Pt and Pd-based catalysts not only can oxidize HMF to HMFCA under relatively mild conditions, but also further oxidize HMFCA to FDCA under relatively harsh conditions (high temperature and long reaction time).^{553,555–559} Since a plethora of original research and review articles have been published, in this section we try to provide a review on the oxidation of HMF with a focus on summarizing the promising reaction pathways, products and catalytic technologies, not aiming to give an exhaustive overview on the results contributed by the scientific community. The oxidation of HMF has already been extensively summarized in previous reviews from different perspectives, and readers can also consult these sources.^34,560–566

Fig. 41 Basic pathways for the oxidation of HMF to FDCA. Adapted from ref. 567.

Table 13 Oxidation of HMF to DFF and FDCA, and oxidative esterification of HMF with methanol to FDMC

Catalyst^a	Solvent	Conditions	Feedstock^a	Conversion	Yield	Ref.
a Relative to solvent. b Reaction conducted in a trickle-bed reactor with weight hourly space velocity (WHSV) of 900 h^−1 and gas hourly space velocity (GHSV) of 1 h^−1. c Electrochemical oxidation of HMF. d The conversion of carbohydrate to DFF was conducted via one-pot, two-step tandem process and the first step was conducted in the absence of catalyst. — Not provided.
M(NO3)x, 5–7.5 mol% (M = Fe, Cu, Al, Zn or H); TEMPO, 5 mol%	AcOH	50 °C, 5 h, 0.1 MPa O2	HMF 6.3 wt%	∼100%	DFF, 94–99%	568
M(NO3)x, 5–7.5 mol% (M = Fe, Cu); TEMPO, 5 mol%; NaNO2, 5 mol%	CH3CN	50 °C, 10–24 h, 0.1 MPa O2	HMF 6.3 wt%	∼100%	DFF, 94–99%	568
Cu/GO, 0.33 wt%; TEMPO, 0.4 wt%	CH3CN	70 °C, 8 h, 0.4 MPa O2	HMF, 0.84 wt%	25.5%	DFF, 18.4%	569
Cu/G-700H, 0.33 wt%; TEMPO, 0.4 wt%	CH3CN	70 °C, 8 h, 0.4 MPa O2	HMF, 0.84 wt%	12.5%	DFF, 8.6%	569
Cu/NG4, 0.33 wt%; TEMPO, 0.4 wt%	CH3CN	50 °C, 8 h, 0.4 MPa O2	HMF, 0.84 wt%	99.5%	DFF, 99.2% (FFCA 0.5%)	569
Cu/NG4, 0.33 wt%; TEMPO, 0.4 wt%	DMSO	70 °C, 8 h, 0.4 MPa O2	HMF, 0.84 wt%	100%	DFF, 92.2% (FFCA 3.2%)	569
Cu/NG4, 0.33 wt%; TEMPO, 0.4 wt%	CH3CN	70 °C, 8 h, 0.4 MPa O2	HMF, 0.84 wt%	99.5%	DFF, 82.5% (FFCA 1.2%)	569
NC-950, 0.2 wt%	CH3CN	100 °C, 14 h, 0.1 MPa air, 14.5 M HNO3 (0.02 mL)	HMF, 0.63 wt%	100%	DFF, 95.1%	570
NC-950, 0.2 wt%	CH3CN	100 °C, 14 h, 0.1 MPa air, 14.5 M HNO3 (0.02 mL)	HMF, 0.63 wt%	100%	DFF, 93.5%	570
I2, 10 mol%; NaOH, 20 mol%	TBPH (6 M)/H2O	70 °C, 36 h, 0.1 MPa air, 14.5 M HNO3 (0.02 mL)	HMF, 10 wt%	—	FDCA, 53%	571
CuO, 0.5 wt%	0.04 M NaOH	40 °C, 2 h, NaClO	HMF, 0.25 wt%	100%	FDCA, 99.80%	572
Co3O4, 0.5 wt%	0.04 M NaOH	40 °C, 2 h, NaClO	HMF, 0.25 wt%	100%	FDCA, 95.7%	572
MnO2, 0.5 wt%	0.04 M NaOH	40 °C, 2 h, NaClO	HMF, 0.25 wt%	100%	FDCA, 34.3%	572
NiO, 0.5 wt%	0.04 M NaOH	40 °C, 2 h, NaClO	HMF, 0.25 wt%	77.7%	FDCA, 18.1%	572
FeOx/C, 20 mol% metal	Toluene	150 °C, 12 h, 1 MPa O2	HMF, 3.1 wt%	100%	DFF, 99%	573
FeNx-900, 1 wt%	N,N-DMF	100 °C, 10 h, 0.5 MPa O2	HMF, 0.32 wt%	99.5%	DFF, 97.3%	574
Co–Al hydrotalcites, 0.5 wt%	DMSO	120 °C, 8 h, 0.3 MPa O2	HMF, 1.26 wt%	100%	DFF, 95%	575
Co–Al hydrotalcites, 0.5 wt%	DMSO	120 °C, 1.5 h, 0.3 MPa O2	HMF, 1.8 wt%	100%	DFF, around 10% (HMF 83%)	575
Co–Al hydrotalcites, 0.5 wt%	DMSO	120 °C, 8 h, 0.3 MPa O2	Fructose, 1.8 wt%	100%	DFF, 77%	575
Co–Al hydrotalcites (0.5 wt%)	DMSO	120 °C, 8 h, 0.3 MPa air	Fructose, 1.8 wt%	100%	DFF, 59%	575
10V2O5@MOR(60) 4 wt%, HCl 0.14 M	DMSO	130 °C, 4 h, 120 °C, 8 h, O2 balloon	Fructose, 2 wt%	100%	DFF, 94.8%	576
DICAT-V, 0.34 wt%	CH3CN	110 °C, 8 h, air 2 MPa	HMF, 1.3 wt%	92%	DFF, 88%	577
Without catalyst	0.1 M NaOH	130 °C, 2.5 h, 0.5 MPa O2	HMF, 0.45 wt%	100%	HMFCA, 4.3%; FDCA 0	578
CeO2-cube, 0.39 wt%	0.1 M NaOH	130 °C, 2.5 h, 0.5 MPa O2	HMF, 0.45 wt%	100%	HMFCA, 4.2%; FDCA 0	578
CeO2-oct, 0.41 wt%	0.1 M NaOH	130 °C, 2.5 h, 0.5 MPa O2	HMF, 0.45 wt%	100%	HMFCA, 4.0%; FDCA 0	578
CeO2-rod, 0.22 wt%	0.1 M NaOH	130 °C, 2.5 h, 0.5 MPa O2	HMF, 0.45 wt%	100%	HMFCA, 6.3%; FDCA 0	578
Au/CeO2-rod, 0.22 wt%	NaOH/H2O	130 °C, 2.5 h, 0.5 MPa O2	HMF, 0.45 wt%	—	FDCA, 87.4%	578
AuNPs-sPSB, 2.5 wt%	Cs2CO3/N,N-DMF	80 °C, 16 h, 1.5 MPa O2	HMF, 1.1 wt%	78%	DFF 62.4%, FFCA 15.6%	579
AuNPs-sPSB, 2.5 wt%	Cs2CO3/N,N-DMF	80 °C, 16 h, 1.5 MPa O2	HMF, 1.1 wt%	78%	DFF 13.9%, FFCA 74.1%	579
AuNPs-sPSB, 2 wt%	MeOH	25 °C, 16 h, 1.5 MPa O2	HMF, 0.88 wt%	78%	MHFCA 77.2%, FDMC 21.8%	579
AuNPs-sPSB, 2 wt%	DMA/MeOH(v/v = 4)	110 °C, 5 h, 3.5 MPa O2	HMF, 0.88 wt%	99%	FDCA 99%	579
Pd–Au/HT, 0.2 wt%	0.064 M NaOH	60 °C, 6 h, 60 mL min^−1 O2 flow	HMF, 0.4 wt%	100%	FDCA, 87.4%	558
Pd/HT, 0.2 wt%	0.064 M NaOH	60 °C, 6 h, 60 mL min^−1 O2 flow	HMF, 0.4 wt%	100%	FDCA <3%; HMFCA, 45%	558
Au/HT, 0.2 wt%	0.064 M NaOH	60 °C, 6 h, 60 mL min^−1 O2 flow	HMF, 0.4 wt%	100%	FDCA 0%; HMFCA 5%	558
AuPd NNWs, 0.05 wt%	0.033 M Na2CO3	60 °C, 6 h, 0.1 MPa O2	HMF, 0.42 wt%	>99%	FDCA 98%	580
AuPd NNWs, 0.05 wt%	0.033 M Na2CO3	60 °C, 6 h, 0.1 MPa O2	HMF, 0.42 wt%	>99%	FDCA 98%	581
AuPd porous, 0.05 wt%	0.033 M Na2CO3	60 °C, 6 h, 0.1 MPa O2	HMF, 0.42 wt%	>99%	FDCA 79% (HMFCA, 14%)	581
AuPd NPs, 0.05 wt%	0.033 M Na2CO3	60 °C, 6 h, 0.1 MPa O2	HMF, 0.42 wt%	>99%	FDCA 44% (HMFCA, 48%)	581
AuPd/TiO2, 0.05 wt%	0.033 M Na2CO3	60 °C, 6 h, 0.1 MPa O2	HMF, 0.42 wt%	>99%	FDCA 61% (HMFCA, 37%)	581
AuPdt NNWs, 0.05 wt%	0.033 M Na2CO3	60 °C, 6 h, 0.1 MPa O2	HMF, 0.42 wt%	>99%	FDCA 98%	581
AuPd-nNiO, metal/HMF molar ratio = 0.01	H2O	90 °C, 6 h, 1 MPa O2	HMF, 0.25 wt%	95%	FDCA 70%	582
Ag-PVP/ZrO2, 0.1 wt%	2.5 g L^−1 NaOH	20 °C, 2 h, 60 mL min^−1 O2	HMF, 0.4 wt%	100%	HMFCA, 98.2%	555
Ag2O, 2 wt%	0.12 mM Na2CO3	90 °C, 1 h, 0.2 mL min^−1 H2O2 (8.1 wt%)	HMF, 0.378 wt%	100%	HMFCA, 98%	556
Ag2O, 2 wt%	0.12 mM Na2CO3	30 W microwave 90 °C, 24 min, 0.2 mL min^−1 H2O2 (8.1 wt%)	HMF, 0.378 wt%	99%	HMFCA, 97%	556
Ag2O (5 runs), 2 wt%	0.12 mM Na2CO3	90 °C, 1 h, 0.2 mL min^−1 H2O2 (8.1 wt%)	HMF, 0.378 wt%	57%	HMFCA, 42%	556
Activated MnO2, 1 wt%	0.12 M NaHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	>99%	FDCA 74%, FFCA 15%	583
β-MnO2, 1 wt%	0.12 M NaHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	>99%	FDCA 28%, FFCA 66%	583
β-MnO2-HS, 1 wt%	0.12 M NaHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	—	FDCA 86%, FFCA 1%	583
λ-MnO2, 1 wt%	0.12 M NaHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	>99%	FDCA 12%, FFCA 66%	583
γ-MnO2, 1 wt%	0.12 M NaHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	93%	FDCA 5%, FFCA 60%	583
α-MnO2, 1 wt%	0.12 M NaHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	>99%	FDCA 59%, FFCA 36%	583
δ-MnO2, 1 wt%	0.12 M NaHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	96%	FDCA 10%, FFCA 69%	583
ε-MnO2, 1 wt%	0.12 M NaHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	>99%	FDCA 63%, FFCA 27%	583
N-MnO2, 7.5 wt%	Toluene	25 °C, 6 h, 16 mL min^−1 0.1 MPa O2	HMF, 3.2 wt%	100%	DFF 99.9%	552
Co3O4-MnO2 (Co/Mn = 0.25), 1 wt%	0.1 M NaHCO3	120 °C, 5 h, 1 MPa O2	HMF, 0.63 wt%	99%	FDCA 95%	584
MnCo2O4, 2 wt%	0.012 M KHCO3	100 °C, 24 h, 1 MPa O2	HMF, 0.5 wt%	99.5%	FDCA, 70.9%	585
Pd-MnO2, 0.1 wt%	H2O	100 °C, 6 h, 25 mL min^−1 0.1 MPa O2	HMF, 0.08 wt%	100%	FDCA 88.1%	586
PdNP/MnO2, 0.1 wt%	H2O	100 °C, 6 h, 25 mL min^−1 0.1 MPa O2	HMF, 0.08 wt%	100%	FDCA 67.8%	586
MnO2, 0.1 wt%	H2O	100 °C, 6 h, 25 mL min^−1 0.1 MPa O2	HMF, 0.08 wt%	42.9%	FDCA 30.1%	586
Commercial Pd/C, 0.1 wt%	H2O	100 °C, 6 h, 25 mL min^−1 0.1 MPa O2	HMF, 0.08 wt%	100%	FDCA 76.6%	586
Cs/MnOx, 0.2 wt%	N,N-DMF	100 °C, 12 h, 1 MPa O2	HMF, 0.6 wt%	98.4%	DFF, 94.7%	587
HPMV6, 5 wt%	[BMIM]Cl	140 °C, 6 h, 1 MPa O2	HMF, 1.3 wt%	100%	FDCA, 88.8%; FFCA 1.1%	588
HPMV6, 5 wt%	[BMIM]Cl	140 °C, 9 h, 1 MPa O2	Fructose, 1.8 wt%	—	FDCA, 30.7%	588
HPMV6, 5 wt%	[BMIM]Cl	140 °C, 9 h, 1 MPa O2	Glucose, 1.8 wt%	—	FDCA, 48.6%	588
Pt-PVB-ACS 800, 0.4 wt%	H2O	110 °C, 5 h, 1 MPa O2	HMF, 0.63 wt%	100%	FDCA, 99%	589
Pt-ACS 800, 0.4 wt%	H2O	110 °C, 5 h, 1 MPa O2	HMF, 0.63 wt%	100%	FDCA, 75%; FFCA 25%	589
Pt-PVB-ACS 800 (10 runs), 0.4 wt%	H2O	110 °C, 5 h, 1 MPa O2	HMF, 0.63 wt%	100%	FDCA, 79%; FFCA 21%	589
Au/HSAG-ox, 0.35 wt%	0.057 M NaHCO3	90 °C, 12 h, 1 MPa O2	HMF, 0.3 wt%	45%	HMFCA 43%, FDCA 0	590
Au/HSAG, 0.35 wt%	0.057 M NaHCO3	90 °C, 12 h, 1 MPa O2	HMF, 0.3 wt%	45%	HMFCA 93%, FDCA 6%	590
Au/HSAG-H, 0.35 wt%	0.057 M NaHCO3	90 °C, 12 h, 1 MPa O2	HMF, 0.3 wt%	45%	HMFCA 56%, FDCA 44%	590
Au/HSAG-H, 0.35 wt%	0.057 M NaHCO3	90 °C, 12 h, 1 MPa O2	HMF, 0.3 wt%	45%	HMFCA 22%, FDCA 75%	590
Mn6Fe1Ox, 1.2 wt%	N,N-DMF	110 °C, 5 h, 1.5 MPa O2	HMF, 2.5 wt%	97%	DFF, 95%	591
CeO2, 0.7 wt%	H2O	110 °C, 15 h, 0.9 MPa O2	HMF, 0.2 wt%	21.3%	DFF, 0.1% (HMFCA 0, FFCA 18.9%, FDCA 0)	592
MnO2, 0.7 wt%	H2O	130 °C, 12 h, 0.9 MPa O2	HMF, 0.2 wt%	99.0%	DFF 0 (HMFCA 0, FFCA 23.3%, FDCA 74.7%)	592
MgO, 0.7 wt%	H2O	110 °C, 15 h, 0.9 MPa O2	HMF, 0.2 wt%	21.3%	DFF, 0.4% (HMFCA 3.4%, FFCA 0, FDCA 0, formic acid 19.4%)	592
MgO·MnO2·CeO2, 0.7 wt%	H2O	110 °C, 10 h, 2 MPa O2	HMF, 0.2 wt%	98.8%	DFF 94.1% (HMFCA 0.8%, FFCA 0, FDCA 0)	592
CuO·MnO2·CeO2, 0.7 wt%	H2O	130 °C, 4 h, 2 MPa O2	HMF, 0.2 wt%	79.0%	DFF 0 (HMFCA 0, FFCA 0, FDCA 77.9%)	592
Ru/Mn6Ce1Oy, 0.7 wt%	H2O	150 °C, 15 h, 10 bar O2	HMF, 1.3 wt%	≥99.0%	DFF 0 (HMFCA 0, FFCA 0, FDCA 77.9%)	593
Pt/NC-CeO2, 0.25 wt%	H2O	110 °C, 8 h, 0.4 MPa O2	HMF, 0.16 wt%	100%	FDCA 100%	594
Ru/ZrO2 H-aero, 0.33 wt%	H2O	120 °C, 16 h, 10 bar O2	HMF, 0.63 wt%	100%	FDCA, 97%	595
Ru(III)-Fe3O4@SiO2, 0.5 wt%	0.25 M n-butylamine	110 °C, 48 h, 10 bar O2	HMF, 1 wt%	92%	FDCA, 74.2%	596
Fe3O4@SiO2-CoOx, 0.5 wt%	0.78 M NaOH	110 °C, 8 h, 10 bar O2	HMF, 1 wt%	78.6%	Succinic acid, 72.9%	596
Fe3O4@SiO2-MnOx, 0.5 wt%	0.25 M n-butylamine	110 °C, 48 h, 10 bar O2	HMF, 1 wt%	5%	FDCA, 3.6%	596
Fe3O4@SiO2-NH2, 0.5 wt%	0.25 M n-butylamine	110 °C, 48 h, 10 bar O2	HMF, 1 wt%	35.6%	FDCA, 30.3%	596
PCNx, 0.2 wt%	0.1 M K2CO3	70 °C, 36 h, O2 flow	HMF, 1.3 wt%	—	DFF, 5%	597
N-Doped graphene, 0.2 wt%	0.1 M K2CO3	70 °C, 36 h, O2 flow	HMF, 1.3 wt%	—	DFF, 83%	597
g-C3N4, 0.2 wt%	0.1 M K2CO3	70 °C, 36 h, O2 flow	HMF, 1.3 wt%	—	DFF, 15%	597
SO3H-PANI-FeVO4, 2.4 wt%	DMSO	140 °C, 24 h, O2 flow	HMF, 2.5 wt%	100%	DFF, 99%	452
RuO2/IL-graphene, 0.33 wt%	Toluene	100 °C, 12 h, O2 flow	HMF, 2.1 wt%	60%	DFF, 60%	598
Rh/C, 0.2 wt%	scCO2	150 °C, 2 h, 8 MPa CO2	HMF, 2.1 wt%	100%	DFF,100%	599
VPO (V : P = 0.25), 0.2 wt%	H2O	140 °C, 24 h, air	HMF, 0.2 wt%	22.0%	DFF, 0	600
VPO (V : P = 0.25), 0.2 wt%	Toluene	140 °C, 24 h, air	HMF, 0.2 wt%	15.0%	DFF, 15%	600
VPO (V : P = 0.25), 0.2 wt%	MIBK	140 °C, 24 h, air	HMF, 0.2 wt%	43.1%	DFF, 19.5%	600
VPO (V : P = 0.25), 0.2 wt%	DMF	140 °C, 24 h, air	HMF, 0.2 wt%	27.4%	DFF, 11.3%	600
MoV2@CP-5.5–400, 2.3 wt%	DMSO	120 °C, 4 h, 1 atm O2	Fructose, 4.5 wt%	—	DFF, 87.3% (HMF, 0.6%)	601
MoV2@CP-5.5, 1 wt%	DMSO	130 °C, 3 h, 1 atm O2	Fructose, 4.5 wt%	—	DFF, 20.0% (HMF, 50.5%)	601
CeCu(OH)6Mo6O18, 1.7 wt%	p-Chlorotoluene	130 °C, 8 h, 1 atm O2	HMF, 0.7 wt%	99%	DFF, 99%	602
P(DVPI-Br), 1 wt%	DMSO	130 °C, 3 h, 1 atm O2	Fructose, 4.5 wt%	—	DFF, 9.6% (HMF, 60.6%)	601
MoV2@CP-5.5-400, 2.3 wt%	DMSO	135 °C, 5 h, 1 atm O2	glucose, 4.5 wt%	—	DFF, 51.2% (HMF, 0.4%)	601
MoV2@CP-5.5-400, 2.3 wt%	DMSO	135 °C, 5 h, 1 atm O2	sucrose, 4.5 wt%	—	DFF, 20.4% (HMF, 0.2%)	601
MoV2@CP-5.5-400, 2.3 wt%	DMSO	135 °C, 5 h, 1 atm O2	inulin, 4.5 wt%	—	DFF, 61.6% (HMF, 0.6%)	601
Octahedral MnO2 molecular sieve, 1.2 wt%	DMSO	120 °C, 10 h, 1 atm O2	HMF, 2.5 wt%	99%	DFF, 99%	551
Without catalyst	DMSO	120 °C, 10 h, 1 atm O2	HMF, 2.5 wt%	99%	DFF, 1.9%	551
H-Beta zeolite, 1.2 wt%	DMSO	120 °C, 10 h, 1 atm O2	HMF, 2.5 wt%	99%	DFF, 1.9%	551
V-CS-800, 1.3 wt%	DMSO	120 °C, 9 h, 20 mL min^−1 O2	HMF, 0.13 wt%	100%	DFF, 99%	603
Mo-HNC, 0.4 wt%	DMSO	150 °C, 9 h, 20 mL min^−1 O2	HMF, 4 wt%	100%	DFF, 77%	604
Ru/MgAlO, 1 wt%	H2O	140 °C, 4 h, 0.62 MPa O2	HMF, 1.3 wt%	100%	FDCA 99% (DFF 0.5%, HMFCA 0, FFCA 0.2%)	605
Ru/MgO, 1 wt%	H2O	160 °C, 4 h, 0.62 MPa O2	HMF, 1.3 wt%	58%	FDCA 90%	605
Ru/La2O3, 1 wt%	H2O	140 °C, 4 h, 0.62 MPa O2	HMF, 1.3 wt%	4.6%	FDCA 0.8% (DFF 0, HMFCA 0, FFCA 1.4%)	605
Ru/CeO2, 1 wt%	H2O	140 °C, 4 h, 0.62 MPa O2	HMF, 1.3 wt%	89%	FDCA 7% (DFF 45%, HMFCA 0, FFCA 35%)	605
Ru/ZrO2, 1 wt%	H2O	140 °C, 4 h, 0.62 MPa O2	HMF, 1.3 wt%	100%	FDCA 0 (DFF 86%, HMFCA 0, FFCA 14%)	605
Ru/Al2O3 ^b	0.1 M Na2CO3	110 °C, 5 h, 3 MPa O2	HMF, 1.3 wt%	64%	FDCA, 63%	606
Au/CeO2, 0.45 wt%	0.16 M NaOH	70 °C, 4 h, 10 bar O2	HMF, 1 wt%	—	FDCA, 63%	607
Au/Ce0.50Zr0.50O2, 0.66 wt%	0.16 M NaOH	70 °C, 4 h, 10 bar O2	HMF, 1 wt%	—	FDCA, 70%	607
Au/Ce0.25Zr0.75O2, 0.66 wt%	0.16 M NaOH	70 °C, 4 h, 10 bar O2	HMF, 1 wt%	—	FDCA, 80%	607
Au/ZrO2, 0.69 wt%	0.16 M NaOH	70 °C, 4 h, 10 bar O2	HMF, 1 wt%	—	FDCA, 58%	607
Au/ZrO2, 0.33 wt%	0.4 M NaOH	125 °C, 5 h, 10 bar O2	HMF, 1.3 wt%	100%	FDCA, 89%	559
Ag/ZrO2, 0.29 wt%	0.4 M NaOH	50 °C, 1 h, 10 bar O2	HMF, 1.3 wt%	100%	HMFCA, 98%	559
PVA-Au/MgF2, 0.5 wt%	H2O	110 °C, 2 h, 20 bar air	HMF, 0.3 wt%	7%	FDCA, 0 (DFF 6.7%, HMFCA 0.2%, FFCA 0.07%)	608
PVA-Au/0.6MgF2-0.4MgO2, 0.5 wt%	H2O	110 °C, 2 h, 20 bar air	HMF, 0.3 wt%	77%	FDCA, 14% (DFF 0, HMFCA 42%, FFCA 22%)	608
PVA-Au/0.4MgF2-0.6MgO2, 0.5 wt%	H2O	110 °C, 2 h, 20 bar air	HMF, 0.3 wt%	98%	FDCA, 63% (DFF 0, HMFCA 53%, FFCA 12%)	608
PVA-Au/MgO2, 0.5 wt%	H2O	110 °C, 2 h, 20 bar air	HMF, 0.3 wt%	99%	FDCA, 91% (DFF 0, HMFCA 7%, FFCA 2%)	608
Pd/C 2.5 mol%; Co(NO3)2 2.5 mol%; Bi(NO3)3, 2.5 mol%	MeOH, K2CO3 (20 mol%)	60 °C, 14 h, 0.1 MPa O2	HMF, 3.1 wt%	∼98%	HMFE, 93%	609
Heterogeneous PdCoBi/C, 2.5 mol%	MeOH, K2CO3 (20 mol%)	60 °C, 14 h, 0.1 MPa O2	HMF, 3.1 wt%	∼98%	HMFE, 96%	609
Au–Fe3O4 (2 mol%)	MeOH, 0.1 M K2CO3	25 °C, 24 h, 0.1 MPa O2	HMF, 1.3 wt%	100%	HMFE, 92%	610
AuPd–Fe3O4 (2 mol%)	MeOH, 0.1 M K2CO3	25 °C, 24 h, 0.1 MPa O2	HMF, 1.3 wt%	92%	FDMC, 92%	610
Co@CN	MeOH	80 °C, 12 h, 0.1 MPa O2	HMF, 1 wt%	100%	FDMC, 95%	611
Co7Cu3-NC	MeOH	80 °C, 4 h, 0.2 MPa O2	HMF, 1 wt%	100%	FDMC, 95%	612
Au (3 wt%)–TiO2, 2.5 wt%	0.13 mL min^−1 1.2 M NaOH	85 °C, 0.6 h, 0.13 mL min^−1 10 wt% H2O2, 50 W microwave	HMF, 2.5 wt%	100%	FDCA, 68%	613
10Co@22Nb@MNP, 0.55 wt%	MeCN	100 °C, 12 h, 70% TBHP	HMF, 0.55 wt%	96.9%	FDCA 93.5%	614
Cu/NG, 0.36 wt%	MeCN	70 °C, 24 h, 70% TBHP (0.1 mL per 2 h)	HMF, 0.6 wt%	100%	FDCA 95.2%	615
CdS nanorod, 2.2 wt%	DMSO	80 °C, 48 h, 70% TBHP	HMF, 1.5 wt%	91.1%	FFCA 91.1%	616
AuPd@Co3O4, 0.07 wt%	0.2 M K2CO3+ 0.1 M NaOH	90 °C, 1 h, 10% H2O2 (adding with a rate of 1.6 mL h^−1)	HMF, 0.26 wt%	100%	FDCA, 95%	617
6-CuO/m-Al2O3, 2.5 wt%	DMSO/H2O (v/v = 3)	100 °C, 20 h, TBHP	HMF, 0.32 wt%	—	FDMC, 96%	618
Pd@Beta, 0.3 wt%	0.15 M Na2CO3	90 °C, 24 h, O2 balloon	HMF, 2.5 wt%	99%	FDMC, 95%	619
Au (3 wt%)-TiO2, 2.5 wt%	0.13 mL min^−1 1.2 M NaOH	97 °C, 0.5 h, 0.13 mL min^−1 10 wt% H2O2, 50 W microwave	HMF, 2.5 wt%	100%	FDCA, 99%	613
Ni3N@C	1 M KOH (pH 13.6)	1.45 V vs. RHE^c	HMF, 10 mM	∼100%	FDCA, 98% (FF 99%)	620
NiCo2O4	1 M KOH (pH 13.6)	1.45 V vs. RHE^c	HMF, 10 mM	—	FDCA, 90% (FF 100%)	621
NiCo2O4	1 M KOH (pH 13.6)	1.5 V vs. RHE; 34.75 C^c	HMF, 5 mM	—	FDCA, 90.4% (FF 87.5%)	622
NiOOH	1 M KOH (pH 13.6)	1.47 V vs. RHE; 4.7 h^c	HMF, 5 mM	99.8%	FDCA, 96.0% (FF of FDCA 96.0%, DFF 0.03%, HMFCA 1.31%, 1.79% FFCA)	567
CoOOH	1 M KOH (pH 13.6)	1.56 V vs. RHE; 22 h^c	HMF, 5 mM	95.5%	FDCA, 35.1% (FF of FDCA 35.1%, DFF 0.56%, HMFCA 13.6%, 30.8% FFCA)	567
FeOOH	1 M KOH (pH 13.6)	1.71 V vs. RHE; 2.3 h^c	HMF, 5 mM	16.0%	FDCA, 1.59% (FF of FDCA 1.59%, DFF 3.64%, HMFCA 2.74%, 4.30% FFCA)	567
NiCoFe-LDHs	1 M KOH	55 °C, 1.71 V vs. RHE; 2.3 h^c	HMF, 10 mM	95.5%	FDCA, 84.9%	623
NiCoFe-LDHs	1 M KOH	55 °C, 1.71 V vs. RHE; 2.3 h^c	HMF, 10 mM	95.5%	FDCA, 84.9%	623
MoO2–FeP@C	10 M KOH	1.486 V vs. RHE	HMF, 10 mM	100%	FDCA, 98.6%	624
Sulfonated MoO3–ZrO2, 0.2 wt%	DMSO	165 °C, 2 h, 20 mL min^−1 O2	Fructose, 4 wt%	100%	DFF, 74%	625
MoOx/CS, 0.6 wt%	DMSO	165 °C, 2 h, 20 mL min^−1 O2	Fructose, 4 wt%	100%	DFF, 78%	626
Ru(3%)/H-Beta, 2.4 wt%	DMSO	120 °C, 1 h, N2 ^d; 140 °C, 24 h, 20 mL min^−1 O2	Fructose, 2.47 wt%	100%	DFF, 80%	627
Ru(3%)/H-Beta, 2.4 wt%	DMSO	120 °C, 4 h, N2 ^d; 140 °C, 24 h, 20 mL min^−1 O2	Sucrose, 2.47 wt%	97%	DFF, 70%	627
Ru(3%)/H-Beta, 2.4 wt%	DMSO	120 °C, 6 h, N2 ^d; 140 °C, 24 h, 20 mL min^−1 O2	Glucose, 2.47 wt%	97%	DFF, 48%	627
Ce0.5Fe0.15Zr0.35O2, 1 wt%	[BMIM]Cl	160 °C, 24 h, 2 MPa O2	Glucose, 1.26 wt%	100%	FDCA, 44.2%	628

---

3.2.1. Selective oxidation of HMF to DFF. Metal oxides and their composites have been widely used for the selective oxidation of HMF to DFF. Bhaskar et al. reported that octahedral MnO₂ molecular sieves can oxidize HMF to DFF selectively without the further oxidation of DFF under relatively mild conditions (120 °C, 10 h, 1 atm O₂).⁵⁵¹ Liu et al. reported that Mn₆Fe₁O_x shows high catalytic activity for the selective oxidation of HMF to DFF, obtaining a DFF yield of up to 95%.⁵⁹¹ They proposed that the improved catalytic activity, compared with separate oxides, is attributed to the high density of catalytic active sites (Mn⁴⁺–O²⁻ pairs) owing to the introduction of α-Fe₂O₃. Ke et al. reported that a nitrogen-doped manganese oxide (N-MnO₂) catalyst exhibited superior catalytic activity for the oxidation of HMF to DFF at room temperature, affording a DFF yield higher than 99.9%.⁵⁵² They proposed that the surface defect sites and coordinatively unsaturated Mn sites resulting from nitrogen doping are crucial for the enhancement of the catalytic performance. Similarly, cesium-doped manganese dioxide (Cs/MnOx) showed much higher catalytic activity than un-doped MnOx for the oxidation of HMF to DFF in the medium of N,N-DMF, affording a DFF yield of 94.7% with the HMF conversion of 98.4%.⁵⁸⁷ Zhao et al. reported that a vanadium dioxide embedded in mesoporous carbon sphere (V-CS) catalyst enabled the selective aerobic oxidation of HMF to DFF under atmospheric oxygen, affording a DFF yield of 99%.⁶⁰³ They suggested that the superior catalytic activity is attributed to the porous structure and the highly dispersed vanadium dioxide species. Pawar et al. showed V supported by polymethylaminosiloxane (abbreviated as DIC_AT-V) was more capable than other transition metals, including Fe, Co and Mn, for the chemoselective oxidation of HMF to DFF using air as the oxidant in the medium of CH₃CN, affording the best DFF yield of 88%.⁵⁷⁷

As a highly effective catalyst for the selective oxidation of alcohols to aldehydes or ketones, (2,2,6,6-tetramethylpiperidin-1-yl)oxyl (TEMPO) has been widely investigated for the oxidation of HMF to DFF. For example, Hong et al. reported that the combination of M(NO₃)_x (M = Fe, Cu, Al, Zn or H) and TEMPO is highly active for the oxidation of HMF to DFF in acetic acid (AcOH), while the combination of M(NO₃)_x (M = Fe or Cu), TEMPO and NaNO₂ is effective for this conversion in CH₃CN, giving a DFF yield higher than 95% at 50 °C using pure O₂ as the oxidant.⁵⁶⁸ Even using air as the oxidant, the combination of Fe(NO₃)₃, TEMPO and NaNO₂ still afforded a DFF yield of 88%.

Nitrogen-doped carbon and its composite materials have also been extensively investigated for the selective oxidation of HMF to DFF. Ren et al. developed a metal-free oxidation strategy using a nitrogen-doped carbon material (NC), HNO₃ and O₂ as the catalyst, co-catalyst and terminal oxidant, respectively, to achieve the conversion of HMF to DFF (Fig. 42) under mild reaction conditions.⁵⁷⁰ In this catalytic system, the oxidation of HMF was initiated by HNO₃ and then HNO₃ could be recovered instantaneously by O₂. A DFF yield of up to 95.1% was obtained over the nitrogen-doped carbon material with a catalytic amount of HNO₃. Similarly, Xu et al. reported that the Anderson-type catalyst CeCu(OH)₆Mo₆O₁₈ enabled the selective oxidation of HMF to DFF (Fig. 43) via a similar mechanism with oxygenase enzymes.⁶⁰² In this procedure, Mo⁵⁺ species play an important role in the oxidation of HMF to DFF, while Ce⁴⁺ species function as electron transfer mediators and [Cu(OH)₆Mo₆O₁₈]⁴⁻ anions serve as electron-donors. Lv et al. reported that the use of Cu nanoparticles supported on pyridinic-nitrogen-dominated nitrogen-doped graphene (Cu/NG₄) as the catalyst with TEMPO as the co-catalyst enabled the efficient oxidation of HMF to DFF under relatively mild conditions (70 °C, 8 h, 0.4 MPa O₂), affording a high DFF yield (99.2%) with a high HMF conversion (99.8%).⁵⁶⁹ The superior catalytic activity and stability of Cu/NG₄ compared with Cu(NO₃)₂, Cu/GO and Cu/G-900H catalysts was mainly attributed to the strong interaction between the Cu nanoparticles and pyridinic N. Zhang et al. reported that the FeN_x/C catalyst afforded a DFF yield of 97.3% from HMF.⁵⁷⁴ They identified the Fe–N₄ species as the main active sites via combined catalyst characterizations and control experiments. However, the FeN_x/C catalyst was deactivated rapidly in the recycling experiment owing to the aggregation of the Fe species. Heat treatment under an NH₃/N₂ flow was required to reactivate the catalyst. Recently, single-atom supported on nitrogen-doped carbon (M–N–C) materials have been intensively developed and exhibit great potential in several fields, in particular CO₂ hydrogeneration, CO oxidation and electrocatalysis.^207,629 The development of these types of single-atom materials is promising for the selective oxidation of HMF.

Fig. 42 Oxidation of HMF to DFF using NC, HNO₃ and O₂ as the catalyst, co-catalyst and terminal oxidant, respectively. Adapted from ref. 570.

Fig. 43 Oxidation of HMF to DFF over biomimetic CeCu(OH)₆Mo₆O₁₈ catalyst. Adapted from ref. 602.

The conversion of carbohydrates to DFF can be either achieved via a one-pot, two-step process or one-pot, one-step process. For example, FeVO₄-supported–SO₃H-functionalized polyaniline (SO₃H–PANI–FeVO₄) enabled the effective conversion of carbohydrates to DFF via a one-pot, two-step process, obtaining DFF yields of 80% and 91% from sucrose and fructose, respectively.⁴⁵² Mo-Containing materials are promising catalysts for the selective oxidation of biomass-derived chemicals.⁶³⁰ Molybdenum oxides supported on carbon spheres (MoO_x/CS), which were prepared using glucose and phosphomolybdic via a hydrothermal carbonization method followed by calcination in air, enabled the one-pot conversion of fructose to DFF in DMSO under atmospheric pressure oxygen, obtaining a DFF yield of 78% with the fructose conversion of 100% at 160 °C within 2 h.⁶²⁶ The acid and oxide sites in the MoO_x/CS catalyst were responsible for the dehydration of fructose to HMF and aerobic oxidation of HMF to DFF, respectively. Wang et al. designed an MoV₂@CP-5.5-400 catalyst via the copolymerization of an ionic liquid bearing carboxylic acid and divinyl benzene and introduction of the heteropolyacid H₅PMo₁₀V₂O₄₀ through ion-exchange followed by partial carbonization, to achieve the one-pot, one-step conversion of carbohydrates to DFF.⁶⁰¹ The catalytic activity for the degradation of fructose to HMF and oxidation of HMF to DFF were both enhanced owing to the increased acid and oxidation property resulting from the partial carbonization. Consequently, the catalyst gave DFF yields of 87.3% and 51.2% from fructose and glucose, respectively. Raut et al. reported that Co–Al hydrotalcite is active for both the dehydration of fructose to HMF and the oxidation of DFF in DMSO, thus enabling the one-pot conversion of fructose with DFF yields of 77% and 59% using 0.3 MPa O₂ and air as oxidant, respectively.⁵⁷⁵ The solvent has an important influence on the conversion of carbohydrates to DFF. For example, Lai et al. demonstrated that DMSO is more suitable than water, toluene, MIBK and N,N-DMF as the reaction medium for the one-pot conversion of fructose to DFF over a vanadium phosphate oxide (VPO) heterogeneous catalyst.⁶⁰⁰

Several studies have shown that bromide salts play an important role in the oxidation of HMF to DFF and the one-pot conversion of fructose to DFF, but the mechanism is still unclear. Laugel et al. reported that the use of bromides, including HBr and NaBr, in DMSO at the evaluated temperature and prolonged time (150 °C, 23 h) could promote the one-pot conversion of fructose to DFF, obtaining DFF yields of 13% and 67%, respectively.⁶³¹ They believed that this conversion involves the generation of 5-(bromomethyl)furan-2-car-baldehyde intermediate followed by Kornblum-type reaction, and the main catalytic species are the strong acid species generated from the in situ thermolysis of DMSO. The combination of KBr and TFP-DABA under mild conditions (100 °C, 12 h) gave DFF yields of 65% and 97% from HMF and fructose, respectively, in the medium of DMSO.⁴³⁸ However, these processes led to the stoichiometric consumption of DMSO, which is impractical for industrial production. In contrast, Verma et al. claimed that sulfonated graphitic carbon nitride (Sg-CN) combined with KBr enabled the one-pot conversion of fructose to DFF via the dehydration of fructose to HMF over Sg-CN followed by the oxidation of HMF to DFF by KBr in the medium of water under mild reaction conditions (100 °C, 0.5 h), while the combination of Sg-CN, DMSO and KBr gave 5-methylsulfanylmethyl-furan-2-carbaldehyde (97%) predominantly with a small amount of DFF (3%).⁴⁵⁸ Nevertheless, it is confusing that KBr is considered an oxidant in this study.

In addition to the oxidation of HMF to DFF with an external oxidant, HMF can also be converted to DFF via the Meerwein–Ponndorf–Verley–Oppenauer (MPVO) reaction or dehydrogenation reaction. For example, DFF and BHMF can be simultaneously produced from HMF (Fig. 44) via the MPVO reaction over the Lewis acid AlMe₃, without the use of an external oxidant or reductant.⁶³² In this process, HMF functions as both an oxidant and reductant, obtaining an DFF/BHMF molar ratio approaching 1 : 1. Under the optimized conditions (microwave irradiation, 80 °C, 1 h), an HMF conversion of up to 44.7% can be obtained within 1 h. Chatterjee et al. reported that the synergistic effect of the Rh/C catalyst with compressed carbon dioxide (scCO₂) enabled the efficient dehydrogenation of HMF to DFF under mild conditions without using an additional hydrogen acceptor or oxidant.⁵⁹⁹ In this catalytic system, the scCO₂ not only accelerated the reaction by removing hydrogen, but also shifted the equilibrium toward DFF as a hydrogen acceptor (Fig. 45).

Fig. 44 Simultaneously production of DFF and BHMF from HMF via the MPVO reaction. Adapted from ref. 632.

Fig. 45 Conversion of HMF to DFF via a dehydrogenation reaction. Adapted from ref. 599.

Photocatalytic oxidation is an effective approach to achieve the conversion of HMF to DFF (Table 14).^633,634 Polymeric carbon nitride (C₃N₄) and C₃N₄-based materials have been widely investigated as photocatalysts for the oxidation of HMF to DFF. However, bulk C₃N₄ usually exhibits a low DFF yield and selectivity. H₂O₂-treated C₃N₄ exhibited higher selectivity toward DFF than bare C₃N₄.^635–637 Similarly, WO₃/g-C₃N₄, which was synthesized from ammonium tungstate hydrate and melamine via a calcination method, exhibited much better photocatalytic performance than g-C₃N₄ for the oxidation of HMF to DFF, attaining the maximum HMF conversion of 27.4% with DFF selectivity of 87.2% under visible-light (>400 nm).⁶³⁸ However, the use of the toxic PhCF₃ and CH₃CN/PhCF₃ as the reaction medium is the main disadvantage of these photocatalytic systems. Yang et al. reported that the combination of a tetraalkylammonium decatungstate (TMADT) catalyst with HBr additive enabled the selective photocatalytic oxidation of HMF in the medium of CH₃CN, attaining a DFF yield of 67.1% with FDCA yield of 5.8% and HMF conversion of 83.1%.⁶³⁹ They suggested that the photo-excited decatungstate can preferentially oxidize HBr to the Br atom free radical, which is crucial for the improvement the DFF yield.

Table 14 Photocatalytic oxidation of HMF

Catalyst	Catalyst loading^a	Solvent	Conditions	Feedstock^a	Conversion	H2 evolution rate	Yield (%)	Ref.
a Relative to solvent. — Not detected.
TiO2-m	—	0.16 M NaOH	30 °C, 8 h, 1 MPa O2, 300 W Xe-lamp, 250–2500 nm	HMF, 0.3 wt%	22%	—	DFF 5% (CO2 8%)	634
Au–TiO2-m	—	0.16 M Na2CO3	30 °C, 8 h, 20 bar air, 300 W Xe-lamp, 250–2500 nm	HMF, 0.3 wt%	15%	—	DFF 3% (CO2 8%)	634
AuNPs/TiO2	2.5 wt%	0.1 M Na2CO3	30 °C, 8 h, atmospheric air, light (λ = 350–400 nm, 0.3 W cm^−2)	HMF, 1.3 wt%	99%	—	HMFCA 90%	557
AuNPs/TiO2	2.5 wt%	0.1 M Na2CO3	30 °C, 8 h, atmospheric air, light (λ = 420–78 nm, 0.3 W cm^−2)	HMF, 1.3 wt%	99%	—	HMFCA 95%	557
Nb2O5	1 wt%	PhCF3	>400 nm, 6 h, 10 mL min^−1 O2	HMF, 1.3 wt%,	19.2%	—	DFF, 17.4%	640
g-C3N4	1 wt%	CH3CN/PhCF3	>400 nm, 6 h, 10 mL min^−1 O2	HMF, 2.5 wt%	12%	—	DFF, 7.5%	638
WO3/g-C3N4	1 wt%	CH3CN/PhCF3	>400 nm, 6 h, 10 mL min^−1 O2	HMF, 2.5 wt%,	27.4%	—	DFF, 23.9%	638
H2O	0.07 wt%	H2O (pH = 7)	Simulated solar light, 6 h, 10 mL min^−1 O2	HMF, 0.006 wt%	73%	—	DFF, 36%	641
TMADT	1 wt%	6 M HBr, MeCN	Visible light from 35 W of tungsten-bromine lamp, 6 h, 1 atom O2	HMF, 0.25 wt%	83.1%	—	DFF, 67.1% (FDCA 5.8%)	639
CoPz/g-C3N4	0.01 wt%	H2O	300–1000 nm light from Xe light, 20 mL min^−1, air, potassium biphthalate/Na2B4O7 buffer solution (pH = 4.01), 14 h	HMF, 0.2 wt%	100%	—	FDCA, 80%	642
CoPz/g-C3N4	0.01 wt%	H2O	300–1000 nm light from Xe light, 20 mL min^−1, air, potassium biphthalate/Na2B4O7 buffer solution (pH = 9.18), 14 h	HMF, 0.2 wt%	99.1%	—	FDCA, 96%	642
ZnxCd1−xS-P	0.05 wt%	H2O	Visible light from LED (30 × 3 W), Ar, 8 h	HMF, 0.2 wt%	—	419 μmol h^−1 g^−1	—	643
ZnxCd1−xS-P	0.05 wt%	H2O	Visible light from LED (30 × 3 W), Ar, 8 h	HMF, 0.2 wt%	40%	623 μmol h^−1 g^−1	DFF, 26%	643
Zn0.5Cd0.5S/1%MnO2	0.1 wt%	H2O	>400 nm, LED (30 W), N2, 6 h	HMF, 0.2 wt%	—	25.4 μmol h^−1 g^−1	DFF, 14%	644
Ni/CdS	0.1 wt%	H2O	Visible light from LED (8 W), N2, 22 h	HMF, 0.2 wt%	20%	334 μmol h^−1 g^−1	DFF, 20%	645
NiS/Zn3In2S6	0.2 wt%	H2O	>400 nm, 300 W xenon lamp, N2	HMF, 1.3 wt%	20%	120 μmol h^−1 g^−1	DFF (selectivity of 94.1%), —	646
Ni/CdS	0.1 wt%	10 M NaOH	Visible light from LED (8 W), N2, 2 h	HMF, 0.2 wt%	100%	—	FDCA, 100%	645

---

The photocatalytic oxidation of HMF to valuable chemicals coupled with H₂ production via water splitting over semiconductors (Fig. 46) is an effective strategy to improve the utilization efficiency of solar energy. Han et al. investigated the photocatalytic decomposition of HMF and furfural alcohol over an ultrathin CdS nanosheet decorated with nickel (Ni/CdS) catalyst.⁶⁴⁵ In neutral aqueous solution, HMF was converted to DFF and H₂, with a DFF yield (20%) and H₂ production rate (334 μmol h⁻¹ g⁻¹) much lower than that (furfural yield 90%, H₂ production rate 4970 μmol h⁻¹ g⁻¹) obtained from furfural alcohol. Ye et al. reported that P-doped Zn_xCd_1−xS with rich S vacancies (Zn_xCd_1−xS–P) exhibited a remarkably higher hydrogen production rate from HMF aqueous solution in comparison with that from pure water.⁶⁴³ After 8 h of irradiation under visible light, a DFF selectivity of 65% was obtained with the HMF conversion of 40%. However, to date, both the reaction efficiency and DFF selectivity of photocatalytic systems are obviously lower than that of traditional thermal catalytic systems. Therefore, further insight into the mechanism of light adsorption, generation and recombination of electrons and holes (h⁺), and regulation of reactive oxygen radicals is required to establish more effective photocatalytic systems.

Fig. 46 Photocatalytic oxidation of HMF coupled with hydrogen evolution from water splitting. Adapted from ref. 643 and 645.

3.2.2. Selective oxidation of HMF to HMFCA. Ag nanoparticles stabilized by poly(vinylpyrrolidone) supported on ZrO₂ (Ag-PVP/ZrO₂) exhibited superior catalytic activity for the aerobic oxidation of HMF to HMFCA in the medium of NaOH, Na₂CO₃ or Ca(OH)₂ aqueous solution.⁵⁵⁵ An HMFCA yield of 98.2% at the HMF conversion of 100% was obtained at 20 °C for 2 h with an oxygen flow of 60 mL min⁻¹. It was observed that the capping agent PVP could weaken the metal–support interaction between the Ag NPs and ZrO₂ support, helping to improve the catalytic activity and selectivity. Zhao et al. reported that the combination of an Ag₂O catalyst with H₂O₂ as the oxidant enabled the selective and efficient oxidation of HMF to HMFCA, but the catalyst was deactivated remarkably due to the reduction of silver oxide to metallic silver and the sintering of the nanoparticles during the reaction.⁵⁵⁶ Zhou et al. reported that the photocatalytic oxidation of HMF using atmospheric air as an oxidant over Au nanoparticles supported on TiO₂ (AuNPs/TiO₂) in Na₂CO₃ aqueous solution also enabled the selective production of HMFCA, under both ultraviolet and visible light radiation.⁵⁵⁷

Brandolese et al. reported that the one-pot, two-step process (Fig. 47) involving the sequential oxidation and self-esterification of HMF into polyester oligomer over a polystyrene-supported triazolium pre-catalyst in the presence of iron(II) phthalocyanine and air, followed by the hydrolysis of the polyester oligomer to HMFCA over a supported base (Ambersep 900 OH) afforded an overall HMFCA yield of 87%.⁶⁴⁷ This strategy also enabled the efficient production of HMFCA-derived ester and amide via nucleophilic depolymerization of the oligomer intermediate in the presence of methanol and butylamine, respectively.

Fig. 47 One-pot, two-step conversion of HMF toward HMFCA. Adapted from ref. 647.

Owing to the specificity of enzymes, the highly selective oxidation of HMF to HMFCA can be achieved via biological methods. For example, recombining 3-succinoylsemialdehyde-pyridine dehydrogenase (SAPDH) from Comamonas testosteroni SC1588 into Escherichia coli cells enabled the efficient conversion of HMF to HMFCA (yield of 95%) at 50 °C for 5 h.⁶⁴⁸ Similarly, resting cells of Gluconobacter oxydans DSM 50049 were active to selectively oxidize HMF into HMFCA, obtaining an HMFCA selectivity of 100% with the HMF conversion of 94%.⁶⁴⁹

3.2.3. Selective oxidation of HMF to FDCA. FDCA, as one of the most desirable oxidized products of HMF, can be used as a building block to displace petroleum-derived terephthalic acid (PTA) for the synthesis of polyethylene furanoate (PEF) or poly(hexylene 2,5-furandicarboxylate) (PHF).^650–652 PEF has better physical properties than the existing polyethylene terephthalate (PET), one of the most widely used plastic products with an annual production of 70 million tons, derived from petroleum.^561,653,654 Recent studies indicated that the use of PEF in fused deposition modeling (FDM) 3D printing resulted in higher chemical resistance than that printed with several available materials, including acrylonitrile butadiene styrene (ABS), polylactic acid (PLA) and glycol-modified poly(ethylene terephthalate) (PETG), with the advantages of optimal adhesion, thermoplasticity, lack of delamination, and low heat shrinkage.⁶⁵⁵ Besides, FDCA can be used as a monomer and cross-linking agent to copolymerize with butyl methacrylate for the synthesis of polymers.⁶⁵⁶

The oxidation of HMF to FDCA can be conducted via thermocatalysis, biocatalysis, electrocatalysis and photocatalysis, and readers can also consult the recent reviews.^{562,563,657–660} Metal oxides have been intensively investigated as catalysts for the selective oxidation of HMF to FDCA. For example, Hayashi et al. demonstrated that MnO₂ can be used as a non-precious-metal catalyst for the selective oxidation of HMF to FDCA in NaHCO₃ aqueous solution using oxygen as an oxidant.⁶⁶¹ Accordingly, they further showed that the aerobic oxidation of HMF to FDCA over MnO₂ depends strongly on its crystal structure and the local environment around the oxygen atoms using combined computational and experimental studies.⁵⁸³ It was observed that the catalytic activity correlates well with the calculated oxygen vacancy formation energy. The reaction rates per surface area for the rate-determining step (FFCA oxidation to FDCA) increased in the order of ε-MnO₂ < δ-MnO₂ < α-MnO₂ ≈ γ-MnO₂ < λ-MnO₂ < β-MnO₂. Moreover, high-surface-area β-MnO₂ (β-MnO₂-HS) nanoparticles⁵⁸³ and high-surface-area mesoporous β-MnO₂ nanoparticles⁶⁶² were successfully prepared via the simple calcination of the precursor prepared from NaMnO₄ and MnSO₄ at 400 °C and low-temperature crystallization of solution-derived Mn⁴⁺ precursor under template-free conditions, respectively, both affording an FDCA yield (>83%) higher than activated MnO₂. Compared with individual MnO₂ and Co₃O₄, the Co–Mn mixed oxide catalyst was more effective for the direct oxidation of HMF to FDCA owing to the enhanced lattice oxygen mobility and presence of different valence states of manganese.^584,585 CeO₂ and MgO mainly convert HMF to FFCA and formic acid, respectively, MgO·MnO₂·CeO₂ catalyzes the selective oxidation of HMF to DFF, while CuO·MnO₂·CeO₂ mainly promotes the oxidation of HMF to FDCA.⁵⁹²

Supported precious metals are also widely used for the selective oxidation of HMF to FDCA. Liao et al. reported that a manganese oxide-supported single-atom Pd (Pd–MnO₂) catalyst exhibit an FDCA yield of 88% for the aerobic oxidation of HMF in aqueous solution, with an FDCA productivity (100.91 mmol h⁻¹ g_Pd⁻¹) much higher than that (45.57 mmol h⁻¹ g_Pd⁻¹) of Pd nanoparticles supported on MnO₂ (PdNP/MnO₂).⁵⁸⁶ Moreover, Pd–MnO₂ showed good recyclability after five times reuse, while PdNP/MnO₂ showed an obvious decrease in catalytic activity. The influence of the morphology of CeO₂, including nanorod, cube and octahedra on the oxidation of HMF over the Au/CeO₂ catalyst was investigated by Li et al.⁵⁷⁸ Au/CeO₂-rod showed abundant oxygen vacancies, cationic gold, interfacial Lewis acidic sites and the highest catalytic activity for the selective conversion of HMF to FDCA. The improved reaction efficiency was attributed to the efficient activation of hydroxyl and aldehyde groups and molecular O₂ owing to the synergistic effect of the interfacial Lewis acid, homogeneous base and neighboring Au particles. Yu et al. reported that Pt supported on activated porous carbon (Pt-PVB-ACS 800) was more active and durable than Pt supported on inactivated carbon (Pt-CS-800) for the basic additive-free aerobic oxidation of HMF to FDCA, mainly owing to its large surface area, high porosity and low density of oxygen groups.⁵⁸⁹ Nevertheless, the FDCA yield decreased from 99% to 79% after ten catalytic runs, owing to the aggregation of Pt nanoparticles. Schade et al. reported that Au/ZrO₂ exhibited a high yield of FDCA (89%) for the aerobic oxidation of HMF at the evaluated temperature (125 °C) with a maximum productivity of 67 mol_FDCA h⁻¹ mol_Au⁻¹, while Ag/ZrO₂ exhibited an HMFCA yield higher than 98% under mild conditions (25 °C) with a maximum productivity of 400 mol_HMFCA h⁻¹ mol_Ag⁻¹.⁵⁵⁹ Their following study showed that the selective conversion of HMF to HMFCA proceeds through the dehydrogenation of the geminal diol by reduced Ag particles and the reduced Ag particles are released by removing hydrogen with oxygen in the catalytic cycle.⁶⁶³

In addition to precious metals, transition metals have also been investigated for the oxidation of HMF to FDCA. For example, cobalt nanoparticles (NPs) encapsulated in Mn/N-doped graphitic carbon (Co–Mn/N@C) were synthesized by co-pyrolyzing of Co/Mn-lignin complex and dicyandiamide.⁶⁶⁴ The Co–Mn/N@C catalyst afforded an FDCA yield of 98.8% in Na₂CO₃ aqueous solution under mild conditions (85 °C, 1 bar O₂, 10 h).

The design of bimetallic catalysts is effective to improve the catalytic performance for the selective oxidation of HMF to FDCA.⁶⁶⁵ Xia et al. reported that bimetallic Pd–Au nanoparticles supported on Mg–Al hydrotalcite are highly effective to catalyze the aerobic oxidation of HMF to FDCA in an NaOH aqueous solution under mild conditions, attaining an FDCA yield of 90% at 60 °C within 6 h.⁵⁵⁸ In contrast, the monometallic Pd or Au catalyst exhibited a low FDCA yield with HMFCA as the main product since they were unable to further oxidize the intermediate HMFCA under this condition. The superior catalytic activity of the bimetallic catalyst mainly resulted from the electron transfer from Pd to Au, which could promote the adsorption and activation of HMF and then accelerate its oxidation. Zhang et al. reported that bimetallic AuPd and PtPd nanowire networks (NNWs) with abundant structural defects, which were prepared via a salt-mediated emulsion method, exhibited much a higher catalytic performance for the selective oxidation of HMF to FDCA in comparison with AuPd NPs, and core–shell and TiO₂-supported AuPd NPs (AuPd/TiO₂).⁵⁸¹ The enhanced catalytic activity of the NNWs was ascribed to their good ability to generate radicals owing to the presence of abundant structural defects. Bonincontro et al. reported that nanosized NiO-supported Au–Pd nanoparticles (AuPd–nNiO) enabled the oxidation of HMF to FDCA under base-free conditions, attaining a relatively high yield of FDCA (70%) at 90 °C.⁵⁸²

The properties of the support have an important influence on the catalytic activity of metals for the oxidation of HMF. Megías-Sayago et al. investigated the role of acid sites in the conversion of HMF over gold catalysts.⁶⁰⁷ They observed that the FDCA yield from the oxidation of HMF over Au/Ce_xZr_1−xO₂ correlates well with the shift in the CO–OH–M⁴⁺ bands in its FTIR spectrum using CO as a probe molecule, indicating that the improved surface hydroxyl acidity is beneficial for a higher FDCA yield. They proposed that the addition of an appropriate amount of Zr⁴⁺ improves the Brønsted acidity of the catalyst, and then the deprotonated Brønsted sites promote the formation of an alkoxy intermediate, thus improving the FDCA yield. Consequently, the Au/Ce_25%Zr_75%O₂ catalyst with a relatively high hydrophilicity and Brønsted acidity gave the highest activity toward FDCA (80%). Ferraz et al. investigated the influence of basic sites on the support on the oxidation of HMF over PVA-stabilized Au catalysts (PVA-Au/xMgF₂–(1 − x)MgO₂) in aqueous solution under base-free conditions.⁶⁰⁸ The Au/MgO catalyst showed a superior performance (FDCA yield of 90%) compared to the other tested catalysts, including PVA-Au/MgF₂–MgO₂, PVA-Au/MgF₂, Au/NiO, Au/TiO₂ and Au/CeO₂, indicating that the basicity of the support is beneficial for the conversion of HMF to FDCA. However, partial leaching of Mg (89 ppm) occurred under the reaction conditions. Donoeva et al. also demonstrated that the Au nanoparticles on a basic carbon support bearing basic functional groups exhibited better catalytic activity for the oxidation of HMF to FDCA and higher stability than Au nanoparticles supported on acidic carbon support since the positively charged surface groups promoted the adsorption of hydroxyl ions, which function as a cocatalyst to promote the gold-catalyzed dehydrogenation process.⁵⁹⁰ The influence of MgAlO, MgO, ZrO₂, and CeO₂ supports on the oxidation of HMF over a supported Ru catalyst in aqueous solution under base-free conditions was investigated by Antonyraj et al.⁶⁰⁵ Although MgAlO-supported Ru (Ru/MgAlO) exhibited the highest catalytic activity, it was deactivated rapidly in the recycling experiment. In contrast, the magnesium oxide (MgO)-supported Ru catalyst (Ru/MgO) showed good catalytic activity and reusability, affording an FDCA yield of 90% with the HMF conversion of 100%.

The construction of Fe₃O₄-based composited can impart magnetic property to the catalyst, which is beneficial for its recovery and reuse. Tirsoaga et al. reported that the magnetic-separable cationic Ru(III)–Fe₃O₄@SiO₂ catalyst showed an FDCA yield of 74.2% with the HMF conversion of 92% in the medium of NaOH solution.⁵⁹⁶ Zhou et al. indicated that the Fe₃O₄-decorated reduced graphene oxide-supported platinum nanocatalyst (Pt/Fe₃O₄/rGO) was more active than Pt/rGO for the conversion of HMF to FDCA.⁶⁶⁶ At a low HMF loading (0.3 wt%), Pt/Fe₃O₄/rGO enabled the selective conversion of HMF to FDCA in pure water, obtaining an FDCA yield of 98% under mild conditions (95 °C, 8 h, 0.5 MPa O₂). They demonstrated that the introduction of Fe₃O₄ not only promoted the dispersion of Pt, but also promoted the easy magnetic recycling of the catalyst.

Owing to its relatively low reaction rate, a long reaction time (4–24 h) is usually required for the conversion of HMF to FDCA. Ji et al. reported that the continuous control of pH coupled with microwave heating could remarkably accelerate the HMF oxidation rate, obtaining an FDCA yield of 99% within 30 min using 3 wt%Au/TiO₂ and H₂O₂ as the catalyst and oxidant, respectively.⁶¹³

The avoidance of using a homogeneous acid and base is very important for the actual production of FDCA. Guo et al. reported that Mn–Ce mixed oxide-supported Ru nanoparticles (Ru/Mn₆Ce₁O_Y) are highly effective and stable for the aerobic oxidation of HMF to FDCA, affording an FDCA yield higher than 99% in the medium of water with productivity as high as 5.3 mol_FDCA mol_Ru⁻¹ h⁻¹ in the absence of any base additive.⁵⁹³ The superior catalytic performance was mainly attributed to the well-dispersed metallic Ru nanoparticles and the improved availability and mobility of active oxygen species owing to the high surface concentration of Mn⁴⁺ and Ce³⁺, and the good stability of catalyst was owing to the strong metal–support interaction. Ke et al. reported that the nitrogen-doped carbon-decorated CeO₂ supported Pt catalyst (Pt/NC-CeO₂) enabled an FDCA yield of 100% in water without the use of a base additive.⁵⁹⁴

The selective oxidation of HMF toward FDCA can also be performed in a packed bed reactor. Danielli et al. demonstrated the feasibility of using a trickle-bed reactor to convert HMF to FDCA, obtaining an FDCA yield of 98% over Ru/Al₂O₃ in Na₂CO₃ solution at 140 °C (GHSV = 900 h⁻¹, WHSV = 1 h⁻¹, 30 bar O₂).⁶⁰⁶ After reactivation via simple hydrothermal treatment, the activity of the Ru/Al₂O₃ catalyst could be retained for 12 cycles. The oxidation of HMF to FDCA was performed over a heterogenous resin-supported Pt catalyst, affording an FDCA yield of 99% continuously in neat water under continuous flow and base-free conditions (120 °C, residence time of 303 s, O₂ flow rate of 1.2 mL min⁻¹ and 7.7 bar O₂ pressure).⁶⁶⁷ In this catalytic system, the FDCA product was obtained with a high space-time-yield of 46.0 g L⁻¹ h⁻¹ without acid/base treatment or purification.

Besides O₂ and air, other oxidants have also been tested for the selective oxidation of HMF to FDCA. For example, Ren et al. reported that CuO and Co₃O₄ exhibit high activity for the oxidation of HMF to FDCA when using NaClO as an oxidant.⁵⁷² Hazra et al. reported that the use of a metal-free I₂/NaOH catalyst enabled the 10 gram-scale synthesis of FDCA from HMF using tert-butyl hydroperoxide (TBHP) as the oxidant.⁵⁷¹ Tirsoaga et al. reported that the selective oxidation of HMF over a multi-functional magnetic nano-composite (10Co@22Nb@MNP) catalyst using TBHP as the oxidant resulted in an FDCA selectivity of 96.5% with the HMF conversion of 96.9%.⁶¹⁴ However, although these oxidants are more powerful than O₂ and air, their large-scale application is limited by their high cost and secondary pollution. Besides, the selective oxidative esterification of HMF in methanol could give furan-2,5-dimethylcarboxylate (FDMC), which is a promising alternative method for the conversion of HMF to FDCA. This reaction will be discussed in detail in section 3.9.

The direct use of the HMF product solution as feedstock reduces the separation and purification steps and then intensifies the process chain of FDCA production. Naim et al. investigated the influence of the HMF synthesis by-products, including unconverted sugars, levulinic acid, formic acid and remaining inorganics on the oxidation of HMF to FDCA over Au/ZrO₂ in aqueous medium.⁶⁶⁸ They found that levulinic acid led to the severe degradation of the formed FDCA due to the poisoning of the catalyst, while other substances decreased the FDCA yield to a lower extent. Therefore, avoiding rehydration of HMF to levulinic acid during the production of HMF is crucial for the subsequent conversion of HMF to FDCA. In a realistic technical scenario, an FDCA yield of up to 74% has been achieved when using impure HMF obtained from unconcentrated sugar syrup as the feedstock.

As discussed in section 3.2.1, the photocatalytic oxidation of HMF not only gives DFF as the main product, but also enables the simultaneous production of H₂via water splitting. Also, similar to the thermocatalytic process, the use of a basic reaction medium is an effective strategy to promote the oxidation of HMF toward FDCA. For example, the Ni/CdS catalyst could convert HMF and furfural to their corresponding carboxylates completely in NaOH solution, but in neutral aqueous solution the Ni/CdS catalyst only converted HMF to DFF.⁶⁴⁵ Xu et al. designed a highly effective photocatalyst by dispersing cobalt thioporphyrazine (CoPz) onto g-C₃N₄ (abbreviated as CoPz/g-C₃N₄), which exhibited superior photocatalytic activity for the selective oxidation of HMF toward DFF at low pH (4.01) or FDCA (96.1% yield) at high pH (9.18) under simulated sunlight using air as a benign oxidant. They demonstrated that the poor catalytic performance of single g-C₃N₄ is owing to the generation of hydroxyl radicals, which could mineralize organics to CO₂ and H₂O, while the strong interaction between CoPz and g-C₃N₄ not only inhibited the generation of hydroxyl radicals over g-C₃N₄, but also accelerated the formation of singlet oxygen (¹O₂) over CoPz sites, thus improving the catalytic performance remarkably.⁶⁴² Similarly, a cross-linked conjugated polymer photosensitizer exhibited strong ¹O₂ generation ability under sunlight and good catalytic performance for the selective oxidation of HMF to FDCA.^669,670

Electrochemical oxidation enables the selective oxidation of HMF to FDCA with high yields. Taitt et al. investigated the electrochemical oxidation of HMF to FDCA in 0.1 M KOH solution using NiOOH, CoOOH, and FeOOH as electrodes.⁵⁶⁷ They found that NiOOH gave an FDCA yield of 96.0% at 1.47 V vs. RHE, while CoOOH and FeOOH were not active for this reaction. The integration of H₂ evolution with the oxidation of HMF (Fig. 48) not only avoided the production of the explosive H₂/O₂ mixture by decoupling the H₂ evolution reaction (HER) and O₂ evolution reaction (OER), but also gave value-added FDCA and H₂ products with a high energy conversion efficiency.^671–674 For example, You et al. reported that the use of hierarchically porous Ni₃S₂/Ni foam as a bifunctional electrocatalyst could reduce the cell voltage by ∼200 mV for integrated H₂ production and HMF oxidation to achieve 100 mA cm⁻², compared with the pure water splitting, with nearly unity Faradaic efficiency.⁶⁷² Cha et al. reported that performing the oxidation of HMF at the cathode of a typical hydrogen-producing photoelectrochemical cell (PEC) under AM 1.5 G illumination with TEMPO as the mediator enabled the simultaneous production of H₂ and FDCA, with near-quantitative FDCA yield and 100% Faradaic efficiency.⁶⁷¹ Li et al. reported that the use of Na₄[Fe(CN)₆] as a proton-independent electron reservoir is an effective approach to decouple water splitting and organic upgrading, thus enabling efficient HER with the oxidation of HMF to FDCA driven either by electricity or a solar cell under sunlight irradiation with the advantages of great flexibility and safety.⁶⁷³

Fig. 48 Electrochemical cell for the integration of H₂ evolution with the oxidation of HMF to FDCA. Adapted from ref. 672 and 674.

The direct conversion of carbohydrates to FDCA via HMF as an intermediate has also been achieved in recent studies. Motagamwala et al. reported the efficient conversion of high concentration fructose to FDCA in the medium of the GVL/H₂O solvent system via a one-pot, two-step process.⁶⁷⁵ Fructose was converted to HMF at high yield (70%) in the solvent system using FDCA as an acid catalyst. The use of FDCA to replace the corrosive HCl as the catalyst improves the process economics and environmental benefits by eliminating the ion exchange operation. The high solubility of FDCA in the solvent system enabled the effective oxidation of HMF over the Pt/C catalyst at a high HMF concentration (7.5 wt%), in the absence of a homogeneous base. Moreover, the FDCA product with a purity of greater than 99% could be obtained from the solvent system via crystallization. Chen et al. reported that the one-pot, two-step process involving the dehydration of fructose to HMF over Amberlyst-15 in DMSO and oxidation of HMF to FDCA over Pt/C in the H₂O(K₂CO₃)/DMSO medium gave an overall FDCA yield of up to 88.4% from fructose under relatively mild conditions.⁴²⁹ The combination of ionic liquids and heteropoly acids is not only effective for the base-free oxidation of HMF to FDCA, but also enables the one-pot conversion of carbohydrates to HMF.⁵⁸⁸ Among the tested heteropoly acids and ionic liquids, the combination of HPMV₆ (HPM = H₃PMo₁₂O₄₀) and BMIMCl gave the highest FDCA yield of 88.8% from HMF. The high solubility of FDCA in BMIMCl is beneficial for the inhibition of the oxidative cleavage of the furan ring. The use of fructose, glucose and cellulose as starting materials gave FDCA yields of 30.7%, 48.6% and 6%, respectively.

Biocatalysis, including enzymatic catalysis (in vitro) and whole-cell catalysis (in vivo), is a promising approach to convert HMF to FDCA.^562,660 Biocatalysis can transform HMF to DFF, HMFCA or FFCA efficiently, but the selective enzymatic oxidation of HMF to FDCA is relatively challenging.^676–679 Whole cells can transform HMF to FDCA with endogenously produced cofactors without using expensive stoichiometric reagents, but they also catabolize FDCA with the generation of many byproducts.⁶⁶⁰ In contrast, isolated enzymes allow better control of the reaction pathway and facile product recovery, but complex cofactors, mediators, immobilization, mutation, and cooperation with other enzymes are required to achieve the efficient conversion of HMF to FDCA. Daou et al. reported that the combination of glyoxal oxidase (PciGLOX) isoenzymes obtained from Pycnoporus cinnabarinus and aryl alcohol oxidase (UmaAAO) could enable the cascade oxidation of HMF toward FDCA.⁶⁸⁰ To avoid the accumulation of HMFCA, HMF was firstly oxidized to DFF and further to FFCA by UmaAAO for 2 h, followed by oxidation with PciGLOX3 in the presence of catalase for 24 h, attaining the maximum FDCA yield of 16%. Yuan et al. demonstrated that selective biocatalytic conversion of HMF to FDCA could be achieved via manipulating the key genes responsible for FDCA synthesis in Raoultella ornithinolytica BF60.^681,682 The overexpression of the AldH gene, which is responsible for the oxidation of FFCA to FDCA, combined with depleting adhP3 and alkR genes responsible for the reduction of HMF to BHMF led to an FDCA yield of up to 96.2%. Wang et al. reported that the co-expression of vanillin dehydrogenase (VDH1) and HMF/furfural oxidoreductase (HmfH) in Escherichia coli enabled the cascade catalytic oxidation of HMF to FDCA without the use of an additional sacrificial substrate, attaining an FDCA yield of 96% at an HMF concentration of 150 mM with a productivity of around 0.4 g L⁻¹ h⁻¹.⁶⁸³

The life cycle assessment (LCA) of PEF plastic production from starchy biomass, which involved the conversion of maize grain to fructose via a well-established industrial process, conversion of fructose to HMF in DMA/LiBr using H₂SO₄ as the catalyst, conversion of HMF to FDCA by KMnO₄ in NaOH aqueous solution and polymerization of FDCA with ethylene glycol to PEF, indicated that the predominant environmental impacts (38–49%) are generated at the stage of the conversion of FDCA to polymer owing to the electricity consumption and use of non-renewable chemicals such as dichloromethane.⁶⁸⁴ Based on LCA, Bello et al. suggested that the search for environmentally friendly substances and the reduction of energy consumption for the production of FDCA from lignocellulosic biomass are necessary to further improve the environmental sustainability.⁶⁸⁵ Byun et al. suggested that the coproduction of bioethanol with FDCA is a more promising pathway to improve the economic feasibility of the biorefinery process compared with the co-production of ethanol with other products, including adipic acid, caprolactam, pentanediol and phthalic anhydride due to the high output and high value of FDCA.⁶⁸⁶ The replacement of 25% of ethanol with FDCA produced 15.3–16.7 MJ of FDCA per gasoline gallon equivalent (GGE) of ethanol, resulting in an economic mitigation potential of up to US$2.40–2.48 per GGE.

The carboxylation of furfural is an alternative approach to obtain FDCA from biomass. Banerjee et al. reported that Cs₂CO₃ molten salts promoted the carboxylation of C–H in 2-furoic acid (Fig. 49a) with CO₂, leading to the formation of carboxylates, which could be readily converted to FDCA via protonation.⁶⁵¹ In this system, Cs₂CO₃ molten salts promoted the deprotonation of C–H bonds by CO₃²⁻ to form carbon-centered nucleophiles followed by reaction with CO₂ to generate carboxylates. Similarly, the carbonylation of furfural-derived 5-bromo-furoic acid, which was obtained via the bromination of furoic acid (Fig. 49b), with CO in aqueous solution over a palladium catalyst gave an FDCA yield of up to 98%.⁶⁸⁷

Fig. 49 Conversion of furfural to FDCA via 2-furoic acid (a) and 5-bromo-furoic acid (b) as the intermediate. Adapted from ref. 651 and 687.

The feasibility of converting HMF to PTA (Fig. 50) was proven by Pacheco et al.⁶⁸⁸ The oxidized derivatives of HMF were reacted with ethylene over solid Lewis acid catalysts that do not contain strong Brønsted acids to synthesize precursors of PTA and its equally important diester, dimethyl terephthalate (DMT). The Diels–Alder reaction of HMFCA with high pressure ethylene over Sn-Beta zeolite generated 4-(hydroxymethyl)benzoic acid (HMBA), attaining an HMBA selectivity of 31% with the HMFA conversion of 61% at 190 °C for 6 h. When HMFCA was protected with methanol to form methyl 5-(methoxymethyl) furan-2-carboxylate (MMFC), MMFC could be converted to methyl 4-(methoxymethyl) benzenecarboxylate (MMBC). HMBA and MMBC can be used for the production of PTA and DMT via oxidation, respectively.

Fig. 50 Conversion of HMF to PTA via oxidation and Diels-Alder/dehydration reaction. Adapted from ref. 688.

3.2.4. Selective oxidation of HMF to succinic acid and maleic acid. In addition to FDCA, other valuable dicarboxylic acids (Fig. 51), including succinic acid and maleic acid (MAc) can be obtained from the oxidation of HMF. For example, Tirsoaga et al. reported that in the medium of n-butylamine aqueous solution, the Fe₃O₄@SiO₂–CoO_x(10 wt% CoO_x) catalyst afforded a succinic acid selectivity of 92.7% with the HMF conversion of 78.6%.⁵⁹⁶ Under the same conditions, the Fe₃O₄@SiO₂–MnO_x catalyst gave an MAc selectivity of 72% with the HMF conversion of 5%, while the Fe₃O₄@SiO₂–NH₂ catalyst gave the MAc selectivity of 85% with the HMF conversion of 35.6%. Lv et al. reported that vanadium-oxo nanosheets (VON) supported on GO could be used as an effective and recyclable heterogeneous catalyst for the aerobic oxidation of HMF and furfural toward maleic anhydride in the medium of acetic acid, affording maleic anhydride yields of 90.9% and 59.9%, respectively.⁶⁸⁹ Jia et al. reported that the use of 5-[(formyloxy)methyl]furfural (FMF) to displace HMF as feedstock could greatly improve the MAc production efficiency over α-MnO₂/Cu(NO₃)₂ catalysts with the assistance of K₂S₂O₈ under atmospheric air or pure oxygen, attaining an MA yield of up to 89%.⁶⁹⁰

Fig. 51 Oxidation of HMF toward succinic acid and maleic acid. Adapted from ref. 596.

In addition to the oxidation of HMF, selective oxidation technologies have also been widely used for the conversion of other biomass-derived platform molecules to value-added chemicals. For instance, the oxidation of glucose can produce gluconic acid, gluconates, glucaric acid and formic acid, and the selective oxidation of levulinic acid and furfural can generate maleic acid and anhydride, succinic acid and furanones. These processes have been extensively reviewed, and the reader is referred to these sources.^{4,34,560,566,691}

3.3. Hydrodeoxygenation of HMF

The hydrodeoxygenation of HMF can give a wide variety of products, including 2,5-bishydroxymethylfuran (BHMF), 2,5-bishydroxymethyltetrahydrofuran (BHMTHF), 1,6-hexanediol (1,6-HD), 2,5-dimethylfuran (DMF) and alkanes. Since the hydrodeoxygenation of HMF involves the tailoring of the aldehyde group, hydroxy group and furan ring, complicated and multiple reaction networks may occur during the conversion of HMF. The reaction pathway and product selectivity of HMF hydrodeoxygenation are influenced by the support, metals, additives, solvents, hydrogen donors and reaction conditions (Fig. 52 and Table 15). Here, we summarize the recent advances in HMF hydrodeoxygenation toward valuable products with a focus on identifying the major factors determining the reaction pathway and product selectivity. For a comprehensive overview in this field, readers can also refer to the previous dedicated review articles.^1,692–694

Fig. 52 Main reaction pathway for the hydrodeoxygenation of HMF to value-added chemicals. Adapted from ref. 695.

Table 15 Hydrogenation of HMF of to BHMF, DMF or DMTHF

Catalyst	Catalyst loading	Solvent	Reaction conditions	Feedstock loading^a	Conversion	Yield (%)	Ref.
a Relative to solvent. b Relative to HMF. c The reaction was conducted in a fixed reactor. — Not provided.
ZrBa-SBA	1 wt%^a	Isopropanol	150 °C, 2.5 h	HMF, 1 wt%	98.3%	BHMF, 96.8%	696
Zr-Benzylphosphonate	1.4 wt%^a	Isopropanol	120 °C, 2 h	HMF, 2.5 wt%	>99%	BHMF, 93%	697
Hf-DTMP	1 wt%^a	2-Butanol	130 °C, 4 h	HMF, 2 wt%	99.1%	BHMF, 96.8%	698
Hf-lignosulfonate	1 wt%^a	Isopropanol	100 °C, 2 h	HMF, 1.26 wt%	∼98%	BHMF, 90%	699
Hf-lignosulfonate	1 wt%^a	Isopropanol	120 °C, 10 h	HMF, 1.26 wt%	∼98%	BHMF, 90%	699
Ir/SiO2	1 mol%^b	THF	60 °C, 5 h, 10 Bar H2	HMF, —	70%	BHMF, 58.1%	700
Ir/SiO2(Cl)	1 mol%^b	THF	60 °C, 5 h, 10 Bar H2	HMF, —	97%	BHMF, 97%	700
Ir/SiO2, H2SO4	1 mol%^b	THF	60 °C, 2 h, 10 Bar H2	HMF, —	78%	BHMF 58.1%; DMF 13.3%	700
Ir/SiO2(Cl), H2SO4	1 mol%^b	THF	60 °C, 2 h, 10 Bar H2	HMF, —	74%	BHMF 38.5%; DMF 17.8%	700
NiBi IMCs	3.3 mM Ni^a	Isopropanol	100 °C, 8 h, 2 MPa H2	HMF, v/v = 1/30^a	100%	BHMF 97.9%	701
MZCCP	0.8 wt%	2-Butanol	140 °C, 5 h	HMF, 2 wt%	98.7%	BHMF, 93.4%	702
MZCCP, 0.8 wt%	0.8 wt%^a	2-Butanol	140 °C, 5 h	Fructose, 2 wt%	98.7%	BHMF, 93.4%	702
Ni-12Cu/SBA-15	1 wt%^a	THF	220 °C, 5 h, formic acid	FMF, 1.8%	100%	DMF, 71%	703
Ni-12Cu/SBA-15	1 wt%^a	THF	220 °C, 5 h, formic acid	HMF, 1.5%	100%	DMF, 60.5%	703
Co/ZrLa0.2Ox	0.5 wt%^a	Water	40 °C, 10 h, 2 MPa H2	HMF, 1 wt%	100%	BHMF, 100%	704
RuCu@NFC	0.45 wt%^a	Isopropanol	210 °C, 12 h, isopropanol	HMF, 0.75 wt%	97.0%	BHMF, 88.8%	705
Mn-NCA	1.1 wt%^a	Isopropanol	160 °C, 1.5 h, isopropanol	HMF, 1.3 wt%	91%	BHMF, 76%	706
MnO@C–N	0.5 wt%^a	Isopropanol	170 °C, 1.5 h, isopropanol	HMF, 1.3 wt%	∼100%	BHMF, 76%	707
Cu(50)-SiO2	0.5 wt%^a	1-Butanol	90 °C, 1.5 MPa H2, 5 h	HMF, 12 wt%	100%	BHMF, 97%	708
NC-Cu/MgAlO	1 wt%^a	Cyclohexanol	220 °C, 1.5 MPa H2, 0.5 h	HMF, 5 wt%	100%	DMF, 96.1%	709
NC-Cu/MgAlO	1 wt%^a	Cyclohexanol	220 °C, 1.5 MPa H2, 3.5 h	HMF, 5 wt%	100%	DMTHF, 94.6%	709
1.5K-Cu/Al2O3	5 g^c	Ethanol	120 °C, 2 MPa H2, WHSV of 1.0 h^−1	HMF, 3 wt%	99.2%	BHMF, 98.9%	710
15CuZr	5 g^c	1-Butanol	200 °C, 1.5 MPa H2, WHSV of 0.15 h^−1 for 4 h	HMF	100%	DMF, 30%	235
1Ru15CuZr	5 g^c	1-Butanol	200 °C, 1.5 MPa H2, WHSV of 0.15 h^−1 for 4 h	HMF	100%	DMF, 45.6%	235
1Ru15CuZr	5 g^c	1-Butanol	200 °C, 1.5 MPa H2, WHSV of 0.15 h^−1 for 4 h	Glucose-derived crude HMF	77.6%	DMF, 16.4%	235
K2CO3	1.4 wt%^a	Me-THF	25 °C, 2 h, Ph2SiH2	HMF, 3.2 wt%	100%	BHMF, 94%	711
Cu/Al2O3	1.4 wt%^a	Methanol	240 °C, 6 h, methanol	HMF, 1.4 wt%	100%	DMF, 73.9%	712
Cu/ZnO	1.4 wt%^a	Methanol	240 °C, 6 h, methanol	HMF, 1.4 wt%	100%	DMF, 59.7% (5-MF 1.5%)	712
Cu/ZrO2	1.4 wt%^a	Methanol	240 °C, 6 h, methanol	HMF, 1.4 wt%	100%	DMF, 24.9%	712
Cu/CeO2	1.4 wt%^a	Methanol	240 °C, 6 h, methanol	HMF, 1.4 wt%	100%	DMF, 23.6%	712
Reused Cu/Al2O3	1.4 wt%^a	Methanol	240 °C, 6 h, methanol	HMF, 1.4 wt%	—	DMF, 0%	712
Reused Cu/ZnO	1.4 wt%^a	Methanol	240 °C, 6 h, methanol	HMF, 1.4 wt%	—	DMF, 3.9%	712
Reused Cu/ZrO2	1.4 wt%^a	Methanol	240 °C, 6 h, methanol	HMF, 1.4 wt%	—	DMF, 3.0%	712
Reused Cu/CeO2	1.4 wt%^a	Methanol	240 °C, 6 h, methanol	HMF, 1.4 wt%	—	DMF, 21%	712
Cu–Co/Al2O3	—	H2O	220 °C, 6 h, 3 MPa H2	HMF, 0.2 wt%	—	DMF, 87%	713
Ru/Co3O4	1 wt%^a	THF	130 °C, 24 h, 0.7 MPa H2	HMF, 2.5 wt%	>99%	DMF, 93.4%	714
Co/rGO	0.25 wt%^a	Ethanol	200 °C, 1 h, 2 MPa H2	HMF, 2.5 wt%	100%	DMF, 94.1%	715
Co-CoOx	0.25 wt%^a	1,4-Dioxane	170 °C, 12 h, 1 MPa H2	HMF, 1.25 wt%	∼100%	DMF, 83.3%	716
11.8%Co-(ZnO-ZnAl2O4)	10 wt%^a	THF	130 °C, 24 h, 7 bar H2	HMF, 2.5 wt%	>99.9%	DMF, 74.2% (polymerization, 22.9%)	717
CuZnCoOx	0.43 wt%^a	Ethanol	200 °C, 5 h	HMF, 2.5 wt%	100%	DMF, 99%	718
Pd-HCl/C	0.5 wt%^a	THF	80 °C, 24 h, 2 MPa H2	HMF, 6.3 wt%	33.0%	DMF, 33.0%	719
Pd-AA/C	0.5 wt%^a	THF	80 °C, 24 h, 2 MPa H2	HMF, 6.3 wt%	72.8%	DMF, 72.5%	719
Pd-EA/C	0.5 wt%^a	THF	80 °C, 24 h, 2 MPa H2	HMF, 6.3 wt%	96.9%	DMF, 93.0%	719
Pd-GVL/C	0.5 wt%^a	THF	80 °C, 24 h, 2 MPa H2	HMF, 6.3 wt%	>99%	DMF, 95.6%	719
Commercial Pd/C	0.5 wt%^a	THF	80 °C, 24 h, 2 MPa H2	HMF, 6.3 wt%	24.9%	DMF, 13.6%	719
CuNi	—	0.2 M sulfate solution	−0.8 V vs. Ag/AgCl, 150 C	HMF, 0.2 g/L	FF 88.0%	DMF 91.1%	720
Fe–Pd/C	1 wt%^a	THF	150 °C, 2 h, 2 MPa H2	HMF, 1.3 wt%	100%	DMF, 85%	721
Ru/C-SiO2@m-SiO2	0.19 wt%^a	THF	140 °C, 2 h, 1 MPa H2	HMF, 0.67 wt%	91%	DMF, 89%	722
Ru/C-SBA-15	0.19 wt%^a	THF	140 °C, 2 h, 1 MPa H2	HMF, 0.67 wt%	93%	DMF, 71%	722
Ru/C-commercial	0.19 wt%^a	THF	140 °C, 2 h, 1 MPa H2	HMF, 0.67 wt%	86%	DMF, 49%	722
PtCo/GC	1 wt%^a	Ethanol	180 °C, 2 h, 1 MPa H2	HMF, 5 wt%	—	DMF, 88%	723
PtCo/GC	1 wt%^a	Toluene	180 °C, 2 h, 1 MPa H2	HMF, 5 wt%	—	DMF, 92%	723
PtCo/GC	1 wt%^a	RON 95 gasoline	180 °C, 2 h, 1 MPa H2	HMF, 5 wt%	—	DMF, 95%	723
PtCo/GC	0.8 wt%^a	RON 95 gasoline	180 °C, 2 h, 2.5 MPa H2	HMF, 10 wt%	—	DMF, 78%	723
PtCo/GC (4 runs)	0.4 wt%^a	E10 gasoline	185 °C, 2 h, 2.5 MPa H2	HMF, 10 wt%	—	DMF, 5%	723
PtCo/GC	0.8 wt%^a	RON 95 gasoline	180 °C, 2 h, 2.5 MPa H2	HMF, 15 wt%	—	DMF, 19%	723
Pt–Co/MWCNTs	0.3 wt%^a	1-Butanol	160 °C, 8 h, 1 MPa H2	HMF, 1.7 wt%	100%	DMF 92.3%	724
RuCo/CoOx	1 wt%^a	4-Dioxane	200 °C, 2 h, 0.5 MPa H2	HMF, 5 wt%	100%	DMF, 96.5%	725
5Ni-7MoS2/mAl2O3	0.5 wt%^a	Isopropanol	130 °C, 6 h, 1 MPa H2	HMF, 1.3 wt%	—	DMF, 95%	726

---

3.3.1. Hydrogenation of HMF to BHMF. The selective hydrogenation of HMF to BHMF has attracted much attention since BHMF is an important starting material for the production of ethers, ketones and polymers.⁶⁹³ In aqueous solution, the commercial Ru/C catalyst enabled the selective hydrogenation of HMF to two furan diols, BHMF and BHMTHF with yields of up to 93.0 and 95.3 mol%, under mild temperature (50 °C, 6 h, 30 bar H₂) and relatively harsh conditions (100 °C, 6 h, 50 bar H₂), respectively.⁷²⁷ The use of diphenylsilane (Ph₂SiH₂) as a hydrogen donor enabled the selective reduction of HMF into BHMF over K₂CO₃ under mild conditions (25 °C, 2 h), obtaining a BHMF yield of 94%.⁷¹¹

Illuminating the roles of single atoms, nanoparticles and supports in the hydrodeoxygenation process is of great importance for the construction of highly effective catalysts. Kuai et al. reported that single-atom palladium (Pd₁) and Pd nanoparticles (PdNPs) supported on TiO₂ worked synergistically for the hydrogenation of ketones and aldehydes to alcohols at room temperature, where PdNPs mainly catalyzed the dissociation of H₂ molecules to H atoms, while Pd₁ was responsible for the activation of the C=O group.⁷²⁸ Owing to the rapid development of single-atom-based materials, this discovery may greatly facilitate the development of superior catalysts for the selective hydrogenation of biomass-derived ketone/aldehydes.

In addition to the hydrogenation of HMF to BHMF using an external hydrogen donor, BHMF can also be obtained via the selective catalytic transfer hydrogenation (CTH) of HMF with alcohol via the Meerwein–Ponndorf–Verley (MPV) reduction reaction (Fig. 53), where alcohols such as 2-butanol or isopropanol function as both the hydrogen donor and solvent.^34,698

Fig. 53 Conversion of HMF to BHMF in the medium of alcohol via the MPV reaction. Adapted from ref. 698.

The Lewis acid and base sites are active for the CTH reaction of HMF, while the Brønsted acid sites can catalyze the etherification of HMF or BHMF with alcohol. Zhou et al. designed a monolithic MnO_x/N-doped carbon aerogel material (Mn-NCA) as a catalyst for the catalytic transfer hydrogenation of HMF to BHMF, affording a BHMF yield of 76% under mild conditions without agitation.⁷⁰⁶ They demonstrated that the high catalytic activity of Mn-NCA is attributed to the synergy between the uniformly dispersed MnO_x nanoparticles (NPs) and the basic sites resulting from urea and the monolithic three-dimensional hierarchical porous architecture. Owing to its monolithic feature, Mn-NCA could be readily recovered by tweezers and then reused for the catalytic reaction with slight loss in its catalytic performance. Feng et al. also demonstrated that the superior catalytic performance of N-doped carbon-supported MnO catalyst (MnO@C–N) for the CTH reaction is attributed to the MnO and nitrogen-doping since the N species can reduce the activation energy of aldehyde conversion and the energy barrier of acetone desorption.⁷⁰⁷

A hafnium-based metal–organic coordination polymer catalyst (Hf-DTMP), which was prepared using hafnium tetrachloride (HfCl₄) and diethylene triaminepenta(methylene phosphonic acid) (DTMP), exhibited strong acid–base bifunctionality and excellent catalytic activity for the transfer hydrogenation of HMF with 2-butanol, attaining a BHMF yield of up to 96.8%.⁶⁹⁸ The blocking of both the Lewis acid and base sites led to a remarkable decrease in the BHMF yield, indicating the synergy of the Lewis acid and base sites in the catalytic transfer hydrogenation process. A zirconium-based organic–inorganic coordination polymer (MZCCP) with magnetic property and acid–base bifunctionality, which was synthesized via the reaction of ZrCl₄ with cyanuric acid (CA) over the surface of Fe₃O₄, exhibited a BHMF yield of 93.4% for the CTH reaction of HMF with 2-butanol.⁷⁰² The reuse of the MZCCP catalyst could be readily achieved by separating it with an external magnet without obvious decrease in its catalytic activity. Wei et al. demonstrated that the introduction of BaO to Zr-SBA could block most of the Brønsted acid sites but retain the Lewis acid sites, which could suppress the further etherification of HMF and BHMF, thus enabling the highly efficient conversion of HMF to BHMF over the ZrBa-SBA catalyst.⁶⁹⁶ The Hf–lignosulfonate (Hf–LigS) nanohybrids, which were prepared from lignosulfonate with Hf⁴⁺via a hydrothermal self-assembly method, exhibited strong Lewis acid–base couple sites and moderate Brønsted acidic sites.⁶⁹⁹ The Hf–LigS catalyst was not only active for the catalytic transfer hydrogenation of HMF with 2-propanol to BHMF (yield: 90%) under mild conditions (100 °C, 2 h), but also enabled the one-step reductive etherification toward 5-[(1-methylethoxy)methyl]-2-furanmethanol (MEFA) (Fig. 54), affording an MEFA yield of up to 96.4% at 120 °C in 8 h. The CTH reaction led to the stoichiometric consumption of alcohols. Besides, the H₂ generated from the in situ decomposition of alcohols can also be used as a hydrogen source for the conversion of HMF to BHMF and the hydrodeoxygenation of HMF to 2,5-dimethylfuran (DMF), as will be discussed in detail in section 3.3.2.

Fig. 54 One-pot reductive etherification of HMF with 2-propanol toward MEFA over Hf–LigS. Adapted from ref. 699.

In the presence of an external H₂ donor, the selective hydrogenation of HMF toward BHMF can be conducted over supported metal catalysts using only alcohols as the reaction medium. For example, in the presence of external H₂ (1.5 MPa), the Cu(50)–SiO₂ nanocomposite exhibited both high catalytic activity and stability for the hydrogeneration of HMF to BHMF in the medium of 1-butanol, attaining a BHMF yield of greater than 97% even at an HMF loading as high as 12 wt%.⁷⁰⁸ Yu showed that the steric hindering effect in NiBi intermetallic compounds (IMCs) due to their well-organized surface atomic arrangement favors the vertical adsorption configuration of unsaturated aldehyde, and then promotes the selective hydrogenation of C=O group without converting the C=C group in the furan ring, leading to a BHMF yield of 97.9% from HMF.⁷⁰¹

3.3.2. Hydrodeoxygenation of HMF to DMF. DMF is generally considered a promising liquid transportation biofuel due to its several obvious advantages, including relatively high energy density (30 MJ L⁻¹), suitable boiling point (92–94 °C) as a liquid fuel, high octane number (119), and low solubility in water (2.3 g L⁻¹).^36,693,729 The elegant regulation of the size and chemical environment of noble metal particles is an effective approach to enhance their catalytic activity and selectivity. Yang et al. reported that the Pd-EA/C and Pd-GVL/C catalysts, which were prepared via a simple impregnation method in the medium of ethyl acetate and GVL, respectively, were highly effective for the hydrogenation of HMF to DMF, obtaining DMF yields of 93.0% and 95.6% at a relatively low reaction temperature (80 °C).⁷¹⁹ They proposed that the use of GVL could not only retain the narrow size distribution of Pd particles, but also prevent the oxidation of Pd, thus leading to superior catalytic activity. Talpade et al. reported that bimetallic Fe–Pd/C containing partially oxidized Fe and reduced Pd atoms could be used as a magnetically separable catalyst for the selective hydrogenation of HMF to DMF, obtaining a DMF yield of 85% with the HMF conversion of 100%.⁷²¹ Compared with commercial Ru/C, Ru clusters in bottleneck and tubular carbon pores (Ru/C–SiO₂@m-SiO₂) exhibited much higher catalytic activity for the hydrogenolysis of HMF to DMF.⁷²² The improved catalytic performance was attributed to the Ru clusters (size around 1 nm) owing to the enhanced confinement effect. Chimentão et al. investigated the influence of the iridium precursor and mineral acid on the hydrogenation of HMF over silica-supported iridium catalysts.⁷⁰⁰ It was found that the Ir/SiO₂(Cl) catalyst obtained from the chlorine-containing iridium precursor exhibited a higher BHMF yield (97%) than the chlorine free Ir/SiO₂ catalyst (58.1%), suggesting that the existence of chlorine species is beneficial for the formation of BHMF. The combination of H₂SO₄ with Ir/SiO₂(Cl), or Ir/SiO₂ promoted the further conversion of HMF to DMF, obtaining DMF yields of 17.8% and 13.3%, respectively.

Transition metal Co-based catalysts have also been widely studied for the conversion of HMF to DMF. Yang et al. reported that a Co-graphene nanomaterial (Co/rGO), which was prepared via a feasible impregnation–calcination method, was active for the hydrogenation of HMF to DMF, affording a DMF yield of up to 94.1%.⁷¹⁵ The excellent catalytic performance was attributed to its superior C=O/C–O hydrogenolysis ability and suppressed C=C/C–C session ability owing to the synergistic effect of single Co atoms, Co clusters, CoO_x nanoparticles and reduced graphene oxide. A bifunctional Co–CoO_x catalyst containing both metallic Co and acidic CoO_x, which was synthesized via the controlled reduction of Co₃O₄ under an H₂ flow, afforded the selective hydrogenolysis of HMF to DMF with a high yield (83%).⁷¹⁶ The excellent catalytic performance was attributed to the synergistic effect of the metallic Co and acidic CoO_x sites, where metallic Co functioned as the hydrogenation sites and the Lewis acidic CoO_x sites were effective to activate the C–O bonds to accelerate the hydrodeoxygenation process. An et al. reported that the 11.8%Co–(ZnO–ZnAl₂O₄) catalyst derived from a layered double hydroxide was effective to catalyze the selective hydrogenation of HMF to DMF, affording a DMF yield of 74.2% at 130 °C.⁷¹⁷ Gao et al. designed a dandelion-like cobalt oxide microsphere-supported bimetallic catalyst (RuCo/CoO_x) for the hydrogenolysis of HMF to DMF, attaining a DMF yield of 96.5% using an HMF/Ru molar ratio of up to 252.7.⁷²⁵ They attributed the superior hydrogenolysis performance to the synergy between the bimetallic RuCo NPs, abundant surface defects, hydrogen spillover effect and dandelion-like superstructure of the catalyst. The strong interaction between the RuCo NPs and the CoO_x support imparted excellent stability to the RuCo/CoO_x catalyst.

Single-atom catalysts have also been investigated for the selective hydrodeoxygenation of HMF to DMF. For example, Gan et al. reported that the Co-alloyed Pt (Pt₁/Co) single-atom alloy catalyst, which was synthesized by a scalable ball milling method on the kilogram level, afforded a DMF yield of 92.9% with the HMF conversion of 100% under 1.0 MPa H₂ at 180 °C for 2 h.⁷³⁰ Moreover, the Pt₁/Co single-atom alloy catalyst showed excellent stability without aggregation in five recycling runs, indicating the high potential of the single-atom catalyst in hydrodeoxygenation. Using the hydrodeoxygenation of furfuryl alcohol to 2-methylfuran (2-MF) as an example, Fu et al. demonstrated that the introduction of an ultralow loading Pt in a highly dispersed form onto TiO₂ could improve the C–O activation rate remarkably, without leading to the bulk reduction of TiO₂ or undesirable ring chemistry.⁷³¹

In addition to the design of the catalyst, the development of suitable reaction media is also very important for the efficient conversion of HMF to DMF. Liu et al. investigated the influence of water content on the hydrogenation and hydrogenolysis of HMF with molecular H₂ over the Cu/γ-Al₂O₃ catalyst in the medium of THF. The reaction kinetic studies indicated that the presence of water in THF remarkably influenced the reaction rate and selectivity. In pure THF, the hydrogenolysis reaction was the dominant process, resulting in the formation of 2-methyl-5-hydroxymethylfuran (MHMF) and DMF.⁷³² In contrast, the addition of 5 wt% H₂O to THF inhibited the hydrogenolysis more than the hydrogenation reaction, leading to a relatively high yield of BHMF and low yield of DMF. Nurenberg et al. reported that the hydrodeoxygenation of HMF could be performed over a platinum/cobalt bimetallic alloy supported on graphitic carbon (PtCo/GC) in the medium of commercial gasoline, obtaining a gasoline/DMF mixture, which is promising for direct use in the current internal combustion engine.⁷²³ Among the tested solvents, the gasoline/ethanol mixture with a mass ratio of 9 : 1 (E10) showed the best catalytic performance for the sequential conversion of a high concentration HMF (10 wt%), giving the final product containing 7–20 wt% DMF with improved knock resistance property (higher octane number than E10). However, the DMF yield decreased rapidly in the recycling experiment owing to the deactivation of the PtCo/GC catalyst. Deng et al. found that the product selectivity of furfural hydrogenation over α-MoC depends highly on the alcohol solvents, since the hydrogen donating ability of solvents can control the hydrogenation rate of reactants and the solvent-induced surface modifications of Mo-based catalysts influence the formation of relevant transient states and the final reaction selectivity via enforcing steric hindrance on its surface.^156,733

Metal sulfides have also been demonstrated as a promising type of catalyst for the selective hydrogenolysis process. For example, Han et al. reported that the ordered mesoporous alumina-supported nickel–molybdenum sulfide catalyst (5Ni–7MoS₂/mAl₂O₃) enabled the selective hydrogenolysis of HMF to DMF with a yield of 95% at 130 °C under 1 MPa H₂ in 2-propanol.⁷²⁶ They attributed the active center to the coordinated unsaturated sites resulting from the substitution of Mo by Ni since the number of coordinated unsaturated sites located at the S-edge correlates well with the proportion of Mo substituted by Ni and a higher proportion of Ni resulted in a higher TOF and lower apparent activation energy (51 kJ mol⁻¹). In this catalytic system, the remarkable synergistic effect of H₂ with 2-propanol for the high reaction efficiency was demonstrated. The conversion of HMF to DMF proceeds via (Fig. 55) the HMF → 5-MF → MFA → DMF and the HMF → ethers → DMF reaction pathway simultaneously. The generation of ethers, which are easier to be hydrogenolyzed, could improve the DMF yield obviously. Similar to the 5Ni–7MoS₂/mAl₂O₃ catalyst, Liu et al. reported that the use of MoS₂ monolayer sheets decorated with isolated Co atoms as a catalyst could greatly improve the catalytic performance for the hydrodeoxygenation of 4-methylphenol to toluene due to the formation of sulfur vacancies.⁷³⁴

Fig. 55 Hydrodeoxygenation of HMF with H₂ generated in situ from the decomposition of alcohol. Adapted from ref. 726.

In addition to H₂, other hydrogen donors have also been attempted for the conversion of HMF to DMF. For example, Zhang et al. developed a facile process to convert HMF to DMF in ethanol using PdCl₂ as the catalyst and non-toxic and low-cost polymethylhydrosiloxane (PMHS) as the hydride source.⁷³⁵ It was demonstrated that the in situ-generated metallic Pd⁰ species and HCl play important roles in the reaction. This catalytic system enabled a DMF yield as high as 89.7% from HMF at room temperature (25 °C) in 0.5 h. In addition, a DMF yield of 41% was obtained from glucose after 2.5 h reaction at 120 °C.

Besides the catalytic transfer hydrogenation of HMF with alcohol toward BHMF, HMF can also be hydrogenated by the H₂ generated in situ from the decomposition of alcohol. For example, the in situ catalytic hydrogenation of HMF to DMF with methanol as the hydrogen donor (Fig. 56) over Cu supported on metal oxide catalysts was investigated by Zhang et al.⁷¹² Among the tested catalysts, Cu/Al₂O₃ gave the maximum DMF yield of 73.9% at 240 °C for 6 h owing to the small Cu crystallite size and strong acidity. However, the catalytic activity of Cu/Al₂O₃, Cu/ZnO, and Cu/CeO₂ was lost almost completely in the recycling experiment, and only Cu/ZrO₂ retained a DMF yield of around 21%. Besides, the in situ catalytic hydrogenation does not seem to be cost-effective owing to the consumption of two equivalents of methanol. They also investigated the one-pot hydrogenation of HMF to DMF over the ternary CuZnCoO_x catalyst with ethanol as the hydrogen donor, affording a maximum DMF yield of 99%.⁷¹⁸ They suggested that the CoO_x species catalyzes the in situ decomposition of ethanol to H₂ and acetaldehyde effectively (Fig. 57), the zinc oxide promotes the hydrogenation of aldehyde group to hydroxymethyl group, and the Cu–Co alloy promotes the hydrogenation of HMF remarkably. In addition, the catalytic performance of CuZnCoO_x could be maintained during six recycling experiment runs.

Fig. 56 Catalytic hydrogenation of HMF to DMF with methanol as the hydrogen donor. Adapted from ref. 712.

Fig. 57 Hydrodeoxygenation of HMF with H₂ generated in situ from the decomposition of alcohol. Adapted from ref. 718.

The drop-in coupling of HMF hydrodeoxygenation with the industrially important dehydrogenation reaction may enable the co-production of DMF with commercial products, thus greatly improving the overall benefit and sustainability. Li et al. reported that the simultaneous production of phenol and DMF with unprecedented high yields (>97%) from HMF and cyclohexanol (CHL) or cyclohexanone (CHN) could be achieved via a vapor-phase dehydrogenation–hydrogenation coupling reaction (Fig. 58) at 240 °C in the atmosphere of N₂ using a bimetallic Ni–Cu alloy catalyst, without the use of an additional hydrogen and oxygen supply.⁷³⁶ Similarly, Gao et al. reported that the use of a nitrogen-doped carbon (NC)-decorated copper-based catalyst (NC-Cu/MgAlO) enabled the selective synthesis of DMF and DMTHF for the transfer hydrogenolysis of HMF using cyclohexanol as the hydrogen source, obtaining a DMF yield as high as 96.1% at 0.5 h and DMTHF yield of up to 94.6% in 3.5 h.⁷⁰⁹ They proposed that the surface basic sites activate the alcohol hydroxyl in cyclohexanol and then promote the release of active hydrogen species, while the Cu⁰ nanoparticles and electrophilic Cu⁺ species promote the hydrogen transfer and the activation of the carbonyl group and hydroxyl group in HMF, respectively.

Fig. 58 Synchronized production of phenol and DMF via dehydrogenation–hydrogenation coupling process. Adapted from ref. 736.

Electrosynthesis is an efficient and environmentally friendly approach to achieve the oxidation or hydrogenation of HMF to useful chemicals. For example, a non-noble CuNi bimetallic alloy was used as a highly effective and stable electrode for the electro-catalytic hydrogenation of HMF toward DMF, achieving a DMF selectivity of 91.1% with a Faradaic efficiency of 88.0%.⁷²⁰

Owing to the higher stability and hydrophobicity of 5-formyloxymethylfurfural (FMF) than HMF, FMF is considered a promising alternative to HMF for the production of DMF. Sun et al. reported that the bimetallic catalyst 36Ni–12Cu/SBA-15 gave a DMF yield of 71.0% from FMF, which is slightly higher than that (60.5%) obtained from HMF under the same reaction conditions.⁷⁰³

The conversion of sugars to DMF via a two-step process is relatively easy to achieve. For example, Thananatthanachon et al. reported a two-step process involving the dehydration of fructose to the formate ester of HMF with formic acid as both the acid catalyst and solvent at high temperature, and hydrogenation of the formate ester to DMF over the Pd/C catalyst with formic acid as the hydrogen donor.⁷³⁷ The one-pot conversion of fructose to DMF (Table 16) was achieved by Li et al. using a multifunctional heterogeneous catalyst consisting of a carbon-based solid acid shell and CuCo bimetal core.⁷³⁸ The acid sites in the external carbon shell promoted the dehydration of fructose to the intermediate HMF, and then the CuCo bimetal catalyzed the hydrogenation of HMF to DMF synergistically. Among the tested catalysts, CuCo@C-TsOH-IM, which was prepared by impregnating CuCo@C-TsOH-IM with TsOH, exhibited the best catalytic performance, obtaining a DMF yield of 71.1% from fructose in THF at 220 °C under 3 MPa H₂ for 10 h. Insyani et al. designed the Pd/UiO-66@SGO catalyst by introducing Pd on a Zr-based metal–organic framework (UiO-66) deposited on sulfonated graphene oxide for the one-pot conversion of sugars, attaining DMF yields of 70.5% and 45.3% from fructose and glucose, respectively, at a relatively low substrate loading (0.5 wt%).⁷³⁹ To produce MF and DMF from hardwood poplar, Seemala et al. constructed an integrated process involving the conversion of poplar wood chips to HMF (66.0%) and furfural (93.5%) using dilute FeCl₃ as the catalyst in the medium of THF/water at subpyrolytic temperature, extraction of HMF and furfural from water to an organic phase, which was composed of toluene and 1,4-dioxane, after treatment with Ca(OH)₂ and hydrodeoxygenation of HMF and furfural to DMF (yield: 87.8%) and MF (yield: 85.6%) over Cu-the Ni/TiO₂ catalyst, attaining an overall fuel and lignin yield of 60%.⁷⁴⁰

Table 16 One-pot conversion of sugars to DMF

Catalyst	Catalyst loading^a	Solvent	Conditions	Feedstock^a	Conversion	DMF yield	Ref.
a Relative to solvent. — Not provided.
CuCo@C	0.5 wt%	THF	220 °C, 10 h, 3 MPa H2	Fructose, 0.5 wt%	56.2%	18.4%	738
CuCo@C-TsOH	0.5 wt%	THF	220 °C, 10 h, 3 MPa H2	Fructose, 0.5 wt%	>99%	55.5%	738
Cu@C-TsOH-IM	0.5 wt%	THF	220 °C, 10 h, 3 MPa H2	Fructose, 0.5 wt%	>99%	23.1%	738
Co@C-TsOH-IM	0.5 wt%	THF	220 °C, 10 h, 3 MPa H2	Fructose, 0.5 wt%	>99%	61.5%	738
CuCo@C-TsOH-IM	0.5 wt%	THF	220 °C, 10 h, 3 MPa H2	Fructose, 0.5 wt%	>99%	71.1%	738
Formic acid; Pd/C	1.3 wt%	Formic acid; +THF	150 °C, 2 h, in formic acid; 70 °C, 15 h, in formic acid + THF	Fructose, 12 wt%	—	51%	737
4.8Pd/UiO-66@SGO	0.5 wt%	THF	130 °C, 3 h, 1 MPa H2	HMF, 0.3 wt%	99.9%	99.2%	739
4.8Pd/UiO-66@SGO	0.5 wt%	THF	180 °C, 3 h, 1 MPa H2	Fructose, 0.50 wt%	91.8%	70.5%	739
4.8Pd/UiO-66@SGO	0.5 wt%	THF	180 °C, 3 h, 1 MPa H2	Glucose, 0.45 wt%	87.3%	45.3%	739

---

In addition to its application as a drop-in fuel, DMF is also a promising starting material for the production of important commodity chemicals. For example, Tao et al. reported that biorenewable para-xylene (PX), an important petroleum-derived commodity chemical with a global production of nearly 40 million tons per year, which is mainly used for the production of PTA, could be produced from HMF and ethylene (Fig. 59) via an integrated one-pot, two-step process involving the conversion of HMF with formic acid to DMF in n-heptane over the Au^Pd_0.2/t-ZrO₂ catalyst and Diels–Alder-like coupling reaction of ethylene with DMF to PX, affording an overall PX yield of 85%.⁷⁴¹ This process is parallel to the conversion of HMFCA to PTA via the Diels–Alder–dehydration reaction described in section 3.2.3. DMF can be transformed to alkylated tetrahydrofuran via a three-step cascade reaction (Fig. 60), composed of ring opening, aldol condensation and hydrogenation–cyclization process using acid, basic and metal catalyst, respectively.⁷⁴²

Fig. 59 Conversion of DMF to PX. Adapted from ref. 741.

Fig. 60 Conversion of DMF to alkylated tetrahydrofuran via a three-step process. Adapted from ref. 742.

3.3.3. Conversion of HMF to 5-MF. Structurally, 5-methylfurfural (5-MF) can be obtained by the selective removal of the hydroxyl group in HMF by hydrogenation. As an important precursor for pharmaceuticals, perfumes and flavoring components, 5-MF is industrially produced from expensive feedstock, including 5-methylfuran and N,N-DMF using the toxic phosgene (COCl₂) (Fig. 61a) or phosphorus oxy-chloride as the catalyst.^743,744 Thus, to avoid the use of these toxic chemicals, much attention has been paid to the environmentally friendly and sustainable synthesis of 5-MF from biomass-derived feedstocks (Table 17). Sun et al. reported that the palladium nanoparticles (Pd NPs) supported on activated carbon with polyvinylpyrrolidone (PVP) as a capping agent enabled the selective hydrogenation of HMF toward 5-MF (Fig. 61b), obtaining a maximum yield of up to 80%.⁷⁴⁴ Using 2.5% Pd-PVP/C (1 : 2), HMF was firstly converted to esterified intermediate, (5-formylfuran-2-yl)methyl formate (FFMF) via acid-catalyzed esterification and then converted to 5-MF via hydrogenolysis. In this process, formic acid not only functions as a reactant for the formation of esterified intermediate, but also serves as a hydrogen donor.

Fig. 61 (a) Industrial 5-MF production process. Adapted from ref. 743 and 744. (b) 5-MF production from HMF. Adapted from ref. 744. (c) Conversion of biomass to 5-MF via CMF as the intermediate. Adapted from ref. 210. (d) Direct conversion of biomass to 5-MF. Adapted from ref. 211.

Table 17 Conversion of HMF and carbohydrate to 5-MF

Catalyst	Catalyst loading^a	Solvent	Conditions	Substrate loading^a	Conversion	Yield	Ref.
a Relative to solvent. — Not provided.
2.5% Pd-PVP/C (1 : 2)	0.5 wt%	THF	200 °C, 7.5 h, HCOOH, 0.5 MPa N2	HMF, 2.3 wt%	87%	5-MF 80%	744
2.5% Pd-PVP/C (1 : 2)	0.5 wt%	THF	200 °C, 7.5 h, HCOOH, 0.5 MPa N2	ClMF, 2.3 wt%	87%	5-MF 80%	744
NaI; H2SO4	0.6 M; 50 mM	H2O/MTHF (v/v = 7/60)	180 °C, 4 h, 2.1 MPa H2	HMF, 1.3 wt%	—	5-MF 80.8%	211
NaI; HCl	0.4 M; 36 mM	H2O/MTHF (v/v = 2/3)	160 °C, 2 h, 2.1 MPa H2	Fructose, 1.3 wt%	—	5-MF 30.2% (LA 22.5%, 2-MF 3.1%)	211
NaI; HCl	0.4 M; 36 mM	H2O/MTHF (v/v = 2/3)	160 °C, 2 h, 2.1 MPa H2	HMF, 1.3 wt%	—	5-MF 30.1% (LA 23.3%, 2-MF 2.4%)	211
NaI; HCl	0.4 M; 36 mM	H2O/MTHF (v/v=2/3)	160 °C, 2 h, 2.1 MPa H2	Starch, 1.3 wt%	—	5-MF 38.0%	211
NaI; HCl	0.4 M; 36 mM	H2O/MTHF (v/v = 2/3)	160 °C, 2 h, 2.1 MPa H2	Cellulose, 1.3 wt%	—	5-MF 24.0% (LA 23.7%, 2-MF 2.0%)	211
Pd/C; HI	0.5 wt%; 0.79 M	H2O/benzene (v/v = 0.9)	90 °C, 0.5 h, 2.1 MPa H2	Fructose, 4.7 wt%	97%	5-MF 68%	210
Pd/C; HI	0.5 wt%; 0.79 M	H2O/benzene (v/v = 0.9)	90 °C, 2 h, 2.1 MPa H2	Glucose, 4.7 wt%	81%	5-MF 31%	210
Pd/C; HI; NaI	0.5 wt%; 0.03 M, 0.3 M	H2O/benzene (v/v = 0.9)	115 °C, 2 h, 2.1 MPa H2	Cellulose, 4.7 wt%	97%	5-MF 42%	210

---

5-MF could also be produced using carbohydrates (including fructose, glucose, cellulose, inulin and starch) and realistic biomass as the starting material via a two-step process, involving the conversion of carbohydrates and biomass to 5-(chloromethyl)furfural (CMF) and hydrodechlorination of CMF to 5-MF (Fig. 61c).^209–211 However, this process is not practical owing to the stoichiometric consumption of HCl. Different from HCl, Yang et al. reported that the combined use of HI with Pd/C (or RuCl₃) enabled the one-pot conversion of carbohydrates to 5-MF via 5-iodomethylfurfural (IMF) as the main reaction intermediate (Fig. 61d) without consuming HI, attaining 5-MF yields of 68%, 31% and 42% from fructose, glucose, and cellulose, respectively.²¹⁰ Similarly, Peng et al. reported that the combination of NaI with HCl (or H₂SO₄) enabled the metal-free hydrogenolysis of biomass to 5-MF, affording 5-MF yields of 80.8% and 38.0% from HMF and starch, respectively.²¹¹

3.3.4. Conversion of biomass to alkane via HMF as an intermediate. As an important milestone of the valorization of biomass to drop-in liquid fuels, Op de Beeck et al. reported that liquid straight-chain alkanes could be directly produced from cellulosic feedstock in a water–organic biphasic catalytic system (water/n-decane = 1/1) via a one-pot process (Fig. 62) involving the conversion of cellulose to HMF by tungstosilicic acid dissolved in the aqueous phase, and then hydrodeoxygenation of HMF to alkanes under H₂ over tungstosilicic acid-modified Ru/C suspended in the organic phase.⁷⁴⁵ In their following work, the use of a biphasic catalytic system consisting of water and light naphtha enabled the one-pot conversion of (hemi)cellulose pulp into C₅–C₆ alkanes via furfural and HMF as main intermediates and then the C₅–C₆ alkane stream enriched with bio-derived carbon, which resembles the so-called light straight run naphtha in a petrorefinery, was subjected to downstream petrorefinery processes for the production of bio-enriched gasolines.⁷⁴⁶ This work provides a promising strategy to integrate biorefinery technology with current petrorefinery processes. Similarly, the combination of the Ir–ReO_x/SiO₂ (Re/Ir = 2) catalyst with the HZSM-5 cocatalyst in the biphasic reaction system (n-dodecane + H₂O) enabled the one-pot conversion of cellulose to n-hexane via sorbitol as the main intermediate (Fig. 63), affording n-alkane yields of 78% and 83% from microcrystalline cellulose and ball-milled cellulose, respectively.⁷⁴⁷ Xia et al. reported that the multifunctional Pt/NbOPO₄ catalyst enabled the simultaneous conversion of cellulose, hemicellulose and lignin fractions in wood sawdust into hexane, pentane and alkylcyclohexanes in the medium of cyclohexane, respectively, with overall mass yields of up to 28.1 wt%.⁷⁴⁸ The superior performance was attributed to the synergistic effect among the Pt and NbOx species and acidic sites for the hydrodeoxygenation of lignocellulose.

Fig. 62 Direct upstream integration of the conversion of cellulose to liquid alkanes with the current light straight run naphtha petrorefinery processes. Adapted from ref. 745.

Fig. 63 Conversion of cellulose to liquid alkanes via sorbitol as an intermediate. Adapted from ref. 745 and 747.

C₅–C₆ alkanes require an additional isomerization process to improve their octane number prior to their utilization as gasoline. Alternatively, coupling HMF with carbonyl or hydroxyl group-containing compounds is an effective approach to extend the carbon chain to produce the precursor of liquid alkanes with a high octane number, as will be discussed in detail in section 3.6. Besides, Luo  et al. demonstrated that the acceptorless dehydrogenative C–C coupling of biomass-derived DMF (or methylfuran) (Fig. 64) over an Ru-doped ZnIn₂S₄ catalyst under visible light could produce H₂ and C_12–16 diesel precursors simultaneously. Subsequently, the desired diesel fuels, straight- and branched-chain alkanes could be synthesized from these precursors via hydrodeoxygenation.⁸

Fig. 64 Conversion of DMF (or MF) to diesel fuels. Adapted from ref. 8.

3.3.5. Hydrodeoxygenation of HMF to other products. As important precursors of furan-based drugs, such as ranitidine, prazosin, furosemide and cefuroxime, 5-hydroxymethyltetrahydro-2-furaldehyde (5-HMTHFF) and tetrahydrofurfural (THFF) are currently synthesized via complex and unsustainable processes in industry. Yang et al. reported that the combination of the Pd/LDH–MgAl–NO₃ catalyst with water as the reaction medium enabled the selective hydrogenation of HMF and furfural to 5-HMTHFF and THFF (Fig. 65), affording the 5-HMTHFF selectivity of 83.7% with the HMF conversion of 97.2% and THFF selectivity of 92.6% with the furfural conversion of 90.3%, respectively.⁷⁴⁹ The control experiment indicated that the use of water as the solvent could reduce the adsorption of the C=O group on the Pd particles and support, while the special nature of LDH–MgAl–NO₃ could prevent the hydrogenation of the C=O group on the support by hydrogen spillover, thus resulting in the selective hydrogenation of the furanyl ring without changing the C=O group.

Fig. 65 Selective hydrogenation of HMF and furfural to 5-HMTHFF and THFF. Adapted from ref. 749.

Similar to BHMF, bishydroxymethyltetrahydrofuran (BHMTF) is considered a promising building block for bio-based polyesters and promising feedstock for the synthesis of 1,6-hexanediol (1,6-HDO) and 2,5-dimethyltetrahydrofuran (DMTHF).⁶⁹² Fulignati et al. reported that at a mild temperature (50 °C, 6 h, 30 bar H₂), the commercial Ru/C catalyst enabled the selective hydrogenation of HMF to BHMF, while under relatively harsh conditions (100 °C, 6 h, 50 bar H₂), HMF was selectively converted to BHMTHF in aqueous solution.⁷²⁷ BHMTHF could be transformed to the high value 1,6-hexanediol (1,6-HDO) over the Pt–WO_x/TiO₂ catalyst via a ring-opening and hydrogenolysis process (Fig. 66), obtaining a 1,6-HDO yield of up to 70%.⁷⁵⁰ As an important monomer to produce polyesters and polyurethanes, the market price of 1,6-HDO is $4400 per ton with a market scale of approximately 138 000 tons per year.⁷⁵¹ Similarly, 1,5-pentanediol (1,5-PDO) can be produced with a high overall yield (80%) from biomass-derived furfural via a consecutive hydrogenation, dehydration, hydration and hydrogenation process.^752,753 Based on these studies, the production of 1,5-PDO and 1,6-HDO with 69% and 28% yield from hemicellulose and cellulose via multiple processes was achieved by He et al. using furfural and HMF as the main reaction intermediates, respectively.⁷⁵⁴ A techno-economic analysis suggested that the MSP of 1,6-HDO and 1,5-PDO is $4090 per ton.⁷⁵⁴

Fig. 66 Conversion of BHMTHF to 1,6-HDO. Adapted from ref. 750.

The hydrogenation of HMF in acidic media can yield 1-hydroxyhexane-2,5-dione (HHD) (Fig. 67) via a ring opening and hydrogenation process.^755,756 For example, bipyridine-coordinated Cp*–iridium(III) complexes enabled the hydrogenation/hydrolytic ring opening reactions of HMF toward HHD.^757,758 Fujita et al. reported that nickel phosphide nanoparticles (Ni₂P NPs) were highly effective for the selective hydrogenation of biofuranic aldehydes to diketones in water without using an additive, attaining an HHD yield of up to 84% from HMF, while the conventional Ni(0), NiO, and other metal phosphide NPs are ineffective.⁷⁵⁹ The spectroscopic analysis showed that the Ni–Ni species function as the H₂ activation sites, while P–OH functions as the surface acid sites. Therefore, the Ni₂P NPs could combine the hydrogen-activating ability and surface acidity to promote selective conversion of biofuranic aldehydes. HHD can be further converted to methylcyclopent-2-enone (MCP), a valuable edible essence widely applied in foods, beverages and flavors, via an intramolecular aldol condensation over base catalysts.⁷⁶⁰ Duan et al. achieved the conversion of HMF to MCP via the hydrogenation of HMF to HHD over the Pd/Nb₂O₅ catalyst and rearrangement of HHD to MCP over the Ca–Al oxide catalyst in the medium of water, attaining an isolated MCP yield of 58%.⁷⁶¹ 3-Hydroxymethyl-cyclopentone (HCPN) is considered a promising intermediate for the production of high-value fragrances, pesticides and polymers. Ramos et al. reported that the Cu–Al₂O₃ catalyst is effective to catalyze the conversion of HMF to HCPN under H₂ in aqueous solution, while Co–Al₂O₃ mainly converts HMF to 3-hydroxymethylcyclopentanol (HCPL).⁷⁶² They observed that the reduced metal phases are responsible for the hydrogenation step. The catalytic activity of Cu–Al₂O₃ and Co–Al₂O₃ both decreased remarkably after the first use, but they could be regenerated via calcination at 500 °C in air followed by reduction under H₂ at 700 °C. Zhang et al. reported that an MOF-derived bimetallic nickel–copper catalyst gave an HCPN yield of 70.3% with a total rearrangement product yield of 99.8% in water without the use of acidic additives.⁷⁶³ They also demonstrated that water not only functions as the reaction medium and reactant, but also serves as a proton donor to form a slightly acidic condition, which accelerates the ring-rearrangement reaction at high temperature (140 °C). Deng et al. reported that Pd nanoparticles supported on Lewis acidic pyrochlore, including La₂Sn₂O₇, Y₂Sn₂O₇, and Y₂(Sn_0.7Ce_0.3)₂O_7−δ, are active for the conversion of HMF to HCPN, attaining an HCPN yield of up to 92.5% over the Pd/Y₂(Sn_0.7Ce_0.3)₂O_7−δ catalyst.⁷⁶⁴

Fig. 67 Conversion of HMF to HCPN and MCP. Adapted from ref. 760, 761 and 763.

The use of high-pressure H₂ not only consumes a large amount of energy, but also induces potential risk, which is a major limiting factor in the commercialization of the hydrodeoxygenation process. Duan et al. reported that the introduction of high-pressure molecular dinitrogen (N₂), which was previously considered as a carrier or inert gas with low cost, is a general strategy to increase the efficiency of catalytic hydrodeoxygenation with a reduced activation energy.⁷⁶⁵ They suggested that the adsorption and activation of N₂ on the metallic ruthenium surface produces N₂H_x species, thus providing protic hydrogen to promote the scission of the carbon–oxygen bond and then the hydrogenation of the hydroxy groups. This strategy may be promising to reduce the consumption of H₂, leading to a reduction in cost.

3.4. Etherification of HMF with alcohols toward alkoxymethyl furfurals

The etherification reaction of aliphatic alcohols with HMF can generate alkoxymethyl furfurals (AMF), such as 5-(methoxymethyl)furfural (MMF) and 5-(ethoxymethyl)furfural (EMF), which are considered promising fuel additives and precursors of drop-in fuel owing to their high energy density, low toxicity, high stability and suitable flow properties.⁷⁶⁶ Here, we summarize the state-of-the-art catalytic systems for the conversion of HMF and carbohydrates to AMF with a focus on the heterogenous catalysts.

The etherification of aliphatic alcohols and HMF usually proceed over Brønsted acid catalysts with a series of side-reactions (Fig. 7). The product selectivity is influenced by the catalyst, solvent and reaction conditions (Table 18). The acid type and intensity have a critical influence on the reaction pathway of HMF in the medium of ethanol. Generally, a strong Brønsted acid is beneficial for the formation of ethyl levulinate (EL), a strong Lewis acid prefers the generation of ether (EMF), while weak acid favors the generation of the acetal, 5-(alkoxymethyl)furfural dialkylacetal (EMFDEA).⁷⁶⁷ For example, a high yield of EMF (71%) was attained at mild temperature (100 °C) within 6 h, while a high yield of EL (73%) was attained at an elevated temperature (140 °C, 6 h) for HMF conversion over sulfonic-acid-functionalized carbon nanomaterial (C–SO₃H) in the medium of ethanol.⁷⁶⁸ Similarly, the EL and EMF selectivity from HMF could be tuned by regulating the reaction conditions and pore structure of the ZSM-5 zeolite.⁷⁶⁹ The use of a high loading of mesoporous ZSM-5 zeolite as the catalyst and n-hexane as the solvent at high temperature is beneficial for the selective formation of EL.

Table 18 Etherification of HMF with alcohols toward alkoxymethyl furfurals

HMF or carbohydrate loading^a	Catalyst loading^a	Solvent	Reaction conditions	HMF or carbohydrate conversion	Yield (%)	Ref.
a Relative to solvent. — Not provided.
HMF, 3.2 wt%	Amberlyst-15, 0.9 wt%	Ethanol/ChCl/MeCN	100 °C, 12 h	99%	82%	770
HMF, 1.6 wt%	[Cu–BTC][HPM], 1 wt%	Ethanol	140 °C, 12 h	—	68.4% (EL 20.2%)	771
HMF, 1.6 wt%	C-SO3H, 1 wt%	Ethanol	100 °C, 6 h	—	71% (EL 22%)	768
HMF, 1.6 wt%	C-SO3H, 1 wt%	Ethanol	140 °C, 6 h	—	<1% (EL 73%)	768
HMF	Graphene oxide, 3 wt%	Ethanol	100 °C, 12 h	—	92%	772
HMF, 6.3 wt%	MHGC-SO3H, 2 wt%	GVL/ethanol (v/v = 1.5)	120 °C, 16 h	95%	95%	773
Fructose, 3.6 wt%	MHGC-SO3H, 2 wt%	GVL/ethanol (v/v = 1.5)	120 °C, 24 h	100%	67.4%	773
Glucose, 3.6 wt%	MHGC-SO3H, 2 wt%	GVL/ethanol (v/v = 1.5)	120 °C, 24 h	98.4%	3%	773
Fructose, 6.3 wt%	Graphene oxide, 3 wt%	Ethanol	100 °C, 24 h	∼99%	71%	772
HMF 6.3 wt%	TaTPA, 3.8 wt%	Ethanol	120 °C, 45 min	100%	88.3%	774
HMF 6.3 wt%	30% TaTPA/SnO2 3.8 wt%	Ethanol	120 °C, 45 min	99.5%	90.2%	774
Fructose 3 wt%	30% TaTPA/SnO2 3.8 wt%	Ethanol	120 °C, 8 h	100%	68%	774
HMF, 6.3 wt%	HPW/NbPO, 3.76 wt%	Ethanol	120 °C, 1 h	95%	89%	775
Fructose, 3.6 wt%	Phosphotungstic acid, 0.72 wt%	Ethanol/DMSO (v/v = 7/3)	140 °C, 1.5 h	∼97%	64%	776
Inulin, 3.6 wt%	[BMIM][HSO4], 40 wt%	Ethanol/H2O (v/v = 125)	130 °C, 0.5 h	—	77%	777
Glucose, 3.6 wt%	CrCl3, 0.02 M	Ethanol	140 °C, 10 h	100%	15.2% (ethyl glucoside 42.6%)	778
Glucose, 4 wt%	AlCl3·6H2O 1.5 wt%; PTSA–POM 2 wt%	Ethanol/H2O (v/v = 9)	150 °C, 0.5 h	97.9%	30.6% (HMF 11.5%)	779
Glucose, 1.8 wt%	AlCl3·6H2O, 0.96 wt%, [BMIM][HSO4], 10 wt%	Ethanol	130 °C, 0.5 h	—	36.7% (HMF 2.7%)	780
Glucose, 3 wt%	H-USY 2.5 wt%; Amberlyst-15 0.5 wt%	Ethanol	96 °C, 11 h	91%	17% (HMF 3%, EL 14 wt%)	781
Glucose, 3 wt%	H-USY 2.5 wt%; Amberlyst-15 0.5 wt%	Ethanol/DMSO (v/v = 7/3)	131 °C, 24 h	98%	26% (HMF 20%, EL 1%)	781
HMF, 2.5 wt%	SO3H-PANI-FeVO4, 1 wt%	Ethanol	120 °C, 6 h	94.1%	80%	452
Fructose, 3.6 wt%	SO3H-PANI-FeVO4, 1 wt%	Ethanol	120 °C, 2 h	86.3%	72.5%	452
Sucrose, 6.8 wt%	SO3H-PANI-FeVO4, 1 wt%	Ethanol	120 °C, 24 h	76.2%	57.2%	452
Fructose, 3.6 wt%	HReO4, 0.02 M	Ethanol/THF	140 °C, 1 h	—	73%	388
Fructose, 3.6 wt%	HReO4, 0.02 M	Ethanol	160 °C, 0.5 h	—	65% (EL 10%)	388
Fructose, 3.6 wt%	HReO4, 0.02 M	Ethanol/THF	160 °C, 16 h	100%	0 (EL 80%)	388
Inulin, 3.6 wt%	HReO4, 0.02 M	Ethanol	160 °C, 16 h	77%	— (EL 65%)	388
Sucrose, 3.6 wt%	HReO4, 0.02 M	Ethanol	160 °C, 16 h	62%	— (EL 52%)	388
Glucose, 3.6 wt%	HReO4, 0.02 M	Ethanol	160 °C, 72 h	39%	— (EL 80%)	388
HMF 5 wt%	Zr-Mont 2 wt%	Isopropanol	Step I: 100 °C, 12 h; step II: 100 °C, 2 h (adding H2O)	100%	iPMF 84%	782
HMF 5 wt%	Zr-Mont-400 2 wt%	Isopropanol	Step I: 100 °C, 12 h; step II: 100 °C, 2 h (adding H2O)	45%	iPMF 34%	782
HMF 5 wt%	ZrO2 2 wt%	Isopropanol	Step I: 100 °C, 12 h; step II: 100 °C, 2 h (adding H2O)	8%	iPMF 3%	782
HMF 5 wt%	Mont 2 wt%	Isopropanol	Step I: 100 °C, 12 h; step II: 100 °C, 2 h (adding H2O)	17%	iPMF 10%	782
HMF, 1 wt%	Zr-SBA-UH, 0.51 wt%	Isopropanol	150 °C, 4 h	—	BPMF 93.6%	783
HMF, 1 wt%	Zr-SBA-UH, 0.51 wt%	Ethanol	150 °C, 4 h	—	BEMF 90.8%	783
HMF, 1 wt%	Zr-SBA-UH, 0.51 wt%	n-Propanol	150 °C, 4 h	—	BPMF 45.4%	783
HMF, 1 wt%	Zr-SBA-UH, 0.51 wt%	n-Butanol	150 °C, 4 h	—	BBMF 27.5%	783
HMF, 1 wt%	Zr-SBA-UH, 0.51 wt%	Isobutanol	150 °C, 4 h	—	BPMF 64.8%	783
HMF, 1 wt%	Zr-SBA-UH, 0.51 wt%	sec-Butanol	150 °C, 4 h	—	BBMF 5.4%	783
HMF, 1.3 wt%	Co-400, 0.1 wt%	Methanol	90 °C, 2 MPa H2, 1 h	∼100%	BHMF 98.5%	784
HMF, 2 wt%	MZSM-5, 4.8 wt%	Ethanol/n-hexane (w/w = 1/3)	150 °C, 12 h	94.2%	EMF 9.1% (EL 90.8%)	769
HMF, 2 wt%	MZSM-5, 2.5 wt%	Ethanol/n-hexane (w/w = 3/1)	150 °C, 12 h	80.3%	EMF 54.1% (EL45.9%)	769
HMF, 2 wt%	HZSM-5, 2.5 wt%	Ethanol	140 °C, 12 h	61.5%	EMF 57.8% (EL4.7%)	769
Corn stover, 2 wt%	USY zeolite, 1 wt%	THF/ethanol	168 °C, 2.9 h	—	EMF, 21.8%	785
Corn stover, 2 wt%	USY zeolite, 1 wt%	THF/ethanol	180 °C, 2.5 h	—	EMF 15.1% (HMF 0.3%, EL 0.8%)	785
Corn stover, 2 wt%	USY zeolite, 1 wt%	DMSO/ethanol	180 °C, 2.5 h	—	EMF, 12.1% (HMF 6.8%, EL 4.7%)	785
Corn stover, 2 wt%	USY zeolite, 1 wt%	Cyclohexane/ethanol	180 °C, 2.5 h	—	EMF, 10.3% (HMF 0.2%, EL 5.1%)	785
Corn stover, 2 wt%	USY zeolite, 1 wt%	n-Hexane/ethanol	180 °C, 2.5 h	—	EMF, 4.4% (HMF 0.2%, EL 9.1%)	785
Corn stover, 2 wt%	USY zeolite, 1 wt%	Ethanol	180 °C, 2.5 h	—	EMF, 4.4% (HMF 0.3%, EL 0.7%)	785
HMF, 1 wt%	HCP-1, 0.7 wt%	Ethanol	100 °C, 10 h	—	EMF, 98.3% (EL 0.5%)	786
Fructose, 1 wt%	HCP-1, 0.7 wt%	Ethanol/DMSO (v/v = 4)	105 °C, 8 h	99.8%	EMF, 78.9% (HMF 15.4%, EL 4.6%)	786
Fructose, 1 wt%	HCP-1, 0.7 wt%, 5 runs	Ethanol/DMSO (v/v = 4)	105 °C, 8 h	—	EMF, 75%	786
Glucose, 1 wt%	HCP-1, 0.7 wt%	Ethanol	125 °C, 16 h	—	EMF, 18.4%	786
Sucrose, 1 wt%	HCP-1, 0.7 wt%	Ethanol	105 °C, 8 h	—	EMF, 28.8% (glucose 7.9%, HMF 14.7%, EL 2.1%)	786
Inulin, 1 wt%	HCP-1, 0.7 wt%	Ethanol	105 °C, 8 h	—	EMF, 75.6% (HMF 8.0%, EL 6.5%)	786
Cellobiose, 1 wt%	HCP-1, 0.7 wt%	Ethanol	105 °C, 8 h	—	EMF <1% (glucose 1.7%, HMF 3.2%, EL <1%)	786
Fructose, 0.9 wt%	Cpolyurethane-SO3H, 0.3 wt%	Ethanol/1,4-dioxane (v/v = 7/3)	140 °C, 2 h	100%	EMF, 66.0%	418
Glucose, 0.9 wt%	Cpolyurethane-SO3H, 0.3 wt%	Ethanol/1,4-dioxane (v/v = 7/3)	140 °C, 4 h	100%	EMF, 9.2%	418
HMF 10 wt%, IB 17 wt%	HY zeolite, 3 wt%	GDE	60 °C, 4 h	59%	tBMF, 55.5%	787
HMF 7 wt%, IB 12.4 wt%	Aquivion/m-SiO2 (25 mM H^+)	THF	90 °C, 3 h	78%	tBMF, 66% (OBMF <4%)	788
CMF 5 wt%	—	Ethanol	50 °C, 3 h	90%	EMF, 90%	789
BrMF 5 wt%	—	Ethanol	50 °C, 3 h	90%	EMF, 90%	789

---

Since Brønsted acids are also effective to convert fructose and inulin to HMF,⁷⁷⁷ the one-pot conversion fructose and inulin to AMF are relatively easy to achieve though a one-pot process over Brønsted acid catalysts.⁷⁷⁵ The conversion of fructose in the medium of alcohols is accompanied by a series of side reactions with EL, resulting in 2-(diethoxymethyl)-5-(ethoxymethyl)furan as the main by-product.⁷⁹⁰ Using the G4 method, Xiang et al. indicated that ethanol promotes the Brønsted acid-catalyzed dehydration of fructose since the [C₂H₅OH₂]⁺ species formed by the solvation of H⁺ on ethanol serves as a more active catalytic species than H⁺ for the preferential dehydration of fructose to HMF and then the etherification of HMF to EMF.⁷⁹¹ The influence of additional organic solvents on the conversion of fructose over Amberlyst-15 catalyst in ethanol was investigated by Guo et al. using combined experimental, spectroscopic and theoretical methods.⁷⁹² They observed that protonation of the polar additive increases in the following order: THF < acetone < CH₃CN < GVL < DMSO, which is proportional to its Kamlet–Taft (KT) π* value, and the preferential protonation of DMSO is beneficial for the selective formation of HMF by preventing the subsequent etherification.

Compared with the one-pot conversion of fructose to EMF, the one-pot conversion of glucose to EMF is more challenging.⁷⁷³ For example, the (HCP-1)-derived hyper-cross-linked polymer exhibited high EMF yields of 98.3%, 78.9% and 75.6% from HMF, fructose and inulin, respectively, whereas the use of glucose resources (glucose and cellulose) as feedstock predominantly gave the ethyl glucoside product with a low EMF yield (18.4%).⁷⁸⁶ A one-pot, two-step process was developed to improve the EMF production efficiency from glucose. Firstly, glucose is isomerized to fructose by the Lewis acid catalyst. After isomerization, the Lewis acid catalyst is separated from the reaction system and then a Brønsted acid catalyst is added to enable the dehydration of fructose to HMF, and then the etherification of HMF with ethanol to EMF. For example, the combined use of H β zeolite and PSDVB–SO₃H in the “two-step, one-pot” process gave an EMF yield of 39.4% with an EL yield of 4.1% from glucose.⁷⁹³ Guo et al. reported that the one-pot, one-step conversion of glucose into EMF could be achieved by combining the acidic ionic liquid 1-butyl-3-methylimidazolium hydrogen sulfate (BMIMHSO₄) with the Lewis acid AlCl₃, affording a maximum EMF yield of 37%.⁷⁸⁰ The direct alcoholysis of corn stover to EMF was achieved by Chen et al. using USY zeolite as the catalyst in ethanol/THF, attaining a maximum EMF yield of 21.8% at 168 °C for 2.9 h.⁷⁸⁵ Compared with other solvents, the use of THF as the solvent could inhibit the formation of EL.

Besides AMF, 2,5-bis(alkoxymethyl)furans (BAMF), a promising biodiesel component or additive, could be synthesized from HMF via reductive etherification (Fig. 68). For example, the use of Zr-SBA prepared via the urea hydrolysis method (Zr-SBA-UH) as a catalyst, with isopropanol as both the solvent and hydrogen source, enabled the highly efficient reductive etherification of HMF to 2,5-bis(isopropoxymethyl)furan (BPMF) with a yield of 93.6%.⁷⁸³ Under 2 MPa H₂, the reductive etherification of HMF with methanol over the Co-400 catalyst, which was synthesized from commercially available Co₃O₄via simple reduction under H₂, could give a 2,5-bis(methoxymethyl)furan (BMMF) yield as high as 98.5% at 140 °C within 1 h.⁷⁸⁴ The catalytic activity of the Co-400 catalyst was retained for use five times without an obvious decrease in the BMMF yield.

Fig. 68 Production of BAMF via the reductive etherification of HMF. Adapted from ref. 783.

The use of 5-(halomethyl)furfural (halo = Cl, Br) to displace HMF (Fig. 69) enables the efficient production of AMFs in excellent isolated yields (>90%) by converting 5-(halomethyl)furfural in monohydric alkyl alcohols, including methanol, ethanol, 1-propanol and 1-butanol under mild conditions without the use of additional catalysts.^544,789 The use of higher primary alcohols, such as 1-pentanol and 1-hexanol, and secondary alcohol, such as 2-propanol, requires a base additive, such as N,N-diisopropylethylamine (DIPEA) to afford a high yield of AMFs. In contrast, at a relatively high temperature, the reaction of CMF with ethanol gives levulinic ester as the main product.⁵⁴⁷

Fig. 69 (a) Production of AMFs from 5-(halomethyl)furfural and alcohols under mild conditions. Adapted from ref. 544 and 789. (b) Production of levulinic ester from 5-(halomethyl)furfural and ethanol at relatively high temperature. Adapted from ref. 547.

Similar to the etherification of HMF with alcohols, Yang et al. reported that tert-butoxymethylfurfural (tBMF), a potential biodiesel additive similar to EMF, could be produced via the etherification of HMF with isobutene (IB) over acid zeolites (Fig. 70). They found that the use of HY zeolite (SiO₂/Al₂O₃ molar ratio = 12) in the medium of glycol dimethyl ether (GDE) could inhibit HMF dimerization and IB oligomerization, thus attaining a tBMF yield of 55.5% with the selectivity of 94% at 60 °C within 3 h.⁷⁸⁷ Dou et al. reported that the reaction between HMF and IB over an Aquivion perfluorosulfonic acid resin (PFSA) modified mesoporous silica (Aquivion/m-SiO₂) solid acid afforded a tBMF yield of 66% with 5,5′-[oxybis(methylene)]bis[2-furaldehyde] (OBMF) yield lower than 4% in the medium of THF.⁷⁸⁸

Fig. 70 Synthesis of tBMF via the etherification of HMF with isobutene. Adapted from ref. 787.

3.5. Acetalization of HMF with alcohols

The acetalization of HMF with alcohols is a major side reaction accompanying the etherification reaction. Kanai et al. reported that the cerium phosphate (CePO₄) catalyst exhibited high catalytic activity for the chemoselective acetalization of HMF with methanol to 5-(dimethoxymethyl)-2-furanmethanol (Fig. 71) in excellent yield (81%) owing to the uniform Lewis acid and weak base sites, which interact with HMF and methanol molecules, respectively.⁷⁹⁴ The mutual valorization of glycerol with HMF toward diol monomers (Fig. 72), which can be used as a potential solvent, lubricant, antifreeze, personal care product and biodegradable and photodegradable polymer, can be achieved via acetalization reaction using acid catalysts, such as laminar zeolite (ITQ-2) and mesoporous aluminosilicate (MCM-41).⁷⁹⁵

Fig. 71 Chemoselective acetalization of HMF with methanol toward 5-(dimethoxymethyl)-2-furanmethanol. Adapted from ref. 794.

Fig. 72 Mutual valorization of HMF with glycerol toward valuable diol monomers. Adapted from ref. 795.

As discussed in section 2.5.4, the stabilization of HMF by protecting group chemistry not only can inhibit the degradation of HMF, but also improve the efficiency and selectivity of upgrading HMF. Recently, a series of studies demonstrated that the protection of the formyl group in HMF via acetylation is an effective strategy to reduce the undesirable oligomerization reactions, thus enabling the upgrading of HMF to high-value products via selective oxidation, oxidative esterification, hydrogenation and Diels–Alder reaction at a high substrate loading with the reduced formation of humins. Kim et al. reported that the acetylation of HMF with 1,3-propanediol (Fig. 73a) gives an HMF acetal derivate (PD-HMF) with obviously higher thermal stability than the corresponding acetal derivates (MeO-HMF, EG-HMF) obtained from HMF with methanol and ethylene glycol, and the aerobic oxidation of PD-HMF (Fig. 73b) over a CeO₂-supported Au catalyst in Na₂CO₃ aqueous solution afforded high FDCA yields (90–95%) even at a high substrate loading (10–20 wt%) without the generation of humins.⁷⁹⁶ Owing to the six-membered acetal ring structure, the thermal decomposition and self-polymerization of HMF at a high substrate loading were remarkably inhibited. The use of an HMF acetal derivate (PD-HMF) to displace HMF as the starting material was also effective for the selective production of FDCA-derived esters via oxidative esterification at a high substrate loading.⁷⁹⁷ The aerobic oxidative esterification of a high concentration PD-HMF (10–20 wt%) with methanol and ethylene glycol over an Au/CeO₂ catalyst attained 80–95% yields of MFDC and bis(2-hydroxyethyl)furan-2,5-dicarboxylate (HEFDC) (Fig. 73c). Since high yield can be obtained at a high substrate loading, this strategy can greatly improve the reaction efficiency and economics for the subsequent production of PEF.

Fig. 73 (a) Acetylation of HMF with methanol, ethylene glycol and 1,3-propanediol, and (b) selective oxidation of PD-HMF to FDCA in Na₂CO₃ aqueous solution. Adapted from ref. 796. (c) Selective oxidation of PD-HMF to bis(2-hydroxyethyl)furan-2,5-dicarboxylate (HEFDC) and methyl furan-2,5-dicarboxylate (MFDC). Adapted from ref. 797. (d) Selective hydrogenation of PD-HMF. Adapted from ref. 798. (e) Selective hydrogenation of PD-HMF to BHMF. Adapted from ref. 799.

Similar to the selective oxidation of PD-HMF, the use of an acetalized HMF derivate to displace HMF for the hydrogenation reaction could also inhibit the undesirable oligomerization. Wiesfeld et al. showed that the hydrogenation of a concentrated PD-HMF aqueous solution over bimetallic Ni–Re supported on TiO₂ at a carefully controlled pH (about 10⁻³ equivalent of Na₂CO₃ with respect to PD-HMF) gave a BHMF yield of 81–89% (Fig. 73d).⁷⁹⁸ At neutral pH, a certain amount of unprotected HMF (9%) is formed since the deprotection rate is faster than the hydrogenation rate, while a high pH leads to a low deprotection rate and essentially promotes the undesirable hydrogenation toward 5-hydroxymethyl tetrahydrofuran (PDHMTHF).⁷⁹⁸

Furthermore, Chang et al. showed that the acetalized HMF derivate could be converted to norcantharimide derivates via Diels–Alder reaction with maleimides, while the hydrolysis of the acetal linkage mismatches the molecular orbital of norcantharimide and then triggers a retro Diels–Alder reaction at ambient temperature, leading to the controlled release of high-value chemicals at 35–60 °C.⁷⁹⁹ This process is promising for the development of drug delivery systems at human body temperature.

3.6. Aldol condensation of HMF with carbonyl or hydroxyl group-containing compounds

Owing to the low octane number of HMF and furfural-derived liquid alkanes, which are determined by their relatively low carbon chain lengths, they cannot be directly used as transportation fuel. Thus, aldol condensation of furan aldehydes with carbonyl or hydroxyl group-containing compounds offers an effective approach for the desirable chain extension toward desired fuel precursors, which can be further converted into liquid alkanes with sufficient energy density and octane number.

Aldol condensation of HMF with acetone using NaOH as a catalyst not only can produce desirable fuel precursors, including (E)-4-[5-(hydroxymethyl)-2-furanyl]-3-butene-2-one (HA) and (E,E)-1,5-bis(5-hydroxymethyl-2-furanyl)-1,4-pentadiene-3-one (HMF-acetone-HMF dimer, HAH) (Fig. 74a), but also lead to undesirable by-products via self- and cross-condensation reactions, resulting in a low selectivity and yield of the desired products.^800,801 The undesirable side reactions between HMF and acetone can be inhibited by operating at low temperature (308 K) and low NaOH concentration (∼110 mM), and the selectivity toward HA or HAH can be tuned by regulating the initial concentration of HMF and acetone.⁸⁰² Chang et al. reported that HAH could be obtained via a two-step process involving the dehydration of fructose to HMF over HCl in water/acetone medium and aldol condensation of HMF with acetone over NaOH, attaining an overall yield of 74.1% at an MSP of $1958 per ton.⁸⁰³ Owing to the low cost, HAH may displace anthraquinone and bisphenol-A as feedstock for the synthesis of organic dye and polyether. The aldol condensation of HMF with acetone toward HA using MgAl and MgZr mixed oxides as catalysts was investigated by Cueto et al.⁸⁰⁴ Owing to its appropriate acid/base site ratio with medium strength, the MgZr catalyst gave the highest whole yield (37%) of aldol condensation products. After three consecutive runs, the MgAl mixed oxide lost its catalytic activity completely, while the HMF conversion over the MgZr mixed oxide decreased to below 20%. Suttipat et al. reported that the composite of hierarchical faujasite nanosheets and zeolitic imidazolate framework-8 (Hie-FAU-ZIF-8) could function as an effective acid–base bifunctional catalyst for the aldol condensation of HMF with acetone, attaining an HA yield of up to 67% owing to the synergistic effect of the Na⁺-stabilized zeolite framework and the imidazolate linker bearing basic nitrogen groups.⁸⁰⁵ Yutthalekha et al. reported that amine-grafted hierarchical basic FAU-type zeolite nanosheets exhibited an excellent catalytic performance for the aldol condensation of HMF and acetone, attaining an HA yield approaching 100%.⁸⁰⁶ Zhang et al. suggested that the coexistence of acidic silanol and basic alkylamine bifunctional groups on the SBA-15 catalyst may generate a synergistic effect for the aldol condensation of HMF with acetone by combining quantum mechanical and molecular mechanical calculations.⁸⁰⁷ They proposed that [–SiOH], as a bridge of H shift, promotes the activation of acetone toward enol-acetone, while [–RNH₂] mainly catalyzes the aldol condensation of enol-acetone with HMF and the subsequent dehydration. Malkar et al. reported that the aluminum-exchanged dodecatungstophosphoric acid encapsulated inside the cage of ZIF-8 (Al_0.66-DTP@ZIF-8) was active for the selective synthesis of HA from HMF and acetone, attaining the maximum HMF conversion of 99% with HA selectively of 84.11%.⁸⁰⁸ The high HA selectivity is attributed to the kinetically controlled reaction pathway since the activation energy (63.1 kJ mol⁻¹) for the formation of HA is remarkably lower than that (102.1 kJ mol⁻¹) for the formation of HAH.

Fig. 74 (a) Aldol condensation of HMF with acetone. Adapted from ref. 800. (b) Aldol condensation of HMF with isophorone. Adapted from ref. 809. (c) Aldol condensation of HMF with levulinic acid. Adapted from ref. 810. (d) Aldol condensation of furfural with MEK. Adapted from ref. 811.

In addition to acetone, other carbonyl or hydroxyl group-containing compounds have also been used for the carbon chain extension of HMF. The aldol condensation of isophorone with HMF (Fig. 74b) over NaOH under solvent-free conditions followed by hydrodeoxygenation could give multi-substituted cycloalkanes with high density and low freezing point.⁸⁰⁹ The aldol condensation between HMF and levulinic acid (Fig. 74c) toward (E)-6-[5-(hydroxymethyl)furan-2-yl]-hex-4-oxo-5-enoic acid and (E)-3-[5-(hydroxymethyl)furan-2-yl]methylene-4-oxo-pentanoic acid can be achieved using NaOH as a catalyst.^810,812 Similarly, the selective aldol condensation of levulinic acid and furfural in the aqueous phase was achieved over MgO and ZnO catalysts.⁸¹³

Similar to HMF, the aldol condensation of furfural with carbonyl or hydroxyl group-containing compounds has also been investigated widely. Jing et al. reported that the Nb₂O₅ catalyst exhibited good catalytic activity and stability in the aldol condensation of furfural with acetone, 4-heptanone or 2,4-pentanedione, which is superior to other common solid acid and base catalysts, including ZrO₂, Al₂O₃, MgO, CaO and magnesium–aluminum hydrotalcite.⁸¹⁴ They proposed that Nb₂O₅ could activate the C=O bond in the carbonyl group-containing molecules, facilitate the formation of a metal enolate intermediate, and then promote the formation of a C–C bond via nucleophilic addition. Moreover, the multifunctional Pd/Nb₂O₅ catalyst enabled a one-pot, two-step process for the conversion of furfural to liquid alkanes. The carbon chain of furfural can also be extended via aldol condensation with methyl ethyl ketone (MEK) (Fig. 74d), the dehydration product of biomass-derived 2,3-butanediol (BD), under alkaline conditions to obtain C₉/C₁₄ fuel precursors.⁸¹¹

The self-coupling reaction is an useful approach to produce value-added chemicals.⁸¹⁵ Both HMF and furfural can be converted to dimers though etherification or aldol condensation reaction.⁸¹⁶ Amarasekara et al. reported that fructose could be converted to 5,5′-[oxybis(methylene)]bis[2-furaldehyde] (OBMF) in DMSO using Dowex 50 W X8 as a solid acid catalyst (76% yield) via HMF as an intermediate (Fig. 75a).⁸¹⁷ Cywar et al. designed polystyrene (PS)-supported benzimidazolium coupled with a dodecyl chain as a catalyst for the self-coupling of furfural and HMF into C_10–14 furoins (Fig. 75b), obtaining the highest C₁₀ furoin yield (monool furoin) of 95% from HMF and C₁₄ furoin (triol DHMF) yield of 88% from furfural, respectively.⁸¹⁸ The monool furoin obtained with the homo-coupling of furfural is a promising precursor for the production of linear and branched alkanes via hydrodeoxygenation, while HMF-derived triol DHMF can be applied for the synthesis of polyesters and PUs. Wilson et al. reported that the cross-coupling of HMF with furfural over the NHC catalyst, 1,3,4-triphenyl-4,5-dihydro-1H-1,2,4-triazol-5-ylidene (TPT), could give difuranic C₁₁ diols yield of 43%.⁸¹⁹ These difuranic C₁₁ diols are promising feedstocks for the production of linear polyesters and polyurethanes.

Fig. 75 (a) Dimerization of HMF via etherification reaction. Adapted from ref. 817. (b) Homo-coupling and cross-coupling of furfural and HMF via aldol condensation. Adapted from ref. 818 and 819.

The one-pot conversion of cellulose to furanic biocrude involving the conversion of cellulose to HMF and subsequent aldol condensation of HMF toward biocrude were carried out under mild conditions (120 °C, 3.5 h) in the medium of acetone, obtaining furanic biocrude yields of 25.7% and 19.3% with acidic ionic liquids 1-(3-propylsulfonic)-3-methylimidazolium chloride and 1-(4-butylsulfonic)-3-methylimidazolium chloride as catalysts, respectively.⁸²⁰ These furanic compounds are promising feedstock for the production of hydrocarbon fuel using the well-established catalytic hydrodeoxygenation process.

3.7. Reductive amination of HMF

N-Substituted-5-(hydroxymethyl)-2-furfuryl amines, one type of important precursor of pharmaceuticals, can be obtained via the reductive amination of HMF (Fig. 76 and Table 19) with different amines and ammonia under high pressure H₂, and the one-pot reaction of HMF with nitrobenzene using Pd/C or Ni_yAlO_x as the catalyst.^821,822 Zhu et al. reported that the abundant and cheaply available CO and water could be use as reductants to displace H₂ for the reductive amination of HMF toward valuable N-containing compounds using rutile titania-supported gold (Au/TiO₂–R) as the catalyst, attaining a broad spectrum of primary and secondary amines and bis(hydroxylmethylfurfuryl)-amines efficiently.⁸²³ Gomes et al. reported that δ-lactone-fused cyclopenten-2-ones could be produced from HMF via the activation of HMF with Meldrum's acid followed by reacting with a secondary amine via consecutive furan ring-opening, cyclization and lactonization process.⁸²⁴ The δ-lactone-fused cyclopenten-2-ones with a quaternary carbon have potential in the synthetic and medicinal fields. Besides, 5-(hydroxymethyl)furfurylamine (HMFA) could be synthesized from HMF over immobilized transaminase via amination transfer reaction.⁸²⁵ Similar to HMF, a series of secondary and tertiary amines can be synthesized using furfural, xylose and xylan as the feedstock.³⁸⁹

Fig. 76 (a) Synthesis of n-substituted-5-(hydroxymethyl)-2-furfuryl amines from furfural via Mannich-type reaction. Indirect (b) and direct method (c) of reductive amination of HMF with primary amines. Adapted from ref. 821.

Table 19 Reductive amination of HMF toward N-substituted-5-(hydroxymethyl)-2-furfuryl amines

Catalyst	Catalyst loading	Solvent	Conditions	Feedstock	Amine Yield (%)	Ref.
a Relative to HMF. b Relative to solvent. — Not provided.
Au/TiO2-R	0.4 mol%^a	Methanol/H2O (v/v = 1)	60 °C, 6 h, 20 bar CO	HMF/R1NH1R2 (molar ratio = 1)	60–99%	823
Au/TiO2-R	0.4 mol%^a	Methanol/H2O (v/v = 1)	60 °C, 6 h, 20 bar CO	HMF, RNH2 (molar ratio = 1), R = n-butyl, n-hexyl or benzyl	bis-(HMF) amines 85–93%	823
Pd/C	0.4 mol%^a	Trifluoro toluene	100 °C, 1 h, 3 bar H2	HMF/aniline (molar ratio = 1)	100%	821
Pd/C	0.4 mol%^a	Trifluoro toluene	100 °C, 3 h, 3 bar H2	HMF/2-methylaniline (molar ratio = 1)	76%	821
Pd/C	0.4 mol%^a	Trifluoro toluene	100 °C, 1 h, 3 bar H2	HMF/4-methylaniline (molar ratio = 1)	98%	821
Pd/C	0.4 mol%^a	Trifluoro toluene	100 °C, 1 h, 3 bar H2	HMF/4-methoxyaniline (molar ratio = 1)	99%	821
Pd/C	0.4 mol%^a	Trifluoro toluene	100 °C, 3 h, 3 bar H2	HMF/4-chloroaniline (molar ratio = 1)	89%	821
Pd/C	0.4 mol%^a	Trifluoro toluene	100 °C, 3 h, 3 bar H2	HMF/4-cyanoaniline (molar ratio = 1)	96%	821
Pd/C	0.4 mol%^a	Trifluoro toluene	100 °C, 3 h, 3 bar H2	HMF/4-acetyl aniline (molar ratio = 1)	86%	821
Pd/C	0.4 mol%^a	Methanol	100 °C, 2 h, 14 bar H2	HMF/NH3(molar ratio = 1/10)	94%	821
Ni1AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 1/49)	32%	822
Ni2AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 1/49)	38%	822
Ni4AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 1/49)	85%	822
Ni6AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 1/49)	99%	822
Ni8AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 1/49)	99%	822
Ni10AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 1/49)	96%	822
NiOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 1/49)	33%	822
RANEY® Ni	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 1/49)	48%	822
Ni6AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/benzylamine (molar ratio = 10/12)	76%	822
Ni6AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/aniline (molar ratio = 10/12)	85%	822
Ni6AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/morpholine (molar ratio = 10/12)	88%	822
Ni6AlOx	1.3 wt%^b	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 10/12)	6%	822
HReO4	5 mol%	H2O	100 °C, 6 h, 1 bar H2	HMF/NH3 (molar ratio = 10/12)	6%	389

---

3.8. Decarbonylation of HMF to furfuryl alcohol

As an important chemical and solvent widely employed in the foundry, refractory, polyester and pharmaceutical industries, the production of furfuryl alcohol utilizes 65% of the global furfural production capacity.³³ The production of furfuryl alcohol from HMF via decarbonylation is a potential alternative approach to the industrially applied furfural hydrogenation process.^826,827

Through theoretical study, Xie et al. suggested that the decarbonylation of HMF to furfuryl alcohol (Fig. 77) over palladium acetate (Pd(OAc)₂) involves migratory extrusion, metal-acetate-co-assisted deprotonation, decarbonylation, metal-assisted deprotonation, migratory extrusion and catalyst regeneration.⁸²⁸ Meng et al. reported that adding an appropriate amount of water to an organic solvent could inhibit side reactions, including hydrogenolysis, dehydrogenation and etherification, thus enabling the highly selective decarbonylation of HMF to furfuryl alcohol over the Pd/Al₂O₃ catalyst.⁸²⁶ Chatterjee et al. reported that the combined use of Pd/Al₂O₃ as the catalyst and compressed CO₂ as the solvent enabled the highly efficient decarbonylation of HMF to furfuryl alcohol, affording a furfuryl alcohol yield as high as 99.4% without the use of any additive, CO surrogate, or organic solvent.⁸²⁹ The excellent catalytic performance is attributed to the synergistic effect between CO₂ and Pd/Al₂O₃, where CO₂ shifts the reaction equilibrium toward furfuryl alcohol by accelerating the diffusion of CO and furfuryl alcohol.

Fig. 77 Decarbonylation of HMF to furfuryl alcohol. Adapted from ref. 826 and 829.

3.9. Conversion of HMF to esters

The esterification of HMF and its derivates, including FDCA, BHMF and levulinic acid can provide a series of useful esters products. For example, the combination of catalyst, O₂, base and methanol enabled the one-pot oxidative esterification of HMF toward FDMC. Compared with the direct oxidation of HMF to FDCA, the one-pot oxidative esterification of HMF to FDMC could be carried out under milder reaction conditions owing to the change in the reaction pathway (Fig. 78), thus providing an alternative approach for the efficient synthesis of FDCA. In the medium of alcohol, HMF is converted to the hemiacetal intermediate easily, and then the hemiacetal intermediate is oxidized to 5-hydroxymethyl-2-methyl-furoate (HMMF) intermediate rapidly owing to the presence of the active hemiacetal structure. The oxidation of HMMF to 5-formyl-2-methyl-furoate (FMF) is the rate-limiting step because the oxidation of the hydroxyl group to aldehyde group is more difficult than the conversion of the aldehyde group in this system. Once FMF is generated, it can be further converted to FDMC easily. For example, Li et al. reported that the heterogeneous PdCoBi/C catalyst is as effective as Pd/C with Co(NO₃)₂ and Bi(NO₃)₃ as homogeneous co-catalysts for the aerobic oxidative esterification of HMF, affording an FDMC yield of 96%.⁶⁰⁹ Sun et al. reported that the hollow yolk–shell Co@CN catalyst, which was derived from ZIF-67@SiO₂@ZIF-8, exhibited an excellent catalytic performance for the oxidative esterification of HMF to FDMC in the medium of MeOH under mild conditions, obtaining an FDMC yield of 95% even at a high HMF loading (2 M).⁶¹¹ Liu et al. reported that an N-doped carbon-supported CoCu bimetallic catalyst (Co₇Cu₃–NC) was more capable than Co–NC since the doping of Cu imparts stronger O₂ activation ability to the Co–N_x species.⁶¹² Buonerba et al. showed that the desired oxidation products, including DFF, FFCA, 5-hydroxymethyl furoic acid methylester (HMFE), FDCA and FDMC could be obtained from the selective oxidation of HMF over gold nanoparticles supported on a semi-crystalline nanoporous multiblock copolymer matrix consisting of syndiotactic poly(styrene)-cis-1,4-poly(butadiene) (AuNPs-sPSB) via judicious manipulation of the reaction conditions and solvents.⁵⁷⁹

Fig. 78 Oxidative esterification of HMF toward FDMC. Adapted from ref. 609–611.

Alloyed AuPd–Fe₃O₄ nanoparticles attained an FDMC yield of up to 92% under atmospheric O₂ at room temperature, while Au–Fe₃O₄ NPs favored the selective formation of HMFE.⁶¹⁰ Kozlov et al. reported that sodium cyanide (NaCN)-promoted oxidative esterification with MnO₂ as a recyclable oxygen carrier enabled the efficient conversion of HMF to FDMC in water-free medium and to HMFE in AcOH with 5% water, respectively.⁸³⁰ Gupta et al. reported that the combination of mesoporous alumina nanospheres-embedded with CuO nanoparticles (CuO/m-Al₂O₃) catalyst with TBHP as both the oxidizing and methylating reagent enabled the efficient oxidative methyl-esterification of HMF toward FDMC, which is an alternative approach to the oxidative esterification of HMF to FDMC in methanol.⁶¹⁸

From the viewpoint of reaction, levulinic acid and formic acid can be obtained directly from the Brønsted acid-catalyzed rehydration of HMF (Fig. 12), which is one of the most ubiquitous side reactions during the synthesis of HMF from carbohydrates and biomass.⁸³¹ Since the Brønsted acid is also involved in the depolymerization of cellulose or lignocellulosic biomass, the direct synthesis of levulinic acid using cellulose or lignocellulosic biomass as a feedstock via a one-pot process generally exhibits higher efficiency than that of converting carbohydrate to HMF followed by decomposition of HMF to levulinic acid and formic acid. Therefore, the use of HMF as the feedstock for the production of levulinic acid and formic acid is not a cost-effective strategy. Accordingly, the transformation of HMF to levulinic acid and formic acid is not specifically discussed in this review. Since levulinic acid is also an important biomass-derived platform chemical for the production of a series of high-value products, including levulinic esters, GVL, valeric acid and ester, butene, 5-nonanone and MeTHF (Fig. 79), the reader is referred to these reviews.^832–837 GVL, MeTHF and valeric ester are promising fuel additives, while butene and 5-nonanone can be further upgraded to liquid alkanes. Formic acid is considered a sustainable hydrogen source due to its high volumetric hydrogen density (53 g of H₂ per liter).^838,839 Readers are referred to the related review on the direct synthesis and application of formic acid from cellulose and biomass.⁸³⁸

Fig. 79 Catalytic upgrading of levulinic acid to chemicals and fuels. Adapted from ref. 836 and 837.

The double esterification of BHMF with fatty acids can give 2,5-bis(hydroxymethyl)furan fatty acid diesters, which are considered promising biodiesel additives. Lăcătuş et al. reported that the lipase Novozym 435 is effective to catalyze the double esterification of BHMF with saturated long-chain fatty acids (Fig. 80a) in the medium of MeTHF with in situ removal of the generated water by molecular sieves, attaining BHMF diester yields between 89% and 99%.⁸⁴⁰ Based on this, they further indicated that the use of a fatty acid mixture resulting from the hydrolysis of commercial sunflower oil as feedstock enabled the solvent-free biocatalytic synthesis of BHMF diesters, with a conversion higher than 97%.⁸⁴¹ In the above catalytic system, the presence of water not only inhibited the activity of the lipases, but also acted as a competitive nucleophilic reagent, thus leading to an obvious decrease in the catalytic performance. Arias et al. reported that the transesterification reaction of BHMF with vinyl esters of acids (Fig. 80b) over lipase not only could avoid the adverse influence of water and fatty acids on the enzymatic activity, but also inhibit the competitive nucleophilic attack, thus leading to a remarkable improvement in the reaction efficiency.⁸⁴² A BHMF diester yield of up to 97% was obtained within only 0.5 h.

Fig. 80 (a) Esterification of BHMF with fatty acids (R₂ = H) or fatty acid esters (R₂ = vinyl group) toward BHMF fatty acid diesters. Adapted from ref. 840 and 841. (b) Transesterification reaction of BHMF with vinyl esters of acids over lipase. Adapted from ref. 842.

Garcia-Suarez et al. reported that the use of HMF as a CO surrogate for the methoxycarbonylation of 1-hexene to methyl hepatanoate (MH) (Fig. 81) could be conducted in the medium of methanol using Pd-(1,2-bis(di-tert-butylphosphinomethyl)benzene) (DTBPMB) as the catalyst and methanesulfonic acid (MSA) as the co-catalyst.⁸⁴³ In this catalytic system, the conversion of HMF to methyl levulinate (ML) via hydration and following esterification, and the hydrogenation of ML to GVL occurred concomitantly with HMF and methanol as the source of CO and H₂, respectively. The MH yield of 50%, ML yield of 12% and GVL yield of 35% were obtained at 120 °C within 20 h. In addition, this catalytic system also enabled the one-pot conversion of glucose, fructose and xylose to ester products, but the yield and selectivity were low.

Fig. 81 Cascade transformation of 1-hexene with HMF to ester products. Adapted from ref. 843.

3.10. Conversion of HMF to other products

The conversion of HMF to vinyl or acetylene-functionalized chemicals can extend the scope of HMF utilization in fine organic synthesis remarkably. For example, Han et al. reported that HMF could be converted to 5-hydroxymethyl-2-vinylfuran (HMVF), a biomass-derived solvent-free adhesive, via the Wittig reaction (Fig. 82).⁸⁴⁴ After heating or acid treatment at room temperature, the versatile adhesive could bond to a series of substrates, including metal, glass, plastic and rubber via the free radical polymerization of the vinyl group and etherification of the hydroxyl group, with an adhesive performance close to that of Krazy Glue (Loctite 401, cyanoacrylate) and superior to that of white glue (Pattex No. 710, PVA) and Pattex PKME15C epoxy glue. Romashov et al. reported that alkynylation of HMF and DFF (Fig. 83) using Ohira-Bestmann reagent (dimethyl 1-diazo-2-oxopropylphosphonate) gave a high yield of 2-hydroxymethyl-5-ethynylfuran (HMEF) and 2,5-diethynylfuran (DEF), respectively.⁸⁴⁵ The terminal alkyne units in HMEF and DEF provide facile access to various chemical transformations, including Glaser–Eglinton–Hay reaction, heterocyclization, Sonogashira coupling and polymerization toward high-value furanic pharmaceuticals and conjugated polymers.

Fig. 82 Conversion of HMF to HMVF via Wittig reaction. Adapted from ref. 844.

Fig. 83 Alkynylation of HMF using Ohira-Bestmann reagent. Adapted from ref. 845.

Besides the conversion of HMF to structurally symmetrical derivates, including FDCA, BHMF, 5,5′-BHMF and HMVF as building blocks of polymers, the direct use of HMF as feedstock for polymer synthesis was achieved via the step-growth copolymerization of HMF (Fig. 84) with dihydrosilane toward linear poly(silyl ether)s, in the presence of B(C₆F₅)₃ and heteroscorpionate zinc hydride complex LZnH [L = (^MePz)₂CP(Ph)₂NPh, ^MePz = 3,5-dimethylpyrazolyl].⁸⁴⁶ A series of butenolide derivates of furans with a good color performance in the yellow to red region of the visible spectrum could be fully synthesized using HMF-derived CMF or DFF as feedstock (Fig. 85).⁸⁴⁷ These butenolide derivates can be used as sustainable synthetic dyes, which are promising to displace polluting petrochemical colorants and expensive plant-based dyes. The Biginelli reaction of HMF, 1,3-dicarbonyl compounds and urea-type building blocks gave moderate to good yields of functionalized dihydropyrimidinones (Fig. 86), indicating the feasibility of using HMF as feedstock in multicomponent reactions (MCRs).⁸⁴⁸ HMF could also be used as feedstock for the preparation of N-doped graphene-like layered carbon (NG) via a metal-free pyrolysis method.⁸⁴⁹ More importantly, the HMF-derived NG exhibited superior catalytic activity for the epoxidation reaction owing to its high activation ability for both alkenes and O-2 resulting from its graphitic layered structure and graphitic N species.

Fig. 84 Copolymerization of HMF with dihydrosilanes into linear poly(silyl ether)s. Adapted from ref. 846.

Fig. 85 Conversion of CMF and DFF to butenolide derivates. Adapted from ref. 847.

Fig. 86 Conversion of HMF to functionalized dihydropyrimidinones via three-component Biginelli reaction. Adapted from ref. 848.

4. Future perspectives

Although considerable progress has been achieved, the high cost of HMF is still the predominant bottleneck in its commercial production and application. Thus, the economical production of HMF from carbohydrates and biomass should be paid continuous attention. To date, many research still focus on the dehydration of fructose to HMF in the medium of DMSO. In these catalytic systems, a fundamental control experiment is necessary to obtain credible catalytic activity because moderate to high HMF yields from fructose can be readily attained in DMSO under an air atmosphere without the use of additional catalysts. The separation and purification of HMF and recycling of DMSO are difficult owing to the high boiling point of DMSO and the instability of both DMSO and HMF under the reaction conditions. Similarly, the use of certain ionic liquids and biomass-derived GVL as the solvent enables the efficient conversion of fructose to HMF, but the reported high HMF yields from fructose are also likely attributed to the solvents, instead of the solid catalysts.⁸⁵⁰ The efficient conversion of fructose to HMF in low-boiling point solvents, such as water/acetone, water/THF, water/butanol and water/1,4-dioxane, have been achieved over a few elegant designed solid acid catalysts, showing the feasibility of the facile separation and purification of HMF product and solvent recycling. Therefore, much attention should be paid to the systematic investigation of the catalytic performance, stability and reusability of heterogeneous catalysts in low-boiling point solvents, the separation and purification of HMF, and the recovery and reusability of solvents, in both batch and continuous reaction, to scale up these catalytic systems.

Transforming glucose into HMF with high efficiency in green solvents is still challenging.⁸⁵¹ The ionic liquid-based homogeneous catalytic systems, such as EMIMCl/CrCl₂, BMIMCl/CrCl₃ and EMIMBr/SnCl₄, are still benchmark catalytic systems for the conversion of glucose to HMF at a high glucose loading (≥10 wt%), with high reaction efficiency, but they suffer from the difficulty of catalyst and ionic liquid recycling and product separation. A few solid catalysts have exhibited both good catalytic performance and reusability for the conversion of glucose to HMF in certain ionic liquids, which can serve as both the reaction medium and highly effective catalyst for the dehydration of fructose, with a reaction efficiency comparable to that of ionic liquid-based homogeneous catalytic systems. Therefore, the search for alternative solvents that can serve as both the solvent and catalyst provides a powerful strategy for the efficient conversion of glucose to HMF. Meanwhile, the development of novel ionic liquid recycling technology is also a promising solution to alleviate the high cost of ionic liquids. In several water–organic biphasic systems, the combination of a Lewis acid (or base and isomerase) with a Brønsted acid has been widely investigated for the conversion of glucose to HMF. The ratio of catalyst, solvent and reaction condition needs to be regulated carefully to inhibit the side reactions, but the degradation of HMF seems to be unavoidable because the activation energy for the degradation of HMF to levulinic and formic acids is usually lower than that of fructose dehydration (Table 1).^13,852 A relatively high HMF yield and selectivity can only be achieved at a low substrate loading (≤5 wt%), seriously impeding the improvement of the intrinsic reaction efficiency. Whether or not the combinations of Lewis acid (or base and isomerase) with Brønsted acid can convert glucose to HMF with high intrinsic efficiency in separable low-boiling point solvents, under batch or continuous condition, is still mostly uncharted territory.

Compared with glucose, the use of cellulose as a starting material is attractive but more challenging. The transfer of HMF from the reaction phase to the extraction phase can inhibit the degradation of HMF to some extent, but this degradation under reaction condition cannot be avoided completely since the activation energy for cellulose hydrolysis to glucose⁶³ is remarkably higher than that of glucose isomerization to fructose, fructose dehydration to HMF and HMF degradation (Table 1). Although unrealistically high yields have been claimed in some reports, these results do not seem to be creditable, as is illustrated by the absence of a receivable reaction mechanism and lack of subsequent work to verify or further improve these catalytic systems. Therefore, the systematic evaluation of previously reported catalysts and new catalysts under standardized reaction conditions and their comparison with the widely validated benchmark catalytic systems, such as DMSO/Brønsted acid catalytic system, for the dehydration of fructose to HMF and BMIMCl/CrCl₃ catalytic system for the dehydration of glucose to HMF, are necessary. To avoid inconspicuous mistakes, HMF degradation experiments under optimized reaction conditions are highly recommended for any new catalyst and solvent. The stabilization or protection of HMF seems to be a more feasible strategy to inhibit the degradation of HMF. For the direct conversion of lignocellulosic biomass to HMF, the reaction is usually accompanied by the conversion of hemicellulose to furfural, and thus the accurate quantitative analysis of HMF and furfural is required. Furthermore, understanding the influence of inorganic elements, heavy metals, nitrogen, phosphorus, sulfur, and chlorine species^9,213,853 on the production and upgrading of HMF is required for the improvement of the reaction efficiency and avoidance of secondary pollution. Besides, food waste is continuously increasing and becoming an important waste biomass resource for the production of HMF.⁸⁵⁴

The development of multifunctional solid catalytic materials, such as Brønsted–Lewis acid, acid–base, and metal particles–acid or base bi-functional catalysts, is very important to achieve the efficient and environmentally-friendly conversion of glucose and glucose-based polymers to HMF and its derivates.⁸⁵⁵ Nevertheless, the targeted regulation of Brønsted acid, Lewis acid and base sites and the morphology and pore structure of heterogenous catalysts is difficult. Thus, to guide the rational design of solid acid catalysts, a detailed, quantitative analysis of their functional groups and stability under the reaction conditions is necessary.⁴⁴⁴ The intensity and nature of acid/base sites, surrounding chemical environment and solvent also have an important influence on the actual catalytic performance.⁸⁵⁶ Therefore, developing advanced ex situ and/or in situ analysis techniques may be beneficial for the precise quantitative and qualitative characterization of the acid/base sites and analysis of the structural features of solid catalysts.^228,857 Based on the detailed characterization of catalysts and carefully designed catalytic experiments, the structure–activity relationship should be established to guide further the design and synthesis of novel catalysts. Besides, the activity, selectivity, durability and reusability of heterogenous catalysts should be tested under actual reaction conditions.

Screening cost-effective, environmentally-friendly and cheap solvents is also important for the efficient production of HMF. Recently, the selective dehydration of fructose to HMF⁴⁸⁹ and isomerization of glucose to fructose²⁸⁴ in readily separable organic solvents, and the efficient conversion of glucose in pure water^239,312,447 have been achieved over a few of well-designed heterogenous catalysts. The effectiveness of the developed heterogenous catalysts and the proposed reaction mechanism should further be tested in low-boiling point solvents. In addition, several solvent systems, especially GVL/water can give moderate HMF yields from glucose with only a Brønsted acid, suggesting that the reaction pathway is different from that in conventional water/organic solvent systems. Therefore, more technologies should be attempted to reveal the actual roles of solvents and their contributions in the production of HMF. Besides, the obvious catalytic effect of trace metal ions in ionic liquids, seawater or biochar for hydrolysis, isomerization and dehydration has been demonstrated extensively. Some anions such as Br⁻, BF₄⁻, PO₄³⁻, HPO₄²⁻, H₂PO₄⁻ and SO₄²⁻, which play important roles in the isomerization and dehydration reaction, may result from the catalyst, solvent or biomass.^130,134,420 Therefore, the comprehensive quantitative analysis of the composition of the catalyst, solvent and biomass is of great importance for understanding the real catalytic species.

The improvement of these catalytic systems not only need the collaborative innovations of catalyst and solvent, but also involve the establishment of effective methods to evaluate intrinsic catalytic activity and actual reaction efficiency. To accurately analyze the HMF yield and selectivity, the concentration of the products should not be analyzed by UV-vis spectrophotometry. UV-vis spectrophotometry cannot reflect the real HMF concentration since furfural and other chemicals also have a strong adsorption at 284 nm. The comparison of the conversion, yield and selectivity of different catalytic systems should consider the reaction conditions, including temperature, time, substrate concentration, catalyst loading, and solvent composition. For instance, a high HMF yield and selectivity are widely reported in previous reports at a very low substrate loading, but their actual reaction efficiency is low. Although initial reaction rates, turnover frequency (TOF) and activation energy are highly recommended to evaluate the intrinsic catalytic performance for many important reactions, the reliable determination of these parameters in biomass conversion is difficult.⁸⁵⁸ Consequently, the application of these parameters in the conversion of carbohydrates to HMF is limited. In contrast, the average conversion rate and the HMF formation rate per catalytic system under optimized conditions seem to be more useful indicators to compare the actual efficiency of different catalytic systems. In these complicated catalytic systems, comprehensive control experiments are required to estimate the contribution of different catalyst species, water and organic solvents for polysaccharide depolymerization, glucose isomerization, fructose dehydration and HMF degradation, or at least to identify their actual influence on the above processes.

In addition to the synthesis of HMF from carbohydrates and biomass, the upgrading of HMF to high-value products via hydrodeoxygenation, oxidation, esterification, etherification, amination, and aldol condensation has also achieved huge progress. However, the efficient upgrading of HMF to target products with high yield is usually achieved when using pure HMF as the starting material at a low initial concentration, limiting the actual efficiency. The design and development of metal nanoparticle catalysts with tailored structures, bi-metal or multi-metal catalysts, single atom catalysts with desirable electronic structures and coordination environment and composite catalysts will be beneficial for the improvement in catalytic performance.⁸⁵⁹ Moreover, HMF stabilization and one-pot catalysis are general strategies to reduce the degradation of HMF during its upgrading, and then to improve the overall yield and efficiency. As an underdeveloped field, understanding the HMF stabilization chemistry and establishing an HMF stabilization toolbox may greatly improve the efficiency and selectivity of HMF upgrading. Currently, the one-pot conversion of carbohydrates and biomass to desired products with HMF as the intermediate only succeeds in limited reaction pathways, such as the conversion of fructose to DFF and AMF. The one-pot conversion of glucose and its polymers suffers from severe side-reactions. Compared with one-pot catalytic systems, the development of tandem catalytic systems ⁴⁸³ consisting of a sequence of precisely tailored catalytic steps over multiple catalysts offers an opportunity to improve the production efficiency and selectivity fundamentally.⁸⁶⁰ By virtue of the precise control of active sites, the accurate cascade of depolymerization, isomerization, dehydration and upgrading reactions in the appropriate sequence with inhibited side reactions will be achieved in the near future. With the rapid development of metal catalysis, organocatalysis, photocatalysis, electrocatalysis, heterocatalysis and biocatalysis, the combination of different catalytic strategies may provide numerous opportunities to achieve higher yields, higher selectivity, lower costs and more environmental benefits.^861,862

The large-scale production and application of HMF and its derivates not only require their efficient production from carbohydrates and biomass, but also upstream and downstream technologies, including biomass pretreatment and fractionation, catalyst synthesis, energy, and by-product utilization.⁸⁶³ For example, digging out low-volume, high-value product streams not only will create new products and market opportunities, but also reduce the overall cost of biorefineries.^25,686 In addition, the integration of the biorefinery process with the current industrial catalytic processes provides an effective strategy for the co-production of value-added products with higher efficiency.⁷⁴⁵ Once one major molecule in the chain can be produced and applied in industry, the economic feasibility of the product tree will improve greatly.¹ Although predicting which product or process will be technically, economically, and environmentally viable at an industrial scale is challenging, the principles of green chemistry and green engineering should be considered in the initial design stage of a biorefinery.⁸⁶⁴ Furthermore, techno-economic analysis, and life cycle assessment combined with uncertainty assessment should be performed to evaluate the feasibility of emerging biorefinery technology.⁶⁸⁶

5. Conclusions

Although considerable progress has been achieve, the catalytic conversion of biomass and their constituent carbohydrates to HMF and the upgrading of HMF towards value-added products still suffer from low reaction efficiency and high cost in the competition against petroleum-derived chemicals and fuels. The commercial production of HMF from carbohydrates and biomass on a large scale requires the collaborative innovation of catalyst and solvent to improve the intrinsic reaction efficiency, and the establishment of economically feasible HMF separation and purification methods. Meanwhile, the development of highly effective catalytic systems and the exploration of novel reaction pathways and products are still necessary to accelerate the upgrading of HMF towards value-added products. The HMF stabilization and tandem catalysis strategy has shown great promise to reduce side reactions, and consequently improve the reaction efficiency. The integration of upstream and downstream technologies, including biomass pretreatment and fractionation, catalyst synthesis, energy, and by-product utilization are necessary to improve the overall output. Furthermore, more efforts are required to evaluate the actual efficiency, economic and technical feasibility and environmental impact of the integrated biorefinery process from raw materials to final commodities using a standardized evaluation methodology to determine which process can be ultimately applied in practical production. We believe that the development of efficient and economic HMF-based biorefinery technologies will greatly promote the establishment of a more sustainable industry in the near future.

Conflicts of interest

There are no conflicts of interest to declare.

Acknowledgements

This work was supported by the National Natural Science Foundation of China (21878163), the China Postdoctoral Science Foundation (2018M640231), the Fundamental Research Funds for the Central Universities, Natural Science Foundation of Tianjin, China (17JCZDJC39500), National Key Research Project (2018YFD080083-03), Science and Technology Demonstration Project of Industrial Integration and Development of Tianjin, China (17ZXYENC00100), National Natural Science Foundation of China (51708301), and Young Elite Scientists Sponsorship Program by Tianjin (TJSQNTJ-2018-06).

References

1. S. Chen, R. Wojcieszak, F. Dumeignil, E. Marceau and S. Royer, Chem. Rev., 2018, 118, 11023–11117.
2. L. T. Mika, E. Cséfalvay and Á. Németh, Chem. Rev., 2018, 118, 505–613.
3. P. Sudarsanam, E. Peeters, E. V. Makshina, V. I. Parvulescu and B. F. Sels, Chem. Soc. Rev., 2019, 48, 2366–2421.
4. X. Zhang, K. Wilson and A. F. Lee, Chem. Rev., 2016, 116, 12328–12368.
5. D. E. Resasco, B. Wang and D. Sabatini, Nat. Catal., 2018, 1, 731–735.
6. Q. Hou, M. Ju, W. Li, L. Liu, Y. Chen and Q. Yang, Molecules, 2017, 22, 490.
7. M. Guo, W. Song and J. Buhain, Renewable Sustainable Energy Rev., 2015, 42, 712–725.
8. N. Luo, T. Montini, J. Zhang, P. Fornasiero, E. Fonda, T. Hou, W. Nie, J. Lu, J. Liu, M. Heggen, L. Lin, C. Ma, M. Wang, F. Fan, S. Jin and F. Wang, Nat. Energy, 2019, 4, 575–584.
9. W.-J. Liu, W.-W. Li, H. Jiang and H.-Q. Yu, Chem. Rev., 2017, 117, 6367–6398.
10. A. R. C. Morais, A. M. da Costa Lopes and R. Bogel-Łukasik, Chem. Rev., 2015, 115, 3–27.
11. N. M. Eagan, M. D. Kumbhalkar, J. S. Buchanan, J. A. Dumesic and G. W. Huber, Nat. Rev. Chem., 2019, 3, 223–249.
12. X. Zhao, H. Zhou, V. S. Sikarwar, M. Zhao, A.-H. A. Park, P. S. Fennell, L. Shen and L.-S. Fan, Energy Environ. Sci., 2017, 10, 1885–1910.
13. R.-J. van Putten, J. C. van der Waal, E. de Jong, C. B. Rasrendra, H. J. Heeres and J. G. de Vries, Chem. Rev., 2013, 113, 1499–1597.
14. A. Corma, S. Iborra and A. Velty, Chem. Rev., 2007, 107, 2411–2502.
15. X. Wu, X. Fan, S. Xie, J. Lin, J. Cheng, Q. Zhang, L. Chen and Y. Wang, Nat. Catal., 2018, 1, 772–780.
16. L. Petridis and J. C. Smith, Nat. Rev. Chem., 2018, 2, 382–389.
17. Z. Zhang, J. Song and B. Han, Chem. Rev., 2017, 117, 6834–6880.
18. B. Mostofian, C. M. Cai, M. D. Smith, L. Petridis, X. Cheng, C. E. Wyman and J. C. Smith, J. Am. Chem. Soc., 2016, 138, 10869–10878.
19. P. Langan, L. Petridis, H. M. O'Neill, S. V. Pingali, M. Foston, Y. Nishiyama, R. Schulz, B. Lindner, B. L. Hanson, S. Harton, W. T. Heller, V. Urban, B. R. Evans, S. Gnanakaran, A. J. Ragauskas, J. C. Smith and B. H. Davison, Green Chem., 2014, 16, 63–68.
20. Y. Jing, Y. Guo, Q. Xia, X. Liu and Y. Wang, Chem, 2019, 5, 2520–2546.
21. C.-C. Chen, L. Dai, L. Ma and R.-T. Guo, Nat. Rev. Chem., 2020, 4, 114–126.
22. S. Y. Lee, H. U. Kim, T. U. Chae, J. S. Cho, J. W. Kim, J. H. Shin, D. I. Kim, Y.-S. Ko, W. D. Jang and Y.-S. Jang, Nat. Catal., 2019, 2, 18–33.
23. V. S. Sikarwar, M. Zhao, P. Clough, J. Yao, X. Zhong, M. Z. Memon, N. Shah, E. J. Anthony and P. S. Fennell, Energy Environ. Sci., 2016, 9, 2939–2977.
24. J. Song, C. Chen, S. Zhu, M. Zhu, J. Dai, U. Ray, Y. Li, Y. Kuang, Y. Li, N. Quispe, Y. Yao, A. Gong, U. H. Leiste, H. A. Bruck, J. Y. Zhu, A. Vellore, H. Li, M. L. Minus, Z. Jia, A. Martini, T. Li and L. Hu, Nature, 2018, 554, 224–228.
25. T. A. Bender, J. A. Dabrowski and M. R. Gagné, Nat. Rev. Chem., 2018, 2, 35–46.
26. C. O. Tuck, E. Pérez, I. T. Horváth, R. A. Sheldon and M. Poliakoff, Science, 2012, 337, 695–699.
27. Y. Li, L. Shuai, H. Kim, A. H. Motagamwala, J. K. Mobley, F. Yue, Y. Tobimatsu, D. Havkin-Frenkel, F. Chen, R. A. Dixon, J. S. Luterbacher, J. A. Dumesic and J. Ralph, Sci. Adv., 2018, 4, eaau2968.
28. T. Renders, S. Van den Bosch, S. F. Koelewijn, W. Schutyser and B. F. Sels, Energy Environ. Sci., 2017, 10, 1551–1557.
29. Q. Hou, W. Li, M. Ju, L. Liu, Y. Chen, Q. Yang and J. Wang, Carbohydr. Polym., 2015, 133, 517–523.
30. L. Shuai, M. T. Amiri, Y. M. Questell-Santiago, F. Heroguel, Y. Li, H. Kim, R. Meilan, C. Chapple, J. Ralph and J. S. Luterbacher, Science, 2016, 354, 329–333.
31. S. S. Wong, R. Shu, J. Zhang, H. Liu and N. Yan, Chem. Soc. Rev., 2020, 49, 5510–5560.
32. I. Delidovich, P. J. C. Hausoul, L. Deng, R. Pfützenreuter, M. Rose and R. Palkovits, Chem. Rev., 2016, 116, 1540–1599.
33. R. Mariscal, P. Maireles-Torres, M. Ojeda, I. Sádaba and M. López Granados, Energy Environ. Sci., 2016, 9, 1144–1189.
34. C. Xu, E. Paone, D. Rodríguez-Padrón, R. Luque and F. Mauriello, Chem. Soc. Rev., 2020, 49, 4273–4306.
35. M. Yabushita, H. Kobayashi and A. Fukuoka, Appl. Catal., B, 2014, 145, 1–9.
36. H. Wang, C. Zhu, D. Li, Q. Liu, J. Tan, C. Wang, C. Cai and L. Ma, Renewable Sustainable Energy Rev., 2019, 103, 227–247.
37. X. Li, R. Xu, J. Yang, S. Nie, D. Liu, Y. Liu and C. Si, Ind. Crops Prod., 2019, 130, 184–197.
38. G. P. Perez, A. Mukherjee and M. J. Dumont, J. Ind. Eng. Chem., 2019, 70, 1–34.
39. K. Kohli, R. Prajapati and B. K. Sharma, Energies, 2019, 12, 40.
40. X. Zou, C. Zhu, Q. Wang and G. Yang, Biofuels, Bioprod. Biorefin., 2019, 13, 153–173.
41. B. Agarwal, K. Kailasam, R. S. Sangwan and S. Elumalai, Renewable Sustainable Energy Rev., 2018, 82, 2408–2425.
42. M. E. Zakrzewska, E. Bogel-Łukasik and R. Bogel-Łukasik, Chem. Rev., 2011, 111, 397–417.
43. J. G. de Vries, in Advances in Heterocyclic Chemistry, ed. E. F. V. Scriven and C. A. Ramsden, Academic Press, 2017, vol. 121, pp. 247–293.
44. L.-L. Zhang, Y. Kong, X. Yang, Y.-Y. Zhang, B.-G. Sun, H.-T. Chen and Y. Sun, J. Sci. Food Agric., 2019, 99, 2340–2347.
45. B. Saha and M. M. Abu-Omar, Green Chem., 2014, 16, 24–38.
46. C. Thoma, J. Konnerth, W. Sailer-Kronlachner, P. Solt, T. Rosenau and H. W. G. van Herwijnen, ChemSusChem, 2020, 13, 3544–3564.
47. F. Menegazzo, E. Ghedini and M. Signoretto, Molecules, 2018, 23, 2201.
48. J. Howard, D. W. Rackemann, J. P. Bartley, C. Samori and W. O. S. Doherty, ACS Sustainable Chem. Eng., 2018, 6, 4531–4538.
49. Y. Nie, Q. Hou, C. Bai, H. Qian, X. Bai and M. Ju, J. Cleaner Prod., 2020, 274, 123023.
50. Y. Su, G. Chang, Z. Zhang, H. Xing, B. Su, Q. Yang, Q. Ren, Y. Yang and Z. Bao, AIChE J., 2016, 62, 4403–4417.
51. J. Wang, J. Xi and Y. Wang, Green Chem., 2015, 17, 737–751.
52. S. S. Chen, D. C. W. Tsang and J.-P. Tessonnier, Appl. Catal., B, 2020, 261, 118126.
53. I. Delidovich and R. Palkovits, ChemSusChem, 2016, 9, 547–561.
54. H. Li, S. Yang, S. Saravanamurugan and A. Riisager, ACS Catal., 2017, 7, 3010–3029.
55. V. Choudhary, S. I. Sandler and D. G. Vlachos, ACS Catal., 2012, 2, 2022–2028.
56. V. Choudhary, S. H. Mushrif, C. Ho, A. Anderko, V. Nikolakis, N. S. Marinkovic, A. I. Frenkel, S. I. Sandler and D. G. Vlachos, J. Am. Chem. Soc., 2013, 135, 3997–4006.
57. J. Tang, L. Zhu, X. Fu, J. Dai, X. Guo and C. Hu, ACS Catal., 2017, 7, 256–266.
58. J. Tang, X. Guo, L. Zhu and C. Hu, ACS Catal., 2015, 5, 5097–5103.
59. J. M. Carraher, C. N. Fleitman and J.-P. Tessonnier, ACS Catal., 2015, 5, 3162–3173.
60. M. A. Mellmer, C. Sanpitakseree, B. Demir, K. Ma, W. A. Elliott, P. Bai, R. L. Johnson, T. W. Walker, B. H. Shanks, R. M. Rioux, M. Neurock and J. A. Dumesic, Nat. Commun., 2019, 10, 1132.
61. E. W. Leng, M. Mao, Y. Peng, X. M. Li, X. Gong and Y. Zhang, ChemistrySelect, 2019, 4, 181–189.
62. B. Girisuta, L. P. B. M. Janssen and H. J. Heeres, Chem. Eng. Res. Des., 2006, 84, 339–349.
63. B. Girisuta, L. P. B. M. Janssen and H. J. Heeres, Ind. Eng. Chem. Res., 2007, 46, 1696–1708.
64. F. Shen, S. Sun, J. Yang, M. Qiu and X. Qi, ACS Omega, 2019, 4, 11756–11759.
65. A. M. Borrero-López, E. Masson, A. Celzard and V. Fierro, Ind. Crops Prod., 2018, 124, 919–930.
66. T.-W. Tzeng, P. Bhaumik and P.-W. Chung, Mol. Catal., 2019, 479, 110627.
67. D. Zhou, D. Shen, W. Lu, T. Song, M. Wang, H. Feng, J. Shentu and Y. Long, Molecules, 2020, 25, 541.
68. C. Sievers, Y. Noda, L. Qi, E. M. Albuquerque, R. M. Rioux and S. L. Scott, ACS Catal., 2016, 6, 8286–8307.
69. A. Farrán, C. Cai, M. Sandoval, Y. Xu, J. Liu, M. J. Hernáiz and R. J. Linhardt, Chem. Rev., 2015, 115, 6811–6853.
70. Y. Yang, C. Zhang and Z. C. Zhang, Wiley Interdiscip. Rev.: Energy Environ., 2018, 7, e284.
71. J. Song, H. Fan, J. Ma and B. Han, Green Chem., 2013, 15, 2619–2635.
72. J. B. Binder and R. T. Raines, Proc. Natl. Acad. Sci. U. S. A., 2010, 107, 4516–4521.
73. B. R. Caes, T. R. Vanoosbree, F. Lu, J. Ralph, C. T. Maravelias and R. T. Raines, ChemSusChem, 2013, 6, 2083–2089.
74. N. Sun, H. Liu, N. Sathitsuksanoh, V. Stavila, M. Sawant, A. Bonito, K. Tran, A. George, K. L. Sale, S. Singh, B. A. Simmons and B. M. Holmes, Biotechnol. Biofuels, 2013, 6, 39.
75. A. S. Amarasekara, L. D. Williams and C. C. Ebede, Carbohydr. Res., 2008, 343, 3021–3024.
76. H. Kimura, M. Nakahara and N. Matubayasi, J. Phys. Chem. A, 2013, 117, 2102–2113.
77. X. Guo, J. Tang, B. Xiang, L. Zhu, H. Yang and C. Hu, ChemCatChem, 2017, 9, 3218–3225.
78. M. R. Whitaker, A. Parulkar, P. Ranadive, R. Joshi and N. A. Brunelli, ChemSusChem, 2019, 12, 2211–2219.
79. L.-K. Ren, L.-F. Zhu, T. Qi, J.-Q. Tang, H.-Q. Yang and C.-W. Hu, ACS Catal., 2017, 7, 2199–2212.
80. J. Zhang, A. Das, R. S. Assary, L. A. Curtiss and E. Weitz, Appl. Catal., B, 2016, 181, 874–887.
81. G. S. Svenningsen, R. Kumar, C. E. Wyman and P. Christopher, ACS Catal., 2018, 8, 5591–5600.
82. G. R. Akien, L. Qi and I. T. Horváth, Chem. Commun., 2012, 48, 5850–5852.
83. S. Jia, Z. Xu and Z. Zhang, Chem. Eng. J., 2014, 254, 333–339.
84. Y. Yang, W. Liu, N. Wang, H. Wang, Z. Song and W. Li, RSC Adv., 2015, 5, 27805–27813.
85. L. Lai and Y. Zhang, ChemSusChem, 2011, 4, 1745–1748.
86. J. Liu, Y. Tang, K. Wu, C. Bi and Q. Cui, Carbohydr. Res., 2012, 350, 20–24.
87. C.-H. Kuo, A. S. Poyraz, L. Jin, Y. Meng, L. Pahalagedara, S.-Y. Chen, D. A. Kriz, C. Guild, A. Gudz and S. L. Suib, Green Chem., 2014, 16, 785–791.
88. R.-J. van Putten, J. C. van der Waal, M. Harmse, H. H. van de Bovenkamp, E. de Jong and H. J. Heeres, ChemSusChem, 2016, 9, 1827–1834.
89. T. W. Walker, A. K. Chew, R. C. Van Lehn, J. A. Dumesic and G. W. Huber, Top. Catal., 2020, 63, 649–663.
90. V. Vasudevan and S. H. Mushrif, RSC Adv., 2015, 5, 20756–20763.
91. M. A. Mellmer, C. Sanpitakseree, B. Demir, P. Bai, K. Ma, M. Neurock and J. A. Dumesic, Nat. Catal., 2018, 1, 199–207.
92. M. A. Mellmer, C. Sener, J. M. R. Gallo, J. S. Luterbacher, D. M. Alonso and J. A. Dumesic, Angew. Chem., Int. Ed., 2014, 53, 11872–11875.
93. M. A. Mellmer, D. Martin Alonso, J. S. Luterbacher, J. M. R. Gallo and J. A. Dumesic, Green Chem., 2014, 16, 4659–4662.
94. T. W. Walker, A. K. Chew, H. Li, B. Demir, Z. C. Zhang, G. W. Huber, R. C. Van Lehn and J. A. Dumesic, Energy Environ. Sci., 2018, 11, 617–628.
95. E. I. Gürbüz, J. M. R. Gallo, D. M. Alonso, S. G. Wettstein, W. Y. Lim and J. A. Dumesic, Angew. Chem., Int. Ed., 2013, 52, 1270–1274.
96. C. Sener, A. H. Motagamwala, D. M. Alonso and J. A. Dumesic, ChemSusChem, 2018, 11, 2321–2331.
97. B. Song, Y. Yu and H. Wu, Fuel, 2019, 238, 225–231.
98. J. S. Luterbacher, J. M. Rand, D. M. Alonso, J. Han, J. T. Youngquist, C. T. Maravelias, B. F. Pfleger and J. A. Dumesic, Science, 2014, 343, 277–280.
99. W. Won, A. H. Motagamwala, J. A. Dumesic and C. T. Maravelias, React. Chem. Eng., 2017, 2, 397–405.
100. F. Cao, T. J. Schwartz, D. J. McClelland, S. H. Krishna, J. A. Dumesic and G. W. Huber, Energy Environ. Sci., 2015, 8, 1808–1815.
101. J. He, M. Liu, K. Huang, T. W. Walker, C. T. Maravelias, J. A. Dumesic and G. W. Huber, Green Chem., 2017, 19, 3642–3653.
102. R. Weingarten, A. Rodriguez-Beuerman, F. Cao, J. S. Luterbacher, D. M. Alonso, J. A. Dumesic and G. W. Huber, ChemCatChem, 2014, 6, 2229–2234.
103. A. H. Motagamwala, K. Huang, C. T. Maravelias and J. A. Dumesic, Energy Environ. Sci., 2019, 12, 2212–2222.
104. M. Li, W. Li, Y. Lu, H. Jameel, H.-M. Chang and L. Ma, RSC Adv., 2017, 7, 14330–14336.
105. O. Oyola-Rivera, J. He, G. W. Huber, J. A. Dumesic and N. Cardona-Martínez, Green Chem., 2019, 21, 4988–4999.
106. S. H. Krishna, T. W. Walker, J. A. Dumesic and G. W. Huber, ChemSusChem, 2017, 10, 129–138.
107. S. Tschirner, E. Weingart, L. Teevs and U. Prüße, Molecules, 2018, 23, 1866.
108. Y. Román-Leshkov, J. N. Chheda and J. A. Dumesic, Science, 2006, 312, 1933–1937.
109. J. E. Romo, N. V. Bollar, C. J. Zimmermann and S. G. Wettstein, ChemCatChem, 2018, 10, 4805–4816.
110. S. Altway, S. C. Pujar and A. B. de Haan, Fluid Phase Equilib., 2018, 475, 100–110.
111. Q. Mei, X. Q. Wei, W. T. Sun, X. H. Zhang, W. Z. Li and L. L. Ma, RSC Adv., 2019, 9, 12846–12853.
112. J. Esteban, A. J. Vorholt and W. Leitner, Green Chem., 2020, 22, 2097–2128.
113. P. Tundo, M. Musolino and F. Aricò, Front. Chem., 2019, 7, 300.
114. A. Dibenedetto, M. Aresta, L. di Bitonto and C. Pastore, ChemSusChem, 2016, 9, 118–125.
115. N. A. S. Ramli and N. A. S. Amin, BioEnergy Res., 2020, 13, 693–736.
116. H. Zhao, J. E. Holladay, H. Brown and Z. C. Zhang, Science, 2007, 316, 1597–1600.
117. H. Li, W. Xu, T. Huang, S. Jia, Z. Xu, P. Yan, X. Liu and Z. C. Zhang, ACS Catal., 2014, 4, 4446–4454.
118. E. A. Khokhlova, V. V. Kachala and V. P. Ananikov, ChemSusChem, 2012, 5, 783–789.
119. Y.-N. Li, J.-Q. Wang, L.-N. He, Z.-Z. Yang, A.-H. Liu, B. Yu and C.-R. Luan, Green Chem., 2012, 14, 2752–2758.
120. P. V. Rathod, R. B. Mujmule, W.-J. Chung, A. R. Jadhav and H. Kim, Catal. Lett., 2019, 149, 672–687.
121. T. Istasse, L. Bockstal and A. Richel, ChemPlusChem, 2018, 83, 1135–1143.
122. K. Enomoto, T. Hosoya and H. Miyafuji, Cellulose, 2018, 25, 2249–2257.
123. X. Qi, M. Watanabe, T. M. Aida and J. R. L. Smith, Green Chem., 2009, 11, 1327–1331.
124. X. Qi, M. Watanabe, T. M. Aida and R. L. Smith Jr, ChemSusChem, 2009, 2, 944–946.
125. Q. Hou, W. Li, M. Zhen, L. Liu, Y. Chen, Q. Yang, F. Huang, S. Zhang and M. Ju, RSC Adv., 2017, 7, 47288–47296.
126. Q. Wang, K. Su and Z. Li, Mol. Catal., 2017, 438, 197–203.
127. J. Li, Y. Yang and D. Zhang, Chem. Phys. Lett., 2019, 723, 175–181.
128. S. Marullo, C. Rizzo and F. Danna, Front. Chem., 2019, 7, 15.
129. Y.-S. Qu, Y.-L. Song, C.-P. Huang, J. Zhang and B.-H. Chen, Ind. Eng. Chem. Res., 2012, 51, 13008–13013.
130. Y. Qu, L. Li, Q. Wei, C. Huang, P. Oleskowicz-Popiel and J. Xu, Sci. Rep., 2016, 6, 26067.
131. S. Siankevich, Z. Fei, R. Scopelliti, P. G. Jessop, J. Zhang, N. Yan and P. J. Dyson, ChemSusChem, 2016, 9, 2089–2096.
132. S. Siankevich, Z. Fei, R. Scopelliti, G. Laurenczy, S. Katsyuba, N. Yan and P. J. Dyson, ChemSusChem, 2014, 7, 1647–1654.
133. A. S. Kashin, K. I. Galkin, E. A. Khokhlova and V. P. Ananikov, Angew. Chem., Int. Ed., 2016, 55, 2161–2166.
134. Y. Qu, Q. Wei, H. Li, P. Oleskowicz-Popiel, C. Huang and J. Xu, Bioresour. Technol., 2014, 162, 358–364.
135. A. A. Ghatta, J. D. E. T. Wilton-Ely and J. P. Hallett, ChemSusChem, 2019, 12, 4452–4460.
136. M. I. Alam, S. De, T. S. Khan, M. A. Haider and B. Saha, Ind. Crops Prod., 2018, 123, 629–637.
137. H. Wang, J. Cui, H. Li, Y. Zhao and J. Wang, J. Mol. Struct., 2019, 1179, 57–64.
138. J. Zhou, T. Huang, Y. Zhao, Z. Xia, Z. Xu, S. Jia, J. Wang and Z. C. Zhang, Ind. Eng. Chem. Res., 2015, 54, 7977–7983.
139. H. Chen, J. Zhou, J. Mao, J. Yin and S. Li, RSC Adv., 2016, 6, 101485–101491.
140. X. Tang, M. Zuo, Z. Li, H. Liu, C. Xiong, X. Zeng, Y. Sun, L. Hu, S. Liu, T. Lei and L. Lin, ChemSusChem, 2017, 10, 2696–2706.
141. P. H. Tran and P. V. Tran, Fuel, 2019, 246, 18–23.
142. C. H. J. T. Dietz, F. Gallucci, M. van Sint Annaland, C. Held and M. C. Kroon, Ind. Eng. Chem. Res., 2019, 58, 4240–4247.
143. C. H. J. T. Dietz, A. Erve, M. C. Kroon, M. van Sint Annaland, F. Gallucci and C. Held, Fluid Phase Equilib., 2019, 489, 75–82.
144. N. Rodriguez Quiroz, A. M. Norton, H. Nguyen, E. Vasileiadou and D. G. Vlachos, ACS Catal., 2019, 9, 9923–9952.
145. D. Liu and E. Y. X. Chen, Appl. Catal., A, 2012, 435–436, 78–85.
146. D. Saang'onyo, S. Parkin and F. T. Ladipo, , Polyhedron, 2018, 149, 153–162.
147. S. Sadula, O. Oesterling, A. Nardone, B. Dinkelacker and B. Saha, Green Chem., 2017, 19, 3888–3898.
148. F. Shen, S. Sun, X. Zhang, J. Yang, M. Qiu and X. Qi, Cellulose, 2020, 27, 3013–3023.
149. J. Amoah, T. Hasunuma, C. Ogino and A. Kondo, Biochem. Eng. J., 2019, 142, 117–123.
150. A. A. Marianou, C. M. Michailof, A. Pineda, E. F. Iliopoulou, K. S. Triantafyllidis and A. A. Lappas, Appl. Catal., A, 2018, 555, 75–87.
151. I. K. M. Yu, D. C. W. Tsang, A. C. K. Yip, S. S. Chen, Y. S. Ok and C. S. Poon, Chemosphere, 2017, 184, 1099–1107.
152. J. B. Mensah, I. Delidovich, P. J. C. Hausoul, L. Weisgerber, W. Schrader and R. Palkovits, ChemSusChem, 2018, 11, 2579–2586.
153. X. Fang, Z. Wang, W. Song, S. Li and W. Lin, J. Taiwan Inst. Chem. Eng., 2019, 97, 105–111.
154. C. G. Yoo, N. Li, M. Swannell and X. Pan, Green Chem., 2017, 19, 4402–4411.
155. J. Fang, W. Zheng, K. Liu, H. Li and C. Li, Chem. Eng. J., 2020, 385, 123796.
156. Y. Deng, R. Gao, L. Lin, T. Liu, X.-D. Wen, S. Wang and D. Ma, J. Am. Chem. Soc., 2018, 140, 14481–14489.
157. T. Stahlberg, S. Rodriguez-Rodriguez, P. Fristrup and A. Riisager, Chem. – Eur. J., 2011, 17, 1456–1464.
158. T. S. Hansen, J. Mielby and A. Riisager, Green Chem., 2011, 13, 109–114.
159. H. Matsumiya and T. Hara, Biomass Bioenergy, 2015, 72, 227–232.
160. L. Hu, Y. Sun, L. Lin and S. Liu, J. Taiwan Inst. Chem. Eng., 2012, 43, 718–723.
161. L. Hu, Y. Sun, L. Lin and S. Liu, Biomass Bioenergy, 2012, 47, 289–294.
162. B. R. Caes, M. J. Palte and R. T. Raines, Chem. Sci., 2013, 4, 196–199.
163. A. Mukherjee, M.-J. Dumont and A. Cherestes, Catal. Lett., 2019, 149, 283–291.
164. H. S. Kim, M.-R. Park, Y. J. Jeon, S.-K. Kim, Y.-K. Hong and G.-T. Jeong, Energy Technol., 2018, 6, 1747–1754.
165. Y. Xuan, R. He, B. Han, T. Wu and Y. Wu, Waste Biomass Valorization, 2018, 9, 401–408.
166. P. Widsten, K. Murton and M. West, Ind. Crops Prod., 2018, 119, 237–242.
167. I. Delidovich and R. Palkovits, Green Chem., 2016, 18, 5822–5830.
168. H. Ma, C. Liao, P. Yang, Y. Qiao, N. Li and J. Teng, ChemistrySelect, 2018, 3, 12113–12121.
169. Y. Feng, M. Li, Z. Gao, X. Zhang, X. Zeng, Y. Sun, X. Tang, T. Lei and L. Lin, ChemSusChem, 2019, 12, 495–502.
170. G. R. Gomes and J. C. Pastre, Sustainable Energy Fuels, 2020, 4, 1891–1898.
171. Z. Cao, M. Li, Y. Chen, T. Shen, C. Tang, C. Zhu and H. Ying, Appl. Catal., A, 2019, 569, 93–100.
172. Y. Zhang, E. A. Pidko and E. J. M. Hensen, Chem. – Eur. J., 2011, 17, 5210–5210.
173. J. Song, B. Zhang, J. Shi, H. Fan, J. Ma, Y. Yang and B. Han, RSC Adv., 2013, 3, 20085–20090.
174. L. Wu, J. Song, B. Zhang, B. Zhou, H. Zhou, H. Fan, Y. Yang and B. Han, Green Chem., 2014, 16, 3935–3941.
175. C. Loerbroks, J. van Rijn, M.-P. Ruby, Q. Tong, F. Schüth and W. Thiel, Chem. – Eur. J., 2014, 20, 12298–12309.
176. H. Nguyen, V. Nikolakis and D. G. Vlachos, ACS Catal., 2016, 6, 1497–1504.
177. S. Jia, K. Liu, Z. Xu, P. Yan, W. Xu, X. Liu and Z. C. Zhang, Catal. Today, 2014, 234, 83–90.
178. G. Yong, Y. Zhang and J. Y. Ying, Angew. Chem., Int. Ed., 2008, 47, 9345–9348.
179. F. P. Malan, E. Singleton, P. H. van Rooyen, J. Conradie and M. Landman, New J. Chem., 2018, 42, 19193–19204.
180. J. He, Y. Zhang and E. Y.-X. Chen, ChemSusChem, 2013, 6, 61–64.
181. J. B. Binder and R. T. Raines, J. Am. Chem. Soc., 2009, 131, 1979–1985.
182. L. Zhou, Y. He, Z. Ma, R. Liang, T. Wu and Y. Wu, Carbohydr. Polym., 2015, 117, 694–700.
183. J. B. Binder, A. V. Cefali, J. J. Blank and R. T. Raines, Energy Environ. Sci., 2010, 3, 765–771.
184. X. Qi, M. Watanabe, T. M. Aida and R. L. Smith Jr, ChemSusChem, 2010, 3, 1071–1077.
185. Y. J. Pagán-Torres, T. Wang, J. M. R. Gallo, B. H. Shanks and J. A. Dumesic, ACS Catal., 2012, 2, 930–934.
186. I. K. M. Yu, D. C. W. Tsang, A. C. K. Yip, S. S. Chen, Y. S. Ok and C. S. Poon, Bioresour. Technol., 2016, 219, 338–347.
187. P. Zhao, C. Zhou, J. Li, S. Xu and C. Hu, ACS Sustainable Chem. Eng., 2019, 7, 5176–5183.
188. Y. Wang, C. M. Pedersen, Y. Qiao, T. Deng, J. Shi and X. Hou, Carbohydr. Polym., 2015, 115, 439–443.
189. M. Dusselier, R. De Clercq, R. Cornelis and B. F. Sels, Catal. Today, 2017, 279, 339–344.
190. Q. Wang, M. Fu, X. Li, R. Huang, R. E. Glaser and L. Zhao, J. Comput. Chem., 2019, 40, 1599–1608.
191. T. Wang, J. A. Glasper and B. H. Shanks, Appl. Catal., A, 2015, 498, 214–221.
192. X. Zhang, P. Murria, Y. Jiang, W. Xiao, H. I. Kenttämaa, M. M. Abu-Omar and N. S. Mosier, Green Chem., 2016, 18, 5219–5229.
193. X. Zhang, B. B. Hewetson and N. S. Mosier, Energy Fuels, 2015, 29, 2387–2393.
194. S. De, S. Dutta and B. Saha, Green Chem., 2011, 13, 2859–2868.
195. Q. Ren, Y. Huang, H. Ma, J. Gao and J. Xu, Chin. J. Catal., 2014, 35, 496–500.
196. S. Jia, X. He and Z. Xu, RSC Adv., 2017, 7, 39221–39227.
197. S. Jia, Y. He and G. Wang, ChemistrySelect, 2017, 2, 2356–2362.
198. S. Xiao, B. Liu, Y. Wang, Z. Fang and Z. Zhang, Bioresour. Technol., 2014, 151, 361–366.
199. C. M. Cai, N. Nagane, R. Kumar and C. E. Wyman, Green Chem., 2014, 16, 3819–3829.
200. L. Yan, R. Ma, H. Wei, L. Li, B. Zou and Y. Xu, Bioresour. Technol., 2019, 279, 84–91.
201. Z. Zhang, Q. Wang, H. Xie, W. Liu and Z. Zhao, ChemSusChem, 2011, 4, 131–138.
202. Z. Zhang, B. Liu and Z. K. Zhao, Starch – Stärke, 2012, 64, 770–775.
203. T. Ståhlberg, M. G. Sørensen and A. Riisager, Green Chem., 2010, 12, 321–325.
204. J. Y. G. Chan and Y. Zhang, ChemSusChem, 2009, 2, 731–734.
205. H. Zhao, H. M. Brown, J. E. Holladay and Z. C. Zhang, Top. Catal., 2012, 55, 33–37.
206. M. Kammoun, T. Istasse, H. Ayeb, N. Rassaa, T. Bettaieb and A. Richel, Front. Chem., 2019, 7, 12.
207. T. Guo, X. Li, X. Liu, Y. Guo and Y. Wang, ChemSusChem, 2018, 11, 2758–2765.
208. Y. Shao, D. C. W. Tsang, D. Shen, Y. Zhou, Z. Jin, D. Zhou, W. Lu and Y. Long, Environ. Res., 2020, 184, 109340.
209. M. Mascal and E. B. Nikitin, ChemSusChem, 2009, 2, 859–861.
210. W. Yang and A. Sen, ChemSusChem, 2011, 4, 349–352.
211. Y. Peng, X. Li, T. Gao, T. Li and W. Yang, Green Chem., 2019, 21, 4169–4177.
212. Q. Liu, Q. Ma, S. Sabnis, W. Zheng, D. G. Vlachos, W. Fan, W. Li and L. Ma, Green Chem., 2019, 21, 5030–5038.
213. P. Körner, S. Beil and A. Kruse, React. Chem. Eng., 2019, 4, 747–762.
214. P. Wrigstedt, J. Keskiväli, M. Leskelä and T. Repo, ChemCatChem, 2015, 7, 501–507.
215. B. J. Graham and R. T. Raines, Biomass Convers. Biorefin., 2019, 9, 471–477.
216. P. Körner, D. Jung and A. Kruse, Green Chem., 2018, 20, 2231–2241.
217. N. Shi, Q. Liu, Q. Zhang, T. Wang and L. Ma, Green Chem., 2013, 15, 1967–1974.
218. G. Tsilomelekis, M. J. Orella, Z. Lin, Z. Cheng, W. Zheng, V. Nikolakis and D. G. Vlachos, Green Chem., 2016, 18, 1983–1993.
219. Z. Cheng, J. L. Everhart, G. Tsilomelekis, V. Nikolakis, B. Saha and D. G. Vlachos, Green Chem., 2018, 20, 997–1006.
220. V. Maruani, S. Narayanin-Richenapin, E. Framery and B. Andrioletti, ACS Sustainable Chem. Eng., 2018, 6, 13487–13493.
221. H. Labauze, S. Camy, P. Floquet, B. Benjelloun-Mlayah and J. S. Condoret, Ind. Eng. Chem. Res., 2019, 58, 92–100.
222. B. Seemala, V. Haritos and A. Tanksale, ChemCatChem, 2016, 8, 640–647.
223. C. Liu, J. M. Carraher, J. L. Swedberg, C. R. Herndon, C. N. Fleitman and J.-P. Tessonnier, ACS Catal., 2014, 4, 4295–4298.
224. Y. Román-Leshkov, M. Moliner, J. A. Labinger and M. E. Davis, Angew. Chem., Int. Ed., 2010, 49, 8954–8957.
225. M. Yabushita, N. Shibayama, K. Nakajima and A. Fukuoka, ACS Catal., 2019, 9, 2101–2109.
226. F. Liu, K. Huang, A. Zheng, F.-S. Xiao and S. Dai, ACS Catal., 2018, 8, 372–391.
227. P. Sudarsanam, R. Zhong, S. Van den Bosch, S. M. Coman, V. I. Parvulescu and B. F. Sels, Chem. Soc. Rev., 2018, 47, 8349–8402.
228. A. Zheng, S. Li, S.-B. Liu and F. Deng, Acc. Chem. Res., 2016, 49, 655–663.
229. A. A. Marianou, C. M. Michailof, A. Pineda, E. F. Iliopoulou, K. S. Triantafyllidis and A. A. Lappas, ChemCatChem, 2016, 8, 1100–1110.
230. A. A. Marianou, C. M. Michailof, D. K. Ipsakis, S. A. Karakoulia, K. G. Kalogiannis, H. Yiannoulakis, K. S. Triantafyllidis and A. A. Lappas, ACS Sustainable Chem. Eng., 2018, 6, 16459–16470.
231. S. S. Chen, Y. Cao, D. C. W. Tsang, J.-P. Tessonnier, J. Shang, D. Hou, Z. Shen, S. Zhang, Y. S. Ok and K. C. W. Wu, ACS Sustainable Chem. Eng., 2020, 8, 6990–7001.
232. J. J. Wiesfeld, R. Gaquere and E. J. M. Hensen, ACS Sustainable Chem. Eng., 2019, 7, 7552–7562.
233. J. Guo, S. Zhu, Y. Cen, Z. Qin, J. Wang and W. Fan, Appl. Catal., B, 2017, 200, 611–619.
234. B. Guo, L. Ye, G. Tang, L. Zhang, B. Yue, S. C. E. Tsang and H. He, Chin. J. Chem., 2017, 35, 1529–1539.
235. J. M. Requies, M. Frias, M. Cuezva, A. Iriondo, I. Agirre and N. Viar, Ind. Eng. Chem. Res., 2018, 57, 11535–11546.
236. J. J. Wiesfeld, N. Sommerdijk and E. J. M. Hensen, Catal. Lett., 2018, 148, 3093–3101.
237. J. Zhong, Y. Guo and J. Chen, J. Energy Chem., 2017, 26, 147–154.
238. A. Gervasini, S. Campisi, P. Carniti, M. Fantauzzi, C. Imparato, N. J. Clayden, A. Aronne and A. Rossi, Appl. Catal., A, 2019, 579, 9–17.
239. C. A. S. Lanziano, S. F. Moya, D. H. Barrett, E. Teixeira-Neto, R. Guirardello, F. de Souto da Silva, R. Rinaldi and C. B. Rodella, ChemSusChem, 2018, 11, 872–880.
240. Z. Tang and J. H. Su, J. Oleo Sci., 2019, 68, 261–271.
241. Q. Hou, M. Zhen, W. Li, L. Liu, J. Liu, S. Zhang, Y. Nie, C. Bai, X. Bai and M. Ju, Appl. Catal., B, 2019, 253, 1–10.
242. S. Pumrod, A. Kaewchada, S. Roddecha and A. Jaree, RSC Adv., 2020, 10, 9492–9498.
243. S. S. Jing, X. F. Cao, L. X. Zhong, X. W. Peng, R. C. Sun and J. C. Liu, Ind. Crops Prod., 2018, 126, 151–157.
244. G. E. Córdova-Pérez, G. Torres-Torres, F. Ortiz-Chi, S. Godavarthi, A. A. Silahua-Pavon, A. Izquierdo-Colorado, P. Da Costa, N. Hernandez-Como, M. Aleman and C. G. Espinosa-Gonzalez, ChemistrySelect, 2018, 3, 12854–12864.
245. B. Guo, L. He, G. Tang, L. Zhang, L. Ye, B. Yue, S. C. E. Tsang and H. He, Chin. J. Catal., 2020, 41, 1248–1260.
246. B. Han, P. Zhao, R. He, T. H. Wu and Y. Wu, Waste Biomass Valorization, 2018, 9, 2181–2190.
247. M. A. Yatoo and S. Saravanamurugan, Appl. Catal., A, 2019, 582, 117094.
248. Z. Wang, T. Li, Y. Jiang, O. Lafon, Z. Liu, J. Trébosc, A. Baiker, J.-P. Amoureux and J. Huang, Nat. Commun., 2020, 11, 225.
249. M. Cui, Z. Wu, R. Huang, W. Qi, R. Su and Z. He, Renewable Energy, 2018, 125, 327–333.
250. C. Chiappe, M. J. Rodriguez Douton, A. Mezzetta, C. S. Pomelli, G. Assanelli and A. R. de Angelis, ACS Sustainable Chem. Eng., 2017, 5, 5529–5536.
251. R. Wang, X. Liang, F. Shen, M. Qiu, J. Yang and X. Qi, ACS Sustainable Chem. Eng., 2020, 8, 1163–1170.
252. C. García-Sancho, I. Fúnez-Núñez, R. Moreno-Tost, J. Santamaría-González, E. Pérez-Inestrosa, J. L. G. Fierro and P. Maireles-Torres, Appl. Catal., B, 2017, 206, 617–625.
253. G. Sampath and K. Srinivasan, Appl. Catal., A, 2017, 533, 75–80.
254. Q. Hou, M. Zhen, L. Liu, Y. Chen, F. Huang, S. Zhang, W. Li and M. Ju, Appl. Catal., B, 2018, 224, 183–193.
255. J. Liu, H. Li, Y.-C. Liu, Y.-M. Lu, J. He, X.-F. Liu, Z.-B. Wu and S. Yang, Catal. Commun., 2015, 62, 19–23.
256. K. Nakajima, Y. Baba, R. Noma, M. Kitano, J. N. Kondo, S. Hayashi and M. Hara, J. Am. Chem. Soc., 2011, 133, 4224–4227.
257. N. T. do Prado, T. E. Souza, A. R. T. Machado, P. P. Souza, R. S. Monteiro and L. C. A. Oliveira, J. Mol. Catal. A: Chem., 2016, 422, 23–34.
258. H. T. Kreissl, K. Nakagawa, Y.-K. Peng, Y. Koito, J. Zheng and S. C. E. Tsang, J. Catal., 2016, 338, 329–339.
259. H. T. Kreissl, M. M. J. Li, Y.-K. Peng, K. Nakagawa, T. J. N. Hooper, J. V. Hanna, A. Shepherd, T.-S. Wu, Y.-L. Soo and S. C. E. Tsang, J. Am. Chem. Soc., 2017, 139, 12670–12680.
260. W. Fan, Q. Zhang, W. Deng and Y. Wang, Chem. Mater., 2013, 25, 3277–3287.
261. A. Takagaki, D. Lu, J. N. Kondo, M. Hara, S. Hayashi and K. Domen, Chem. Mater., 2005, 17, 2487–2489.
262. Q. Wu, Y. Yan, Q. Zhang, J. Lu, Z. Yang, Y. Zhang and Y. Tang, ChemSusChem, 2013, 6, 820–825.
263. P. Carniti, A. Gervasini, S. Biella and A. Auroux, Catal. Today, 2006, 118, 373–378.
264. K. Kawamura, T. Yasuda, T. Hatanaka, K. Hamahiga, N. Matsuda, M. Ueshima and K. Nakai, Chem. Eng. J., 2017, 307, 1066–1075.
265. N. K. Gupta, A. Fukuoka and K. Nakajima, ACS Catal., 2017, 7, 2430–2436.
266. C. García-Sancho, J. M. Rubio-Caballero, J. M. Mérida-Robles, R. Moreno-Tost, J. Santamaría-González and P. Maireles-Torres, Catal. Today, 2014, 234, 119–124.
267. K. Nakajima, T. Fukui, H. Kato, M. Kitano, J. N. Kondo, S. Hayashi and M. Hara, Chem. Mater., 2010, 22, 3332–3339.
268. P. Sudarsanam, H. Li and T. V. Sagar, ACS Catal., 2020, 10, 9555–9584.
269. K. Nakajima, R. Noma, M. Kitano and M. Hara, J. Mol. Catal. A: Chem., 2014, 388–389, 100–105.
270. M. Kitano, K. Nakajima, J. N. Kondo, S. Hayashi and M. Hara, J. Am. Chem. Soc., 2010, 132, 6622–6623.
271. G. Li, E. A. Pidko, E. J. M. Hensen and K. Nakajima, ChemCatChem, 2018, 10, 4084–4089.
272. R. Noma, K. Nakajima, K. Kamata, M. Kitano, S. Hayashi and M. Hara, J. Phys. Chem. C, 2015, 119, 17117–17125.
273. G. Li, E. A. Pidko and E. J. M. Hensen, ACS Catal., 2016, 6, 4162–4169.
274. C. Tagusagawa, A. Takagaki, A. Iguchi, K. Takanabe, J. N. Kondo, K. Ebitani, S. Hayashi, T. Tatsumi and K. Domen, Angew. Chem., 2010, 122, 1146–1150.
275. T. Murayama, K. Nakajima, J. Hirata, K. Omata, E. J. M. Hensen and W. Ueda, Catal. Sci. Technol., 2017, 7, 243–250.
276. H. Jiao, X. Zhao, C. Lv, Y. Wang, D. Yang, Z. Li and X. Yao, Sci. Rep., 2016, 6, 34068.
277. Y. Wang, X. Tong, Y. Yan, S. Xue and Y. Zhang, Catal. Commun., 2014, 50, 38–43.
278. R. M. de Almeida, N. J. A. de Albuquerque, F. T. C. Souza and S. M. P. Meneghetti, Catal. Sci. Technol., 2016, 6, 3137–3142.
279. E. L. S. Ngee, Y. Gao, X. Chen, T. M. Lee, Z. Hu, D. Zhao and N. Yan, Ind. Eng. Chem. Res., 2014, 53, 14225–14233.
280. S. Yu, E. Kim, S. Park, I. K. Song and J. C. Jung, Catal. Commun., 2012, 29, 63–67.
281. I. Delidovich and R. Palkovits, J. Catal., 2015, 327, 1–9.
282. G. Lee, Y. Jeong, A. Takagaki and J. C. Jung, J. Mol. Catal. A: Chem., 2014, 393, 289–295.
283. I. Delidovich and R. Palkovits, Catal. Sci. Technol., 2014, 4, 4322–4329.
284. P. P. Upare, A. Chamas, J. H. Lee, J. C. Kim, S. K. Kwak, Y. K. Hwang and D. W. Hwang, ACS Catal., 2020, 10, 1388–1396.
285. A. Takagaki, M. Ohara, S. Nishimura and K. Ebitani, Chem. Commun., 2009, 6276–6278.
286. S. Xu, X. Yan, Q. Bu and H. Xia, RSC Adv., 2016, 6, 8048–8052.
287. S. Xu, D. Pan, Y. Wu, X. Song, L. Gao, W. Li, L. Das and G. Xiao, Fuel Process. Technol., 2018, 175, 90–96.
288. S. Q. Xu, D. H. Pan, W. Q. Li, P. X. Shen, Y. F. Wu, X. H. Song, Y. L. Zhu, N. N. Xu, L. J. Gao and G. M. Xiao, Fuel Process. Technol., 2018, 181, 199–206.
289. D.-M. Gao, B. Zhao, H. Liu, K. Morisato, K. Kanamori, Z. He, M. Zeng, H. Wu, J. Chen and K. Nakanishi, Catal. Sci. Technol., 2018, 8, 3675–3685.
290. K. Saravanan, K. S. Park, S. Jeon and J. W. Bae, ACS Omega, 2018, 3, 808–820.
291. W. X. Ni, D. F. Li, X. G. Zhao, W. B. Ma, K. Kong, Q. W. Gu, M. Y. Chen and Z. S. Hou, Catal. Today, 2019, 319, 66–75.
292. C. Zhu, C. Cai, Q. Liu, W. Li, J. Tan, C. Wang, L. Chen, Q. Zhang and L. Ma, ChemistrySelect, 2018, 3, 10983–10990.
293. Z. Cao, Z. Fan, Y. Chen, M. Li, T. Shen, C. Zhu and H. Ying, Appl. Catal., B, 2019, 244, 170–177.
294. L. Yang, X. Yan, S. Xu, H. Chen, H. Xia and S. Zuo, RSC Adv., 2015, 5, 19900–19906.
295. Y. Liu, Z. Li, Y. You, X. Zheng and J. Wen, RSC Adv., 2017, 7, 51281–51289.
296. A. Dutta, D. Gupta, A. K. Patra, B. Saha and A. Bhaumik, ChemSusChem, 2014, 7, 925–933.
297. K. T. V. Rao, S. Souzanchi, Z. Yuan, M. B. Ray and C. Xu, RSC Adv., 2017, 7, 48501–48511.
298. W. Li, T. Yang, M. Su and Y. Liu, Catal. Lett., 2020, 150, 3304–3313.
299. K. Li, M. Du and P. Ji, ACS Sustainable Chem. Eng., 2018, 6, 5636–5644.
300. F. Huang, Y. Su, Z. Long, G. Chen and Y. Yao, Ind. Eng. Chem. Res., 2018, 57, 10198–10205.
301. L. Yang, X. Yan, S. Xu, H. Chen, H. Xia and S. Zuo, RSC Adv., 2015, 5, 19900–19906.
302. H. Xia, S. Xu, X. Yan and S. Zuo, Fuel Process. Technol., 2016, 152, 140–146.
303. L. Zhang, G. Xi, Z. Chen, Z. Qi and X. Wang, Chem. Eng. J., 2017, 307, 877–883.
304. J. E. Romo, T. Wu, X. Huang, J. Lucero, J. L. Irwin, J. Q. Bond, M. A. Carreon and S. G. Wettstein, ACS Omega, 2018, 3, 16253–16259.
305. Y. Zhang, J. Wang, X. Li, X. Liu, Y. Xia, B. Hu, G. Lu and Y. Wang, Fuel, 2015, 139, 301–307.
306. Y. Zhang, J. Wang, J. Ren, X. Liu, X. Li, Y. Xia, G. Lu and Y. Wang, Catal. Sci. Technol., 2012, 2, 2485–2491.
307. P. Carniti, A. Gervasini, F. Bossola and V. Dal Santo, Appl. Catal., B, 2016, 193, 93–102.
308. A. Dutta, A. K. Patra, S. Dutta, B. Saha and A. Bhaumik, J. Mater. Chem., 2012, 22, 14094–14100.
309. L. Atanda, S. Mukundan, A. Shrotri, Q. Ma and J. Beltramini, ChemCatChem, 2015, 7, 781–790.
310. L. Atanda, A. Shrotri, S. Mukundan, Q. Ma, M. Konarova and J. Beltramini, ChemSusChem, 2015, 8, 2907–2916.
311. L. Atanda, M. Konarova, Q. Ma, S. Mukundan, A. Shrotri and J. Beltramini, Catal. Sci. Technol., 2016, 6, 6257–6266.
312. M. Hattori, K. Kamata and M. Hara, Phys. Chem. Chem. Phys., 2017, 19, 3688–3693.
313. M. Zhang, X. Tong, R. Ma and Y. Li, Catal. Today, 2016, 264, 131–135.
314. H. Ning, J. Song, M. Hou, D. Yang, H. Fan and B. Han, Sci. China: Chem., 2013, 56, 1578–1585.
315. P. Daorattanachai, P. Khemthong, N. Viriya-empikul, N. Laosiripojana and K. Faungnawakij, Chem. Eng. J., 2015, 278, 92–98.
316. I. Jiménez-Morales, A. Teckchandani-Ortiz, J. Santamaría-González, P. Maireles-Torres and A. Jiménez-López, Appl. Catal., B, 2014, 144, 22–28.
317. I. Jiménez-Morales, M. Moreno-Recio, J. Santamaría-González, P. Maireles-Torres and A. Jiménez-López, Appl. Catal., B, 2014, 154–155, 190–196.
318. F. Yang, Q. Liu, M. Yue, X. Bai and Y. Du, Chem. Commun., 2011, 47, 4469–4471.
319. A. Dibenedetto, M. Aresta, C. Pastore, L. Di Bitonto, A. Angelini and E. Quaranta, RSC Adv., 2015, 5, 26941–26948.
320. S. Jia, X. He, J. Ma, Z. Xu, K. Wang and Z. C. Zhang, RSC Adv., 2018, 8, 32533–32537.
321. I. Delidovich, M. S. Gyngazova, N. Sánchez-Bastardo, J. P. Wohland, C. Hoppe and P. Drabo, Green Chem., 2018, 20, 724–734.
322. P. Bhanja and A. Bhaumik, ChemCatChem, 2016, 8, 1607–1616.
323. M. Shamzhy, M. Opanasenko, P. Concepcion and A. Martinez, Chem. Soc. Rev., 2019, 48, 1095–1149.
324. M. Moliner, Y. Román-Leshkov and M. E. Davis, Proc. Natl. Acad. Sci. U. S. A., 2010, 6164–6168.
325. J. Dijkmans, J. Demol, K. Houthoofd, S. Huang, Y. Pontikes and B. Sels, J. Catal., 2015, 330, 545–557.
326. J. W. Harris, M. J. Cordon, J. R. Di Iorio, J. C. Vega-Vila, F. H. Ribeiro and R. Gounder, J. Catal., 2016, 335, 141–154.
327. R. Bermejo-Deval, M. Orazov, R. Gounder, S.-J. Hwang and M. E. Davis, ACS Catal., 2014, 4, 2288–2297.
328. S. K. Brand, T. R. Josephson, J. A. Labinger, S. Caratzoulas, D. G. Vlachos and M. E. Davis, J. Catal., 2016, 341, 62–71.
329. T. R. Josephson, S. K. Brand, S. Caratzoulas and D. G. Vlachos, ACS Catal., 2017, 7, 25–33.
330. R. Bermejo-Deval, R. Gounder and M. E. Davis, ACS Catal., 2012, 2, 2705–2713.
331. J. Dijkmans, D. Gabriëls, M. Dusselier, F. de Clippel, P. Vanelderen, K. Houthoofd, A. Malfliet, Y. Pontikes and B. F. Sels, Green Chem., 2013, 15, 2777–2785.
332. D. Garcés, L. Faba, E. Díaz and S. Ordóñez, ChemSusChem, 2019, 12, 924–934.
333. E. Nikolla, Y. Román-Leshkov, M. Moliner and M. E. Davis, ACS Catal., 2011, 1, 408–410.
334. J. M. R. Gallo, D. M. Alonso, M. A. Mellmer and J. A. Dumesic, Green Chem., 2013, 15, 85–90.
335. S. Marullo, C. Rizzo, A. Meli and F. D'Anna, ACS Sustainable Chem. Eng., 2019, 7, 5818–5826.
336. L. Wang, L. B. Zhang, H. Y. Li, Y. B. Ma and R. H. Zhang, Composites, Part B, 2019, 156, 88–94.
337. P. Bhanja, A. Modak, S. Chatterjee and A. Bhaumik, ACS Sustainable Chem. Eng., 2017, 5, 2763–2773.
338. Y. Zhang, Y. Chen, J. Pan, M. Liu, P. Jin and Y. Yan, Chem. Eng. J., 2017, 313, 1593–1606.
339. Q. Wu, F. Liu, X. Yi, Y. Zou and L. Jiang, Green Chem., 2018, 20, 1020–1030.
340. X. Huang, S. Kudo, J. Sperry and J.-I. Hayashi, ACS Sustainable Chem. Eng., 2019, 7, 5892–5899.
341. L. K. Rihko-Struckmann, O. Oluyinka, A. Sahni, K. McBride, M. Fachet, K. Ludwig and K. Sundmacher, RSC Adv., 2020, 10, 24753–24763.
342. N. Candu, M. El Fergani, M. Verziu, B. Cojocaru, B. Jurca, N. Apostol, C. Teodorescu, V. I. Parvulescu and S. M. Coman, Catal. Today, 2019, 325, 109–116.
343. Y. Feng, G. Yan, T. Wang, W. Jia, X. Zeng, J. Sperry, Y. Sun, X. Tang, T. Lei and L. Lin, ChemSusChem, 2019, 12, 978–982.
344. Y. D. Xie, W. W. Yuan, Y. Huang, C. Y. Wu, H. J. Wang, Y. M. Xia and X. Liu, Chin. Chem. Lett., 2019, 30, 359–362.
345. N. D. Kalane, R. A. Krishnan, V. D. Yadav, R. Jain and P. Dandekar, Cellulose, 2019, 26, 2805–2819.
346. Y. Zhang, Q. Xiong, E. Zhu, M. Liu, J. Pan and Y. Yan, Energy Technol., 2018, 6, 1941–1950.
347. Y. Wang, L. W. Zhu, Y. Zhang, H. Y. Cui, W. M. Yi, F. Song, P. P. Zhao, X. Y. Sun, Y. J. Xie, L. H. Wang and Z. H. Li, ChemistrySelect, 2018, 3, 3555–3560.
348. Y. C. Feng, M. Zuo, T. Wang, W. L. Jia, X. Y. Zhao, X. H. Zeng, Y. Sun, X. Tang, T. Z. Lei and L. Lin, J. Taiwan Inst. Chem. Eng., 2019, 96, 431–438.
349. J. Cao, M. Ma, J. Liu, Y. Yang, H. Liu, X. Xu, J. Huang, H. Yue, G. Tian and S. Feng, Appl. Catal., A, 2019, 571, 96–101.
350. G. Qiu, C. Huang, X. Sun and B. Chen, Green Chem., 2019, 21, 3930–3939.
351. L. Wang, H. Guo, Q. Xie, J. Wang, B. Hou, L. Jia, J. Cui and D. Li, Appl. Catal., A, 2019, 51–60.
352. Y. Wang, G. Ding, X. Yang, H. Zheng, Y. Zhu and Y. Li, Appl. Catal., B, 2018, 235, 150–157.
353. P. Saxena, B. Velaga and N. R. Peela, ChemistrySelect, 2017, 2, 10379–10386.
354. Q. Xu, Z. Zhu, Y. Tian, J. Deng, J. Shi and Y. Fu, BioResources, 2014, 9, 303–315.
355. J. Su, M. Qiu, F. Shen and X. Qi, Cellulose, 2018, 25, 17–22.
356. J. Wang, J. Ren, X. Liu, J. Xi, Q. Xia, Y. Zu, G. Lu and Y. Wang, Green Chem., 2012, 14, 2506–2512.
357. G. M. Lari, P. Y. Dapsens, D. Scholz, S. Mitchell, C. Mondelli and J. Pérez-Ramírez, Green Chem., 2016, 18, 1249–1260.
358. Q. Guo, L. Ren, S. M. Alhassan and M. Tsapatsis, Chem. Commun., 2019, 55, 14942–14945.
359. N. Rajabbeigi, A. I. Torres, C. M. Lew, B. Elyassi, L. Ren, Z. Wang, H. Je Cho, W. Fan, P. Daoutidis and M. Tsapatsis, Chem. Eng. Sci., 2014, 116, 235–242.
360. D. Padovan, S. Tolborg, L. Botti, E. Taarning, I. Sádaba and C. Hammond, React. Chem. Eng., 2018, 3, 155–163.
361. D. Padovan, L. Botti and C. Hammond, ACS Catal., 2018, 8, 7131–7140.
362. R. Bermejo-Deval, R. S. Assary, E. Nikolla, M. Moliner, Y. Roman-Leshkov, S. J. Hwang, A. Palsdottir, D. Silverman, R. F. Lobo, L. A. Curtiss and M. E. Davis, Proc. Natl. Acad. Sci. U. S. A., 2012, 109, 9727–9732.
363. R. Gounder and M. E. Davis, ACS Catal., 2013, 3, 1469–1476.
364. M. J. Cordon, J. C. Vega-Vila, A. Casper, Z. Huang and R. Gounder, Angew. Chem., Int. Ed., 2020, 132, 19264–19269.
365. S. Xu, D. Pan, F. Hu, Y. Wu, H. Wang, Y. Chen, H. Yuan, L. Gao and G. Xiao, Fuel Process. Technol., 2019, 190, 38–46.
366. R. Otomo, T. Yokoi and T. Tatsumi, ChemCatChem, 2015, 7, 4180–4187.
367. X. Jia, I. K. M. Yu, D. C. W. Tsang and A. C. K. Yip, Microporous Mesoporous Mater., 2019, 284, 43–52.
368. I. Jiménez-Morales, M. Moreno-Recio, J. Santamaría-González, P. Maireles-Torres and A. Jiménez-López, Appl. Catal., B, 2015, 164, 70–76.
369. M. Moreno-Recio, I. Jiménez-Morales, P. L. Arias, J. Santamaría-González and P. Maireles-Torres, ChemistrySelect, 2017, 2, 2444–2451.
370. M. Moreno-Recio, J. Santamaría-González and P. Maireles-Torres, Chem. Eng. J., 2016, 303, 22–30.
371. I. Jiménez-Morales, J. Santamaría-González, A. Jiménez-López and P. Maireles-Torres, Fuel, 2014, 118, 265–271.
372. L. Hu, Z. Wu, J. Xu, Y. Sun, L. Lin and S. Liu, Chem. Eng. J., 2014, 244, 137–144.
373. B. Velaga and N. R. Peela, Microporous Mesoporous Mater., 2019, 279, 211–219.
374. W. Mamo, Y. Chebude, C. Márquez-Álvarez, I. Díaz and E. Sastre, Catal. Sci. Technol., 2016, 6, 2766–2774.
375. H. Abou-Yousef and E. B. Hassan, J. Ind. Eng. Chem., 2014, 20, 1952–1957.
376. H. Xia, H. Hu, S. Xu, K. Xiao and S. Zuo, Biomass Bioenergy, 2018, 108, 426–432.
377. S. Xu, L. Zhang, K. Xiao and H. Xia, Carbohydr. Res., 2017, 446–447, 48–51.
378. A. Osatiashtiani, A. F. Lee, M. Granollers, D. R. Brown, L. Olivi, G. Morales, J. A. Melero and K. Wilson, ACS Catal., 2015, 5, 4345–4352.
379. T. D. Swift, H. Nguyen, Z. Erdman, J. S. Kruger, V. Nikolakis and D. G. Vlachos, J. Catal., 2016, 333, 149–161.
380. S. Shunmugavel, M. Paniagua, J. A. Melero and A. Riisager, J. Am. Chem. Soc., 2013, 5246–5249.
381. L. Ren, Q. Guo, P. Kumar, M. Orazov, D. Xu, S. M. Alhassan, K. A. Mkhoyan, M. E. Davis and M. Tsapatsis, Angew. Chem., Int. Ed., 2015, 54, 10848–10851.
382. L. Hu, L. Lin, Z. Wu, S. Zhou and S. Liu, Appl. Catal., B, 2015, 174–175, 225–243.
383. Y. Tang, Y. Cheng, H. Xu, Y. Wang, L. Ke, X. Huang, X. Liao and B. Shi, Catal. Commun., 2019, 123, 96–99.
384. N. Pasha, P. K. Kumari, N. Vamsikrishna, N. Lingaiah, N. J. P. Subhashini and Shivaraj, Indian J. Chem., Sect. A: Inorg., Bio-inorg., Phys., Theor. Anal. Chem., 2019, 58, 313–320.
385. F. M. Huang, Y. W. Su, Y. Tao, W. Sun and W. T. Wang, Fuel, 2018, 226, 417–422.
386. S. Q. Xu, D. H. Pan, Y. F. Wu, N. N. Xu, H. M. Yang, L. J. Gao, W. Q. Li and G. M. Xiao, Ind. Eng. Chem. Res., 2019, 58, 9276–9285.
387. P. P. Zhao, H. Y. Cui, Y. Y. Zhang, Y. Zhang, Y. Wang, Y. L. Zhang, Y. J. Xie and W. M. Yi, ChemistryOpen, 2018, 7, 824–832.
388. J. R. Bernardo, M. C. Oliveira and A. C. Fernandes, Mol. Catal., 2019, 465, 87–94.
389. J. A. T. Caetano and A. C. Fernandes, Green Chem., 2018, 20, 2494–2498.
390. P. Zhao, Y. Zhang, Y. Wang, H. Cui, F. Song, X. Sun and L. Zhang, Green Chem., 2018, 20, 1551–1559.
391. X. Zhang, D. Zhang, Z. Sun, L. Xue, X. Wang and Z. Jiang, Appl. Catal., B, 2016, 196, 50–56.
392. F. Lai, F. Yan, P. Wang, S. Wang, S. Li and Z. Zhang, Chem. Eng. J., 2020, 396, 125282.
393. X. Wang, T. Lv, M. Wu, J. Sui, Q. Liu, H. Liu, J. Huang and L. Jia, Appl. Catal., A, 2019, 574, 87–96.
394. S. Zhao, M. Cheng, J. Li, J. Tian and X. Wang, Chem. Commun., 2011, 47, 2176–2178.
395. L. J. Konwar, P. Mäki-Arvela and J.-P. Mikkola, Chem. Rev., 2019, 119, 11576–11630.
396. J. Wang, W. Xu, J. Ren, X. Liu, G. Lu and Y. Wang, Green Chem., 2011, 13, 2678–2681.
397. H. Guo, X. Qi, L. Li and R. L. Smith, Bioresour. Technol., 2012, 116, 355–359.
398. X. Qi, N. Liu and Y. Lian, RSC Adv., 2015, 5, 17526–17531.
399. M. Qiu, C. Bai, L. Yan, F. Shen and X. Qi, ACS Sustainable Chem. Eng., 2018, 6, 13826–13833.
400. C. Bai, L. Zhu, F. Shen and X. Qi, Bioresour. Technol., 2016, 220, 656–660.
401. X. Qi, L. Yan, F. Shen and M. Qiu, Bioresour. Technol., 2019, 273, 687–691.
402. Y. Ni, Z. Bi, H. Su and L. Yan, Green Chem., 2019, 21, 1075–1079.
403. N. V. Gromov, T. B. Medvedeva, O. P. Taran, A. V. Bukhtiyarov, C. Aymonier, I. P. Prosvirin and V. N. Parmon, Top. Catal., 2018, 61, 1912–1927.
404. L. Shuai and X. Pan, Energy Environ. Sci., 2012, 5, 6889–6894.
405. S. Suganuma, K. Nakajima, M. Kitano, D. Yamaguchi, H. Kato, S. Hayashi and M. Hara, J. Am. Chem. Soc., 2008, 130, 12787–12793.
406. X. H. Liao, Y. Liu, X. Peng, C. Mi and X. G. Meng, Catal. Lett., 2016, 146, 1249–1255.
407. A. Charmot, P. W. Chung and A. Katz, ACS Sustainable Chem. Eng., 2014, 2, 2866–2872.
408. P. W. Chung, A. Charmot, O. M. Gazit and A. Katz, Langmuir, 2012, 28, 15222–15232.
409. A. T. To, P.-W. Chung and A. Katz, Angew. Chem., Int. Ed., 2015, 54, 11050–11053.
410. F. Liu, B. Li, C. Liu, W. Kong, X. Yi, A. Zheng and C. Qi, Catal. Sci. Technol., 2016, 6, 2995–3007.
411. L. Gan, L. Lyu, T. Shen and S. Wang, Appl. Catal., A, 2019, 574, 132–143.
412. K. Li, J. Chen, Y. Yan, Y. Min, H. Li, F. Xi, J. Liu and P. Chen, Carbon, 2018, 136, 224–233.
413. J. Ma, W. Li, S. Guan, Q. Liu, Q. Li, C. Zhu, T. Yang, A. T. Ogunbiyi and L. Ma, RSC Adv., 2019, 9, 10569–10577.
414. M. M. Songo, R. Moutloali and S. S. Ray, Catalysts, 2019, 9, 15.
415. F. Yang, X. Tong, F. Xia, C. Zheng, L. Qin and X. Jiang, Catal. Lett., 2018, 148, 1848–1855.
416. Z. Chen, Q. Li, Y. Xiao, C. Zhang, Z. Fu, Y. Liu, X. Yi, A. Zheng, C. Li and D. Yin, Cellulose, 2019, 26, 751–762.
417. F. Huang, W. Li, Q. Liu, T. Zhang, S. An, D. Li and X. Zhu, Fuel Process. Technol., 2018, 181, 294–303.
418. J. Li, Y. Wang, B. Lu, Y. Wang, T. Deng and X. Hou, Appl. Catal., A, 2018, 566, 140–145.
419. F. Huang, W. Z. Li, T. W. Zhang, D. W. Li, Q. Y. Liu, X. F. Zhu and L. L. Ma, Res. Chem. Intermed., 2018, 44, 5439–5453.
420. C. Zhang, Z. Cheng, Z. Fu, Y. Liu, X. Yi, A. Zheng, S. R. Kirk and D. Yin, Cellulose, 2017, 24, 95–106.
421. B. M. Matsagar, C. Van Nguyen, M. S. A. Hossain, M. T. Islam, Y. Yamauchi, P. L. Dhepe and K. C. W. Wu, Sustainable Energy Fuels, 2018, 2, 2148–2153.
422. F. Shen, J. Fu, X. Zhang and X. Qi, ACS Sustainable Chem. Eng., 2019, 7, 4466–4472.
423. K. N. Sorokina, O. P. Taran, T. B. Medvedeva, Y. V. Samoylova, A. V. Piligaev and V. N. Parmon, ChemSusChem, 2017, 10, 562–574.
424. I. K. M. Yu, X. Xiong, D. C. W. Tsang, L. Wang, A. J. Hunt, H. Song, J. Shang, Y. S. Ok and C. S. Poon, Green Chem., 2019, 21, 1267–1281.
425. X. Yang, I. K. M. Yu, D.-W. Cho, S. S. Chen, D. C. W. Tsang, J. Shang, A. C. K. Yip, L. Wang and Y. S. Ok, ACS Sustainable Chem. Eng., 2019, 7, 4851–4860.
426. M. Nahavandi, T. Kasanneni, Z. S. Yuan, C. C. Xu and S. Rohani, ACS Sustainable Chem. Eng., 2019, 7, 11970–11984.
427. L. C. Cao, I. K. M. Yu, D. C. W. Tsang, S. C. Zhang, Y. S. Ok, E. E. Kwon, H. Song and C. S. Poon, Bioresour. Technol., 2018, 267, 242–248.
428. L. Cao, I. K. M. Yu, S. S. Chen, D. C. W. Tsang, L. Wang, X. Xiong, S. Zhang, Y. S. Ok, E. E. Kwon, H. Song and C. S. Poon, Bioresour. Technol., 2018, 252, 76–82.
429. G. Y. Chen, L. B. Wu, H. Fan and B. G. Li, Ind. Eng. Chem. Res., 2018, 57, 16172–16181.
430. S. S. Chen, I. K. M. Yu, D.-W. Cho, H. Song, D. C. W. Tsang, J.-P. Tessonnier, Y. S. Ok and C. S. Poon, ACS Sustainable Chem. Eng., 2018, 6, 16113–16120.
431. X. Qi, Y. Lian, L. Yan and R. L. Smith Jr., Catal. Commun., 2014, 57, 50–54.
432. L. Yang, M. Wang, X. Yan, Y. Wang and H. Xia, Asian – J. Chem., 2015, 27, 2979–2982.
433. A. Villa, M. Schiavoni, P. F. Fulvio, S. M. Mahurin, S. Dai, R. T. Mayes, G. M. Veith and L. Prati, J. Energy Chem., 2013, 22, 305–311.
434. Y. Zhang, J. Wang, J. Wang, Y. Wang, M. Wang, H. Cui, F. Song, X. Sun, Y. Xie and W. Yi, ChemistrySelect, 2019, 4, 5724–5731.
435. N. T. Thanh, N. H. Long, L. Q. Dien, G. T. Phuong Ly, P. H. Hoang, N. T. Minh Phuong and N. T. Hue, Energy Sources, Part A, 2019, 1–10.
436. R. Fang, A. Dhakshinamoorthy, Y. Li and H. Garcia, Chem. Soc. Rev., 2020, 49, 3638–3687.
437. Z. Hu, Y. Peng, Y. Gao, Y. Qian, S. Ying, D. Yuan, S. Horike, N. Ogiwara, R. Babarao, Y. Wang, N. Yan and D. Zhao, Chem. Mater., 2016, 28, 2659–2667.
438. Y. Peng, Z. Hu, Y. Gao, D. Yuan, Z. Kang, Y. Qian, N. Yan and D. Zhao, ChemSusChem, 2015, 8, 3208–3212.
439. R. Oozeerally, D. L. Burnett, T. W. Chamberlain, R. I. Walton and V. Degirmenci, ChemCatChem, 2018, 10, 706–709.
440. Y. L. Zhang, B. Li, Y. A. Wei, C. H. Yan, M. J. Meng and Y. S. Yan, J. Taiwan Inst. Chem. Eng., 2019, 96, 93–103.
441. Y. Zhang, P. Jin, M. Meng, L. Gao, M. Liu and Y. Yan, Nano, 2018, 13, 1850132.
442. Y. Zhang, J. Zhao, K. Wang, L. Gao, M. Meng and Y. Yan, ChemistrySelect, 2018, 3, 9378–9387.
443. J. Zhao, Y. Zhang, K. Wang, C. Yan, Z. Da, C. Li and Y. Yan, ChemistrySelect, 2018, 3, 11476–11485.
444. Z. Hu and D. Zhao, CrystEngComm, 2017, 19, 4066–4081.
445. G. Zi, Z. Yan, Y. Wang, Y. Chen, Y. Guo, F. Yuan, W. Gao, Y. Wang and J. Wang, Carbohydr. Polym., 2015, 115, 146–151.
446. Q. Guo, L. Ren, P. Kumar, V. J. Cybulskis, K. A. Mkhoyan, M. E. Davis and M. Tsapatsis, Angew. Chem., Int. Ed., 2018, 57, 4926–4930.
447. D. L. Burnett, R. Oozeerally, R. Pertiwi, T. W. Chamberlain, N. Cherkasov, G. J. Clarkson, Y. K. Krisnandi, V. Degirmenci and R. I. Walton, Chem. Commun., 2019, 55, 11446–11449.
448. C. D. Malonzo, S. M. Shaker, L. Ren, S. D. Prinslow, A. E. Platero-Prats, L. C. Gallington, J. Borycz, A. B. Thompson, T. C. Wang, O. K. Farha, J. T. Hupp, C. C. Lu, K. W. Chapman, J. C. Myers, R. L. Penn, L. Gagliardi, M. Tsapatsis and A. Stein, J. Am. Chem. Soc., 2016, 138, 2739–2748.
449. M. Du, A. M. Agrawal, S. Chakraborty, S. J. Garibay, R. Limvorapitux, B. Choi, S. T. Madrahimov and S. T. Nguyen, ACS Sustainable Chem. Eng., 2019, 7, 8126–8135.
450. K. Cho, S. M. Lee, H. J. Kim, Y. J. Ko and S. U. Son, Chem. Commun., 2019, 55, 3697–3700.
451. K. Cho, S. M. Lee, H. J. Kim, Y.-J. Ko and S. U. Son, J. Mater. Chem. A, 2018, 6, 15553–15557.
452. A. Kumar and R. Srivastava, Mol. Catal., 2019, 465, 68–79.
453. W. Li, T. Zhang, H. Xin, M. Su, L. Ma, H. Jameel, H.-M. Chang and G. Pei, RSC Adv., 2017, 7, 27682–27688.
454. T. Zhang, W. Li, S. An, F. Huang, X. Li, J. Liu, G. Pei and Q. Liu, Bioresour. Technol., 2018, 264, 261–267.
455. H. Tang, N. Li, G. Y. Li, W. T. Wang, A. Q. Wang, Y. Cong and X. D. Wang, ACS Sustainable Chem. Eng., 2018, 6, 5645–5652.
456. S. Ravi, Y. Choi and J. K. Choe, Appl. Catal., B, 2020, 271, 118942.
457. Z. Li, K. Su, J. Ren, D. Yang, B. Cheng, C. K. Kim and X. Yao, Green Chem., 2018, 20, 863–872.
458. S. Verma, R. B. N. Baig, M. N. Nadagouda, C. Len and R. S. Varma, Green Chem., 2017, 19, 164–168.
459. T. Chhabra, A. Bahuguna, S. S. Dhankhar, C. M. Nagaraja and V. Krishnan, Green Chem., 2019, 21, 6012–6026.
460. X. Cao, S. P. Teong, D. Wu, G. Yi, H. Su and Y. Zhang, Green Chem., 2015, 17, 2348–2352.
461. H. Pawar and A. Lali, Ind. Eng. Chem. Res., 2018, 57, 14428–14439.
462. A. S. Wagh and H. S. Pawar, Energy Fuels, 2020, 34, 9643–9653.
463. F. D. Bobbink, Z. Huang, F. Menoud and P. J. Dyson, ChemSusChem, 2019, 12, 1437–1442.
464. Z. Babaei, A. N. Chermahini, M. Dinari, M. Saraji and A. Shahvar, Sustainable Energy Fuels, 2019, 3, 1024–1032.
465. S. Xu, C. Yin, D. Pan, F. Hu, Y. Wu, Y. Miao, L. Gao and G. Xiao, Sustainable Energy Fuels, 2019, 3, 390–395.
466. Z. Babaei, A. Najafi Chermahini, M. Dinari, M. Saraji and A. Shahvar, J. Cleaner Prod., 2018, 198, 381–388.
467. K. Wang, C. Liang, Q. Zhang and F. Zhang, ACS Omega, 2019, 4, 1053–1059.
468. L. Jiao, S. Sun, X. Meng and P. Ji, Catalysts, 2019, 9, 739.
469. G. Qiu, B. Chen, C. Huang, N. Liu and X. Sun, Fuel, 2020, 268, 117136.
470. P. Gogoi and R. Borah, J. Chem. Sci., 2018, 130, 170.
471. J. Dai, L. Zhu, D. Tang, X. Fu, J. Tang, X. Guo and C. Hu, Green Chem., 2017, 19, 1932–1939.
472. H. Tang, N. Li, F. Chen, G. Li, A. Wang, Y. Cong, X. Wang and T. Zhang, Green Chem., 2017, 19, 1855–1860.
473. R. L. Johnson, F. A. Perras, M. P. Hanrahan, M. Mellmer, T. F. Garrison, T. Kobayashi, J. A. Dumesic, M. Pruski, A. J. Rossini and B. H. Shanks, ACS Catal., 2019, 9, 11568–11578.
474. Y. Nishimura, M. Suda, M. Kuroha, H. Kobayashi, K. Nakajima and A. Fukuoka, Carbohydr. Res., 2019, 486, 107826.
475. S.-H. Pyo, M. Sayed and R. Hatti-Kaul, Org. Process Res. Dev., 2019, 23, 952–960.
476. L. Zhu, J. Dai, M. Liu, D. Tang, S. Liu and C. Hu, ChemSusChem, 2016, 2174–2181.
477. M.-P. Ruby and F. Schüth, Green Chem., 2016, 18, 3422–3429.
478. F. Yu, M. Smet, W. Dehaen and B. F. Sels, Chem. Commun., 2016, 52, 2756–2759.
479. F. Yu, J. Thomas, M. Smet, W. Dehaen and B. F. Sels, Green Chem., 2016, 18, 1694–1705.
480. M. Tyufekchiev, P. Duan, K. Schmidt-Rohr, S. Granados Focil, M. T. Timko and M. H. Emmert, ACS Catal., 2018, 8, 1464–1468.
481. R. Ye, J. Zhao, B. B. Wickemeyer, F. D. Toste and G. A. Somorjai, Nat. Catal., 2018, 1, 318–325.
482. T. L. Lohr and T. J. Marks, Nat. Chem., 2015, 7, 477.
483. H. Huang, C. A. Denard, R. Alamillo, A. J. Crisci, Y. Miao, J. A. Dumesic, S. L. Scott and H. Zhao, ACS Catal., 2014, 4, 2165–2168.
484. F. Lin, K. Wang, L. Gao and X. Guo, Appl. Organomet. Chem., 2019, 33, e4821.
485. B. Liu and Z. Zhang, ACS Catal., 2016, 6, 326–338.
486. M. Shaikh, M. Sahu, K. K. Atyam and K. V. S. Ranganath, RSC Adv., 2016, 6, 76795–76801.
487. M. Shaikh, S. K. Singh, S. Khilari, M. Sahu and K. V. S. Ranganath, Catal. Commun., 2018, 106, 64–67.
488. R. T. Woodward, M. Kessler, S. Lima and R. Rinaldi, Green Chem., 2018, 20, 2374–2381.
489. Q. Sun, S. Wang, B. Aguila, X. Meng, S. Ma and F.-S. Xiao, Nat. Commun., 2018, 9, 3236.
490. J. Yang, K. De Oliveira Vigier, Y. Gu and F. Jérôme, ChemSusChem, 2015, 8, 269–274.
491. T. W. Walker, A. H. Motagamwala, J. A. Dumesic and G. W. Huber, J. Catal., 2019, 369, 518–525.
492. A. P. S. Brogan, L. Bui-Le and J. P. Hallett, Nat. Chem., 2018, 10, 859–865.
493. N. Mimura, O. Sato, M. Shirai and A. Yamaguchi, ChemistrySelect, 2017, 2, 1305–1310.
494. N. Sweygers, D. E. C. Depuydt, A. W. Van Vuure, J. Degrève, G. Potters, R. Dewil and L. Appels, Chem. Eng. J., 2020, 386, 123957.
495. S. Amirjalayer, H. Fuchs and D. Marx, Angew. Chem., Int. Ed., 2019, 58, 5232–5235.
496. F. Shen, X. Xiong, J. Fu, J. Yang, M. Qiu, X. Qi and D. C. W. Tsang, Renewable Sustainable Energy Rev., 2020, 130, 109944.
497. D. W. Wakerley, M. F. Kuehnel, K. L. Orchard, K. H. Ly, T. E. Rosser and E. Reisner, Nat. Energy, 2017, 2, 17021.
498. M. F. Kuehnel and E. Reisner, Angew. Chem., Int. Ed., 2018, 57, 3290–3296.
499. X. Liu, X. Duan, W. Wei, S. Wang and B.-J. Ni, Green Chem., 2019, 21, 4266–4289.
500. K. Tsutsumi, N. Kurata, E. Takata, K. Furuichi, M. Nagano and K. Tabata, Appl. Catal., B, 2014, 147, 1009–1014.
501. B. Zhang, J. Li, L. Guo, Z. Chen and C. Li, Appl. Catal., B, 2018, 237, 660–664.
502. F. Delbecq and C. Len, Molecules, 2018, 23, 1973.
503. N. Sweygers, N. Alewaters, R. Dewil and L. Appels, Sci. Rep., 2018, 8, 7719.
504. N. Sweygers, J. Harrer, R. Dewil and L. Appels, J. Cleaner Prod., 2018, 187, 1014–1024.
505. T. Ji, R. Tu, L. Mu, X. Lu and J. Zhu, ACS Sustainable Chem. Eng., 2017, 5, 4352–4358.
506. T. Ji, R. Tu, L. Mu, X. Lu and J. Zhu, Appl. Catal., B, 2018, 220, 581–588.
507. T. Ji, Z. Li, C. Liu, X. Lu, L. Li and J. Zhu, Appl. Catal., B, 2019, 243, 741–749.
508. C. Xiouras, N. Radacsi, G. Sturm and G. D. Stefanidis, ChemSusChem, 2016, 9, 2159–2166.
509. S. Marullo, A. Sutera, G. Gallo, F. Billeci, C. Rizzo and F. D’Anna, ACS Sustainable Chem. Eng., 2020, 8, 11204–11214.
510. R. Galaverna, M. C. Breitkreitz and J. C. Pastre, ACS Sustainable Chem. Eng., 2018, 6, 4220–4230.
511. J. Tacacima, S. Derenzo and J. G. R. Poco, Mol. Catal., 2018, 458, 180–188.
512. F. J. Morales-Leal, J. Rivera de la Rosa, C. J. Lucio-Ortiz, D. A. De Haro-Del Rio, C. Solis Maldonado, S. Wi, L. B. Casabianca and C. D. Garcia, Appl. Catal., B, 2019, 244, 250–261.
513. C. Sonsiam, A. Kaewchada, S. Pumrod and A. Jaree, Chem. Eng. Process., 2019, 138, 65–72.
514. E. Weingart, S. Tschirner, L. Teevs and U. Prüße, Molecules, 2018, 23, 1802.
515. J. M. R. Gallo, R. Alamillo and J. A. Dumesic, J. Mol. Catal. A: Chem., 2016, 422, 13–17.
516. C. Tempelman, U. Jacobs, T. Hut, E. Pereira de Pina, M. van Munster, N. Cherkasov and V. Degirmenci, Appl. Catal., A, 2019, 588, 117267.
517. Y. Muranaka, H. Nakagawa, R. Masaki, T. Maki and K. Mae, Ind. Eng. Chem. Res., 2017, 56, 10998–11005.
518. K. I. Galkin, E. A. Krivodaeva, L. V. Romashov, S. S. Zalesskiy, V. V. Kachala, J. V. Burykina and V. P. Ananikov, Angew. Chem., Int. Ed., 2016, 55, 8338–8342.
519. R. F. A. Gomes, Y. N. Mitrev, S. P. Simeonov and C. A. M. Afonso, ChemSusChem, 2018, 11, 1612–1616.
520. Y. M. Questell-Santiago, R. Zambrano-Varela, M. T. Amiri and J. S. Luterbacher, Nat. Chem., 2018, 10, 1222–1228.
521. Y. M. Questell-Santiago, M. V. Galkin, K. Barta and J. S. Luterbacher, Nat. Rev. Chem., 2020, 4, 311–330.
522. C. Zhou, C. Shen, K. Ji, J. Yin and L. Du, ACS Sustainable Chem. Eng., 2018, 6, 3992–3999.
523. J. Lueckgen, L. Vanoye, R. Philippe, M. Eternot, P. Fongarland, C. de Bellefon and A. Favre-Réguillon, J. Flow Chem., 2018, 8, 3–9.
524. A. Sarwono, Z. Man, A. Idris, A. S. Khan, N. Muhammad and C. D. Wilfred, J. Ind. Eng. Chem., 2019, 69, 171–178.
525. P. Yan, M. Xia, S. Chen, W. Han, H. Wang and W. Zhu, Green Chem., 2020, 22, 5274–5284.
526. S. Gajula, K. Inthumathi, S. R. Arumugam and K. Srinivasan, ACS Sustainable Chem. Eng., 2017, 5, 5373–5381.
527. S. Alipour, Green Chem., 2016, 18, 4990–4998.
528. A. Gimbernat, M. Guehl, N. Lopes Ferreira, E. Heuson, P. Dhulster, M. Capron, F. Dumeignil, D. Delcroix, J.-S. Girardon and R. Froidevaux, Catalysts, 2018, 8, 335.
529. M. Yabushita, P. Li, H. Kobayashi, A. Fukuoka, O. K. Farha and A. Katz, Chem. Commun., 2016, 52, 11791–11794.
530. H. Jin, Y. Li, X. Liu, Y. Ban, Y. Peng, W. Jiao and W. Yang, Chem. Eng. Sci., 2015, 124, 170–178.
531. C. Detoni, C. H. Gierlich, M. Rose and R. Palkovits, ACS Sustainable Chem. Eng., 2014, 2, 2407–2415.
532. H. S. Pawar, ChemistrySelect, 2020, 5, 6851–6855.
533. K. Schute, Y. Louven, C. Detoni and M. Rose, Chem. Ing. Tech., 2016, 88, 355–362.
534. M. León, T. D. Swift, V. Nikolakis and D. G. Vlachos, Langmuir, 2013, 29, 6597–6605.
535. Y.-B. Zhang, Q.-X. Luo, M.-H. Lu, D. Luo, Z.-W. Liu and Z.-T. Liu, Chem. Eng. J., 2019, 358, 467–479.
536. M. Yabushita, N. A. Grosso-Giordano, A. Fukuoka and A. Katz, ACS Appl. Mater. Interfaces, 2018, 10, 39670–39678.
537. K. I. Galkin and V. P. Ananikov, ChemSusChem, 2019, 12, 2976–2982.
538. E. G. L. de Carvalho, F. D. Rodrigues, R. S. Monteiro, R. M. Ribas and M. J. da Silva, Biomass Convers. Biorefin., 2018, 8, 635–646.
539. M. A. Kougioumtzis, A. Marianou, K. Atsonios, C. Michailof, N. Nikolopoulos, N. Koukouzas, K. Triantafyllidis, A. Lappas and E. Kakaras, Waste Biomass Valorization, 2018, 9, 2433–2445.
540. K. Gupta, R. K. Rai and S. K. Singh, ChemCatChem, 2018, 10, 2326–2349.
541. F. A. Kucherov, L. V. Romashov, K. I. Galkin and V. P. Ananikov, ACS Sustainable Chem. Eng., 2018, 6, 8064–8092.
542. X. Kong, Y. Zhu, Z. Fang, J. A. Kozinski, I. S. Butler, L. Xu, H. Song and X. Wei, Green Chem., 2018, 20, 3657–3682.
543. B. Liu and Z. Zhang, ChemSusChem, 2016, 9, 2015–2036.
544. M. Mascal and E. B. Nikitin, Angew. Chem., Int. Ed., 2008, 47, 7924–7926.
545. M. Mascal, ACS Sustainable Chem. Eng., 2019, 7, 5588–5601.
546. M. Mascal, ChemSusChem, 2015, 8, 3391–3395.
547. M. Mascal and E. B. Nikitin, Green Chem., 2010, 12, 370–373.
548. E.-S. Kang, Y.-W. Hong, D. W. Chae, B. Kim, B. Kim, Y. J. Kim, J. K. Cho and Y. G. Kim, ChemSusChem, 2015, 8, 1179–1188.
549. M. Bicker, D. Kaiser, L. Ott and H. Vogel, J. Supercrit. Fluids, 2005, 36, 118–126.
550. L. Gavilà and D. Esposito, Green Chem., 2017, 19, 2496–2500.
551. B. Sarmah and R. Srivastava, Mol. Catal., 2019, 462, 92–103.
552. Q. Ke, Y. Jin, F. Ruan, M. N. Ha, D. Li, P. Cui, Y. Cao, H. Wang, T. Wang, V. N. Nguyen, X. Han, X. Wang and P. Cui, Green Chem., 2019, 21, 4313–4318.
553. Y. Liu, H.-Y. Ma, D. Lei, L.-L. Lou, S. Liu, W. Zhou, G.-C. Wang and K. Yu, ACS Catal., 2019, 9, 8306–8315.
554. L. Ardemani, G. Cibin, A. J. Dent, M. A. Isaacs, G. Kyriakou, A. F. Lee, C. M. A. Parlett, S. A. Parry and K. Wilson, Chem. Sci., 2015, 6, 4940–4945.
555. J. An, G. Sun and H. Xia, ACS Sustainable Chem. Eng., 2019, 7, 6696–6706.
556. D. Zhao, D. Rodriguez-Padron, R. Luque and C. Len, ACS Sustainable Chem. Eng., 2020, 8, 8486–8495.
557. B. Zhou, J. Song, Z. Zhang, Z. Jiang, P. Zhang and B. Han, Green Chem., 2017, 19, 1075–1081.
558. H. Xia, J. An, M. Hong, S. Xu, L. Zhang and S. Zuo, Catal. Today, 2019, 319, 113–120.
559. O. R. Schade, K. F. Kalz, D. Neukum, W. Kleist and J.-D. Grunwaldt, Green Chem., 2018, 20, 3530–3541.
560. P. L. Arias, J. A. Cecilia, I. Gandarias, J. Iglesias, M. L. Granados, R. Mariscal, G. Morales, R. Moreno-Tost and P. Maireles-Torres, Catal. Sci. Technol., 2020, 10, 2721–2757.
561. K. R. Hwang, W. Jeon, S. Y. Lee, M. S. Kim and Y. K. Park, Chem. Eng. J., 2020, 390, 13.
562. H. B. Yuan, H. L. Liu, J. K. Du, K. Q. Liu, T. F. Wang and L. Liu, Appl. Microbiol. Biotechnol., 2020, 104, 527–543.
563. K. Li and Y. J. Sun, Chem. – Eur. J., 2018, 24, 18258–18270.
564. P. Pal and S. Saravanamurugan, ChemSusChem, 2019, 12, 145–163.
565. Z. Zhang and G. W. Huber, Chem. Soc. Rev., 2018, 47, 1351–1390.
566. R. A. Sheldon, Front. Chem., 2020, 8, 132.
567. B. J. Taitt, D. H. Nam and K. S. Choi, ACS Catal., 2019, 9, 660–670.
568. M. Hong, J. Min, S. Wu, H. Cui, Y. Zhao, J. Li and S. Wang, ACS Omega, 2019, 4, 7054–7060.
569. G. Lv, S. Chen, H. Zhu, M. Li and Y. Yang, Appl. Surf. Sci., 2018, 458, 24–31.
570. Y. Ren, Z. Yuan, K. Lv, J. Sun, Z. Zhang and Q. Chi, Green Chem., 2018, 20, 4946–4956.
571. S. Hazra, M. Deb and A. J. Elias, Green Chem., 2017, 19, 5548–5552.
572. J. Ren, K. H. Song, Z. Li, Q. Wang, J. Li, Y. X. Wang, D. B. Li and C. K. Kim, Appl. Surf. Sci., 2018, 456, 174–183.
573. P. Tan, G. Li, R. Fang, L. Chen, R. Luque and Y. Li, ACS Catal., 2017, 7, 2948–2955.
574. J. P. Zhang, S. Nagamatsu, J. M. Du, C. L. Tong, H. H. Fang, D. H. Deng, X. Liu, K. Asakura and Y. Z. Yuan, J. Catal., 2018, 367, 16–26.
575. A. B. Raut and B. M. Bhanage, ChemistrySelect, 2018, 3, 11388–11397.
576. W. Zhang, T. Meng, J. Tang, W. Zhuang, Y. Zhou and J. Wang, ACS Sustainable Chem. Eng., 2017, 5, 10029–10037.
577. H. S. Pawar, ChemistrySelect, 2020, 5, 7417–7426.
578. Q. Q. Li, H. Y. Wang, Z. P. Tian, Y. J. Weng, C. G. Wang, J. R. Ma, C. F. Zhu, W. Z. Li, Q. Y. Liu and L. L. Ma, Catal. Sci. Technol., 2019, 9, 1570–1580.
579. A. Buonerba, S. Impemba, A. D. Litta, C. Capacchione, S. Milione and A. Grassi, ChemSusChem, 2018, 11, 3139–3149.
580. Q. Wang, W. Hou, S. Li, J. Xie, J. Li, Y. Zhou and J. Wang, Green Chem., 2017, 19, 3820–3830.
581. P. Zhang, X. Zhang, X. Kang, H. Liu, C. Chen, C. Xie and B. Han, Chem. Commun., 2018, 54, 12065–12068.
582. D. Bonincontro, A. Lolli, A. Villa, L. Prati, N. Dimitratos, G. M. Veith, L. E. Chinchilla, G. A. Botton, F. Cavani and S. Albonetti, Green Chem., 2019, 21, 4090–4099.
583. E. Hayashi, Y. Yamaguchi, K. Kamata, N. Tsunoda, Y. Kumagai, F. Oba and M. Hara, J. Am. Chem. Soc., 2019, 141, 890–900.
584. K. T. V. Rao, J. L. Rogers, S. Souzanchi, L. Dessbesell, M. B. Ray and C. Xu, ChemSusChem, 2018, 11, 3323–3334.
585. S. Zhang, X. Sun, Z. Zheng and L. Zhang, Catal. Commun., 2018, 113, 19–22.
586. X. Liao, J. Hou, Y. Wang, H. Zhang, Y. Sun, X. Li, S. Tang, K. Kato, M. Yamauchi and Z. Jiang, Green Chem., 2019, 21, 4194–4203.
587. Z. Yuan, B. Liu, P. Zhou, Z. Zhang and Q. Chi, Catal. Sci. Technol., 2018, 8, 4430–4439.
588. R. Chen, J. Xin, D. Yan, H. Dong, X. Lu and S. Zhang, ChemSusChem, 2019, 12, 2715–2724.
589. H. Yu, K.-A. Kim, M. J. Kang, S. Y. Hwang and H. G. Cha, ACS Sustainable Chem. Eng., 2019, 7, 3742–3748.
590. B. Donoeva, N. Masoud and P. E. de Jongh, ACS Catal., 2017, 7, 4581–4591.
591. H. Liu, X. J. Cao, J. N. Wei, W. L. Jia, M. Z. Li, X. Tang, X. H. Zeng, Y. Sun, T. Z. Lei, S. J. Liu and L. Lin, ACS Sustainable Chem. Eng., 2019, 7, 7812–7822.
592. F. Nocito, M. Ventura, M. Aresta and A. Dibenedetto, ACS Omega, 2018, 3, 18724–18729.
593. T. Gao, J. Chen, W. Fang, Q. Cao, W. Su and F. Dumeignil, J. Catal., 2018, 368, 53–68.
594. C. Ke, M. Li, G. Fan, L. Yang and F. Li, Chem. - Asian J., 2018, 13, 2714–2722.
595. C. M. Pichler, M. G. Al-Shaal, D. Gu, H. Joshi, W. Ciptonugroho and F. Schüth, ChemSusChem, 2018, 11, 2083–2090.
596. A. Tirsoaga, M. El Fergani, V. I. Parvulescu and S. M. Coman, ACS Sustainable Chem. Eng., 2018, 6, 14292–14301.
597. S. Verma, M. N. Nadagouda and R. S. Varma, Sci. Rep., 2017, 7, 13596.
598. J. M. Jeong, S. B. Jin, J. H. Yoon, J. G. Yeo, G. Y. Lee, M. Irshad, S. Lee, D. Seo, B. E. Kwak, B. G. Choi, D. H. Kim and J. W. Kim, Catalysts, 2019, 9, 9.
599. M. Chatterjee, T. Ishizaka, A. Chatterjee and H. Kawanami, Green Chem., 2017, 19, 1315–1326.
600. J. Lai, K. Liu, S. Zhou, D. Zhang, X. Liu, Q. Xu and D. Yin, RSC Adv., 2019, 9, 14242–14246.
601. Q. Wang, W. Hou, T. Meng, Q. Hou, Y. Zhou and J. Wang, Catal. Today, 2019, 319, 57–65.
602. J. Xu, T. Su, Z. Zhu, N. Chen, D. Hao, M. Wang, Y. Zhao, W. Ren and H. Lü, Chem. Eng. J., 2020, 396, 125303.
603. J. Zhao, X. Chen, Y. Du, Y. Yang and J.-M. Lee, Appl. Catal., A, 2018, 568, 16–22.
604. J. Zhao, A. Jayakumar, Z.-T. Hu, Y. Yan, Y. Yang and J.-M. Lee, ACS Sustainable Chem. Eng., 2018, 6, 284–291.
605. C. A. Antonyraj, N. T. T. Huynh, K. W. Lee, Y. J. Kim, S. Shin, J. S. Shin and J. K. Cho, J. Chem. Sci., 2018, 130, 9.
606. A. Danielli da Fonseca Ferreira, M. Dorneles de Mello and M. A. P. da Silva, Ind. Eng. Chem. Res., 2019, 58, 128–137.
607. C. Megías-Sayago, K. Chakarova, A. Penkova, A. Lolli, S. Ivanova, S. Albonetti, F. Cavani and J. A. Odriozola, ACS Catal., 2018, 8, 11154–11164.
608. C. P. Ferraz, M. Zielinski, M. Pietrowski, S. Heyte, F. Dumeignil, L. M. Rossi and R. Wojcieszak, ACS Sustainable Chem. Eng., 2018, 6, 16332–16340.
609. F. Li, X.-L. Li, C. Li, J. Shi and Y. Fu, Green Chem., 2018, 20, 3050–3058.
610. A. Cho, S. Byun, J. H. Cho and B. M. Kim, ChemSusChem, 2019, 12, 2310–2317.
611. K.-K. Sun, S.-J. Chen, Z.-L. Li, G.-P. Lu and C. Cai, Green Chem., 2019, 21, 1602–1608.
612. H. Liu, N. Ding, J. N. Wei, X. Tang, X. H. Zeng, Y. Sun, T. Z. Lei, H. Y. Fang, T. Y. Li and L. Lin, ChemSusChem, 2020, 13, 4151–4158.
613. T. Ji, C. Liu, X. Lu and J. Zhu, ACS Sustainable Chem. Eng., 2018, 6, 11493–11501.
614. A. Tirsoaga, M. El Fergani, N. Nuns, P. Simon, P. Granger, V. I. Parvulescu and S. M. Coman, Appl. Catal., B, 2020, 278, 119309.
615. C. Yang, X. Li, Z. Zhang, B. Lv, J. Li, Z. Liu, W. Zhu, F. Tao, G. Lv and Y. Yang, J. Energy Chem., 2020, 50, 96–105.
616. K. Afroz, M. Ntambwe and N. Nuraje, Inorg. Chem., 2020, 59, 13335–13342.
617. Y.-T. Liao, V. C. Nguyen, N. Ishiguro, A. P. Young, C.-K. Tsung and K. C. W. Wu, Appl. Catal., B, 2020, 270, 118805.
618. S. S. R. Gupta, A. Vinu and M. L. Kantam, J. Catal., 2020, 389, 259–269.
619. W. Zhuang, X. Liu, L. Chen, P. Liu, H. Wen, Y. Zhou and J. Wang, Green Chem., 2020, 22, 4199–4209.
620. N. Zhang, Y. Zou, L. Tao, W. Chen, L. Zhou, Z. Liu, B. Zhou, G. Huang, H. Lin and S. Wang, Angew. Chem., Int. Ed., 2019, 58, 15895–15903.
621. L. Gao, Y. Bao, S. Gan, Z. Sun, Z. Song, D. Han, F. Li and L. Niu, ChemSusChem, 2018, 11, 2547–2553.
622. M. J. Kang, H. Park, J. Jegal, S. Y. Hwang, Y. S. Kang and H. G. Cha, Appl. Catal., B, 2019, 242, 85–91.
623. M. Zhang, Y. Liu, B. Liu, Z. Chen, H. Xu and K. Yan, ACS Catal., 2020, 10, 5179–5189.
624. G. Yang, Y. Jiao, H. Yan, Y. Xie, A. Wu, X. Dong, D. Guo, C. Tian and H. Fu, Adv. Mater., 2020, 32, 2000455.
625. J. Zhao, A. Jayakumar and J.-M. Lee, ACS Sustainable Chem. Eng., 2018, 6, 2976–2982.
626. C. Zhou, J. Zhao, H. Sun, Y. Song, X. Wan, H. Lin and Y. Yang, ACS Sustainable Chem. Eng., 2019, 7, 315–323.
627. B. Sarmah, B. Satpati and R. Srivastava, Catal. Sci. Technol., 2018, 8, 2870–2882.
628. D. Yan, J. Xin, C. Shi, X. Lu, L. Ni, G. Wang and S. Zhang, Chem. Eng. J., 2017, 323, 473–482.
629. Y. Cheng, S. Zhao, B. Johannessen, J.-P. Veder, M. Saunders, M. R. Rowles, M. Cheng, C. Liu, M. F. Chisholm, R. De Marco, H.-M. Cheng, S.-Z. Yang and S. P. Jiang, Adv. Mater., 2018, 30, 1706287.
630. Z.-M. Wang, L.-J. Liu, B. Xiang, Y. Wang, Y.-J. Lyu, T. Qi, Z.-B. Si, H.-Q. Yang and C.-W. Hu, Catal. Sci. Technol., 2019, 9, 811–821.
631. C. Laugel, B. Estrine, J. Le Bras, N. Hoffmann, S. Marinkovic and J. Muzart, ChemCatChem, 2014, 6, 1195–1198.
632. G. Li, Z. Sun, Y. Yan, Y. Zhang and Y. Tang, ChemSusChem, 2017, 10, 494–498.
633. I. Krivtsov, E. I. García-López, G. Marcì, L. Palmisano, Z. Amghouz, J. R. García, S. Ordóñez and E. Díaz, Appl. Catal., B, 2017, 204, 430–439.
634. A. Lolli, V. Maslova, D. Bonincontro, F. Basile, S. Ortelli and S. Albonetti, Molecules, 2018, 23, 2792.
635. G. Marcì, E. I. García-López, F. R. Pomilla, L. Palmisano, A. Zaffora, M. Santamaria, I. Krivtsov, M. Ilkaeva, Z. Barbieriková and V. Brezová, Catal. Today, 2019, 328, 21–28.
636. M. Ilkaeva, I. Krivtsov, J. R. Garcia, E. Diaz, S. Ordonez, E. I. Garcia-Lopez, G. Marci, L. Palmisano, M. I. Maldonado and S. Malato, Catal. Today, 2018, 315, 138–148.
637. M. Ilkaeva, I. Krivtsov, E. I. García-López, G. Marcì, O. Khainakova, J. R. García, L. Palmisano, E. Díaz and S. Ordóñez, J. Catal., 2018, 359, 212–222.
638. H. Zhang, Z. Feng, Y. Zhu, Y. Wu and T. Wu, J. Photochem. Photobiol., A, 2019, 371, 1–9.
639. B. Yang, W. Hu, F. Wan, C. Zhang, Z. Fu, A. Su, M. Chen and Y. Liu, Chem. Eng. J., 2020, 396, 125345.
640. H. Zhang, Q. Wu, C. Guo, Y. Wu and T. Wu, ACS Sustainable Chem. Eng., 2017, 5, 3517–3523.
641. E. I. Garcia-Lopez, F. R. Pomilla, E. Bloise, X. F. Lu, G. Mele, L. Palmisano and G. Marci, Top. Catal., 2020, 14 DOI:10.1007/s11244-020-01293-0.
642. S. Xu, P. Zhou, Z. Zhang, C. Yang, B. Zhang, K. Deng, S. Bottle and H. Zhu, J. Am. Chem. Soc., 2017, 139, 14775–14782.
643. H.-F. Ye, R. Shi, X. Yang, W.-F. Fu and Y. Chen, Appl. Catal., B, 2018, 233, 70–79.
644. S. Dhingra, T. Chhabra, V. Krishnan and C. M. Nagaraja, ACS Appl. Energy Mater., 2020, 3, 7138–7148.
645. G. Han, Y.-H. Jin, R. A. Burgess, N. E. Dickenson, X.-M. Cao and Y. Sun, J. Am. Chem. Soc., 2017, 139, 15584–15587.
646. S. Meng, H. Wu, Y. Cui, X. Zheng, H. Wang, S. Chen, Y. Wang and X. Fu, Appl. Catal., B, 2020, 266, 118617.
647. A. Brandolese, D. Ragno, G. Di Carmine, T. Bernardi, O. Bortolini, P. P. Giovannini, O. G. Pandoli, A. Altomare and A. Massi, Org. Biomol. Chem., 2018, 16, 8955–8964.
648. S.-S. Shi, X.-Y. Zhang, M.-H. Zong, C.-F. Wang and N. Li, Mol. Catal., 2019, 469, 68–74.
649. M. Sayed, S.-H. Pyo, N. Rehnberg and R. Hatti-Kaul, ACS Sustainable Chem. Eng., 2019, 7, 4406–4413.
650. J. H. Zhang, Q. D. Liang, W. X. Xie, L. C. Peng, L. He, Z. B. He, S. P. Chowdhury, R. Christensen and Y. H. Ni, Polymers, 2019, 11, 14.
651. A. Banerjee, G. R. Dick, T. Yoshino and M. W. Kanan, Nature, 2016, 531, 215.
652. B. M. Stadler, C. Wulf, T. Werner, S. Tin and J. G. de Vries, ACS Catal., 2019, 9, 8012–8067.
653. S. H. Krishna, K. Huang, K. J. Barnett, J. He, C. T. Maravelias, J. A. Dumesic, G. W. Huber, M. De Bruyn and B. M. Weckhuysen, AIChE J., 2018, 64, 1910–1922.
654. V. Tournier, C. M. Topham, A. Gilles, B. David, C. Folgoas, E. Moya-Leclair, E. Kamionka, M. L. Desrousseaux, H. Texier, S. Gavalda, M. Cot, E. Guémard, M. Dalibey, J. Nomme, G. Cioci, S. Barbe, M. Chateau, I. André, S. Duquesne and A. Marty, Nature, 2020, 580, 216–219.
655. F. A. Kucherov, E. G. Gordeev, A. S. Kashin and V. P. Ananikov, Angew. Chem., Int. Ed., 2017, 56, 15931–15935.
656. V. A. Klushin, V. P. Kashparova, I. S. Kashparov, Y. A. Chus, A. A. Chizhikova, T. A. Molodtsova and N. V. Smirnova, Russ. Chem. Bull., 2019, 68, 570–577.
657. M. Sajid, X. B. Zhao and D. H. Liu, Green Chem., 2018, 20, 5427–5453.
658. C. Chen, L. Wang, B. Zhu, Z. Zhou, S. I. El-Hout, J. Yang and J. Zhang, J. Energy Chem., 2021, 54, 528–554.
659. A. D. K. Deshan, L. Atanda, L. Moghaddam, D. W. Rackemann, J. Beltramini and W. O. S. Doherty, Front. Chem., 2020, 8, 659.
660. D. Troiano, V. Orsat and M. J. Dumont, ACS Catal., 2020, 10, 9145–9169.
661. E. Hayashi, T. Komanoya, K. Kamata and M. Hara, ChemSusChem, 2017, 10, 654–658.
662. Y. Yamaguchi, R. Aono, E. Hayashi, K. Kamata and M. Hara, ACS Appl. Mater. Interfaces, 2020, 12, 36004–36013.
663. O. R. Schade, A. Gaur, A. Zimina, E. Saraçi and J.-D. Grunwaldt, Catal. Sci. Technol., 2020, 10, 5036–5047.
664. H. Zhou, H. Xu and Y. Liu, Appl. Catal., B, 2019, 244, 965–973.
665. S. K. Singh, Asian J. Org. Chem., 2018, 7, 1901–1923.
666. C. Zhou, W. Shi, X. Wan, Y. Meng, Y. Yao, Z. Guo, Y. Dai, C. Wang and Y. Yang, Catal. Today, 2019, 330, 92–100.
667. F. Liguori, P. Barbaro and N. Calisi, ChemSusChem, 2019, 12, 2558–2563.
668. W. Naim, O. R. Schade, E. Saraci, D. Wust, A. Kruse and J. D. Grunwaldt, ACS Sustainable Chem. Eng., 2020, 8, 11512–11521.
669. W. Wu, S. Xu, G. Qi, H. Zhu, F. Hu, Z. Liu, D. Zhang and B. Liu, Angew. Chem., Int. Ed., 2019, 58, 3062–3066.
670. W. B. Wu, D. Mao, S. D. Xu, Kenry, F. Hu, X. Q. Li, D. L. Kong and B. Liu, Chem, 2018, 4, 1937–1951.
671. H. G. Cha and K.-S. Choi, Nat. Chem., 2015, 7, 328–333.
672. B. You, X. Liu, N. Jiang and Y. Sun, J. Am. Chem. Soc., 2016, 138, 13639–13646.
673. W. Li, N. Jiang, B. Hu, X. Liu, F. Song, G. Han, T. J. Jordan, T. B. Hanson, T. L. Liu and Y. Sun, Chem, 2018, 4, 637–649.
674. M. Park, M. Gu and B.-S. Kim, ACS Nano, 2020, 14, 6812–6822.
675. A. H. Motagamwala, W. Won, C. Sener, D. M. Alonso, C. T. Maravelias and J. A. Dumesic, Sci. Adv., 2018, 4.
676. Y. Mathieu, W. A. Offen, S. M. Forget, L. Ciano, A. H. Viborg, E. Blagova, B. Henrissat, P. H. Walton, G. J. Davies and H. Brumer, ACS Catal., 2020, 10, 3042–3058.
677. L. Hu, A. Y. He, X. Y. Liu, J. Xia, J. X. Xu, S. Y. Zhou and J. M. Xu, ACS Sustainable Chem. Eng., 2018, 6, 15915–15935.
678. C. Zhang, X. Chang, L. Zhu, Q. Xing, S. You, W. Qi, R. Su and Z. He, Int. J. Biol. Macromol., 2019, 128, 132–139.
679. T. K. Godan, R. O. Rajesh, P. C. Loreni, A. Kumar Rai, D. Sahoo, A. Pandey and P. Binod, Bioresour. Technol., 2019, 282, 88–93.
680. M. Daou, B. Yassine, S. Wikee, E. Record, F. Duprat, E. Bertrand and C. B. Faulds, Fungal Biol. Rev., 2019, 6, 4.
681. H. B. Yuan, Y. F. Liu, X. Q. Lv, J. H. Li, G. C. Du, Z. P. Shi and L. Liu, J. Microbiol. Biotechnol., 2018, 28, 1999–2008.
682. H. Yuan, Y. Liu, J. Li, H.-D. Shin, G. Du, Z. Shi, J. Chen and L. Liu, Biotechnol. Bioeng., 2018, 115, 2148–2155.
683. X. Wang, X.-Y. Zhang, M.-H. Zong and N. Li, ACS Sustainable Chem. Eng., 2020, 8, 4341–4345.
684. C. Isola, H. L. Sieverding, R. Raghunathan, M. P. Sibi, D. C. Webster, J. Sivaguru and J. J. Stone, J. Cleaner Prod., 2017, 142, 2935–2944.
685. S. Bello, I. Salim, P. Mendez-Trelles, E. Rodil, G. Feijoo and M. T. Moreira, Holzforschung, 2019, 73, 105–115.
686. J. Byun and J. Han, Energy Environ. Sci., 2020, 13, 2233–2242.
687. S. C. Zhang, G. F. Shen, Y. Y. Deng, Y. Lei, J. W. Xue, Z. Q. Chen and G. C. Yin, ACS Sustainable Chem. Eng., 2018, 6, 13192–13198.
688. J. J. Pacheco and M. E. Davis, Proc. Natl. Acad. Sci. U. S. A., 2014, 111, 8363–8367.
689. G. Lv, S. Chen, H. Zhu, M. Li and Y. Yang, J. Cleaner Prod., 2018, 196, 32–41.
690. W. Jia, Z. Si, Y. Feng, X. Zhang, X. Zhao, Y. Sun, X. Tang, X. Zeng and L. Lin, ACS Sustainable Chem. Eng., 2020, 8, 7901–7908.
691. W.-J. Liu, Z. Xu, D. Zhao, X.-Q. Pan, H.-C. Li, X. Hu, Z.-Y. Fan, W.-K. Wang, G.-H. Zhao, S. Jin, G. W. Huber and H.-Q. Yu, Nat. Commun., 2020, 11, 265.
692. X. Tang, J. Wei, N. Ding, Y. Sun, X. Zeng, L. Hu, S. Liu, T. Lei and L. Lin, Renewable Sustainable Energy Rev., 2017, 77, 287–296.
693. L. Hu, J. Xu, S. Zhou, A. He, X. Tang, L. Lin, J. Xu and Y. Zhao, ACS Catal., 2018, 8, 2959–2980.
694. M. J. Gilkey and B. J. Xu, ACS Catal., 2016, 6, 1420–1436.
695. Z. Wei, J. Lou, Z. Li and Y. Liu, Catal. Sci. Technol., 2016, 6, 6217–6225.
696. J. Wei, X. Cao, T. Wang, H. Liu, X. Tang, X. Zeng, Y. Sun, T. Lei, S. Liu and L. Lin, Catal. Sci. Technol., 2018, 8, 4474–4484.
697. H. Li, Z. Fang, J. He and S. Yang, ChemSusChem, 2017, 10, 681–686.
698. L. Hu, X. Dai, N. Li, X. Tang and Y. Jiang, Sustainable Energy Fuels, 2019, 3, 1033–1041.
699. S. H. Zhou, F. T. Dai, Y. A. Chen, C. Dang, C. Z. Zhang, D. T. Liu and H. S. Qi, Green Chem., 2019, 21, 1421–1431.
700. R. J. Chimentão, H. Oliva, J. Belmar, K. Morales, P. Mäki-Arvela, J. Wärnå, D. Y. Murzin, J. L. G. Fierro, J. Llorca and D. Ruiz, Appl. Catal., B, 2019, 241, 270–283.
701. J. Yu, Y. Yang, L. Chen, Z. Li, W. Liu, E. Xu, Y. Zhang, S. Hong, X. Zhang and M. Wei, Appl. Catal., B, 2020, 277, 119273.
702. L. Hu, T. Li, J. Xu, A. He, X. Tang, X. Chu and J. Xu, Chem. Eng. J., 2018, 352, 110–119.
703. Y. Sun, C. Xiong, Q. Liu, J. Zhang, X. Tang, X. Zeng, S. Liu and L. Lin, Ind. Eng. Chem. Res., 2019, 58, 5414–5422.
704. Y. Ma, G. Xu, H. Wang, Y. Wang, Y. Zhang and Y. Fu, ACS Catal., 2018, 8, 1268–1277.
705. J. Zhang, W. Xie, Q. Liang, L. Peng and L. He, ChemistrySelect, 2019, 4, 2846–2850.
706. S. Zhou, G. Chen, X. Feng, M. Wang, T. Song, D. Liu, F. Lu and H. Qi, Green Chem., 2018, 20, 3593–3603.
707. Y. Feng, S. Long, G. Yan, B. Chen, J. Sperry, W. Xu, Y. Sun, X. Tang, X. Zeng and L. Lin, J. Catal., 2020, 389, 157–165.
708. P. P. Upare, Y. K. Hwang and D. W. Hwang, Green Chem., 2018, 20, 879–885.
709. Z. Gao, C. Li, G. Fan, L. Yang and F. Li, Appl. Catal., B, 2018, 226, 523–533.
710. D. Hu, H. Hu, H. Zhou, G. Li, C. Chen, J. Zhang, Y. Yang, Y. Hu, Y. Zhang and L. Wang, Catal. Sci. Technol., 2018, 8, 6091–6099.
711. J. X. Long, W. F. Zhao, Y. F. Xu, H. Li and S. Yang, Catalysts, 2018, 8, 12.
712. Z. Zhang, C. Wang, X. Gou, H. Chen, K. Chen, X. Lu, P. Ouyang and J. Fu, Appl. Catal., A, 2019, 570, 245–250.
713. S. Srivastava, G. C. Jadeja and J. K. Parikh, Int. J. Chem. React. Eng., 2018, 16, 20170197.
714. Y. Zu, P. Yang, J. Wang, X. Liu, J. Ren, G. Lu and Y. Wang, Appl. Catal., B, 2014, 146, 244–248.
715. F. Yang, J. B. Mao, S. M. Li, J. M. Yin, J. X. Zhou and W. Liu, Catal. Sci. Technol., 2019, 9, 1329–1333.
716. D. Li, Q. Y. Liu, C. H. Zhu, H. Y. Wang, C. H. Cui, C. G. Wang and L. L. Ma, J. Energy Chem., 2019, 30, 34–41.
717. Z. An, W. L. Wang, S. H. Dong and J. He, Catal. Today, 2019, 319, 128–138.
718. Z. Zhang, S. Yao, C. Wang, M. Liu, F. Zhang, X. Hu, H. Chen, X. Gou, K. Chen, Y. Zhu, X. Lu, P. Ouyang and J. Fu, J. Catal., 2019, 373, 314–321.
719. Y. D. Yang, H. Y. Liu, S. P. Li, C. J. Chen, T. B. Wu, Q. Q. Mei, Y. Y. Wang, B. F. Chen, H. Z. Liu and B. X. Han, ACS Sustainable Chem. Eng., 2019, 7, 5711–5716.
720. Y.-R. Zhang, B.-X. Wang, L. Qin, Q. Li and Y.-M. Fan, Green Chem., 2019, 21, 1108–1113.
721. A. D. Talpade, M. S. Tiwari and G. D. Yadav, Mol. Catal., 2019, 465, 1–15.
722. R. Xu, L. Kang, J. Knossalla, J. Mielby, Q. Wang, B. Wang, J. Feng, G. He, Y. Qin, J. Xie, A.-C. Swertz, Q. He, S. Kegnæs, D. J. L. Brett, F. Schüth and F. R. Wang, ACS Nano, 2019, 13, 2463–2472.
723. E. Nurenberg, P. Schulze, F. Kohler, M. Zubel, S. Pischinger and F. Schuth, ACS Sustainable Chem. Eng., 2019, 7, 249–257.
724. X. Wang, Y. Liu and X. Liang, Green Chem., 2018, 20, 2894–2902.
725. Z. Gao, G. L. Fan, M. R. Liu, L. Yang and F. Li, Appl. Catal., B, 2018, 237, 649–659.
726. W. Han, M. Tang, J. Li, X. Li, J. Wang, L. Zhou, Y. Yang, Y. Wang and H. Ge, Appl. Catal., B, 2020, 268, 118748.
727. S. Fulignati, C. Antonetti, D. Licursi, M. Pieraccioni, E. Wilbers, H. J. Heeres and A. M. Raspolli Galletti, Appl. Catal., A, 2019, 578, 122–133.
728. L. Kuai, Z. Chen, S. Liu, E. Kan, N. Yu, Y. Ren, C. Fang, X. Li, Y. Li and B. Geng, Nat. Commun., 2020, 11, 48.
729. Y. Román-Leshkov, C. J. Barrett, Z. Y. Liu and J. A. Dumesic, Nature, 2007, 447, 982–985.
730. T. Gan, Y. Liu, Q. He, H. Zhang, X. He and H. Ji, ACS Sustainable Chem. Eng., 2020, 8, 8692–8699.
731. J. Fu, J. Lym, W. Zheng, K. Alexopoulos, A. V. Mironenko, N. Li, J. A. Boscoboinik, D. Su, R. T. Weber and D. G. Vlachos, Nat. Catal., 2020, 3, 446–453.
732. Y. Liu, M. A. Mellmer, D. M. Alonso and J. A. Dumesic, ChemSusChem, 2015, 8, 3983–3986.
733. Y. Deng, Y. Ge, M. Xu, Q. Yu, D. Xiao, S. Yao and D. Ma, Acc. Chem. Res., 2019, 52, 3372–3383.
734. G. Liu, A. W. Robertson, M. M.-J. Li, W. C. H. Kuo, M. T. Darby, M. H. Muhieddine, Y.-C. Lin, K. Suenaga, M. Stamatakis, J. H. Warner and S. C. E. Tsang, Nat. Chem., 2017, 9, 810.
735. J. Zhang, K. Dong and W. Luo, Chem. Eng. Sci., 2019, 201, 467–474.
736. W. Li, G. Fan, L. Yang and F. Li, Green Chem., 2017, 19, 4353–4363.
737. T. Thananatthanachon and T. B. Rauchfuss, Angew. Chem., 2010, 122, 6766–6768.
738. J. F. Li, Z. Song, Y. F. Hou, Z. Y. Li, C. L. Xu, C. L. Liu and W. S. Dong, ACS Appl. Mater. Interfaces, 2019, 11, 12481–12491.
739. R. Insyani, D. Verma, S. M. Kim and J. Kim, Green Chem., 2017, 19, 2482–2490.
740. B. Seemala, X. Meng, A. Parikh, N. Nagane, R. Kumar, C. E. Wyman, A. Ragauskas, P. Christopher and C. M. Cai, ACS Sustainable Chem. Eng., 2018, 6, 10587–10594.
741. L. Tao, T.-H. Yan, W. Li, Y. Zhao, Q. Zhang, Y.-M. Liu, M. M. Wright, Z.-H. Li, H.-Y. He and Y. Cao, Chem, 2018, 4, 2212–2227.
742. J. Li, E. Muller, M. Pera-Titus, F. Jérôme and K. De Oliveira Vigier, Green Chem., 2019, 21, 2601–2609.
743. M. E. Jung and G. Y. J. Im, J. Org. Chem., 2009, 74, 8739–8753.
744. G. Sun, J. An, H. Hu, C. Li, S. Zuo and H. Xia, Catal. Sci. Technol., 2019, 9, 1238–1244.
745. B. Op de Beeck, M. Dusselier, J. Geboers, J. Holsbeek, E. Morré, S. Oswald, L. Giebeler and B. F. Sels, Energy Environ. Sci., 2015, 8, 230–240.
746. A. Deneyer, E. Peeters, T. Renders, S. Van den Bosch, N. Van Oeckel, T. Ennaert, T. Szarvas, T. I. Korányi, M. Dusselier and B. F. Sels, Nat. Energy, 2018, 3, 969–977.
747. S. Liu, M. Tamura, Y. Nakagawa and K. Tomishige, ACS Sustainable Chem. Eng., 2014, 2, 1819–1827.
748. Q. Xia, Z. Chen, Y. Shao, X. Gong, H. Wang, X. Liu, S. F. Parker, X. Han, S. Yang and Y. Wang, Nat. Commun., 2016, 7, 11162.
749. Y. Yang, Y. Wang, S. Li, X. Shen, B. Chen, H. Liu and B. Han, Green Chem., 2020, 22, 4937–4942.
750. J. He, S. P. Burt, M. Ball, D. Zhao, I. Hermans, J. A. Dumesic and G. W. Huber, ACS Catal., 2018, 8, 1427–1439.
751. S. P. Burt, K. J. Barnett, D. J. McClelland, P. Wolf, J. A. Dumesic, G. W. Huber and I. Hermans, Green Chem., 2017, 19, 1390–1398.
752. K. Huang, Z. J. Brentzel, K. J. Barnett, J. A. Dumesic, G. W. Huber and C. T. Maravelias, ACS Sustainable Chem. Eng., 2017, 5, 4699–4706.
753. Z. J. Brentzel, K. J. Barnett, K. Huang, C. T. Maravelias, J. A. Dumesic and G. W. Huber, ChemSusChem, 2017, 10, 1351–1355.
754. J. He, K. Huang, K. J. Barnett, S. H. Krishna, D. M. Alonso, Z. J. Brentzel, S. P. Burt, T. Walker, W. F. Banholzer, C. T. Maravelias, I. Hermans, J. A. Dumesic and G. W. Huber, Faraday Discuss., 2017, 202, 247–267.
755. B. Wozniak, S. Tin and J. G. de Vries, Chem. Sci., 2019, 10, 6024–6034.
756. Z. Xu, P. Yan and Z. Zongconrad, Chinese J. Org. Chem., 2017, 37, 40–46.
757. Z. Xu, P. Yan, W. Xu, X. Liu, Z. Xia, B. Chung, S. Jia and Z. C. Zhang, ACS Catal., 2015, 5, 788–792.
758. Z. Xu, P. Yan, H. Li, K. Liu, X. Liu, S. Jia and Z. C. Zhang, ACS Catal., 2016, 6, 3784–3788.
759. S. Fujita, K. Nakajima, J. Yamasaki, T. Mizugaki, K. Jitsukawa and T. Mitsudome, ACS Catal., 2020, 10, 4261–4267.
760. B. Wozniak, A. Spannenberg, Y. Li, S. Hinze and J. G. de Vries, ChemSusChem, 2018, 11, 356–359.
761. Y. Duan, M. Zheng, D. Li, D. Deng, L.-F. Ma and Y. Yang, Green Chem., 2017, 19, 5103–5113.
762. R. Ramos, A. Grigoropoulos, N. Perret, M. Zanella, A. P. Katsoulidis, T. D. Manning, J. B. Claridge and M. J. Rosseinsky, Green Chem., 2017, 19, 1701–1713.
763. S. Zhang, H. Ma, Y. Sun, Y. Luo, X. Liu, M. Zhang, J. Gao and J. Xu, Green Chem., 2019, 21, 1702–1709.
764. Q. Deng, R. Gao, X. Li, J. Wang, Z. L. Zeng, J. J. Zou and S. G. Deng, ACS Catal., 2020, 10, 7355–7366.
765. H. Duan, J.-C. Liu, M. Xu, Y. Zhao, X.-L. Ma, J. Dong, X. Zheng, J. Zheng, C. S. Allen, M. Danaie, Y.-K. Peng, T. Issariyakul, D. Chen, A. I. Kirkland, J.-C. Buffet, J. Li, S. C. E. Tsang and D. O'Hare, Nat. Catal., 2019, 2, 1078–1087.
766. S. Alipour, H. Omidvarborna and D.-S. Kim, Renewable Sustainable Energy Rev., 2017, 71, 908–926.
767. M. J. Climent, A. Corma and S. Iborra, Green Chem., 2014, 16, 516–547.
768. Z. Wang and Q. Chen, Nanomaterials, 2018, 8, 492.
769. C. P. A. and S. Darbha, Catal. Commun., 2020, 140, 105998.
770. M. Zuo, K. Le, Y. Feng, C. Xiong, Z. Li, X. Zeng, X. Tang, Y. Sun and L. Lin, Ind. Crops Prod., 2018, 112, 18–23.
771. Z. Wang and Q. Chen, Green Chem., 2016, 18, 5884–5889.
772. H. Wang, T. Deng, Y. Wang, X. Cui, Y. Qi, X. Mu, X. Hou and Y. Zhu, Green Chem., 2013, 15, 2379–2383.
773. Y.-Y. Bai, S. Su, S. Wang, B. Wang, R.-C. Sun, G. Song and L.-P. Xiao, Energy Technol., 2018, 6, 1951–1958.
774. P. K. Kumari, B. S. Rao, D. Padmakar, N. Pasha and N. Lingaiah, Mol. Catal., 2018, 448, 108–115.
775. P. K. Kumari, B. S. Rao, D. D. Lakshmi, N. R. S. Paramesh, C. Sumana and N. Lingaiah, Catal. Today, 2019, 325, 53–60.
776. H. Wang, T. Deng, Y. Wang, Y. Qi, X. Hou and Y. Zhu, Bioresour. Technol., 2013, 136, 394–400.
777. H. X. Guo, X. H. Qi, Y. Hiraga, T. M. Aida and R. L. Smith, Chem. Eng. J., 2017, 314, 508–514.
778. X. Yu, X. Gao, R. Tao and L. Peng, Catalysts, 2017, 7, 182.
779. H. Xin, T. Zhang, W. Li, M. Su, S. Li, Q. Shao and L. Ma, RSC Adv., 2017, 7, 41546–41551.
780. H. Guo, A. Duereh, Y. Hiraga, X. Qi and R. L. Smith, Energy Fuels, 2018, 32, 8411–8419.
781. H. Li, S. Saravanamurugan, S. Yang and A. Riisager, Green Chem., 2016, 18, 726–734.
782. S. Shinde and C. Rode, ChemSusChem, 2017, 10, 4090–4101.
783. J. Wei, T. Wang, H. Liu, M. Li, X. Tang, Y. Sun, X. Zeng, L. Hu, T. Lei and L. Lin, Energy Technol., 2019, 7, 1801071.
784. X.-L. Li, K. Zhang, S.-Y. Chen, C. Li, F. Li, H.-J. Xu and Y. Fu, Green Chem., 2018, 20, 1095–1105.
785. B. Chen, G. Xu, Z. Zheng, D. Wang, C. Zou and C. Chang, Ind. Crops Prod., 2019, 129, 503–511.
786. J. Zhang, K. Dong, W. Luo and H. Guan, Fuel, 2018, 234, 664–673.
787. F. Yang, S. Zhang, Z. C. Zhang, J. Mao, S. Li, J. Yin and J. Zhou, Catal. Sci. Technol., 2015, 5, 4602–4612.
788. Y. W. Dou, M. Y. Zhang, S. Zhou, C. Oldani, W. H. Fang and Q. Cao, Eur. J. Inorg. Chem., 2018, 3706–3716.
789. S. B. Onkarappa and S. Dutta, ChemistrySelect, 2019, 4, 5540–5543.
790. R. Zhong, F. Yu, W. Schutyser, Y. Liao, F. de Clippel, L. Peng and B. F. Sels, Appl. Catal., B, 2017, 206, 74–88.
791. B. Xiang, Y. Wang, T. Qi, H.-Q. Yang and C.-W. Hu, J. Catal., 2017, 352, 586–598.
792. H. Guo, A. Duereh, Y. Su, E. J. M. Hensen, X. Qi and R. L. Smith, Appl. Catal., B, 2020, 264, 118509.
793. L. Zhang, Y. Zhu, L. Tian, Y. He, H. Wang and F. Deng, Fuel Process. Technol., 2019, 193, 39–47.
794. S. Kanai, I. Nagahara, Y. Kita, K. Kamata and M. Hara, Chem. Sci., 2017, 8, 3146–3153.
795. K. S. Arias, A. Garcia-Ortiz, M. J. Climent, A. Corma and S. Iborra, ACS Sustainable Chem. Eng., 2018, 6, 4239–4245.
796. M. Kim, Y. Q. Su, A. Fukuoka, E. J. M. Hensen and K. Nakajima, Angew. Chem., Int. Ed., 2018, 57, 8235–8239.
797. M. Kim, Y. Su, T. Aoshima, A. Fukuoka, E. J. M. Hensen and K. Nakajima, ACS Catal., 2019, 9, 4277–4285.
798. J. J. Wiesfeld, M. Kim, K. Nakajima and E. J. M. Hensen, Green Chem., 2020, 22, 1229–1238.
799. H. C. Chang, G. W. Huber and J. A. Dumesic, ChemSusChem, 8 DOI:10.1002/cssc.202001471.
800. A. D. Sutton, F. D. Waldie, R. Wu, M. Schlaf, L. A. ‘Pete’ Silks III and J. C. Gordon, Nat. Chem., 2013, 5, 428.
801. G. W. Huber, J. N. Chheda, C. J. Barrett and J. A. Dumesic, Science, 2005, 308, 1446–1450.
802. H. Chang, A. H. Motagamwala, G. W. Huber and J. A. Dumesic, Green Chem., 2019, 21, 5532–5540.
803. H. C. Chang, I. Bajaj, G. W. Huber, C. T. Maravelias and J. A. Dumesic, Green Chem., 2020, 22, 5285–5295.
804. J. Cueto, L. Faba, E. Díaz and S. Ordóñez, Appl. Catal., B, 2017, 201, 221–231.
805. D. Suttipat, W. Wannapakdee, T. Yutthalekha, S. Ittisanronnachai, T. Ungpittagul, K. Phomphrai, S. Bureekaew and C. Wattanakit, ACS Appl. Mater. Interfaces, 2018, 10, 16358–16366.
806. T. Yutthalekha, D. Suttipat, S. Salakhum, A. Thivasasith, S. Nokbin, J. Limtrakul and C. Wattanakit, Chem. Commun., 2017, 53, 12185–12188.
807. J.-F. Zhang, Z.-M. Wang, Y.-J. Lyu, H. Xie, T. Qi, Z.-B. Si, L.-J. Liu, H.-Q. Yang and C.-W. Hu, J. Phys. Chem. C, 2019, 123, 4903–4913.
808. R. S. Malkar, H. Daly, C. Hardacre and G. D. Yadav, ACS Sustainable Chem. Eng., 2019, 7, 16215–16224.
809. J. Xie, L. Zhang, X. Zhang, P. Han, J. Xie, L. Pan, D.-R. Zou, S.-H. Liu and J.-J. Zou, Sustainable Energy Fuels, 2018, 2, 1863–1869.
810. A. S. Amarasekara, T. B. Singh, E. Larkin, M. A. Hasan and H.-J. Fan, Ind. Crops Prod., 2015, 65, 546–549.
811. X. Cui, X. Zhao and D. Liu, Green Chem., 2018, 20, 2018–2026.
812. L. Zhao, N. Elechi, R. Qian, T. B. Singh, A. S. Amarasekara and H.-J. Fan, J. Phys. Chem. A, 2017, 121, 1985–1992.
813. G. Liang, A. Wang, X. Zhao, N. Lei and T. Zhang, Green Chem., 2016, 18, 3430–3438.
814. Y. Jing, Y. Xin, Y. Guo, X. Liu and Y. Wang, Chin. J. Catal., 2019, 40, 1168–1177.
815. M. Decostanzi, R. Auvergne, B. Boutevin and S. Caillol, Green Chem., 2019, 21, 724–747.
816. H. Zang, K. Wang, M. Zhang, R. Xie, L. Wang and E. Y. X. Chen, Catal. Sci. Technol., 2018, 8, 1777–1798.
817. A. S. Amarasekara, L. H. Nguyen, N. C. Okorie and S. M. Jamal, Green Chem., 2017, 19, 1570–1575.
818. R. M. Cywar, L. Wang and E. Y. X. Chen, ACS Sustainable Chem. Eng., 2019, 7, 1980–1988.
819. J. F. Wilson and E. Y. X. Chen, ACS Sustainable Chem. Eng., 2019, 7, 7035–7046.
820. A. S. Amarasekara and C. D. Gutierrez Reyes, Renewable Energy, 2019, 136, 352–357.
821. A. Garcia-Ortiz, J. D. Vidal, M. J. Climent, P. Concepcion, A. Corma and S. Iborra, ACS Sustainable Chem. Eng., 2019, 7, 6243–6250.
822. H. K. Yuan, J. P. Li, F. Z. Su, Z. Yan, B. T. Kusema, S. Streiff, Y. J. Huang, M. Pera-Titus and F. Shi, ACS Omega, 2019, 4, 2510–2516.
823. M. M. Zhu, L. Tao, Q. Zhang, J. Dong, Y. M. Liu, H. Y. He and Y. Cao, Green Chem., 2017, 19, 3880–3887.
824. R. F. A. Gomes, J. A. S. Coelho and C. A. M. Afonso, ChemSusChem, 2019, 12, 420–425.
825. A. Petri, G. Masia and O. Piccolo, Catal. Commun., 2018, 114, 15–18.
826. Q. Meng, D. Cao, G. Zhao, C. Qiu, X. Liu, X. Wen, Y. Zhu and Y. Li, Appl. Catal., B, 2017, 212, 15–22.
827. S. Zhang, X. Yang, K. Zheng, R. Xiao, Q. Hou, B. Liu, M. Ju and L. Liu, Appl. Catal., A, 2019, 581, 103–110.
828. H. Xie, T. Qi, Y. J. Lyu, J. F. Zhang, Z. B. Si, L. J. Liu, L. F. Zhu, H. Q. Yang and C. W. Hu, Phys. Chem. Chem. Phys., 2019, 21, 3795–3804.
829. M. Chatterjee, T. Ishizaka and H. Kawanami, Green Chem., 2018, 20, 2345–2355.
830. K. S. Kozlov, L. V. Romashov and V. P. Ananikov, Green Chem., 2019, 21, 3464–3468.
831. J. Li, Y. Jing, C. Liu and D. Zhang, New J. Chem., 2017, 41, 8714–8720.
832. H. C. Genuino, H. H. Van De Bovenkamp, E. Wilbers, J. G. M. Winkelman, A. Goryachev, J. P. Hofmann, E. J. M. Hensen, B. M. Weckhuysen, P. C. A. Bruijnincx and H. J. Heeres, ACS Sustainable Chem. Eng., 2020, 8, 5903–5919.
833. S. Kang, J. Fu and G. Zhang, Renewable Sustainable Energy Rev., 2018, 94, 340–362.
834. D. Ding, J. Wang, J. Xi, X. Liu, G. Lu and Y. Wang, Green Chem., 2014, 16, 3846–3853.
835. D. Ding, J. Xi, J. Wang, X. Liu, G. Lu and Y. Wang, Green Chem., 2015, 17, 4037–4044.
836. A. Seretis, P. Diamantopoulou, I. Thanou, P. Tzevelekidis, C. Fakas, P. Lilas and G. Papadogianakis, Front. Chem., 2020, 8, 221.
837. R. G. Weng, Z. H. Yu, J. Xiong and X. B. Lu, Green Chem., 2020, 22, 3013–3027.
838. F. Valentini, V. Kozell, C. Petrucci, A. Marrocchi, Y. Gu, D. Gelman and L. Vaccaro, Energy Environ. Sci., 2019, 12, 2646–2664.
839. P. Zhang, Y.-J. Guo, J. Chen, Y.-R. Zhao, J. Chang, H. Junge, M. Beller and Y. Li, Nat. Catal., 2018, 1, 332–338.
840. M. A. Lăcătuş, L. C. Bencze, M. I. Tosa, C. Paizs and F. D. Irimie, ACS Sustainable Chem. Eng., 2018, 6, 11353–11359.
841. M. A. Lacatus, A. J. Dudu, L. C. Bencze, G. Katona, F. D. Irimie, C. Paizs and M. I. Tosa, ACS Sustainable Chem. Eng., 2020, 8, 1611–1617.
842. K. S. Arias, J. M. Carceller, M. J. Climent, A. Corma and S. Iborra, ChemSusChem, 2020, 13, 1864–1875.
843. E. J. Garcia-Suarez, D. Paolicchi, H. Li, J. He, S. Yang, A. Riisager and S. Saravanamurugan, Appl. Catal., A, 2019, 569, 170–174.
844. M. Han, X. Liu, X. Zhang, Y. Pang, P. Xu, J. Guo, Y. Liu, S. Zhang and S. Ji, Green Chem., 2017, 19, 722–728.
845. L. V. Romashov and V. P. Ananikov, Chem. – Asian J., 2017, 12, 2652–2655.
846. C. Li, L. Wang, M. Wang, B. Liu, X. Liu and D. Cui, Angew. Chem., Int. Ed., 2019, 58, 11434–11438.
847. J. Saska, Z. Li, A. L. Otsuki, J. Wei, J. C. Fettinger and M. Mascal, Angew. Chem., Int. Ed., 2019, 58, 17293–17296.
848. W. Fan, Y. Queneau and F. Popowycz, Green Chem., 2018, 20, 485–492.
849. G. D. Wen, Q. Q. Gu, Y. F. Liu, R. Schlogl, C. X. Wang, Z. J. Tian and D. S. Su, Angew. Chem., Int. Ed., 2018, 57, 16898–16902.
850. Z. Ma, H. Hu, Z. Sun, W. Fang, J. Zhang, L. Yang, Y. Zhang and L. Wang, ChemSusChem, 2017, 10, 1669–1674.
851. S. Kunnikuruvan and N. N. Nair, ACS Catal., 2019, 9, 7250–7263.
852. K. R. Enslow and A. T. Bell, Catal. Sci. Technol., 2015, 5, 2839–2847.
853. H. Wang, Y. Duan, Q. Zhang and B. Yang, ChemSusChem, 2018, 11, 2562–2568.
854. C. Xu, M. Nasrollahzadeh, M. Selva, Z. Issaabadi and R. Luque, Chem. Soc. Rev., 2019, 48, 4791–4822.
855. H. Li, Z. Fang, R. L. Smith and S. Yang, Prog. Energy Combust. Sci., 2016, 55, 98–194.
856. K. M. H. Mohammed, A. Chutia, J. Callison, P. P. Wells, E. K. Gibson, A. M. Beale, C. R. A. Catlow and R. Raja, J. Mater. Chem. A, 2016, 4, 5706–5712.
857. Y. G. Kolyagin, A. V. Yakimov, S. Tolborg, P. N. R. Vennestrøm and I. I. Ivanova, J. Phys. Chem. Lett., 2016, 7, 1249–1253.
858. M. Zhang, M. Wang, B. Xu and D. Ma, Joule, 2019, 3, 2876–2883.
859. C. Mondelli, G. Gozaydin, N. Yan and J. Perez-Ramirez, Chem. Soc. Rev., 2020, 8, 11204–11214.
860. J. Kang, S. He, W. Zhou, Z. Shen, Y. Li, M. Chen, Q. Zhang and Y. Wang, Nat. Commun., 2020, 11, 827.
861. F. Rudroff, M. D. Mihovilovic, H. Gröger, R. Snajdrova, H. Iding and U. T. Bornscheuer, Nat. Catal., 2018, 1, 12–22.
862. H. Chen, F. Dong and S. D. Minteer, Nat. Catal., 2020, 3, 225–244.
863. Z. Cheng, B. Saha and D. G. Vlachos, ChemSusChem, 2018, 11, 3609–3617.
864. J. B. Zimmerman, P. T. Anastas, H. C. Erythropel and W. Leitner, Science, 2020, 367, 397–400.

---