Recent advances in heterogeneous catalysts for biocrude hydrodeoxygenation

Abstract

Hydrodeoxygenation (HDO) of biocrude into chemicals and transportation fuels represents a promising avenue for the sustainable utilization of biomass-derived biocrude oil, obtained through pyrolysis or liquefaction. Catalysts play a pivotal role in this process, providing active metal sites for hydrogenation and hydrogenolysis, alongside acid sites for ring-opening, cracking, and C–O bond cleavage. Despite its potential, previous studies have often reported low HDO rates, leading to rapid catalyst deactivation and the formation of undesirable byproducts. Thus, the careful selection of catalysts that achieve an optimal balance between metal and acid functionality is critical. This review systematically examines the properties of biocrude produced by various techniques and the catalysts used in HDO of biocrude and its model compounds. Particular attention is given to the roles of sulfided metals, noble metals, non-noble metals as catalysts as well as various supports in HDO reactions. The influence of catalyst characteristics, including metal particle size, acid type and strength, and support structure, on HDO activity and product distribution is thoroughly analyzed. Additionally, factors contributing to catalyst deactivation are discussed. Finally, the review addresses current technical challenges and offers future perspectives on the development of catalysts with improved HDO activity and stability.

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Penghui Yan

Penghui Yan obtained his MSc from University of Chinese Academy of Sciences in 2013 and PhD degree in Chemical Engineering from the University of Newcastle in 2020. He worked at Shaanxi Yanchang Petroleum Group in 2013–2014, contributing to the pilot projects involving fast pyrolysis of coal and biomass. Currently, he is a research fellow at the University of Queensland. In the past 10 years, he has been engaged in the field of heterogeneous catalysis, with a specific focus on renewable energy. His research interests encompass natural zeolite utilisation, CO2 hydrogenation, biomass catalytic pyrolysis, and biofuel upgrading.

Eric M. Kennedy

Eric Kennedy obtained his BSc (Pure and Applied Chemistry) with 1st Class Honours (1985) and PhD in Physical Chemistry (1989) from the University of NSW. He was a Research Fellow at Macquarie University before moving to the USA as a Research Fellow at Texas A&M and Yale. He joined the Department of Chemical Engineering at The University of Newcastle in 1994 and became a professor in 2006. His research focuses on process safety, environmental protection, and energy-related projects, including CO2 storage via mineral carbonation, non-destructive treatment of fluorinated gases, biomass conversion, plasma-based methane utilisation, and polyfluoroalklysubstances (PFAS) decomposition.

Michael Stockenhuber

Michael Stockenhuber did his PhD at the Institute of Physical Chemistry TU Vienna which he was awarded with distinction in 1994 (Dr Techn.) He is now full Professor of Chemical Engineering at the University of Newcastle, Australia. Prof Stockenhuber is president of the Australian Catalysis Society and was awarded the Catalysis Development Excellence Award by APACS in 2023. His main research interest is heterogeneous catalysis with a special emphasis on structure-function relationships and process scaleup in particular for environmental catalysis applications. He is an expert in the use of spectroscopy using synchrotron radiation, XPS, IR and in situ techniques.

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Green foundation

1. This review comprehensively examines the latest advancements in designing and developing heterogeneous catalysts for hydrodeoxygenation (HDO) of biocrude oil. Key topics include the role of bifunctional catalysts, novel supports, and strategies to enhance catalyst activity and stability while addressing environmental and economic concerns.

2. HDO of biocrude oil is vital for converting renewable biomass into sustainable, oxygen-free fuels and chemicals, reducing reliance on fossil fuels. This review connects laboratory-scale innovations to industrial applications, offering practical solutions to global energy and environmental challenges.

3. Advancing green chemistry in this field requires the development of cost-effective, earth-abundant catalysts and the integration of renewable hydrogen sources. Research should also focus on scalable processes operating under mild conditions to improve energy efficiency and sustainability. Insights provided here will guide future catalyst design and process optimization.

1. Introduction

Fossil fuels, including coal, natural gas, and petroleum, remain the cornerstone of global energy consumption, accounting for 81.5% of the world's energy use in 2023.¹ However, the increasing reliance on these non-renewable resources poses significant challenges, including the depletion of fossil fuel reserves and environmental concerns, particularly the emission of CO₂ during combustion.² To address these issues, substantial efforts have been dedicated to the development of alternative, renewable energy sources capable of playing a critical role in future energy systems.

Biomass has emerged as a promising candidate for renewable liquid energy carriers due to its widespread availability and potential for carbon neutrality. Pyrolysis^3–8 and liquefaction^9,10 are the primary methods for converting biomass into organic liquid products known as biocrude oil. However, biocrude oil mostly is not a suitable fuel in its raw form, as it possesses undesirable properties such as high oxygen content, corrosiveness, high viscosity, and low heating value.¹¹ Consequently, an upgrading process is necessary to remove oxygen and transform biocrude oil into hydrocarbons or green chemicals suitable for further use.

Hydrodeoxygenation (HDO) has emerged as a promising technology for upgrading biocrude oil, enabling the production of valuable chemicals, gasoline, diesel, and aviation fuel under relatively mild conditions (150–400 °C, atmospheric pressure to 5 MPa).¹² During HDO, various reactions including hydrogenation, decarboxylation, decyclization, dehydration, hydrogenolysis, demethoxylation, transalkylation, ring-opening, and hydrocracking occur, leading to the removal of oxygen atoms such as CO₂, CO, and H₂O. Simultaneously, condensed aromatics undergo hydrogenation, ring-opening, and cracking, yielding lighter liquid hydrocarbons.

Bifunctional catalysts are critical in the HDO process, optimizing the yield of target products under suitable conditions. As summarized in Fig. 1, both noble metals (e.g., Ru,^13,14 Pt,¹⁵ Pd,¹⁶ Rh¹⁷) and non-noble metals (e.g., Ni,^18,19 Fe,²⁰ Mo,²¹ Cu²²) have been extensively studied for biocrude oil upgrading. Supports such as carbon,²³ SiO₂,^24,25 metal oxides (e.g., Al₂O₃,²⁶ MgO,²⁷ Nb₂O₅,^28,29 and mixed Nb₂O₅-ZrO₂³⁰), synthetic zeolites (e.g., ZSM-5,^31–33 Y,³⁴ BEA,^35,36 MOR,³⁷ MCM-41,^38–40 SBA-15⁴¹), and natural zeolites^42,43 are widely used to provide large surface areas for active metal species and offer acidic sites that promote reactions like cracking, deoxygenation, ring-opening, and isomerization.

Fig. 1 Overview of metals and supports used in the hydrodeoxygenation (HDO) of biocrude and its model compounds, focusing on the production of benzene, toluene and xylene (BTX) and cycloalkanes.

Numerous studies have explored the upgrading of biocrude oil, with particular attention to bifunctional catalysts,^44–46 nickel metal,⁴⁷ operating parameters,⁴⁸ and reaction mechanisms.⁴⁹ However, to our knowledge, the specific relationships between biocrude oil model compounds and catalyst properties (such as pore size and metal particle size) have not been thoroughly examined. This review aims to fill this gap by providing a comprehensive analysis of catalysts used for biocrude oil and its model compounds. We also explore in detail the effects of zeolite structure, acidity and metal particle size on catalyst HDO activity and product distribution. Additionally, we address key challenges, limitations, and current gaps in catalyst development for biocrude upgrading, offering insights for the advancement of more efficient catalysts applicable in both scientific research and industrial settings.

2. Biocrude properties and its upgrading approaches

Various types of biomass feedstocks, including sugars, vegetables oils and lignocellulosic biomass, are available for the production of biofuels.⁵⁰ Bioethanol and biodiesel, which are produced from edible feedstock (sugar and vegetable oil) and known as first-generation biofuels, have seen an increase in production in some countries, attributing to the direct blending with conventional fossil fuels.⁵¹ The development of first-generation biofuel is restricted due to the competition with food supply and the limited feedstock source. An increasing number of studies have thus focused on second-generation biocrude oil, which are derived from pyrolysis or liquefaction of lignocellulosic biomass.^52,53 A comparison of first-generation biofuel and second-generation biocrude is shown in Fig. 2.

Fig. 2 A comparison of first-generation biofuel and second-generation biocrude.

Biocrude is a dark brown, oxygenated organic liquid produced through the depolymerization and fragmentation of the three primary biomass components: hemicellulose, cellulose, and lignin. This process occurs at high temperatures and/or via hydrothermal liquefaction under high pressure.¹¹ Biocrude contains over 300 oxygenated compounds categorized by their predominant functional groups, including phenols, acids, esters, linear ketones, cyclic ketones, furans, cyclic hydrocarbons, and aromatics.^54–56 The structures of the main compounds in biocrude are illustrated in Fig. 3. Among these, phenolic compounds are particularly abundant, accounting for up to 50 wt%. However, these phenolic species tend to polymerize, leading to low product stability, which in turn increases transportation and downstream processing costs. The phenolic components (e.g., phenol, guaiacol, cresols, and catechol) are primarily derived from the decomposition of lignin.

Fig. 3 Major components of biocrude oil derived from pyrolysis or liquefaction of (1) hemicellulose, cellulose, and (2) lignin components of lignocellulosic biomass, the R can be H or methyl. This figure is drawn on the basis of information from ref. 71 and 72.

Although biocrude oil can be produced through relatively simple and efficient methods, its properties differ markedly from those of petroleum-derived oil, limiting its direct use as a transportation fuel in engines, combustion boilers, and similar equipment.⁶⁷ The most significant difference between petroleum and biocrude is the latter's high oxygen content.^54,68–70 Biocrude typically contains 20–40% oxygen, which imparts several undesirable properties, including high viscosity, high acidity, low heating value, corrosiveness, and low stability.⁶⁶ Despite these challenges, biocrude also offers several advantageous characteristics, such as high lubricity, low sulfur content, biodegradability compared to fossil fuels.

Additionally, biocrude oil contains a substantial amount of water (15–35 wt%), which originates from the moisture present in the biomass feedstock and is further produced during dehydration reactions. The oxygen content in biocrude varies depending on the feedstock source and processing conditions, as summarized in Table 1. Given the complexity of biocrude oil, various model compounds (e.g., guaiacol,⁷³ anisole,⁷⁴ phenols,⁷⁵ cresol,⁷⁶ furfural,⁷⁷ benzofuran,⁷⁸ vanillin,⁷⁹ acetic acid⁸⁰) are used to elucidate reaction mechanisms and assess catalyst activity, selectivity, and stability.

Table 1 Composition of biocrude obtained from varying feedstocks

Feedstock	C (%)	H (%)	O (%)	N (%)	Conversion technique	Ref.
Euphorbia rigida	75.5	11.1	10.6	2.8	Pyrolysis	57
Sunflower pressed bagasse	74.5	9.9	10.3	5.3	Pyrolysis	57
Hazelnut shells	64.9	6.5	27.9	0.7	Pyrolysis	57
Cashew nutshell	76.4	10.5	12.9	<0.2	Pyrolysis	58
Rapeseed cake	73.7	10.7	10.5	4.6	Pyrolysis	59
Olive husks	66.5	8.1	21.9	3.2	Pyrolysis	60
Soybean	67.9	7.8	13.5	10.8	Pyrolysis	61
Apricot pulp	61.5	7.8	29.0	1.7	Pyrolysis	62
Peach pulp	59.6	7.9	32.0	0.6	Pyrolysis	63
Algae	76.1	9.7	8.3	5.3	Liquefaction	64
Corn stover	76.9	8.9	11.7	1.9	Pyrolysis	65
Forest residues	53.7	7.1	39.1	0.1	Pyrolysis	66
Forest residues	72.0	6.8	20.7	0.5	Catalytic pyrolysis	66

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The high oxygen content in biocrude oil is a significant limitation for its use as a transportation fuel due to the resulting poor thermal and chemical stability, high viscosity, and corrosiveness.⁸¹ Various studies have focused on upgrading the biocrude oil to improve its quality, with a primary emphasis on reducing its oxygen content.^44,82–84

Several methods are available for upgrading biocrude, including hydrodeoxygenation,^85–89 catalytic cracking,^64,90,91 thermal cracking,⁹¹ esterification,^2,92 and molecular distillation.^65,93 Among these, HDO is the most effective method for upgrading pyrolysis biocrude oils into high-value oil and chemicals, as it uses heterogeneous catalysts and hydrogen gas to remove oxygen atoms and convert aromatic rings into alkanes under relatively mild conditions (150–400 °C). This process minimizes coke formation and reduces the risk of catalyst deactivation. In contrast, catalytic cracking, which is more prone to high coke formation, is primarily used for first-generation biofuel that contain fewer aromatic and condensed-aromatic compounds. During HDO, C–O bonds are cleaved via hydrogenolysis, with oxygen removed as water, CO, CO₂, and methanol. Additionally, ring-opening, hydrocracking and aromatic ring saturation through hydrogenation can occur.^94,95 The typical reactions involved in the hydrodeoxygenation process are illustrated in Fig. 4.

Fig. 4 Typical reactions in hydrodeoxygenation process.

3. Hydrodeoxygenation catalysts

Since HDO reactions involve deoxygenation, hydrogenation and cracking, bifunctional catalysts are commonly employed, which exhibited enhanced HDO activity compared to the monofunctional catalysts. Hellinger⁹⁶ compared a bifunctional catalyst (Pt/HMFI-90) with a monofunctional catalyst (Pt/SiO₂) for the HDO of guaiacol in flow reactor. A high amount of 2-methoxycyclohexanol was produced over Pt/SiO₂ catalyst, indicating strong hydrogenation but weak deoxygenation activity, likely due to the absence of acid sites. In contrast, Pt/H-MFI demonstrated high deoxygenation activity, with cyclohexane as the main product. Similarly, bifunctional (Pt/H-BEA) and monofunctional (HBEA, Pt/SiO₂) catalysts were tested for the HDO of anisole.⁹⁷ The acidic sites provided by HBEA catalyzed the transalkylation from anisole to the phenolic ring, resulting in phenol, cresols, and xylenols as the primary products. The metal sites facilitate demethylation, hydrodeoxygenation, and hydrogenation in sequence, yielding phenol, benzene, and cyclohexane. The bifunctional Pt/H-BEA catalyst promoted both hydrodeoxygenation and transalkylation more effectively than the monofunctional catalysts, producing a high yield of benzene, toluene, and xylenes. Unsupported molybdenum nitride catalysts were also studied for the HDO of guaiacol in a batch reactor.⁹⁸ The É£-Mo₂N catalyst demonstrated high activity and selectivity for phenol, suggesting a direct demethoxylation pathway, while other Mo catalysts (β-Mo₂N_0.78, MoO₂, and Mo) showed a reduced HDO activity.

The bifunctional catalysts, which typically include materials like Al₂O₃, TiO₂, ZrO₂, zeolites, and carbon supported by sulfided metals, reduced non-noble metals, or noble metals, feature metal sites for hydrogenation/dehydrogenation and acid sites for C–O bond cleavage and ring-opening. The reaction pathways and product distributions in HDO reactions vary depending on the catalyst used and the reaction conditions. Generally, noble metal-based catalysts exhibit high hydrogenation activity, leading to the initial saturation of aromatic rings during the HDO of model compounds, followed by deoxygenation reactions. This results in the production of saturated hydrocarbons as the main products. Conversely, sulfided metal catalysts, which have lower hydrogenation activity, typically initiate the reaction with deoxygenation, resulting in aromatic compounds as the primary products.^99,100

3.1. Sulfided metals

Sulfided CoMo and NiMo catalysts supported on Al₂O₃ have been extensively used in oil refineries for the hydrotreatment of crude oil, aiding in the removal of oxygen, nitrogen and sulfur through hydrogenation, hydrodeoxygenation, hydrodesulfurization, hydrodenitrogenation, and hydrocracking reactions,^101,102 These sulfided catalysts, including sulfided CoMo/ZrO₂,¹⁰³ have also been employed in the HDO of biocrude oil and its model compounds. Studies by Hurff¹⁰⁴ observed that catechol and phenol were the primary products from the HDO of guaiacol over the sulfided CoMo/Al₂O₃ catalyst.

Bui¹⁰⁵ investigated the effect of cobalt on the HDO of guaiacol over Mo-based catalysts, finding that the direct deoxygenation (DDO) pathway was significantly enhanced with a CoMoS catalyst compared to a non-promoted MoS₂ catalyst. A similar promoting effect was observed in the HDO of 2-ethylphenol,¹⁰⁶ where cobalt facilitated hydrogenation and direct hydrogenolysis of 2-ethylphenol, while nickel predominantly promoted the hydrogenation of the aromatic ring. Commercially available sulfided CoMo/Al₂O₃ catalysts have been extensively studied for the HDO of model compounds.²¹ These studies revealed that HDO, demethylation, and hydrogenation occurred simultaneously within the proposed reaction network, though complete HDO of guaiacol was challenging, as phenol and cresols were the main products. Liu¹⁰⁷ reported on a MoS₂ monolayer catalyst doped with isolated Co atoms for the HDO of 2-methylphenol, achieving 98.4% toluene selectivity at 97.6% conversion at 180 °C in a batch reactor. The enhanced HDO activity was attributed to the increased number of Co–S–Mo interfacial sites.

Additionally, it has been suggested that oxygen elimination reactions in the HDO of guaiacol over CoMo/Al₂O₃ and NiMo/Al₂O₃ catalysts are inhibited by the presence of water and ammonia.^108–111 Sulfur detachment from the surface of these sulfided catalysts during HDO reactions leads to catalyst deactivation. The addition of sulfiding agents to maintain catalyst activity presents significant environmental concerns.^104,112,113 Moreover, H₂S, as a sulfiding agent, has been observed to suppress direct hydrogenolysis and hydrogenation reactions due to the competitive adsorption of phenol and H₂S.^112,113

Hong¹¹⁴ utilized sulfided Ni/W/TiO₂ (anatase) catalysts for the guaiacol HDO, highlighting that the sulfidation degree of the tungstate layer, its molecular structure, and the number and intrinsic activity of sulfur sites were critical factors in determining the HDO activity of these catalysts. Furthermore, nickel was found to have a more pronounced promoting effect than cobalt in the HDO of guaiacol, due to enhanced sulfidation of the tungstate layer facilitated by Ni. A 16% yield of cyclohexane was observed over the sulfided 2 wt% Ni–12 wt% W/TiO₂ catalyst.

The influence of phosphorus content on guaiacol conversion over sulfided Mo/γ-Al₂O₃ catalysts was studied at 300 °C and 5 MPa H₂ in a batch reactor.¹¹⁵ Results showed that guaiacol conversion initially increased with rising phosphorus content but decreased once the phosphorus content exceeded 1.0 wt%. This promoting effect was attributed to the formation of two-dimensional MoS₂ induced by PO₄³⁻ coverage when the phosphorus content was below 1 wt%. In contrast, higher phosphorus loadings (>1.0 wt%) led to reduced guaiacol conversion due to the loss of active sites and the formation of three-dimensional MoS₂.

The challenges discussed above are further amplified in the HDO of biocrude oil. The large-scale use of these sulfided catalysts in refineries poses significant challenges, driving research efforts toward developing non-sulfided catalysts for upgrading biocrude oil.

3.2. Non-noble metals

Due to the various drawbacks associated with sulfided catalysts, the development of reduced catalysts based on metals such as Ni, Fe, Mo, W, Co, and Cu has gained significant interest in recent decades.¹¹⁶Table 2 provides a summary of several non-noble metal catalysts used in HDO reactions.

Table 2 Summary of biocrude compound upgrading using non-noble catalysts

Catalyst	Feed	Conversion (%)	Main products	P (MPa)	T (°C)	Reactor type	Ref.
Co/Al2O3	Phenol	100	38% cyclohexane	3	300	Batch	123
Co/K-Al2O3	Guaiacol	99.9	96.7% cyclohexanol	1	220	Batch	117
Co/CeO2	Guaiacol	97.1	91.7% cyclohexanol	2	220	Flow	120
Co/TiO2	Eugenol	100	99.9% propylcyclohexanol	1	200	Batch	119
Co/ZSM-5	Phenol	6	58% cyclohexanone, 19% cyclohexene, 23% cyclohexane	5	250	Batch	134
Cu/PSNT	Phenol	100	97.8% cyclohexane	5	230	Batch	22
Cu/ZnO-Al2O3	5-HMF	100	90.1% 2,5-dimethylfuran	1.2	180	Batch	126
Cu/ZSM-5	Phenol	3	38% cyclohexene, 30% cyclohexane	5	250	Batch	134
Co@Cu/3CoAlOx	5-HMF	100	98.5% 2,5-dimethylfura	1.5	180	Batch	127
Fe/BEA	Guaiacol	1.2	Anisole, toluene, cresol	4	230	Flow	135
Fe-CeO2	Guaiacol	52	63% phenol	0.1	400	Flow	136
Fe/CNT	Guaiacol	17.2	40.4% phenol	3	300	Flow	137
Fe-SiO2	Guaiacol	74	38% BTX	0.9	300	Flow	138
Fe/SiO2	m-Cresol	8.8	60.2% toluene, 39.8% transalkylation products	atm.	300	Flow	20
Ni	Phenol	12	82% cyclohexanol, 18% cyclohexanone	5	250	Batch	134
Ni/Al2O3	Anisole	66.1	72.5% benzene, 14.9% cyclohexane	1	300	Flow	125
Ni/Al2O3	Guaiacol	75.1	41.1% cyclohexanol, 39.1% 2-methoxycyclohexanol	4	200	Batch	139
Ni/CNT	Guaiacol	73.0	53.0% cyclohexane	3	300	Flow	137
Ni/SiO2	Phenol	100	90% cyclohexanol, 4% cyclohexane	10	275	Batch	129
Ni/SiO2	Phenol	—	Benzene	atm.	300	Flow	140
Ni/SiO2	m-Cresol	16.2	33.3% 3-methylcyclohexanone, 14.2% toluene	atm.	300	Flow	20
Ni/ZrO2	Phenol	100	97% cyclohexanol	3	150	Batch	141
Ni/β-Mo2C	Guaiacol	100	94.1% cyclohexane	4	260	Batch	142
Ni/MgAlOx	Guaiacol	87.9	74.4% cyclohexanol	4	200	Batch	139
Ni/BEA-DP	Microalgae oil	100	100% liquid alkanes	4	260	Batch	87
Ni/BEA	Guaiacol	8.8	28.4% cyclohexane	4	230	Flow	36
Ni/HZSM-5	Phenol	79.0	81% cyclohexane	5	200	Batch	143
Ni/ZSM-5	Phenol	97.0	88% cyclohexane	5	250	Batch	134
Ni/Al2O3-HZSM-5	Phenol	98.0	98% cyclohexane	5	200	Batch	143
NiCu/Al2O3	Anisole	73.8	43.5% benzene, 32.8% cyclohexane	1	300	Flow	125
NiCu/ZSM-5	Phenol	48.0	76% cyclohexane	5	250	Batch	134
NiCo/ZSM-5	Phenol	100	91% cyclohexane	5	250	Batch	134
NiFe/SiO2	m-Cresol	13.7	52.6% toluene, 45.3% transalkylation products	atm.	300	Flow	20
NiFe/BEA	Guaiacol	14.0	35.0% cyclohexane	4	230	Flow	36
NiFe/CNT	Guaiacol	96.8	83.4% cyclohexane	3	300	Flow	137
NiFe/MgAlOx	Guaiacol	99.0	82.7% cyclohexanol	4	200	Batch	139
NiMo/ZrO2	Guaiacol	99.9	75% phenol	2	325	Batch	100
NiMo/BEA	Guaiacol	11.9	28.5% cyclohexane	4	230	Flow	36
Ni/WO3-ZrO2	Phenol	100	98% cyclohexane	3	150	Batch	141
NiW/BEA	Guaiacol	4.9	22.4% cyclohexane	4	230	Flow	36
ReCo/Al2O3	Phenol	100	60% cyclohexane	3	300	Batch	123

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Co-based catalysts have also been employed in HDO reactions.^117,118 The Co/TiO₂ catalyst promoted demethoxylation and aromatic ring hydrogenation, yielding 100% selectivity for propylcyclohexanol from the HDO of eugenol at 1 MPa and 200 °C.¹¹⁹ CeO₂ supported Co catalysts have also been used for the HDO of guaiacol, with cyclohexanol observed as the main product at 2 MPa and 160–220 °C.¹²⁰ Cyclohexane was not detected over CeO₂ and TiO₂-supported Co catalysts, likely due to the low concentration of Brønsted acid sites (BASs), which play a key role in the deoxygenation of hydroxyl groups.

Co catalysts modified with promoters like Mo show enhanced performance.^121,122 The MoO_x-decorated Co structures exhibit excellent catalytic activity for the hydrodeoxygenation (HDO) of biomass-derived platform molecules. This improvement is attributed to the adsorption of C=O bonds promoted by Co^δ+ species. Additionally, the MoO_x overlayer reduces byproduct formation by shielding the Co⁰ sites (Fig. 5A and B). Ghampson¹²³ studied phenol HDO using metal oxides (Al₂O₃, SiO₂-Al₂O₃, ZrO₂, TiO₂) supported Co–Re catalysts in a batch reactor. The addition of Re to Co catalysts had a beneficial effect on HDO activity, attributed to improved reducibility and the addition of hydrogenation sites, with the beneficial effect being most pronounced over the TiO₂ support.

Fig. 5 (A) Reaction pathways for hydrodeoxygenation of LA. (B) Schematic representation for the preparation of Co–MoO_x catalysts.¹²¹ (C) Copper phyllosilicate nanotube catalysts for chemosynthesis of cyclohexane via hydrodeoxygenation of phenol.²² (D) HDO of guaiacol over BEA supported Ni, Ni–Mo, Ni–Fe and Ni–W catalysts.³⁶ (E) HDO of cresol over Ni–Mo/SiO₂ catalyst.¹²⁸

Zhang¹²⁴ studied the influence of Cu on Ni catalysts supported by ZrO₂-SiO₂ in the HDO of guaiacol to hydrocarbons. The addition of Cu significantly improved the acidic properties and enhanced the reduction of NiO, resulting in a Ni-Cu/ZrO₂-SiO₂ catalyst that exhibited high selectivity toward methyl-substituted compounds such as methylcyclohexane and toluene. Ardiyanti¹²⁵ investigated the influence of the Ni/Cu ratio on the HDO of biocrude oil and its model compound (anisole) over Al₂O₃-supported NiCu bimetallic catalysts. The Al₂O₃-supported Ni/Cu catalyst, with a Ni/Cu weight ratio of 8, showed high hydrogenation activity for both biocrude oil and anisole, and it also displayed the lowest levels of leaching and coking among the prepared catalysts. While most studies used Cu as a promoter, some studies have also explored its role as the primary active site in HDO reactions.^126,127 For example, Wang²² utilized copper phyllosilicate nanotubes for the HDO of phenol to produce cyclohexane in a batch reactor (as shown in Fig. 5C). The process achieved a cyclohexane yield of 97.8% with complete conversion. The Cu⁺ species facilitate the adsorption of phenol and cyclohexanol, while the Cu⁰ species facilitate the hydrogenation process.

Metallic Ni is the most widely used metal in HDO reactions due to its low cost, strong hydrogenation activity, and wide availability.^{19,35,39,42,86,89,129} Ni-based catalysts exhibit superior HDO activity for phenolic compounds (e.g., anisole, phenols, guaiacols) and biocrude oil, promoting the formation of saturated hydrocarbons (e.g., cycloalkanes) at higher pressures^130,131 and the production of aromatics monomers at lower pressure.^132,133

The influence of promoters (Mo, W, and Ta) on Ni catalysts supported by Al₂O₃, ZrO₂, TiO₂, and SiO₂ in the HDO of guaiacol was investigated.¹⁰⁰ The ZrO₂-supported Ni–Mo catalyst exhibited higher activity and selectivity for desired products among the prepared catalysts. NiO was fully reduced to metallic Ni, while the MoO₃ was partially reduced to Mo₄⁺ species, which were regarded as Lewis acid sites. We further explored the impact of promoters (Mo, W, Fe) on Ni/BEA catalysts for guaiacol HDO using both experimental methods and DFT calculations.³⁶ As depicted in Fig. 5D, Mo improved the dispersion of Ni species, thereby enhancing the cycloalkane formation rate (14.1 × 10⁻⁴ mol min⁻¹ g⁻¹) compared to Ni/BEA (9.6 × 10⁻⁴ mol min⁻¹ g⁻¹), while the addition of W reduced the hydrogenation activity of Ni/BEA, resulting in a lower HDO rate (4.4 × 10⁻⁴ mol min⁻¹ g⁻¹). Yang¹²⁸ used a Ni–Mo/SiO₂ catalyst for m-cresol HDO, which showed high toluene selectivity at reaction temperatures of 250–350 °C and 1 atm of H₂. The enhanced effect was attributed to the decoration of the Ni surface by reduced MoO_x species (as displayed in Fig. 5E).

Monometallic Fe catalysts have also been used for guaiacol HDO, though they show low activity for aromatic ring hydrogenation. For instance, a 38% BTX yield at 74% guaiacol conversion was achieved over Fe/SiO₂,¹³⁸ and a 62% phenol selectivity at 52% guaiacol conversion was observed over Fe/CeO₂,¹³⁶ indicating lower HDO activity for Fe based catalysts. The oxygen functional groups could be readily adsorbed on Fe species due to its oxophilicity.¹⁴⁴ The strong hydrogenolysis activity of the Fe species, however, makes them excellent promoters for HDO reactions. Nie²⁰ compared the activity of SiO₂ supported bimetallic Ni–Fe and monometallic Ni catalysts using the HDO of m-cresol as a model reaction. The monometallic Ni catalyst exhibited higher 2-methylcyclohexanol selectivity compared to the bimetallic Ni–Fe catalyst, where toluene became the main product. These results align with our studies, where we observed that adding Fe species improved the HDO activity of Ni/BEA catalysts due to the strong hydrogenolysis activity of Fe species.¹³⁵ Moreover, when the catalyst was initially impregnated with Fe, the metal-support interaction was relatively weak, promoting the formation of Ni–Fe species. The Ni–(Fe/BEA) catalyst exhibited a significantly higher turnover frequency (TOF) of 11.2 min⁻¹ in guaiacol HDO compared to the Fe–(Ni/BEA) catalyst (7.0 min⁻¹). It is also determined that the Ni/Fe mass ratio of 3.3 exhibited the highest cyclohexane formation rate in guaiacol HDO.

3.3. Noble metals

Noble metal catalysts, known for their high aromatic hydrogenation activity, are promising alternatives for HDO reactions. Catalysts based on Pt,^145,146 Pd,^15,147,148 Ru^14,149 and Rh¹⁵⁰ supported on catalytic frameworks such as metal oxides, silica, zeolites, and activated carbon have been extensively investigated in recent HDO studies. A summary of the literature on HDO reactions using noble metal-based catalysts is presented in Table 3.

Table 3 Summary of biocrude compound upgrading using noble metal catalysts

Catalyst	Feed	Conversion (%)	Main products	T (°C)	P (MPa)	Reactor	Ref.
Au/TiO2	Guaiacol	57.8	71% phenol, 8% cresol	280	4	Flow	153
Au-Rh/TiO2	Guaiacol	94.0	61% cyclohexane	280	4	Flow	153
Ir/ZSM-5	Phenol	60.0	96% cyclohexane	200	3	Batch	154
Pd/C, HZSM-5	Phenol/methanol	99.0	80% cyclohexyl methyl ether	200	4	Batch	23
Pd/C,	Phenol, H3PO4, H2O	100	85% cyclohexane	200	5	Batch	155
Pd/C	Phenol–H2O	100	98% cyclohexanol	200	5	Batch	155
Pt/C	Phenol H3PO4, H2O	100	86% cyclohexane	200	5	Batch	155
Pt/Al2O3	m-Cresol	30.0	11% methylcyclohexane, 65% toluene, 20% 2-methylcyclohexanol	260	4.5	Flow	26
Pt–Co/Al2O3	m-Cresol	30.0	52.4% methylcyclohexane, 44.7% toluene	260	4.5	Flow	26
Pt–Ni/Al2O3	m-Cresol	30.0	43.2% methylcyclohexane, 51% toluene	260	4.5	Flow	26
Pt/ZSM-5	Phenol	60.0	60% cyclohexane, 32% cyclohexanol	200	3	Batch	154
Pt/TiO2	Guaiacol	70.0	44% cyclohexane	285	4	Flow	156
Pt–Ir/ZSM-5	Phenol	60.0	95% cyclohexane	200	3	Batch	154
Rh/C	Phenol H3PO4, H2O	100	92% cyclohexane	200	5	Batch	155
Pt-Mo/TiO2	Guaiacol	93.0	69% cyclohexane	285	4	Flow	156
Ru/C	Phenol H3PO4, H2O	100	88% cyclohexane	200	5	Batch	155
Ru/TiO2	Phenol	4.0	61% cyclohexanone, 20% cyclohexanol	300	4.5	Batch	149
Ru/TiO2 (uncalcined)	Phenol	12.0	85% benzene	300	4.5	Batch	149
Ru/H-BEA	Guaiacol	98.8	97.9% cyclohexane	140	4	Batch	14
Ru/H-ZSM-5	Guaiacol	98.9	48.6% cyclohexane, 51.4% 2-methoxycyclohexanol	140	4	Batch	14
Ru/MCM-41	Phenol	6.0	58% cyclohexanone	300	4.5	Batch	149
Ru@ITQ	Guaiacol	60.0	83% 2-methoxycyclohexanol	160	3	Batch	157
Ru@HMCM-22	Guaiacol	99.0	34% cyclohexane, 40% 2-methoxycyclohexanol	160	3	Batch	157
Rh/ZrO2	Guaiacol, tetradecane	—	cyclohexane	400	5	Batch	150

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Ru-based catalysts are particularly common HDO reactions.^149,151 For instance, the HDO of biocrude derived from the fast pyrolysis of lignocellulosic biomass was investigated using a carbon supported Ru catalyst in a batch reactor at 350 °C and 20 MPa H₂.¹⁵² Under these conditions, various reactions including decarboxylation, polymerization, HDO, methanation, demethylation, decarbonylation, and pyrolytic lignin depolymerization occurred. Product analysis revealed that the oxygen content decreased from 34 wt% to 13 wt%, with a corresponding increase in heating value from 24.2 to 35.5 MJ kg⁻¹. The optimal yield of desired deoxygenated products was achieved when the catalyst loading was equal to or less than 5.0 wt% Ru.

Lin¹⁵⁰ compared the catalytic activities of Rh-based, CoMoS, and NiMoS catalysts in the HDO of guaiacol using a batch reactor. The noble-metal catalyst exhibited a significantly different product distribution compared to the sulfided catalysts, with a notable increase in cyclohexane yield. In contrast, anisole and phenol were the main products over sulfided CoMo and NiMo catalysts at 350 °C. Additionally, the Rh-based catalyst demonstrated superior reactivity, producing a lower amount of coke compared to the sulfided catalysts.

The combination of noble and non-noble metals for HDO reactions has also been studied. He¹⁵⁶ investigated the promoting effect of Mo and Mg on the HDO of guaiacol over a TiO₂-supported Pt catalyst in a fixed-bed reactor. The incorporation of Mg or Mo improved the dispersion of Pt particles while reducing the surface acidity of the Pt/TiO₂ catalyst. Although both Mo and Mg additions increased guaiacol conversion, an improved cyclohexane yield was only observed with the Pt–Mo/TiO₂ catalyst. It is suggested that Mo species favor hydrogenolysis and suppress dehydration to cyclohexene by decreasing the concentration of acid sites and increasing the amount of active hydrogen species. Hong¹⁵⁸ studied the effect of the Pd/W mass ratio on the HDO of guaiacol over Pd/WO_x/γ-Al₂O₃ catalysts at 300 °C in a batch reactor. The highest HDO activity was achieved with 2 wt% Pd and 32 wt% tungsten loadings, resulting in an 88% yield of cyclohexane. It is proposed that guaiacol was hydrodeoxygenated via a bifunctional mechanism: (1) full hydrogenation of the aromatic ring over Pd sites and (2) deoxygenation of methoxy and hydroxyl groups by Pd-acid sites.

In addition to combining noble and non-noble metals, noble bimetals have also been used in HDO reactions, demonstrating enhanced activity.¹⁵⁴ TiO₂-supported Au, Rh, and Au–Rh catalysts were studied in the HDO of guaiacol in a fixed-bed reactor.¹⁵³ Au/TiO₂ exhibited stable activity and high selectivity toward phenol. The main product shifted from phenol to cyclohexane with the bimetallic Au–Rh catalyst, which may be attributed to a cooperative effect between the metals. Partial substitution of Pt with Ir (Pt-50-Ir50/ZSM-5) also results in an improved HDO rate (0.066 mol_Phenol mol_Metal⁻¹ s⁻¹) and higher selectivity toward oxygen-free products compared to the Ir/ZSM-5 (0.028 mol_Phenol mol_Metal⁻¹ s⁻¹) and Pt/ZSM-5 (0.054 mol_Phenol mol_Metal⁻¹ s⁻¹).¹⁵⁴ This enhancement is attributed to the surface enrichment of Ir species, which are more effective in dehydration reactions than Pt species.

In summary, noble metals such as Ru and Pd, along with metallic Ni-based catalysts, demonstrated strong hydrodeoxygenation activity due to their high selectivity towards hydrogenation products. Fe and Mo also proved to be effective promoters, with Fe particularly enhancing hydrogenolysis activity and promoting efficient adsorption of oxygen-containing functional groups. Given the complex composition of biocrude oil, using cost-effective catalysts with high metal loading, especially those with Ni, is essential. This approach not only facilitates the upgrading of phenolics, guaiacols, and heavy polyaromatics from biocrude oil but also helps to mitigate catalyst deactivation by enhanced hydrogen spillover.

4. Impact of catalyst metal particle size on hydrodeoxygenation: single atoms vs. nanoparticles

HDO is considered a structure-sensitive reaction, where the structure and particle size of metal species not only affect the catalytic activity but also alter the product distribution.^24,25 The size of metal particles plays a crucial role in determining the efficiency of the HDO reaction. The small Ni nanoparticles (2 nm) with highly coordinatively unsaturated Ni sites enhance C–O cleavage by facilitating the adsorption and stabilization of OH in the transition state. This leads to a higher yield of toluene in the hydrodeoxygenation (HDO) of cresol²⁴ (Fig. 6a).

Fig. 6 (a) Relationship between particle size and product distribution,²⁴ (b)turnover frequency for hydrogenation (TOF_Hyd) and deoxygenation (TOF_Deox) over Ni/SiO₂ catalysts as a function of metal dispersion, and the fraction of sites as a function of dispersion.²⁵ (c) HDO of anisole over Ni/BEA-DP with Ni clusters to produce aromatics and Ni/BEA-ORPH with small Ni nanoparticles to form cyclohexane.³⁵ (d) relationship of mean Ni particle size (d_Ni) and normalized cycloalkanes formation rate (TOF) in toluene hydrogenation; TOF_Ni: cycloalkane formation rate based on the mole of surface Ni (metal loading × dispersion), unit: min⁻¹, TOF-_{high T desorbed H2}: cycloalkanes formation rate based on the peak area of high-temperature desorbed H₂, unit: mol min⁻¹ × 10⁻⁴ per a.u.¹⁶² (e) Investigation of anisole HDO by in situ FTIR and TPST-MS.¹⁶⁰ (f) Single atoms for HDO of 2-HMF.¹⁶³

Mortensen²⁵ studied the effect of nickel particle size on the HDO of phenol over a Ni/SiO₂ catalyst in a batch reactor (Fig. 6b). The Ni/SiO₂ catalyst with larger metal particles (22 nm) displayed hydrogenation activity 85 times higher than that of the catalyst with smaller particles (5 nm). However, the bulk Ni catalyst's deoxygenation activity was 20 times lower than that of the catalyst with 5 nm particles. Similarly, Song⁸⁷ investigated the impact of Ni particle size on the HDO of microalgae oil using Ni/BEA catalysts prepared by different methods. The smallest Ni particles (2.5 nm), prepared by the deposition-precipitation method, showed the highest initial rate (4.6 g_oil g_cat⁻¹ h⁻¹) compared to larger Ni particles (24.7 nm) prepared by the impregnation method, which had a much lower rate (0.6 g_oil g_cat⁻¹ h⁻¹).

Teles¹⁵⁹ studied the HDO of cresol over different Ni particle sizes and concluded that Ni particles of 1 nm favor direct deoxygenation to produce toluene, while bulk Ni nanoparticles promote hydrogenation and hydrogenolysis, forming methylcyclohexanone and phenol. These findings align with our previous studies,^35,160,161 which showed that small nickel particles promote deoxygenation, while larger particles favor hydrogenation (Fig. 6c and e). In our work on the HDO of anisole, small Ni clusters and nanoparticles enhanced hydrogenolysis activity to form BTX (benzene, toluene, and xylene), while larger Ni particles facilitated hydrogenation to saturated products (e.g., cyclohexane and 2-methoxycyclohexanol).

Ni nanoparticles sized 3–4 nm exhibited higher hydrogenation activity in toluene hydrogenation compared to Ni nanoclusters smaller than 2 nm and Ni nanoparticles larger than 4 nm (as shown in Fig. 6d).¹⁶² This is attributed to their balanced surface metal concentration, relatively weak interaction with the support, and increased concentration of high-temperature surface hydrogen species. In another study,⁴² we observed that smaller Ni particles (4.5–6.4 nm) had a lower cyclohexane formation rate in the HDO of anisole compared to larger Ni particles (9.0–13.5 nm) supported on natural zeolites. Although single Ni atoms supported on β-Mo₂C have been reported to achieve 100% conversion of guaiacols with over 90% yield of cyclohexane,¹⁴² it is important to note that this reaction occurs in a batch process with an exceptionally long residence time.¹⁰⁷ As a result, the intrinsic reaction rate for the single Ni or Co atoms cannot be accurately determined.

While we concluded that Ni nanoparticles exhibited higher HDO and hydrogenation rates, the scenario may differ with noble metals. Ru/ZSM-5 and Ru/BEA catalysts prepared by ion exchange and incipient wetness impregnation methods were used for HDO of guaiacol.¹⁶⁴ The ion exchange catalysts with atomically dispersed Ru exhibited higher intrinsic HDO rates (2.03–2.67 × 10⁻⁵ mol_cyclohexane min⁻¹ g⁻¹) compared to impregnated Ru/ZSM-5 (0.10–0.18 × 10⁻⁵ mol_cyclohexane min⁻¹ g⁻¹). The sub-nanometric Ru metal clusters (<1.5 nm) confined within MWW zeolite thin layers showed significantly enhanced HDO activity for guaiacol compared to larger Ru particles.¹⁵⁷ Moreover, the close proximity of single noble metal atoms to acid sites allows for synergistic interactions between the metal and acid, further contributing to the enhanced HDO activity¹⁶³ (as depicted in Fig. 6f).

In summary, the choice of particle size depends on the type of metal and the desired target products. As displayed in Fig. 7, single atoms and clusters of noble metals exhibit higher hydrogenation and HDO rates compared to nanoparticles, owing to the enhanced metal surface area and weaker metal–support interactions. In contrast, single atoms and clusters of non-noble metals (e.g., Ni) display lower hydrogenation activity and HDO rates due to strong metal–support interactions, which promote enhanced electron transfer from the metal to the support. However, these non-noble metal single atoms and clusters show strong hydrogenolysis activity, resulting in the formation of aromatics without ring saturation.

Fig. 7 Schematic illustrating the particle sizes of (a) noble metals and (b) non-noble metals in the HDO reaction.

5. Supports

5.1. Carbon, SiO₂ and metal oxides

Activated carbon, metal oxides, and zeolites are commonly used as supports.^14,18,97,165 These materials not only provide a large surface area but also offer a high concentration of acid sites, which are crucial for catalytic activity. Additionally, they enhance the diffusion of reactants to the active sites, a feature particularly important in the HDO of biocrude, where large and complex molecules must access the active sites within the pores. The choice of support not only affects metal dispersion and HDO activity but also influences product distribution and catalyst stability. A summary of studies on HDO reactions using different supports is provided in Table 4.

Table 4 Summary of biocrude compound upgrading using different supports

Support	Metal	P (MPa)	T (°C)	Feed	Conversion (100%)	Products	Reactor	Ref.
a SAR = silica-to-alumina ratio.
Al2O3	CoMoS	5.5	250	Guaiacol	33	36% phenol, 46% catechol	Batch	168
Al2O3	Co	3	300	Phenol	100	38% cyclohexane	Batch	123
A2O3	Ru	4	250	Guaiacol	100	74.8% 2-methoxycyclohexanol	Batch	95
K-Al2O3	CoMoS	5.5	250	Guaiacol	30	70% phenol, 16.6% catechol, 13% benzene	Batch	168
Carbon	Ni1Mo3N	1	260	Anisole, phenol, guaiacol	99.9	95.8% cyclohexane	Batch	167
Carbon nanotube	Ru	5	220	Eugenol	100	94% propylcyclohexane	Batch	171
CeO2	Ru	5	220	Eugenol	100	61% 4-propylcyclohexanol, 26% 2-methoxy-4-propyl-cyclohexanol	Batch	171
CeO2	Pd	0.1	300	Guaiacol	9.6	32.5% phenol, 8.8% cyclohexone	Flow	172
Nb2O5	Pd	0.1	300	Guaiacol	25.0	15.7% benzene, 27.1% phenol	Flow	172
L3-Nb2O3	Ru	0.5	250	4-Methylphenol	72.3	68.1% toluene	Batch	173
SiO2	Ru	4	250	Guaiacol	97.2	68.2% 2-methoxycyclohexanol	Batch	95
SiO2	Pt	10	250	Guaiacol/1-octanol	52	78.8% 2-methoxy-cyclohexanol	Flow	96
SiO2	Pd	0.1	300	Guaiacol	14.3	41.8% phenol	Flow	172
SiO2-ZrO2	Ni5Cu	5	300	Guaiacol	100	80.8% cyclohexane	Batch	124
SiO2-Al2O3	Co	3	300	Phenol	100	60% cyclohexane	Batch	123
TiO2	Co	3	300	Phenol	10.5	67% cyclohexane	Batch	123
TiO2	CoMoS	4	300	Guaiacol	100	60% phenol	Flow	169
TiO2	Pd	0.1	300	Guaiacol	15.5	13.4% benzene, 26.8% phenol	Flow	172
ZrO2	CoMoS	4	300	Guaiacol	100	58% phenol	Flow	105
ZrO2	Co	3	300	Phenol	98.5	80% cyclohexane	Batch	123
ZrO2	Ru	5	220	Eugenol	100	45.7% propylcyclohexane, 40% 2-methoxy-4-propyl-cyclohexanol	Batch	171
ZrO2	Pd	0.1	300	Guaiacol	11.1	15.0% benzene, 22.7% phenol	Flow	172
BEA	Ni	4	230	Guaiacol	11.1	48.6% cyclohexane	Flow	39
BEA (SAR^a = 12.5)	Ru	4	230	Guaiacol	100	64.4% cyclohexane, 7.6% 2-methoxycyclohexanol	Batch	95
BEA (SAR = 25)	Ru	4	230	Guaiacol	100	43.2% cyclohexane, 26.4% 2-methoxycyclohexanol	Batch	95
BEA (SAR = 175)	Ru	4	230	Guaiacol	92.9	26.6% cyclohexane, 45.7% 2-methoxycyclohexanol	Batch	95
HBEA	Pt	4	250	Guaiacol	97	45.2% cyclohexane	Batch	174
Meso-BEA	Pt	4	250	Guaiacol	98	26.1% cyclohexane, 9.0% tetramethylphenol	Batch	174
HY	Ni	4	230	Guaiacol	7.0	17.1% cyclohexane	Flow	39
HY (SAR = 2.6)	Pt	4	250	Guaiacol	82	58.2% cyclohexane	Batch	175
HY (SAR = 40)	Pt	4	250	Guaiacol	70	62.6% cyclohexane	Batch	175
HY (SAR = 100)	pt	4	250	Guaiacol	22	81.8% cyclohexane	Batch	175
MOR	Ni	4	230	Guaiacol	8.4	32.1% cyclohexane	Flow	39
MCM-41	Ni	4	230	Guaiacol	10.6	41.5% cyclohexane	Flow	39
MFI	Pt	10	250	Guaiacol/1-octanol	93	52% cyclohexane, octane	Flow	96
ZSM-5	Ni	4	230	Guaiacol	5.3	20.7% cyclohexane	Flow	39

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While activated carbon lacks intrinsic acid sites, it has been widely employed as support for both noble and non-noble catalysts. Ruiz¹⁶⁶ investigated the effect of activated carbon on the HDO of guaiacol over a MoS₂ catalyst. The porosity of activated carbon had a negligible impact on product selectivity; however, the surface chemistry of the support significantly influenced the dispersion of sulfide Mo species. Specifically, acidic oxygen surface functionalities hindered the dispersion of MoO₄²⁻ and Mo₇O₂₄⁶⁻ species. Jiang¹⁶⁷ used a Ni₁Mo₃N/C catalyst for the HDO of phenol and guaiacol at 260 °C, achieving a 95.8% selectivity for cyclohexane. The synergistic catalysis of Ni₂Mo₃N and β-Mo₂C promoted efficient hydrogen activation and C–O bond hydrogenolysis.

Metal oxides are also frequently used as support in HDO catalysts. For instance, Al₂O₃, modified by K, was used to support NiMoS and CoMoS catalysts in the HDO of guaiacol.¹⁶⁸ The addition of K to γ-Al₂O₃ increased the formation of partially or completely deoxygenated compounds (e.g., benzene, phenol), an effect attributed to reduced acidity. However, a decrease in HDO reaction rate was observed with K-modified Al₂O₃, likely due to the formation of Co and Ni aluminate spinel and a reduction in the number of active sulfided metal sites.

ZrO₂ is considered an excellent support for HDO catalysts due to its higher catalytic activity in activating oxygen-containing compounds and its lower carbon deposition compared to Al₂O₃.^73,169 When ZrO₂, TiO₂ and the traditional γ-Al₂O₃ were used as supports for CoMoS and NiMoS catalysts in the HDO of guaiacol.¹⁶⁹ The results showed that ZrO₂ significantly promoted the conversion of guaiacol into deoxygenated hydrocarbons. Additionally, SiO₂-ZrO₂ composite oxides exhibited higher and stronger acidity than ZrO₂ alone, facilitating the formation of methyl-substituted compounds during guaiacol HDO.¹²⁴ Ghampson¹²³ studied the HDO of phenol using various metal oxide supports. The catalytic activity varied with the metal species and acid properties at the metal-support interface, with activity decreasing in the order: Co/Al₂O₃ > Co/SiO₂-Al₂O₃ > Co/ZrO₂ > Co/TiO₂. Strong metal-support interactions led to the coverage of active metal species by a thin TiO_x layer on Co/TiO₂, resulting in the lowest HDO activity. It is proposed that the support could control the reaction pathway: (i) Co/Al₂O₃ and Co/SiO₂-Al₂O₃ followed the sequential ring hydrogenation–dehydration–hydrogenation route typical of metal/acidic support catalysts; (ii) Co/ZrO₂ favored phenol tautomerization followed by hydrogenation/dehydration; and (iii) Co/TiO₂ predominantly followed the direct deoxygenation–hydrogenation pathway. The total acidity of the Co-support interface influenced activity, while product selectivity was a function of the nature of acid sites, as determined by cyclohexanol dehydration. Ni catalysts supported on Nb₂O₅ have also been utilized in HDO reactions.¹⁷⁰ Specifically, the Ni_0.9Nb_0.1 catalyst exhibited a tenfold increase in cyclohexane selectivity compared to the pure Ni catalyst during the HDO of anisole. This significant improvement is attributed to the enhanced dispersion of Ni on the Nb₂O₅ support.²⁸

In summary, the roles of carbon and metal oxides in HDO are highly complex. The choice of support material significantly influences HDO activity through varying metal–support interactions. Moreover, different metal species demonstrate diverse synergistic effects with these supports during HDO reactions. Typically, using carbon and metal oxides as supports leads to a slightly reduced HDO rate, resulting in higher yields of partially deoxygenated compounds such as phenol, catechol, and various alcohols, as illustrated in Table 4. This decrease in HDO rate is likely due to strong metal-support interactions, which diminish hydrogenation activity and reduce the availability of Brønsted acid sites. Therefore, greater emphasis should be placed on exploring zeolite supports to enhance HDO performance.

5.2. Influence of zeolite topology on HDO

Zeolite-based catalysts have gained attention due to their large surface area and porosity, which play a crucial role in product selectivity. Lee¹⁷⁴ studied the influence of support pore size on the HDO of guaiacol in a batch reactor. Although Pt/ZSM-5 possessed a high concentration of acid sites, it showed lower guaiacol conversion compared to a mesoporous Pt/HBEA catalyst, likely due to the smaller pore diameter restricting guaiacol molecule access. In contrast, the Pt/Si-MCM-48 catalyst, which lacks acid sites, displayed weak HDO activity despite its large pore size.

Our group has studied BEA and ZSM-5 supported Ru catalysts for the HDO of biocrude oil.⁹⁵ While Ru/BEA and Ru/ZSM-5 catalysts with lower Si/Al ratios both exhibited good HDO activity for guaiacol, only Ru/BEA demonstrated high selectivity toward cycloalkanes when processing complex biocrude oil. This enhanced performance is attributed to the greater mesoporosity and higher concentration of Brønsted acid sites in BEA zeolite, which provide accessible active sites for bulky aromatic molecules. The hierarchical pore system in BEA facilitates the diffusion, hydrogenation, deoxygenation, and ring-opening of large components in biocrude oil, leading to more efficient conversion. Enhanced HDO activity in phenolic monomers and lignin oils was also observed with BEA-supported Pd catalysts compared to Pd/ZSM-5.¹⁷⁶

The structures of various zeolites (Y, ZSM-5, BEA, MOR, MCM-41) and their shape selectivity in the HDO of biocrude oil and its model compounds have been extensively studied.³⁹ Ni/beta and Ni/Y catalysts, which feature 12 × 12-ring channels, exhibit higher hydrogenation activity for phenanthrene compared to Ni/ZSM-5 and Ni/MOR catalysts, which have smaller 10 × 10-ring and 12 × 8-ring channels. Notably, only Ni/Y, with the largest pore-limiting diameter (7.4 Å), is effective in hydrogenating pyrene (6.7 Å). Although all catalysts demonstrated good HDO activity for guaiacol, Ni/ZSM-5 showed a lower cyclohexane formation rate at reduced residence times, attributed to its smaller pore diameter (5.0 Å), which restricts guaiacol diffusion (∼4.9 Å). Despite having the largest pore size, Ni/AlMCM-41 exhibited a lower cyclohexane formation rate (16.3 × 10⁻⁴ mol_cyclohexane g_cat⁻¹ min⁻¹) compared to Ni/beta (18.9 × 10⁻⁴ mol_cyclohexane g_cat⁻¹ min⁻¹), likely due to its low metal dispersion. Mao¹⁷⁷ studied the role of zeolite structures in the disproportionation and transalkylation of phenol and dimethylphenol, finding that ZSM-5 exhibited the lowest activity due to microporous limitations. In contrast, MCM-22 (MWW-type) achieved the highest 2,6-DMP conversion and cresol production due to its strong Brønsted acidity and mesoporous volume. FAU-type zeolite HY, with its wider 3D channels (7.4 × 7.4 Å) and the largest sphere diameter (11.2 Å), was the most selective, achieving up to 60% cresol selectivity by minimizing spatial constraints for bimolecular reactions.

Hierarchical USY,⁷⁴ ZSM-5,^178–180 BEA,¹⁸¹ when used as supports for Ni catalysts in HDO reactions, have shown improved reaction rates. This improvement is attributed to the enhanced mesoporosity, increased accessibility, optimized acid strength, and the formation of bulky external Ni particles.

As summarized in Fig. 8, the zeolite structure significantly influences not only the diffusion of feedstock, intermediates, and products, but also the distribution of metals. ZSM-5 and MOR, with their microporous structures, restrict the diffusion of larger molecules of biocrude oil (e.g., retene), resulting in a lower HDO reaction rate. Conversely, while MCM-41 and SBA-15 facilitate the diffusion of larger molecules in biocrude oil, they also promote the formation of bulk metal particles, which limits the HDO rate. In contrast, BEA, with its combination of micropores and mesopores, not only enhances metal dispersion but also facilitates the diffusion of biocrude molecules. Therefore, BEA is a more suitable support for HDO catalysts.

Fig. 8 Shape selectivity of zeolites supported metals catalysts in HDO of anisole, guaiacol, phenanthrene.

5.3. Unveiling the role of acidity in hydrodeoxygenation

The acid concentration and type (Brønsted and Lewis acid sites) provided by zeolites,¹⁷⁵ Al₂O₃,¹⁸² WO_x/ZrO₂,¹⁸³ TiO₂,¹⁸⁴ Nb₂O₅,¹⁸⁵ H₃PO₄¹⁵⁵ and other metal oxides play a significant role in deoxygenation, ring-opening, cracking, coupling reactions, and transalkylation.¹⁸⁶ Ru/CHPW, with its excellent synergistic effect between active metal and acid sites, offers high efficiency in the HDO of guaiacol, achieving over 98% yield of cycloalkanes in the HDO of phenol, anisole, cresols, and guaiacols¹⁸⁷ (as shown in Fig. 9a and b).

Fig. 9 The synergistic effect of active metal-acid sites and the guaiacol HDO reaction pathway (a) Ru/C-Al₂O₃ (b) Ru/C-HPW.¹⁸⁷ (c) Tuning acid-metal synergy in m-cresol hydrodeoxygenation over Pt/Aluminosilicate catalysts.¹⁸⁹ (d) Proposed reaction pathways for HDO of guaiacol over Ni/AlPO₄ catalyst.¹⁹⁰ (e) Pore structure properties of MFI coupled with abundant LAS (TS-1) as well as Ru NPs on the orifice of pores of TS-1 promotes the hydrogenolysis of lignin to aromatics, while BAS on Ru/MFI promotes the hydrogenation rates of aromatics.¹⁹² (f) Reaction pathway for HDO of guaiacol over ion-exchanged Ni/Beta-12.5 catalyst and acid-containing supports.¹³⁰

In our previous study,¹³⁰ Ni/BEA catalysts with varying acid site concentrations exhibited a similar normalized rate (TOF_-B = cyclohexane formation rate based on Brønsted acid sites) in the HDO of guaiacol, indicating that Brønsted acid sites are key to the reaction. High concentrations of BASs facilitate the HDO reactions. This was further demonstrated by a similar TOF_-B value observed over Ru/ZSM-5 and Ru/BEA catalysts,¹⁶⁴ suggesting that BASs play a crucial role in the deoxygenation of 2-methoxycyclohexanol to cyclohexane. Ru catalysts supported on BEA, ZSM-5, Al₂O₃, and SiO₂ were used for the HDO of guaiacol in batch reactor at 250 °C.⁹⁵ Ru/ZSM-5 and Ru/BEA, with strong BASs, predominantly produced cyclohexane, while Ru/Al₂O₃ and Ru/SiO₂ primarily yielded 2-methoxycyclohexanol (68.2–74.8%) and cyclohexanol (15.9–24.1%), suggesting that BASs favor deoxygenation reactions. Lewis acid sites (LASs) on Al₂O₃ played a limited role in deoxygenating hydroxyl bonds, consistent with findings by He.¹⁵⁷ Ru@MCM-22 exhibited a majority of cyclohexane, whereas Ru@ITQ-1, which lacks acid sites, mainly produced 2-methoxycyclohexanol and cyclohexanol. Moreover, the role of BASs was investigated in the HDO of acetophenone over Pd/ZSM-5 catalysts.¹⁸⁸ The dehydration of alcohol, which was the rate-limiting step in the reaction, was efficiently accelerated by the BASs. Moreover, the BASs are also provided by catalysts such as Pt/amorphous silica-alumina¹⁸⁹ (Fig. 9c) and Ni/AlPO₄¹⁹⁰ (Fig. 9d), which facilitate the dehydration of alcohols.

Similar results were observed with Pt/HBeta, which enhanced the formation of toluene and xylene during the HDO of guaiacol.¹⁸⁶ While LASs play a limited role in deoxygenation of alcohols, they act as electron acceptors, facilitating the adsorption of oxygen-containing intermediates and, in turn, enhancing the HDO of dibenzofuran.¹⁹¹ In addition, LAS on Ru/TS-1 led to a linear increase of guaiacol hydrogenolysis rate to benzene, which was attributed to the enhanced absorbance capability of guaiacol and phenol on the LAS¹⁹² (as show in Fig. 9e). Ni/BEA, HBEA, and Al₂O₃ all exhibited high yields of 1,2-dimethoxybenzene and catechol in the HDO of guaiacol (as shown in Fig. 9f), suggesting that both BASs and LASs promote transalkylation reactions.¹³⁰

WO_x/TiO₂-supported Pt catalysts have also been employed in the HDO of polyols.¹⁹³ The strong interaction between secondary WO_x species facilitates the formation of Brønsted acid sites (H–O–WO_x), which promote the acid-catalyzed dehydration of secondary hydroxyl groups.

Acid strength also plays a key role in HDO reactions. Levia¹¹⁵ suggested that catalysts with strong acid sites favor demethylation reactions, while those with weaker acid sites promote direct demethoxylation. Strong acid sites promote the condensation of 2,5-hexanedione in HDO of 2,5-dimethylfuran, leading to reduced yield of p-xylene.¹⁹⁴ The addition of H₃PO₄ to guaiacol and H₂O over Pd/C results in cyclohexane as the main product (85%), whereas without H₃PO₄ the primary product is cyclohexanol (98%). This suggests that the acid sites provided by H₃PO₄ facilitate the dehydration reaction.¹⁵⁵ Nb₂O₅, known for its abundant water-tolerant Lewis acid sites (LASs), was used as a support for HDO catalysts.¹⁸⁵ Ni/Nb₂O₅ was used for the HDO of vanillin, achieving 79.2% selectivity for 2-methoxy-4-methylphenol at a conversion rate of 96.1%.¹⁹⁵ The Nb₂O₅ support effectively lowered the dissociation energy of the aromatic C–OH bonds.

In summary, guaiacol, phenols, and anisole are first hydrogenated on metal sites with strong hydrogenation activity, such as Ni nanoparticles and noble metals, to produce saturated alcohols (e.g., cyclohexanol, 2-methoxycyclohexanol) and ethers (e.g., methoxycyclohexane). These intermediates are then dehydrated and demethoxylation to olefins via BASs, as illustrated in Fig. 9 and 10. The resulting olefins undergo further hydrogenation on metal sites with dissociated hydrogen species to form cycloalkanes. LASs from the supports or unreduced Ni species facilitate transalkylation, leading to the formation of alkylphenols (as shown in Fig. 9f). While LASs aid in the deoxygenation of ethers like methoxycyclohexane, they are ineffective for dehydrating alcohols such as cyclohexanol, which are key intermediates in HDO reactions. Additionally, acid sites promote coupling reactions during biocrude upgrading, enhancing the production of cycloalkanes suitable for jet fuel. Moreover, BASs play a crucial role in the ring-opening and cracking of heavy compounds in biocrude oil. Therefore, optimizing HDO performance requires a balanced combination of BASs and LASs to effectively manage each step of the reaction process.

Fig. 10 (a) Proposed reaction pathways for HDO reactions over Ni-based catalysts.⁴² (b) Role of BASs and LASs in HDO reactions.

6. Key factors contributing to catalyst deactivation

Catalyst deactivation is one of the primary challenges in HDO reactions, often attributed to the formation of condensed-ring products,^196,197 sintering,¹⁴⁸ coking,¹⁹⁸ water,¹⁹⁹ and the loss of sulfide from sulfided catalysts⁵⁴ (as shown in Fig. 11).

Fig. 11 Factors contributing to catalyst deactivation in the HDO process.

Among the causes of deactivation, carbon deposition is considered the most significant.^200,201 Carbon forms via polycondensation and polymerization reactions at acid sites, leading to pore blockage and a reduction in the number of active sites. The rate of catalyst deactivation depends on reaction conditions, catalyst characteristics, and the functional groups of the model compounds. Gonzalez-Borja²⁰² studied the deactivation behavior of a Pt–Sn catalyst in the HDO of anisole and guaiacol. The catalyst exhibited a higher deactivation rate with guaiacol than with anisole, and the Pt–Sn bimetallic catalyst displayed greater activity and stability compared to monometallic Sn or Pt catalysts. Metal leaching is another factor leading to catalysts deactivation.²⁰³ Zhao¹⁴³ used Ni catalysts supported on ZSM-5 and HZSM-5-Al₂O₃ for the HDO of phenol, finding a significantly lower Ni leaching rate over Ni/HZSM-5-Al₂O₃, likely due to a strong metal-Al₂O₃ interaction. Additionally, the dehydration of Brønsted acid sites (as Al-OH-Si) to coordinately unsaturated Al atoms forming LASs during recycling may be irreversible. The water produced during HDO can lead to catalyst deactivation by altering the support or metal phases,¹¹⁰ such as by modifying the active sulfide phase through the formation of nickel aluminate or by transforming Al₂O₃ into boehmite (AlO(OH)).¹⁶⁵

Recent studies have shown that certain metals can mitigate catalyst deactivation. Zhou²⁰⁴ compared the deactivation rates of noble metals (Rh, Pt) and sulfided NiMo catalysts in the HDO of microalgae oil, concluding that coke accumulation decreased in the order of sulfided NiMo > Pt > Rh. Additionally, coke formation on sulfided NiMo catalysts increased with time on stream, while no such time dependence was observed for Rh and Pt. Gao¹⁹⁷ examined the deactivation mechanisms of carbon-supported noble metal catalysts for the HDO of guaiacol, finding that Rh/C, Pd/C, and Ru/C catalysts experienced rapid deactivation during 300 minutes of time on stream, whereas Pt/C showed negligible activity loss. Zhao¹⁴³ investigated used catalysts for phenol hydrodeoxygenation, attributing the gradual decrease in catalytic activity to the agglomeration of Ni particles.

The nature of the support also plays a crucial role in catalyst deactivation. Souza¹⁴⁸ explored the role of various supports (SiO₂, Al₂O₃, TiO₂, ZrO₂, CeO₂ and CeZrO₂) in deactivation process during the HDO of phenol. Pd catalysts supported on Al₂O₃, SiO₂, ZeO₂ and TiO₂ showed rapid deactivation over time, while Pd/CeO₂ and Pd/CeZrO₂ exhibited only a slight decrease in activity. It is suggested that intermediates formed during the reaction remained on the Lewis acid sites, inhibiting further adsorption of reactants. However, during the Hellinger studies,⁹⁶ the SiO₂ supported Pt catalyst showed a high stability compared to Pt/HMFI-90 catalyst during the HDO of guaiacol. Moreover, it is reported that the Lewis acidic Nb₂O₅ favored coke formation via lignin species condensation.¹⁷⁰ We also investigated catalyst deactivation in HDO reactions, finding that phenolic-hydroxyl groups from phenols and guaiacols contribute to catalyst deactivation by facilitating the formation of condensed-ring compounds. This was evidenced by decreased cycloalkane yields and subsequent pore blockage in the catalyst.¹⁹⁶ Among the catalysts tested, Ni/ZSM-5 exhibited the fastest deactivation, followed by Ni/BEA, while Ni/AlMCM-41 showed the slowest deactivation. The lower deactivation rate of Ni/AlMCM-41 can be attributed to its mesoporous structure, which enhances the diffusion of condensed-ring products (e.g., 2-cyclohexylphenol and 1,2,3,4-tetrahydro-1-ethyl-6-hydroxy-7-methoxy-naphthalene).

Overall, noble metals and bulk Ni catalysts with strong hydrogenation activity demonstrate better resistance to coking compared to non-noble metal catalysts, while less acidic oxides generate less coke than Al₂O₃. Additionally, zeolites with enhanced mesopores improve catalyst stability by facilitating the entry and diffusion of bulky feedstock, intermediates, and products.

7. Concluding remarks and perspectives

Significant research has been conducted to develop renewable energy systems that could replace traditional fossil fuels. Lignocellulosic biomass, considered the most abundant and readily available biomass, holds great potential for biocrude production via hydrothermal liquefaction or fast pyrolysis. Biocrude oil, derived from the pyrolysis or liquefaction of biomass, cannot be directly used as a transportation fuel due to its high oxygen content. Therefore, an upgrading process is necessary to remove oxygen and convert poly-aromatic compounds into saturated hydrocarbons, making them comparable to petroleum fuels. Hydrodeoxygenation, which removes oxygen through deoxygenation and saturates double bonds or aromatic rings via hydrogenation, is the most common method for upgrading biocrude oil. Bifunctional catalysts play a critical role in HDO reactions, requiring high deoxygenation activity, water tolerance, and resistance to coke formation. The current HDO catalysts face several key challenges: (1) limited understanding of the role of metal species (sulfided metals, noble metals, reduced non-noble metals) in HDO reactions; (2) limited exploration of catalyst deactivation mechanisms and regeneration strategies; (3) conflicting conclusions regarding the impact of metal particle size on HDO activity; (4) insufficient studies on the influence of zeolite structure on HDO of biocrude with varying dimensions; and (5) a lack of comprehensive understanding of the role of acid type, concentration, and strength in HDO reactions.

This article provides an in-depth review of commonly used sulfided, noble metal, and non-noble metal catalysts for HDO reactions. Among these, non-noble Ni-based catalysts, particularly those with Fe promoters supported on zeolites, emerge as promising candidates due to the strong hydrogenation capability of Ni species, the high hydrogenolysis activity of Fe species, and their superior deoxygenation performance. We also present a detailed analysis of the role of metal particle size in HDO reactions (Fig. 12). Single noble metal atoms exhibit enhanced reaction rates compared to noble metal clusters, attributed to their high atomic efficiency; however, they are prone to deactivation via metal sintering with limited metal loading. Non-noble metal single atoms and clusters demonstrate lower HDO and hydrogenation rates than nanoparticles due to strong metal–support interactions, which create electron-deficient Ni species. Nonetheless, these Ni single atoms and clusters show significant hydrogenolysis and transalkylation activities, particularly selective for the formation of aromatics.

Fig. 12 Potential suitable catalysts for HDO reactions.

Furthermore, this review addresses the influence of acid types and concentrations on HDO reactions. High concentrations of acid sites favor deoxygenation, leading to higher hydrocarbon yields, whereas reduced acid sites result in greater yields of alcohols and ethers. While Lewis acid sites (LAS) promote hydrogenolysis and transalkylation, their role in the HDO of saturated alcohols and ethers is limited. In contrast, Brønsted acid sites (BAS) play a crucial role in hydrodehydration and hydrodemethoxylation reactions.

We also examine factors contributing to catalyst deactivation. Microporous catalysts often exhibit higher deactivation rates compared to mesoporous ones due to pore blockages by larger feedstock compounds, intermediates, and condensed-ring compounds. Bulk Ni can mitigate deactivation, presumably through spillover hydrogen, which reduces coke formation.

Finally, we offer challenges and recommendations for future research on biocrude upgrading, which are outlined below.

• Bifunctional catalysts: most current research focuses on developing bifunctional catalysts, which have shown significant improvements in catalytic activity and product selectivity for model compounds, typically studied in batch reactors. More studies should include the HDO of model compounds in continuous flow reactors.

• Biocrude oils as feedstock: investigating biocrude oils derived from lignocellulosic biomass, which are expected to be more viscous and heavier than commonly used model compounds (e.g., phenols, guaiacols), is crucial. These oils must be hydrotreated using improved catalysts and stringent reaction conditions.

• Catalyst pore size: the shape selectivity of zeolites in catalytic pyrolysis and cracking has been studied, but the influence of catalyst pore size on biocrude oil with different weight distributions is rarely investigated. It is necessary to elucidate the effect of pore size on the HDO of biocrude oil.

• Develop natural zeolite-based catalyst: modified natural zeolites, with their medium acidity and large pore sizes, offer an ideal environment for the formation of nanoparticle catalysts, making them well-suited for processing real biocrude oil. Further research should be dedicated to exploring and optimizing natural zeolite-based catalysts to enhance their effectiveness and applicability in industrial biocrude upgrading.

• Catalyst deactivation mechanisms: a deeper understanding of the factors influencing catalyst deactivation during HDO is essential. It is insufficient to attribute deactivation solely to carbon deposition. The fundamental mechanisms should be clarified by modifying factors such as functional groups, temperature, WHSV, and active site concentration. Designing optimal catalysts with high stability will depend on better understanding these deactivation mechanisms.

Data availability

No new primary data were generated or analyzed in this review. The data supporting the results and analysis presented in this article are derived from previously published works, which have been duly cited in the reference section.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

Funding from the Australian Research Council and Licella Holdings is gratefully acknowledged.

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