Biomass valorization with metal-free catalysts: innovations in thermocatalytic, photocatalytic, and electrocatalytic approaches
Arzoo
Chauhan
and
Rajendra
Srivastava
*
Catalysis Research Laboratory, Department of Chemistry, Indian Institute of Technology Ropar, Rupnagar, Punjab-140001, India. E-mail: rajendra@iitrpr.ac.in; Tel: +91-1881-242175
First published on 7th July 2025
Abstract
The catalytic valorization of biomass into high-value chemicals and sustainable fuels is critical for addressing global environmental challenges and advancing a bio-based circular economy. Traditional metal-based catalysts, though effective, face major limitations, including resource scarcity, toxicity, leaching, and cost, underscoring the need for alternative catalytic paradigms. Metal-free catalytic systems have emerged as promising sustainable solutions due to their environmental compatibility, cost-effectiveness, and material abundance. This review comprehensively evaluates recent progress in metal-free catalysis for biomass valorization, uniquely integrating and comparing thermal, photocatalytic, and electrocatalytic methodologies. We systematically discuss diverse classes of metal-free catalysts, including carbon-only materials, heteroatom-doped carbons, and emerging non-carbon frameworks, while highlighting advanced synthesis strategies, tailored active site engineering, mechanistic insights, and catalyst recyclability under varying operational conditions. The comparative analysis reveals distinct advantages and limitations inherent to each catalytic route, emphasizing the tunability and versatility of metal-free systems. Importantly, future proposed directions are rooted in the synergistic integration of photothermal and photoelectrochemical pathways, paving the way for next-generation multifunctional catalytic systems. By identifying persistent challenges such as active site localization, long-term stability, reaction selectivity, and scalability, the review advocates for interdisciplinary efforts incorporating advanced heterostructure design and AI-driven catalyst optimization to realize the full potential of metal-free catalysis in sustainable biomass valorization.
 Arzoo Chauhan | Arzoo Chauhan received her Master's degree in Chemistry from Hemwati Nandan Bahuguna Garhwal University, Srinagar, Uttarakhand, India, in July 2018. She qualified for the CSIR Junior Research Fellowship (JRF-LS) in Chemical Sciences in June 2019. Later, she joined the PhD program in the Department of Chemistry at the Indian Institute of Technology Ropar, Punjab, India, under the supervision of Prof. Rajendra Srivastava. Her research focuses on the design and synthesis of carbon-based catalytic materials for biomass valorization and organic transformations. She is also involved in developing and applying metal-free catalysts derived from carbon materials for sustainable chemical processes. |
 Rajendra Srivastava | Dr Rajendra Srivastava is a Professor at the Department of Chemistry, IIT Ropar. He acquired a PhD from CSIR-NCL Pune. He worked as a Postdoctoral fellow at KAIST, South Korea, and as a JSPS fellow at Hokkaido University, Japan. He received the Best Thesis award, the NASI-SCOPUS Young Scientist, the Young Scientist award from the Catalysis Society of India, and the Research awards from IIT Ropar and Punjab University. His research interest includes the synthesis of nanostructured functional materials for fuels and chemical production. He has published more than 205 peer-reviewed research articles in reputed international journals and nine patents. |
1. Introduction
The global transition to a bio-based circular economy demands innovative solutions for sustainable chemical production. Among these, the catalytic valorization of biomass, nature's most abundant renewable resource, emerges as a promising pathway to address the dual challenges of depleting fossil fuel reserves and rising carbon emissions.1 The Energy Institute's yearly report shows that renewable energy sources contributed 14.6% to global primary energy consumption, a 0.4% increase from the previous year.2 Alongside nuclear energy, renewables collectively accounted for over 18% of the total energy mix. Fossil fuels, while still dominant, saw a slight decline, dropping to 81.5% of primary energy consumption. This trend highlights the gradual shift toward cleaner energy sources, yet the continued reliance on fossil fuels underscores the urgency for a more accelerated and sustainable energy transition. It is the time to embrace innovative strategies and technologies to achieve a sustainable energy future.Biomass, derived from photosynthesis, is a carbon-neutral and renewable feedstock, with an estimated global production exceeding 181.5 billion tonnes annually.3 As depicted in Fig. 1, lignocellulosic biomass, which forms the majority of biomass feedstocks, contains cellulose (40–50%), hemicellulose (15–30%), and lignin (15–30%)-each component offering unique chemical properties. Cellulose and hemicellulose, rich in carbohydrates, provide pathways to platform chemicals like glucose, furfural (FAL), and levulinic acid (LA). Lignin, an aromatic polymer containing ∼30% of the energy content in biomass, presents a promising source for high-value aromatic compounds.4 Integrated biorefineries are being developed to simultaneously produce fuels and chemicals, reducing costs and environmental impacts by replacing petroleum-based products with sustainable alternatives. Its transformation into biofuels, platform chemicals, and materials can escalate the transition to a greener future while aligning with global sustainability initiatives such as the European Green Deal's net-zero carbon goals5 and the UN's Sustainable Development Goal 7 (Affordable and Clean Energy).6
 | ||
| Fig. 1 Overview of lignocellulosic biomass valorisation, showing conversion of cellulose, hemicellulose, and lignin into value-added products using catalytic processes powered by thermal, solar, and electrochemical energy. | ||
Catalysis lies at the heart of biomass valorization, enabling selective and efficient transformations of complex biomass feedstocks.7 However, traditional metal-based catalysts, though highly active, face critical limitations such as high costs, resource scarcity, leaching under reaction conditions, and the generation of toxic residues. For instance, Pd catalysts often suffer from leaching in aqueous systems,8 leading to activity loss and contamination, while Ru- and Ni-based systems generate hazardous residues, complicating downstream processes.9,10 These challenges underscore the urgent need for alternative catalytic approaches. Metal-free catalysts, composed of earth-abundant elements such as carbon, nitrogen, boron, and phosphorus, have emerged as transformative materials for biomass valorization (Fig. 2a). Offering advantages such as low toxicity, cost-effectiveness, and environmental compatibility, these catalysts provide a sustainable solution to the challenges posed by metal-based systems. As shown in Fig. 2b, the academic interest in metal-free catalysts has grown significantly over the years, reflecting their rising importance in catalysis research and sustainable applications. A comparative evaluation of key features underscores their superiority in sustainability and environmental impact over traditional metal-based systems.
 | ||
| Fig. 2 (a) Comparison of metal-based and metal-free catalysts features and research trends. (b) Annual publications highlight the increasing focus on metal-free catalysts for catalysis in recent years, underscoring their growing importance in sustainable chemistry (derived from Scopus by searching keywords-metal-free catalyst, and biomass valorization. | ||
Recent breakthroughs highlight thermocatalytic, photocatalytic, and electrocatalytic potential to achieve selective transformations under mild conditions. For instance, N-doped carbon catalysts, leveraging basic nitrogen sites and high surface area, have achieved remarkable fructose selectivity (84%) at moderate temperatures.11 Similarly, in lignin depolymerization, heteroatom-doped carbons, derived from bio-inspired polydopamine or marine biomass, have demonstrated exceptional activity and simultaneous H2 evolution reactions (HER), paving the way for the efficient conversion of aromatic polymers.12 In the electrocatalysis, defect-rich carbon frameworks and polymer-supported electrodes have enabled selective electrooxidation of HMF to FDCA and electroreduction of levulinic acid to γ-valerolactone under ambient conditions, providing energy-efficient alternatives to thermocatalytic and photodriven processes.13,14 Despite these advancements, challenges remain in achieving high turnover frequencies, ensuring stability under harsh conditions, and understanding active site distribution. Innovative materials like covalent organic frameworks (COFs) and advanced heterostructures are emerging as promising solutions to address these gaps. For example, multifunctional catalysts with hierarchical porosity are overcoming mass transport limitations, enabling efficient transformations of bulky biomass molecules. Moreover, electrode-integrated architectures, redox mediator-assisted systems, and defect-engineered carbon networks are expanding the versatility of metal-free catalysts in electrochemical applications, offering new routes for selective and scalable biomass valorization.15,16
Several recent reviews have addressed the development and application of metal-free catalysts for various chemical transformations. Some focus on carbonaceous materials derived from biomass for general catalytic applications, with limited emphasis on the specific valorization of biomass feedstocks.17 Others provide detailed insights into doped graphene systems, discussing their electronic structures and catalytic behavior without expanding to broader material classes or biomass valorization pathways.18 Reviews highlighting the broader role of metal-free heterogeneous catalysts in sustainable chemistry typically address a range of organic transformations but do not systematically treat lignocellulosic or platform biomass molecules.19 Similarly, reviews dedicated to carbon-based systems such as graphene, CNTs, and g-C3N4 emphasize material synthesis and physical properties but often do not delve into biomass-specific applications.20 Some recent articles on covalent organic frameworks (COFs) focus on their emerging role in photocatalysis and organic synthesis but rarely explore their function in biomass transformation.21 Hence, it opens an avenue to explicitly focus on metal-free catalysts for biomass valorization.
1.1. Scope and structure of the review
This review is the first to explicitly focus on metal-free catalysts for biomass valorization. Unlike previous reviews, which broadly cover catalytic approaches or material classes, this work narrows its focus to the use of metal-free systems in transforming biomass-derived feedstocks. This specificity highlights the unique potential of these sustainable catalysts to address the challenges of energy-efficient and selective biomass valorization. Furthermore, the review is the first to comprehensively discuss the role of these catalysts in thermocatalytic, photocatalytic, and electrocatalytic pathways. This review systematically bridges the gap between these three processes by systematically comparing these strategies, providing new insights into their complementary roles in achieving efficient biomass valorization. The review aims to critically assess recent progress in metal-free catalysts for biomass valorization, focusing on their synthesis, mechanisms, and applications. It identifies and addresses critical knowledge gaps, including mechanistic pathways underlying catalytic activity, challenges in scalability, and performance optimization under diverse reaction conditions.The review is organized into five major sections. The first section highlights the preparation and properties of various classes of metal-free catalysts, including carbon-based materials (e.g., graphene, activated carbon, graphitic carbon nitride), non-carbon-based materials (e.g., boron nitride, boron oxide, sulphur–nitrogen polymers), polymeric frameworks, and covalent organic frameworks (COFs). Special attention is given to advanced modification techniques, such as heteroatom doping (e.g., with N, B, P, S), surface functionalization, and morphological control, which are critical in tailoring catalytic activity, stability, and selectivity. The section also explores synthesis methodologies like pyrolysis, sol–gel processes, and chemical vapor deposition, linking these routes to catalyst properties and scalability. Key advancements in structural and electronic modifications that enhance catalytic performance are critically reviewed to provide a holistic understanding of catalyst design.
The second section delves into the role of metal-free catalysts in high-temperature biomass valorization processes. Reactions such as oxidation, deoxygenation, depolymerization, hydrogenation, and selective cleavage of C–O and C–C bonds are analyzed in detail. The influence of catalyst properties, such as thermal stability, porosity, and active site accessibility, on reaction kinetics, product yields, and selectivity is thoroughly explored. Recent advancements in thermally stable carbon-based and polymeric catalysts are highlighted, focusing on their ability to withstand harsh reaction conditions. Challenges in achieving controlled reaction pathways and selectivity in complex biomass matrices are critically examined, and strategies for catalyst optimization and scalability are discussed.
The third section focuses on light-driven reactions facilitated by metal-free catalysts, where visible-light absorption and charge separation play pivotal roles. Materials such as graphitic carbon nitride (g-C3N4), carbon dots, and conjugated polymers are examined with an emphasis on bandgap engineering, photostability, and charge carrier dynamics. Mechanisms of charge generation, separation, and radical formation are analyzed, with attention to how these properties influence photocatalytic performance in biomass valorization. Advancements in heterojunction design, such as combining g-C3N4 with carbon nanostructures, are reviewed for their ability to enhance charge transfer and minimize recombination losses. Applications in lignin depolymerization, cellulose hydrolysis, and upgrading biomass-derived molecules are critically reviewed, showcasing the advantages of metal-free catalysts in achieving selective and sustainable transformations under mild conditions.
The fourth section highlights the role of metal-free catalysts in electrochemical biomass valorization. Carbon-based materials such as doped porous carbons, boron-doped diamond, graphite, and conductive polymers are discussed for their tunable surface properties, heteroatom doping, and charge transport behavior. Reactions, including electrooxidation of HMF to FDCA, electroreduction of levulinic acid to GVL and valeric acid, and electrodimerization of FAL and HMF, into biofuel intermediates are reviewed. Mechanistic pathways involving outer- and inner-sphere electron transfer, suppression of hydrogen evolution, and the influence of surface hybridization on selectivity are examined. The section also covers redox mediators and flow-cell systems to improve reaction efficiency and scalability, positioning metal-free electrocatalysis as a sustainable approach for biomass valorization.
The final section summarises the key insights discussed throughout the review, focusing on the transformative potential of metal-free catalysts in biomass valorization. It highlights challenges like scalability, long-term stability, and mechanistic understanding while offering forward-looking recommendations for future research. This section emphasizes the critical role of interdisciplinary approaches, hybrid catalytic systems, and emerging technologies such as AI/ML in advancing the field.
The main goal of this review is to provide a comprehensive evaluation of the current state of metal-free catalysis for biomass valorization. By systematically identifying persistent challenges, this review aims to guide innovation in catalyst design and application. This analysis highlights opportunities for developing economically viable and environmentally sustainable pathways for bio-based fuel and chemical production.
2. Key considerations in the design of metal-free catalysts
Understanding the fundamental design principles of metal-free catalysts is crucial before delving into specific categories, as these principles underpin their performance across thermal, photocatalytic, and electrocatalytic biomass valorization. While the mechanistic environments differ, these systems share standard requirements in material structure, surface chemistry, and sustainability, with additional process-specific adaptations.Utilizing biomass-derived or waste materials as precursors promotes a circular economy by converting waste into value-added catalysts. The synthesis of metal-free catalysts emphasizes methods such as pyrolysis, hydrothermal processes, and chemical vapor deposition (CVD), limiting the use of various additional solvents and reagents.22 These scalable techniques reduce dependence on finite resources and facilitate the industrial implementation of sustainable catalytic systems. Practical applications of metal-free catalysts demand scalability, reproducibility, reusability, and long-term stability. Catalysts must resist fouling and deactivation while retaining their activity and selectivity over multiple cycles. For instance, N-doped carbon derived from pyrolyzed chitosan exhibits exceptional stability in hydrolysis reactions under acidic conditions.23 Simple recovery and regeneration methods, such as thermal reactivation or washing, enhance the economic feasibility of these systems for large-scale biomass valorization. Fig. 3 illustrates these considerations, highlighting both shared and exclusive features relevant to each catalytic route.
 | ||
| Fig. 3 Key design principles for metal-free catalysts, highlighting shared features and specific requirements for thermocatalytic, photocatalytic, and electrocatalytic biomass valorization. | ||
Equally important is the selection of raw materials, which directly influences cost-effectiveness and catalytic performance. Renewable, abundant, low-cost precursors such as lignin, cellulose, and agricultural residues are frequently employed to align with circular economy principles. For example, carbon-based catalysts derived from biomass via pyrolysis provide high carbon content while minimizing environmental impact.24 Additionally, N-doped carbons and g-C3N4 are synthesized using inexpensive feedstocks like urea and melamine. Boron and phosphorus-based catalysts utilize accessible inorganic salts such as boric acid and phosphates.25 Its emphasis on affordable and renewable precursors ensures large-scale production with a minimal ecological footprint.
To further enhance performance, tuning of textural and electronic properties is essential. Properties like surface area, pore volume, and meso-porosity,26 should be taken care to improve reactant diffusion, active site accessibility, and catalyst–substrate interaction.27 Heteroatom doping creates functional acidic, basic, or redox-active sites and modulates the electronic structure, influencing charge transport, Fermi level, and redox potential alignment.28,29 Dopant configuration, spatial distribution, and defect chemistry collectively influence activity and product selectivity, which is particularly important for complex biomass-derived feedstocks. Surface functionalization with basic (–NH2), and acidic (–SO3H) groups also assists in achieving a highly interactive catalyst surface.30
Finally, practical implementation often requires integration into modular reactor systems. The compatibility of metal-free catalysts with flow reactors, photoreactors, and electrochemical cells expands their applicability under continuous processing conditions, which is a key requirement for future biorefinery-scale deployment. Together, these considerations provide a rational foundation for designing metal-free catalysts that can operate effectively under diverse reaction environments. Collectively, these strategies ensure that metal-free catalysts achieve high catalytic efficiency and alignment with the principles of green chemistry, cost-effectiveness, and scalable deployment.
Thermal catalysts in biomass valorization must withstand harsh operating conditions, including elevated temperatures and chemically aggressive environments such as acidic, basic, or hydrothermal media. Stability under these conditions is essential for maintaining catalytic performance during transformations like hydrogenation, deoxygenation, hydrolysis, and lignin depolymerization. Effective thermal catalysts typically feature robust porous carbon frameworks and stable acidic or basic functional groups (e.g., –SO3H, –COOH, quinones, P–OH) that promote bond activation and facilitate hydrogen transfer pathways.
Photocatalytic systems, in contrast, rely on visible-light absorption and efficient charge separation. Materials like graphene and g-C3N4 exhibit efficient light absorption, charge transport, and separation, which are vital for photocatalytic applications.31 Bandgap engineering through doping, defect creation, or hybridization adjusts light absorption and positions the conduction and valence bands relative to target redox reactions. Strategies such as heterojunction formation or π-conjugated carbon integration minimize electron–hole recombination. Photostability is also critical; prolonged irradiation can degrade catalyst frameworks unless π-conjugation and dopant anchoring stabilize the structure.
Electrocatalysts introduce the need for high electrical conductivity and low interfacial charge resistance. Graphitized carbon, N-doped graphene, and CNTs offer excellent conductivity.32 The catalyst's Fermi level must align with the redox potential of the reaction facilitated by doping with N or O-containing groups. High electrochemical surface area, optimal wettability, and structural defect density further enhance current density and selectivity.33 Hydrophilic functional groups and porous frameworks improve ion diffusion and electrolyte contact in aqueous systems. Moreover, low charge transfer resistance (Rct) and stable performance across redox cycles are essential for long-term operation. These characteristics make metal-free catalysts categorized according to their suitability in thermal, photo, and electrocatalysis (Table 1).
: highly suited, 
: moderately suited, 
: less suited), with brief justifications
| S.no. | Metal-free catalyst | Thermocatalysis | Photocatalysis | Electrocatalysis |
|---|---|---|---|---|
| 1 | g-C3N4 |
 Moderate thermal stability, limited to ∼550 °C |
 Semiconductor (bandgap ∼2.7 eV) |
 Insulating without conductive hybrid |
| 2 | Graphene/rGO |
 Chemically inert, needs activation |
 Enhances charge transfer & stability |
 High conductivity and ECSA |
| 3 | Graphene oxide (GO) |
 Thermally unstable at high T |
 O-rich surface promotes charge separation |
 Low conductivity limits performance |
| 4 | Activated carbon/biochar |
 Robust acid/base catalysis |
 No optical absorption |
 Conductivity varies with structure |
| 5 | Carbon nanotubes (CNTs) |
 Requires functionalization |
 Mainly as support |
 Excellent electron transport |
| 6 | Carbon dots |
 Low thermal durability |
 Strong light absorption & PL |
 Low active site density |
| 7 | Heteroatom-doped carbons |
 Strong acid/base/redox sites |
 Doping tunes band structure |
 Enhanced conductivity and selectivity |
| 8 | Hexagonal-boron nitride |
 Excellent thermal stability |
 Wide bandgap, low absorption |
 Insulating, limited e− transfer |
| 9 | Polymeric materials/COF |
 Tunable pore/acid sites |
 Bandgap-engineered frameworks |
 Low conductivity unless hybridized |
| 10 | g-Phosphorus |
 Air-sensitive, needs protection |
 Narrow bandgap, visible-light active |
 Oxidation susceptibility |
| 11 | Sulfur–nitrogen polymers |
 π-Backbone offers redox activity |
 π-Conjugated light harvesting |
 Poor intrinsic conductivity |
2.1. Raw materials and synthetic strategies for metal-free catalysts
The design of high-performance metal-free catalysts for biomass valorization relies heavily on the judicious selection of raw materials and their transformation via robust and tunable synthetic strategies. These two aspects, material origin, and synthesis route, determine the final physicochemical properties of the catalyst (e.g., surface area, porosity, acidity/basicity, conductivity) and impact sustainability, scalability, and application-specific performance. This section provides a focused overview of the natural and synthetic precursor categories and the key synthetic strategies employed to convert them into advanced metal-free catalysts suitable for thermocatalytic, photocatalytic, and electrocatalytic applications.2.1. Classification of catalyst precursors
Biopolymers like chitosan, due to their nitrogen-rich backbone, yield porous carbons with basic sites and tunable conductivity upon pyrolysis. These properties make them suitable for thermocatalysis, photocatalysis and electrocatalysis. For instance, N-doped activated carbon derived from chitosan has demonstrated remarkable selectivity and efficiency in biomass valorization reactions, such as hydrogenation and oxidation.35,36 Chitosan-derived activated carbon, for instance, has shown high selectivity in biomass valorization due to abundant –NH2, –OH, and graphitic-N functionalities.37–40
2.2. Synthesis strategies and their influence on material properties
Calcination is a thermal treatment process carried out in an air atmosphere (typically at 400–900 °C) to decompose organic precursors, remove volatile components, and induce carbonization. It is widely used to convert biomass, biopolymers, or synthetic polymers into porous carbon materials. During calcination, oxygenated surface functionalities are induced, which are helpful in thermal and electrocatalytic oxidation reactions,46 but the only problem is the loss of carbon due to CO2 formation, which limits the use of calcination in carbon material synthesis.
2.3. Influence of precursor and synthesis strategy on catalyst properties
The physicochemical characteristics of metal-free catalysts, such as porosity, acidity/basicity, conductivity, optical absorption, and stability, are intricately governed by the nature of the precursor and the method of synthesis. Understanding and controlling these links are key to engineering tailored catalysts for thermal, photo, and electrocatalytic biomass valorization.2.4. Sustainability and scalability considerations
The transformation of waste-derived materials into high-value carbon catalysts exemplifies the sustainable “waste-to-wealth” paradigm.34 Utilizing marine waste (e.g., chitin, chitosan), agricultural residues, and plastic waste as precursors offers both environmental benefits and resource circularity. Innovative processes like pyrolysis, hydrothermal treatment, and CVD enable the conversion of these feedstocks into functional carbon materials with desirable properties such as porous structure, large surface area, and tailored functionality.62 A schematic representation of this strategy is shown in Fig. 4. | ||
| Fig. 4 Schematic representation of the waste-to-wealth paradigm, showcasing diverse feedstocks (marine, agricultural, and plastic waste) converted into functional carbon materials via pyrolysis, carbonization, and activation, for catalytic biomass valorization. | ||
Challenges that remain include ensuring batch reproducibility, optimizing surface functionalities for application-specific needs, and scaling synthesis methods for industrial use. However, by leveraging these sustainable resources and scalable methods, metal-free catalysts can be designed to align with green chemistry and circular economy goals.
In conclusion, a rational combination of appropriate precursors and synthetic methods enables the tailoring of metal-free catalysts for specific catalytic functions. A deeper understanding of how the choice of starting material and preparation route affects key structural and electronic properties is critical to advancing the field of sustainable catalysis.
3. Types of metal-free catalysts
A defining characteristic of metal-free catalysts is their reliance on carbon as a structural backbone, owing to their exceptional versatility, chemical stability, and tunable properties. These attributes directly benefit biomass valorization by providing a robust platform for designing catalysts with high stability, optimized reactivity, and enhanced interactions. The sp2-hybridized structure of carbon enables the formation of conjugated systems, such as graphene and g-C3N4, which exhibit excellent electron mobility and conductivity.63 These attributes make carbon-based materials ideal for catalytic applications. Furthermore, carbon's ability to bond with various elements allows the design of functional materials with tailored properties.Standalone carbon-based catalysts, such as activated carbon, graphene, and carbon nanotubes, feature high surface areas, thermal stability, and porous architectures that promote interactions with biomass-derived molecules. These materials can be further enhanced through heteroatom doping (e.g., N, B, P, S) or surface functionalization, introducing active sites to control acidity, basicity, or redox behaviour. Beyond carbon-only materials, heteroatom-doped carbons, and non-carbon-based frameworks such as boron nitride (BN), sulfur–nitrogen polymers (SNx), and covalent organic frameworks (COFs) extend the range of metal-free catalysts. These materials combine tailored surface chemistry with tunable electronic and structural properties, making them particularly effective in thermal and photocatalytic processes.
3.1. Carbon-only materials
 | ||
| Fig. 5 Catalytic features and synthesis of graphene. (a) Reactive prismatic sites at zigzag and armchair edges, and stable basal planes. (b) Oxygen-functional groups in graphene oxide influencing catalytic properties. Reproduced with permission from ref. 67, Copyright 2018 Royal Society of Chemistry. (c) Hummers' method for the synthesis of graphene oxide and reduced graphene oxide. | ||
Graphene oxide (GO), a derivative of graphene, features abundant O-containing functional groups, such as hydroxyl (–OH), carbonyl (C![[double bond, length as m-dash]](https://www.rsc.org/images/entities/char_e001.gif)
O), and carboxyl (–COOH), primarily located at its edges and defect sites (Fig. 5b).67 These groups act as active centres for key catalytic reactions, including oxidation, deoxygenation, and depolymerization. Zhang et al. demonstrated the remarkable ability of GO to catalyze the depolymerization of lignin under mild conditions, achieving a β-O-4 bond cleavage efficiency of over 80%, which resulted in a high yield of phenolic monomers.68 When compared to traditional catalysts, GO's efficiency was approximately 20% higher, highlighting the impact of its abundant O-functional groups in facilitating β-O-4 bond cleavage for effective biomass valorization. This result underscores the potential of GO as a sustainable, metal-free catalyst for biomass valorization.
The synthesis of graphene and its derivatives is central to enhancing their catalytic performance and scalability. Techniques such as CVD, epitaxial growth on silicon carbide, and chemical reduction of graphite oxide are widely adopted. Among these, Hummers' method (Fig. 5c) is a prominent approach involving the oxidation of graphite into graphite oxide (GO) using strong oxidizers (e.g., sulfuric acid, nitric acid, potassium permanganate). It is followed by chemical or thermal reduction to produce reduced graphene oxide (rGO).69 However, challenges such as re-stacking graphene sheets due to π–π interactions can hinder performance, necessitating functionalization with hydrophilic or hydrophobic groups to preserve catalytic properties. The synthesis of rGO has received significant attention due to its superior electronic conductivity and catalytic performance. Greener reduction methods have gained prominence for their alignment with sustainable chemistry principles. Palomba et al. reviewed using L-ascorbic acid as an eco-friendly reducing agent for converting GO to rGO.70 This method effectively removes O-containing functional groups while preserving graphene's properties, offering a non-toxic and efficient alternative to traditional reductants like hydrazine.
The synergy between graphene's reactive prismatic edge sites, O-functionalized basal planes, and scalable synthesis methods strengthens its role as a vital material for biomass valorization. Graphene's ability to catalyze essential transformations, such as lignin depolymerization,68 highlights its versatility as a metal-free catalyst, advancing the pursuit of sustainable and green chemical processes.
In biomass valorization, CNTs are rarely utilized as standalone catalysts but play a critical role as active supports and promoters.73 Their tubular morphology and high electron mobility facilitate mass transport and electron transfer, enabling efficient catalytic transformations. Functionalization and heteroatom doping further enhance their catalytic properties, introducing active sites tailored for specific reactions. N-doping, for instance, imparts basicity, enhancing activity in base-catalyzed reactions, while B-doping introduces Lewis acidic sites, facilitating reactions such as hydrodeoxygenation and oxidative dehydrogenation.74,75
Synthesis methods for CNTs include high-temperature techniques like arc discharge, laser ablation, and CVD, which allow precise control over alignment, purity, and morphology. Sustainable approaches, such as using biogenic catalysts derived from agricultural residues or natural materials like garnet sand, have gained attention to reduce environmental impact.76 Functionalization, such as the addition of hydroxyl (–OH) or carboxyl (–COOH) groups, improves dispersibility in polar solvents and enhances interactions with biomass-derived molecules.
CNTs’ role as promoters in biomass valorization has been particularly well-documented. In a study by Faba et al., CNTs were used as mass-transfer promoters for converting cellulose into 5-hydroxymethylfurfural (5-HMF) in a water–organic biphasic system.77 The authors demonstrated that CNTs enhanced about 20% cellulose conversion, accelerating the liquid-phase mass transfer of 5-HMF and improving extraction kinetics by 3.7 times. This displacement of equilibrium steps increased productivity by 270 times under acidic conditions, showcasing CNTs’ efficacy in enhancing catalytic performance. In addition to promoting mass transfer, CNTs have demonstrated catalytic activity in lignin depolymerization.
Despite their potential, challenges remain in scaling up CNT production and achieving uniform doping. The re-stacking of layers and variability in synthesis conditions limit their widespread adoption. Continued advancements in cost-effective synthesis, functionalization, and biogenic catalyst use could address these challenges, paving the way for broader applications of CNTs in biomass valorization.
Commercial AC catalysts, such as NORIT® (Cabot Corporation) and DARCO® activated carbon (Imerys), have been widely adopted in industrial processes. NORIT® AC, for instance, is used in hydroprocessing to remove impurities from bio-oil, improving its quality for fuel production.80 Similarly, DARCO® activated carbon is employed in catalytic applications for decolorization and purification in biorefineries, demonstrating its versatility in biomass processing.81 AC also serves as a robust support for metal-loaded catalysts, enabling the dispersion of metal nanoparticles while maintaining structural stability under harsh reaction conditions. For instance, metal-supported AC is frequently used in hydrodeoxygenation and hydrogenation reactions to upgrade bio-oil and produce high-quality biofuels.82
Beyond biomass valorization, AC finds applications across diverse fields, including water purification, gas adsorption, supercapacitors, and environmental remediation. Its porous architecture and functionalized surfaces enable efficient removal of contaminants, selective gas separation, and energy storage, further emphasizing its adaptability.
The surface chemistry of AC plays a critical role in its catalytic applications. O-containing functional groups, including –COOH, –OH, and CO, are introduced during activation and can be selectively tuned to influence acidity, basicity, or redox activity.83 For example, Shrotri et al. demonstrated that air-oxidized AC derived from eucalyptus biomass could catalyze cellulose hydrolysis without prior lignocellulose fractionation.84 The material, prepared by heating eucalyptus biomass at 300 °C, exhibited aromatic structures and carboxylic groups (2.1 mmol g−1), converting cellulose to glucose (78%) and xylose (94%). The oxidation temperature influenced functional group formation, with higher temperatures favoring carboxyl groups and improved catalytic efficiency.
The hierarchical porosity of AC enhances its performance by concentrating biomass-derived reactants at active sites and facilitating mass transport. This property is particularly beneficial in biphasic systems, where AC acts as an interfacial stabilizer, promoting efficient mass transfer between aqueous and organic phases. Furthermore, AC's robustness under harsh conditions, including high temperatures and acidic or basic environments, enables its application in diverse catalytic processes, such as hydrolysis, dehydration, and depolymerization.85
Despite its versatility, challenges such as the energy-intensive activation process and the need for precise control of functional group distribution remain. Recent advances in sustainable synthesis approaches, including hydrothermal synthesis, and renewable feedstocks, offer promising solutions for reducing production costs and environmental impact. With its adaptability, renewable origins, and scalability, AC remains a cornerstone material in metal-free catalysis, driving innovations in sustainable biomass valorization while maintaining its significance across other key industrial domains.
 | ||
| Fig. 6 Comparative illustration of semiconductor quantum dots and CQDs, emphasizing the tunable surface functional groups of CQDs and their diverse applications in photocatalysis, including CO2 conversion, biomass valorization, water splitting, degradation, and chemical reactions. Reproduced with permission from ref. 86, Copyright 2020 Royal Society of Chemistry. | ||
The synthesis of CDs is broadly categorized into top-down and bottom-up approaches. Top-down methods involve exfoliating larger carbon structures, such as graphite, candle soot, and carbon nanotubes, into nanoscale CDs using techniques like laser ablation, electrochemical etching, and arc discharge.89,90 These methods yield CDs with high crystallinity and clean surfaces. Conversely, bottom-up approaches rely on carbonizing small molecules or polymeric precursors containing functional groups like –OH, –COOH, and –NH2 groups. Techniques such as hydrothermal synthesis, microwave-assisted synthesis, thermal decomposition, templated routes, and plasma treatment allow for precise control over the size, morphology, and surface chemistry of the resulting CDs.91
Due to their tunable bandgap, up-converted PL, and photoinduced electron transfer capabilities, CDs have demonstrated exceptional potential in photocatalytic biomass valorization.92,93 For instance, CDs have been employed in the oxidative depolymerization of lignin, facilitating its breakdown into valuable aromatic monomers under light irradiation. In a study by Jiang et al., carbon quantum dots modified WO3 nanosheets were utilized for the photocatalytic depolymerization of lignin, achieving significant conversion rates and yielding high-value chemicals such as vanillin and vanillic acid under visible light illumination at room temperature.94 In composite systems, CDs enhance light absorption and promote electron–hole (e−/h+) pair separation, thereby improving the efficiency of semiconductor photocatalysts in biomass valorization reactions. For example, Zhao et al. developed carbon quantum dots modified TiO2 composites for H2 production and selective glucose photo-reforming, demonstrating enhanced photocatalytic activity and selectivity under neutral conditions.95
Despite their numerous advantages, CDs face challenges such as scalability, synthesis uniformity, and the optimization of surface chemistry for specific catalytic reactions.
3.2. Heteroatom-doped carbon materials
Heteroatom-doped carbon catalysts present a major advancement in carbon-based catalysis by incorporating heteroatoms such as N, B, S, P, or O into the carbon framework. The doping process alters the electronic structure of the carbon matrix, generating active sites that enhance catalytic activity, selectivity, and stability for a range of reactions. Surface chemistry plays a crucial role in the catalytic performance of these materials, with studies demonstrating that introducing electron-donating or electron-withdrawing elements produces functional groups on the carbon surface, thereby boosting catalytic properties.96Fig. 7 provides an overview of single heteroatom doping and the synergistic benefits of dual and ternary doping.97 It illustrates how doping multiple heteroatoms enables fine-tuning band gaps and surface polarity, enhancing thermal and photocatalytic performance across various applications. | ||
| Fig. 7 Overview of heteroatom-doped carbon catalysts including their synthesis methods, precursors used, properties induced, and applications. N, B, P, S, and D/T corresponds to nitrogen, boron, phosphorus, sulphur, and dual/ternary doped carbon catalysts. | ||
Surface oxidation is one of the most widely used methods for modifying carbon surfaces. Researchers use it to incorporate oxygen-containing groups, which affect carbon materials’ hydrophobic or hydrophilic nature. It enhances its wettability and dispersibility, which is beneficial for polar solvent-based reactions.83 Insights from surface oxidation studies have also laid the foundation for heteroatom doping, which goes a step further by embedding heteroatoms into the carbon framework. This functionalization introduces electron-modulating groups and creates high-reactive sites that substantially improve catalytic performance.98,99
Nitrogen doping, in particular, is highly favored due to its ability to enhance basic and base-catalyzed reactions while improving charge transfer properties and adsorption of organic compounds.28,100,101 The adaptability of heteroatom doping elevates catalytic efficiency and broadens the scope of applications, making these materials invaluable in thermal and photocatalytic processes, especially in metal-free catalytic systems.
Building on this foundation, the following section delves deeper into the properties, synthesis strategies, and applications of hetero-doped carbon catalysts. The focus will be on how each hetero-atom species influences catalytic activity and how advanced synthesis techniques enable the tailored design of these highly effective materials.
Researchers incorporate N atoms in various forms, including edge-bound functional groups such as amino or cyano groups, substitutional doping within graphene layers, or disordered regions containing sp3-hybridized carbon (Fig. 8a).103 Different types of N atoms, such as pyridinic, pyrrolic, and graphitic-N, contribute uniquely to catalytic properties.104 Pyridinic-N enhances charge transfer and acid–base properties, pyrrolic-N improves redox activity, and graphitic-N increases conductivity and stability. Pyridinic and pyrrolic-N structures contribute significantly to catalytic activity by modifying the material's acid–base properties.105 The critical role of specific N species was explored by Xiong et al. in the hydrogenation of nitrobenzene using N-CNTs prepared through in situ synthesis and post-nitridation treatments.106 Their work identified pyrrolic-N as the key active site for catalytic activity, facilitating the chemisorption and dissociation of H2 molecules. Catalysts with abundant pyrrolic-N species achieved a conversion of 60% and a selectivity of 90% for aniline, at 170 °C with excellent recyclability over six cycles. The proposed mechanism, supported by DFT calculations, highlights pyrrolic-N as the primary centre for hydrogen activation, while defects were deemed catalytically inactive.
 | ||
| Fig. 8 (a) Schematic representation of different types of N in the N-doped carbon framework, highlighting their potential catalytic roles. Reproduced with permission from ref. 103. Copyright 2018 Elsevier. (b) Chemical structures of common N-rich precursors (Kevlar, Nomex, Kapton, Melamine, and Urea) used for N-doped carbon synthesis, and (c) a process flow for synthesizing N-doped carbons from biopolymer precursors using pyrolysis and thermal treatments under an inert atmosphere. Reproduced with permission from ref. 37. Copyright 2024 John Wiley and Sons. | ||
Researchers synthesize N-doped carbon materials through versatile methods, including the carbonization of N-rich organic precursors or the high-temperature treatment of carbon materials with N-containing gases. Plasma-assisted synthesis has recently emerged to achieve precise N-doping configurations, allowing for greater control over catalytic properties. The common precursors include natural sources such as peat and synthetic polymers like poly(acrylonitrile) (PAN), vinylpyridine resins, and polyimides such as Kevlar, Nomex, and Kapton (Fig. 8b).107 N-rich biopolymers such as chitin and chitosan, derived from crustacean shells and fungal cell walls, are also widely used as sustainable precursors. Chitosan, for example, is thermally treated to produce N-doped carbons with active sites suitable for base-catalysed reactions (Fig. 8c).37,108 These biopolymer-derived materials exhibit significant catalytic activity due to the inherent N content and the presence of functional groups that enhance surface chemistry. Furthermore, N-doped carbons derived from disordered structures, such as activated carbons, allow higher N incorporation than well-crystallized graphite, making them more effective in catalytic applications. The combination of enhanced electrical properties, acid–base activity, and synthesis flexibility positions N-doped carbon materials as promising catalysts for various applications, including photocatalysis, electrocatalysis, thermo-catalysis, and biomass valorization.
3.2.1.1. Graphitic-carbon nitride: a nitrogen-rich photocatalyst. Graphitic-C3N4 has attracted significant attention as a promising metal-free photocatalyst due to its unique properties, sustainable composition, and broad applicability. As an N-rich, two-dimensional polymeric material, g-C3N4 is composed of earth-abundant C and N, with trace amounts of H associated with surface terminations and uncondensed –NH2 groups. An ideal C/N stoichiometric ratio of 0.75 is challenging to achieve in practice, as the synthesized samples typically show a slightly lower ratio (∼0.72) and around 2% residual hydrogen.109 The simple composition, combined with its non-toxic and biocompatible nature, positions g-C3N4 as a strong candidate for green chemistry and visible-light-driven photocatalysis.
The history of carbon nitrides dates back to 1834 when Liebig synthesized “melon”, a linear polymer of connected tri-s-triazine units.110 However, the remarkable potential of this material was not fully realized until recent decades. g-C3N4 is the most stable among the five structural forms of C3N4 – α, β, cubic, pseudo-cubic, and graphitic, because of its layered, graphene-like structure. Its framework is built from tri-s-triazine (heptazine) units connected by planar tertiary amines, forming a highly conjugated network. This structure is about 30 kJ mol−1 more stable than s-triazine-based configurations, making tri-s-triazine favoured building block.111 A single layer of g-C3N4 contains s-triazine (C3N3) units with single carbon vacancies alongside tri-s-triazine/heptazine (C6N7) units, connected by tertiary amines.112
As depicted in Fig. 9a, the synthesis of g-C3N4 is cost-effective and straightforward, typically involving the thermal polymerization of N-rich precursors such as urea, melamine, dicyandiamide, or cyanamide. These precursors enable scalability and accessibility, making g-C3N4 a practical choice for widespread applications.
 | ||
| Fig. 9 (a) Melamine derived tri-s-triazine-based layered structure of g-C3N4, illustrating its conjugated polymeric framework and N-rich architecture. Reproduced with permission from ref. 112. Copyright 2012 IOP Publishing. (b) Comparative UV-visible absorption spectra of g-C3N4 and N-doped carbon highlighting g-C3N4's visible-light absorption edge (∼420–450 nm) enabled by its ∼2.7 eV bandgap, (c) PL spectra comparing g-C3N4 and N-doped carbon, (d) mechanism of visible-light-driven photocatalysis on g-C3N4, depicting photogenerated e−/h+ separation. (b)–(d) Reproduced with permission from ref. 121. Copyright 2025 American Chemical Society. | ||
The difference between g-C3N4 and N-doped carbon materials underscores their advantages in photocatalytic applications. While N-doped carbons rely on localized catalytic activity from N species like pyridinic and pyrrolic N, g-C3N4's tri-s-triazine-based polymeric framework provides intrinsic N functionality with evenly distributed active sites, which enhances its photocatalytic performance.113 N-doped carbons also exhibit excellent electrical conductivity, making them useful in electrocatalysis. However, they lack the broad photocatalytic versatility and visible-light absorption of g-C3N4 due to their larger bandgap, which confines their absorption primarily to the UV region.114 These distinctions arise from their contrasting structures: sp2-hybridized carbon lattices in N-doped carbons versus N-rich conjugated frameworks in g-C3N4. Fig. 9b highlights the UV-visible absorption spectra of g-C3N4 and N-doped carbon materials. g-C3N4 displays a distinct absorption edge in the visible-light region (420–450 nm) due to its moderate bandgap (∼2.7 eV), making it highly effective for solar-driven photocatalysis. In contrast, N-doped or other heteroatom-doped carbon materials primarily absorb in the UV region, limiting their applicability in solar-driven processes. Similarly, Fig. 9c compares the PL spectra of g-C3N4 and N-doped carbon materials. PL intensity reflects the recombination rate of photogenerated charge carriers. The enhanced charge dynamics of g-C3N4 significantly boost its photocatalytic efficiency, enabling it to outperform N-doped carbon materials in light-driven chemical transformations as N-doped carbon is incapable of visible light absorption. Upon light irradiation, g-C3N4 absorbs visible light, exciting electrons from the valence band (VB) to the conduction band (CB) and leaving photogenerated holes in the VB. These charge carriers drive photocatalytic reactions, where holes catalyze oxidation processes and electrons facilitate reduction reactions (Fig. 9d).
Studies have demonstrated that doping or structural modifications can further reduce the bandgap of g-C3N4 to ∼2.3 eV, enhancing its light absorption efficiency and extending its activity into the visible-light spectrum. Additionally, the absorption edge of g-C3N4 exhibits a red shift with increasing polymerization temperatures, further lowering the bandgap and improving its photocatalytic performance.115 These optical properties are complemented by excellent electronic conductivity because of the well-aligned tri-s-triazine units and the conjugated framework, making it a suitable for H2 evolution during photocatalytic water splitting under visible light, underscoring its potential for green energy applications.116
Modifications such as heteroatom doping, structural engineering (e.g., nanosheets and mesoporosity), and surface functionalization have proven effective in enhancing photocatalytic efficiency by improving charge carrier mobility, light absorption, and active site exposure. For instance, doping with elements like N, P, O, or B can tune its electronic structure, as detailed in Table 2, which outlines the properties and applications of heteroatom-doped g-C3N4 systems.117 Despite its numerous advantages, g-C3N4 faces challenges such as charge recombination, limited solar spectrum utilization, and achieving ideal stoichiometric control. To address these issues, innovative hybrid systems, such as Z-scheme photocatalysts, carbon channelizers coupled with g-C3N4, have been developed to enhance charge separation and broaden light absorption.118–121 Additionally, the development of advanced co-catalysts is crucial for mitigating charge recombination and further boosting the practical efficiency of g-C3N4-based systems.
| S. no. | Heteroatom-doped g-C3N4 system | Precursors | Synthesis techniques | Properties | Applications |
|---|---|---|---|---|---|
| 1 | Bulk-g-C3N4 | Melamine, urea, dicyandiamide, cyanamide | Thermal polymerization | Bandgap ∼2.7 eV; stable, layered structure; moderate photocatalytic activity | Photocatalytic water splitting, pollutant degradation, CO2 reduction, H2 evolution |
| 2 | B-g-C3N4 | Bulk g-C3N4 + boric acid, boron oxide, boron-containing organics | Thermal annealing, chemical vapor deposition (CVD), or solvothermal methods | Lower bandgap (∼2.5 eV), enhanced visible-light absorption, improved charge separation, Lewis acidic sites | Photocatalytic lignin depolymerization, selective oxidation, CO2 reduction, water splitting, enhanced solar-driven reactions |
| 3 | S-g-C3N4 | Bulk g-C3N4 + thiourea, elemental S, sodium thiosulfate | Thermal annealing or hydrothermal treatment | Narrowed bandgap (∼2.4–2.6 eV), strong visible-light absorption, S-containing active sites | Photocatalytic H2 production, pollutant degradation, CO2 photoreduction |
| 4 | P-g-C3N4 | Bulk g-C3N4 + phosphoric acid, phosphonates, or ammonium phosphates | Thermal condensation or post-treatment annealing | Narrowed bandgap (∼2.5 eV); improved charge mobility; higher photostability; active phosphorus sites | Photocatalytic selective oxidation, water splitting, CO2 reduction, biomass valorization |
| 5 | N-g-C3N4 | Bulk g-C3N4 (inherently nitrogen-rich); modified precursors with additional nitrogen (e.g., NH3 gas) | Thermal polymerization, post-treatment with N-rich precursors | Enhanced nitrogen active sites (e.g., pyridinic-N, pyrrolic-N); improved acid–base catalysis | Redox catalysis, ORR/HER electrocatalysis, biomass valorization |
| 6 | O-g-C3N4 | Bulk g-C3N4 + oxygen-containing agents (e.g., H2O2, K2Cr2O7, KMnO4) | Post-synthesis oxidation or hydrothermal treatment | Enhanced hydrophilicity, improved charge separation, additional O-containing functional groups | Photocatalytic selective oxidation, pollutant degradation, biomass valorization, enhanced photocatalytic H2 production |
| 7 | B–N–g-C3N4 | Bulk g-C3N4+ precursors containing B and N (e.g., boric acid + urea) | Co-doping via thermal annealing or solvothermal methods | Synergistic effects from B and N, reduced bandgap (∼2.3–2.5 eV), high visible-light activity, improved photostability | Photocatalytic enhanced H2 production, pollutant degradation, CO2 reduction, hybrid catalysts for high efficiency biomass valorization reactions |
The synthesis of B-doped carbons employs methodologies such as high-temperature annealing with boron precursors (e.g., boric acid), CVD, and hydrothermal techniques. These methods enable precise control over doping levels and configurations, introducing graphitic sp2 B (BC3) species or interstitial B into the carbon matrix.126 This tailored doping improves catalytic activity by modulating electronic properties and creating uniformly distributed active sites. For instance, Wang et al. synthesized B-doped polymeric carbon nitride (B-PCN) and demonstrated selective oxidation of aliphatic C–H bonds at 150 °C, achieving stability and reactivity enhancements without metal-based catalysts.127 Similarly, Cheng et al. highlighted the advantages of B-doped graphene for the gas-phase oxidation of benzyl alcohol to benzaldehyde in a fixed-bed reactor operating in 473–523 K, reporting a 2.35-fold increase in conversion rates and remarkable selectivity of 99.2% for benzaldehyde compared to undoped graphene.128
The surface chemistry of B-doped carbons further enhances their catalytic performance. B-doping improves hydrophilicity, facilitating stronger interactions with aqueous biomass-derived reactants, and also reduces e−/h+ recombination in composite photocatalytic systems. In a photocatalytic set-up irradiated by a 300 W Xe-lamp, Wang et al. developed g-C3N4/BCNQDs heterojunction photocatalysts through hydrothermal synthesis, achieving HER that was 11 times higher than g-C3N4 alone.129 This enhanced photocatalytic activity was attributed to the adjusted band structure of BCNQDs via B doping and efficient charge separation across the heterojunction interface. These findings emphasize the potential of B-doped carbons in optimizing electronic and photocatalytic properties for sustainable hydrogen production, pollutant degradation, and oxidation reactions.
The synthesis of P-doped carbon materials relies on various P-containing precursors, such as orthophosphoric acid (H3PO4), ammonium phosphate, and phytic acid, paired with carbon-rich materials like lignin, cellulose, and glucose.133,134 During thermal treatment, P reacts with carbon to form chemically bonded dopants, which modify electronic properties and introduce defects. High-temperature annealing in inert environments is commonly employed, although it poses challenges for energy efficiency and sustainability. Alternative synthesis strategies, including microwave-assisted and hydrothermal methods, have been explored to overcome these limitations. For example, Patel et al. utilized phytic acid in a microwave-assisted synthesis of P-doped graphitic porous carbon, yielding materials with well-defined PO and P–OH functionalities that acted as active sites for selective aerobic oxidation.133 Similarly, Hu et al. developed porous P-doped carbons using soluble starch and phosphoric acid, achieving complete conversion (>99%) of benzyl alcohol and demonstrating the critical role of P–O–C species in catalysis.131
The distinct characteristics of P doping arise from its lower electronegativity than C and N (2.19 versus 2.55 and 3.04, respectively). This polarity difference in C–P bonds results in partially positively charged P atoms acting as catalytic sites. Additionally, the large atomic diameter of P, induces structural distortions, introducing defects that enhance catalytic functionality.18 These effects are further amplified by co-doping with O, forming functional groups like PO, P–O, and P–O–C that significantly influence electronic properties and catalytic performance.135 P-doped carbons exhibit excellent charge separation, enabling efficient redox reactions and enhancing photocatalytic activity.
P doping also plays a significant role in photocatalysis. For instance, Qian et al. synthesized P-doped graphene quantum dots (P-GQDs) that formed stable p–n junctions with g-C3N4, resulting in enhanced photocatalytic performance in Rhodamine B degradation.128 The P-GQDs introduced efficient charge separation mechanisms, demonstrating the potential of P doping to optimize electronic and optical properties for photocatalytic applications. Beyond catalysis, P-doped carbon materials find applications in energy storage, such as supercapacitors and batteries, due to enhanced conductivity and pseudo-capacitance. Their high surface area, porosity, and functional groups make them valuable for sensors, water treatment, and air purification.
For instance, a study reported that sulfur-doped g-C3N4 achieved a hydrogen evolution rate of 133.12 μmol h−1, which is nearly eight times higher than that of pure g-C3N4.146 The combination of S and O functionalities further improves hydrophilicity and promotes hydroxyl radical formation, which is beneficial for reactive adsorption and photocatalysis under solar irradiation.
Beyond photocatalysis, S-doped carbons excel as solid acid catalysts for biomass valorization. Functionalized with sulfonic acid groups (–SO3H), these materials provide robust acidic sites for esterification, transesterification, and hydrolysis reactions. For example, cellulose-derived carbon-bearing –Cl and –SO3H groups have been shown to achieve high glucose selectivity (95.8%) at moderate temperatures (155 °C) under hydrothermal conditions.147 Their stability and reusability make them ideal for green chemistry applications, such as biodiesel production. The synergy between S and O functionalities amplifies catalytic efficiency, enabling high-performance applications in biomass valorization.
Synthesis methods for dual heteroatom-doped carbons include thermal annealing, hydrothermal treatment, and CVD. These techniques allow the precise incorporation of more than one dopant while maintaining control over material morphology. For instance, N, S and N, P co-doped carbon materials have demonstrated exceptional performance in catalysis due to their synergistic electronic and structural properties. Hu et al. synthesized hierarchically porous N,S co-doped carbon (PDNSC-800) via polymer precursor carbonization, achieving superior catalytic activity in the reduction of nitroarenes employing hydrazine hydrate. This performance was attributed to the high surface area, hierarchical porosity, and active sites derived from N and S doping, with pyrrolic N playing a key role in selectivity.149 Similarly, N,P co-doped porous carbon materials derived from biomass waste have shown outstanding catalytic efficiency in the aerobic oxidation of alcohols. Yin et al. prepared NPC-800 using peanut shells and phosphoric acid, achieving 100% benzyl alcohol conversion with 99.7% selectivity to benzaldehyde under mild conditions along with a high TOF value of 2.49 × 10−3 mol g−1 h−1.150 The synergistic effect of N and P, along with the amount of graphitic-N and C3PO species on the surface of the catalyst, was shown to be correlated with catalytic activity. These examples highlight the versatility and scalability of dual-doped carbon materials in sustainable catalysis.
In photocatalysis, dual doping narrows the bandgap, enhances visible-light absorption, and facilitates charge separation, making these materials effective for applications like water splitting, CO2 reduction, and pollutant degradation.151,152 For example, P,S and N,S co-doped g-C3N4 (PSCN) have shown significantly enhanced photocatalytic activity. Hu et al. demonstrated that P atoms substituted carbon in the heptazine units of g-C3N4, forming covalent P–N bonds, while S occupied interstitial sites with weaker bonding to N. This dual doping improved charge transfer by creating an N–S–N–C–N–P pathway across the heptazine structure, enhancing charge separation and reducing the recombination of photogenerated carriers.153 These findings underscore the potential of dual doping to optimize the photocatalytic efficiency of g-C3N4 for environmental applications.
3.3. Non-carbon materials
While carbon-based catalysts have firmly established their dominance in metal-free catalysis, alternative frameworks such as graphitic phosphorus, boron nitride derivatives, and sulfur–nitrogen polymers are promising systems for diverse catalytic applications. These materials leverage the unique electronic, chemical, and structural properties of non-metal elements, offering functionalities that often complement or, in specific scenarios, surpass those of carbon-based systems. Many of these materials have been theoretically predicted to exhibit exceptional catalytic activity and reactivity, particularly through computational studies involving density functional theory (DFT). However, their practical realization remains limited, and experimental synthesis and characterization have proven challenging. This gap between theory and practice opens new avenues for exploration, with the potential to unlock transformative applications if these materials can be synthesized and validated under real-world conditions. Below is a detailed discussion of these materials, focusing on their synthesis, properties, and emerging applications. | ||
| Fig. 10 Structures of (a) g-phosphorous/layered phosphorene. Reproduced with permission from ref. 158. Copyright 2015 American Chemical Society. (b) Boron nitride, (c) S–N polymer chain, (d) boron phosphide framework, (e) α, β, γ forms of P–N materials. Reproduced with permission from ref. 179. Copyright 2015 Royal Society of Chemistry. | ||
Despite its theoretical potential, the practical utilization of phosphorene faces significant challenges, primarily due to its environmental sensitivity, such as rapid oxidation in air and chemical degradation under ambient conditions, and difficulties in scalable synthesis. Top-down methods, including mechanical and liquid-phase exfoliation, are employed but often result in limited yields or structural defects.158,159 Recent advances, such as plasma-assisted and microwave-assisted exfoliation, have improved scalability and product quality,160 while bottom-up approaches like CVD and wet-chemical synthesis remain underexplored yet hold immense potential.
Various strategies have been proposed to address rapid degradation under ambient conditions. Surface passivation using alkylamines and polymer coatings has created a protective barrier against oxidation. Encapsulation in inert matrices, such as hexagonal boron nitride (h-BN) or Al2O3,161 significantly enhances stability, while solvent engineering with ionic liquids or aprotic solvents minimizes degradation by suppressing interactions with moisture and oxygen. Furthermore, integrating phosphorene with other 2D materials leverages its anisotropic properties, creating synergies that enhance stability and functionality. By overcoming these limitations, phosphorene has the potential to revolutionize sustainable catalytic and optoelectronic technologies. Further advancements in synthesis techniques and stability enhancement will be critical to bridging the gap between its theoretical promise and practical applications.
Since BN does not occur naturally, its synthesis relies on precursors like boric acid (H3BO3) or boron trioxide (B2O3). Early synthesis efforts, such as Balmain's reaction of molten H3BO3 with KCN in 1842, laid the groundwork for modern production techniques.165 While early stabilization of BN powders remained challenging until the 1960s, advancements have rendered h-BN an affordable material available in various forms for industrial applications.163
The production of h-BN employs top-down and bottom-up techniques.166,167 Top-down methods, including mechanical and liquid-phase exfoliation, are cost-effective but often result in smaller, defect-prone flakes. In contrast, bottom-up approaches, such as CVD and pulsed laser deposition (PLD), produce high-quality, large-area films but face scalability and energy consumption challenges. Recent innovations, such as plasma-enhanced CVD, have improved synthesis efficiency, enabling better control over thickness and crystallinity.168
Functionalization and defect engineering are critical for activating the catalytic potential of h-BN. O-functionalized h-BN has shown promise in oxidative dehydrogenation and acid–base co-operative catalysis.163 By creating controlled defects or doping with elements like O or S, researchers have enhanced h-BN's reactivity and broadened its applicability in catalysis.
Polythiazyl [(SN)x], the first electrically conductive inorganic polymer, is a gold- or bronze-colored material with a metallic luster and exceptional electronic and structural properties, including anisotropic conductivity, high room-temperature conductivity (∼103 S cm−1), and unique S–N bonding that stabilizes its structure while enabling superconductivity (Fig. 10c).169 Discovered by F. P. Burt in 1910, it is the first non-metallic compound to demonstrate superconductivity at very low temperatures (below 0.26 K).170 Structurally, (SN)x is a fibrous solid with golden or dark blue-black faces, depending on orientation, and features alternating S and N atoms in its chains. This arrangement facilitates anisotropic conductivity, with metallic behavior along the chains and insulating properties perpendicular to them. The unique bonding conditions in the S–N chain involve π electrons forming two-center 3π electron bonds, which differ from conventional covalent bonds by delocalizing electrons over the S and N atoms. In 1975, Mikulski et al. synthesized analytically pure (SN)x crystals through the controlled solid-state polymerization of disulfur dinitride (S2N2), demonstrating that the material maintains thermal stability in a vacuum up to 150 °C.171 Kennett et al. explored alternative synthesis methods using azides and sulfur–nitrogen chlorides, achieving yields of up to 65% for high-purity (SN)x in solution.172
As its potential for catalysis remains unexplored, high conductivity and unique S–N bonding might enable (SN)x to act as an efficient electron donor and acceptor, enhancing reaction kinetics in energy-related catalytic processes.
B–P material, a III–V semiconductor with a cubic zinc blende structure, has emerged as a stable and chemically inert material with promising photocatalytic properties. The cubic zinc blende structure facilitates efficient charge separation and transport due to its symmetrical arrangement of tetrahedrally coordinated B and P atoms (Fig. 10d). This configuration minimizes recombination losses and enhances photocatalytic efficiency, particularly under visible light.173
Tetrahedrally coordinated B and P atoms form covalent bonds, imparting resistance to corrosion by concentrated acids and exceptional structural stability under harsh conditions. While B–P was first synthesized in 1957 through the reaction of B and red-P at elevated temperatures, recent advances include a solid-state thermal treatment of B and zinc phosphide, yielding pure n-type B–P.174 Woo et al. present a pioneering approach for synthesizing a novel Na2BP2 crystalline solid featuring one-dimensional B–P chains via oxidative polymerization of PBP anionic monomers.175 Recently, the material was characterized by an indirect bandgap of 2.0 eV and exhibits moderate photocatalytic activity for hydrogen evolution under visible light.176
Their tunable electronic properties and chemical stability make them excellent candidates for catalytic and electronic applications. However, challenges remain in controlling synthesis and addressing air sensitivity, which restricts scalability and practical deployment. Advances in heterostructure integration and surface modification are expected to unlock the full potential of B–P materials for sustainable technologies.
P–N frameworks, comprising covalently bonded P and N atoms, represent a promising class of materials due to their unique structural, electronic, and catalytic properties (Fig. 10e). Unlike other covalent networks, P–N frameworks combine high thermal and chemical stability with tunable electronic properties, such as variable bandgaps and ferromagnetic behavior.177 This duality arises from the strong P–N bonds and their ability to form diverse structures, including amorphous and crystalline forms, which enable precise control over active sites and electron transfer pathways, which are critical for catalytic and electronic applications.178 However, to date, most studies on P–N frameworks remain theoretical, relying heavily on computational approaches to explore their structural stability, electronic properties, and potential applications.179 These studies predict exceptional characteristics for both amorphous and crystalline forms, with 2D P–N monolayers and nitro-phosphorene emerging as standout candidates for advanced applications in catalysis, optoelectronics, and energy systems.180
P–N frameworks exhibit robust covalent bonds between P and N atoms, forming extended networks that display remarkable thermal and chemical stability. The amorphous P–N frameworks are highly disordered, which enables the exposure of active sites critical for catalytic performance. On the other hand, crystalline 2D P–N monolayers, such as orthorhombic or puckered honeycomb structures, offer well-defined electronic properties with tunable bandgaps ranging from 1.2 to 2.8 eV.177 The 2015 study by Shuang et al. analyzed three P–N monolayer sheets, α-PN, β-PN, and γ-PN, featuring quasi-2D hexagonal frameworks with a 1![[thin space (1/6-em)]](https://www.rsc.org/images/entities/char_2009.gif)
:1 P-to-N ratio and proposed CVD on silver substrates as a feasible synthesis route.179 In 2017, Tan et al. introduced g-PN, a novel two-dimensional material designed as a metal-free visible-light-driven photocatalyst for water splitting.178 The study highlights the role of tensile strain in reducing the bandgap to 2.27 eV, significantly enhancing visible-light absorption (400–800 nm) and photocatalytic performance. g-PN surpasses conventional photocatalysts like g-C3N4 in visible light absorption and stability. Although theoretical, the proposed CVD synthesis on silver substrates provides a plausible pathway for experimental realization, emphasizing the environmentally friendly and scalable potential of g-PN. Collectively, these studies underscore the possible broad applicability and future potential of P–N materials across cutting-edge technologies.
3.4. Polymeric materials and covalent organic frameworks
Polymeric materials and COFs have emerged as transformative platforms in metal-free catalysis due to their structural versatility, tunable properties, and ability to incorporate diverse functional groups.181 Functional groups such as hydroxyl (–OH) and carboxyl (–COOH) enhance hydrophilicity and catalytic activity in aqueous systems,182 while thiol (–SH) groups improve interactions with metal ions for pollutant removal.183 Similarly, hydrophobic groups like methyl (–CH3) and trifluoromethyl (–CF3) tailor compatibility with organic substrates, enabling selective transformations in nonpolar environments.184 These materials, ranging from polymeric photocatalysts such as conjugated polymers and covalent triazine frameworks (CTFs) to crystalline COFs, offer thermal and chemical stability, high porosity, and efficient light-harvesting capabilities.183,185 Such features make them ideal candidates for applications in energy conversion, environmental remediation, and sustainable chemical production through thermal and photocatalytic pathway.The synthesis of polymeric photocatalysts is centered on creating extended π-conjugated networks that promote light absorption and charge transport.186,187 Thermal polymerization, a widely used approach, involves heating N-rich precursors such as melamine, urea, or thiourea at 500–600 °C under inert conditions to produce polymeric g-C3N4 and related derivatives.188–191 Alternatively, chemical polymerization methods, including Suzuki and Stille coupling, are employed to construct conjugated polymers with tailored optical and electronic properties.192,193 Monomers such as thiophene, benzothiadiazole, and carbazole are commonly used to form π-conjugated systems that enable precise control over bandgap and charge mobility.
COFs are synthesized through covalent bonding between organic monomers, resulting in highly crystalline, porous frameworks. Schiff base chemistry provides a versatile and efficient approach for synthesizing COFs via the condensation of aldehydes and amines, forming stable imine (–CN–) linkages (Fig. 11a).28,194 Solvothermal synthesis is the most common method, where monomers such as boronic acids, aldehydes, or amines react under controlled temperature and pressure to yield ordered structures. Ionothermal synthesis, which uses ionic liquids as solvents, offers a sustainable and eco-friendly alternative. Interfacial polymerization, meanwhile, allows the formation of ultrathin COF layers with enhanced surface area and reactivity (Fig. 11b).195 An essential feature of COF synthesis is the ability to tune functional groups, incorporating moieties such as –OH, –COOH, and –SH to improve charge transport, light absorption, and catalytic activity.
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| Fig. 11 (a) COF synthesis based on Schiff base chemistry method. Reproduced with permission from ref. 28. Copyright 2017 Elsevier. (b) Synthesis of COF based membrane on the polymeric substrate through interfacial polymerization. Reproduced with permission from ref. 195. Copyright 2018 Elsevier. (c) Synthesis of functionalised polymer with high charge efficiency for H2O2 production. Reproduced with permission from ref. 200. Copyright 2024 John Wiley and Sons. | ||
In thermal catalysis, polymeric materials and COFs excel due to their stability and diversity in active site functionality. Polymeric photocatalysts, such as S-doped conjugated polymers, facilitate key biomass valorization reactions like lignin depolymerization and cellulose conversion to platform chemicals such as levulinic acid and furfural.196,197 Similarly, CTFs demonstrate strong activity in thermal oxidation and reduction reactions, driven by their robust frameworks and redox-active sites.198 Functionalized COFs with acidic groups such as –COOH or –SO3H and basic groups like –NH2 are widely used in hydrolysis, dehydration, and condensation reactions, enabling the efficient transformation of complex reactants under high-temperature conditions. For instance, COFs functionalized with –COOH groups have demonstrated superior performance in the hydrolysis of cellulose to glucose, achieving high conversion rates and selectivity under mild reaction conditions.199
The photocatalytic applications of these materials are equally promising. Conjugated polymers, especially donor–acceptor systems, are effective in water splitting, with benzothiadiazole–thiophene copolymers achieving high hydrogen evolution rates under visible light (Fig. 11c).200–202 Similarly, COFs with tunable band structures have shown great potential in CO2 photoreduction, converting CO2 into valuable products like methane and methanol.203 The porous structures of COFs enhance CO2 adsorption, while their functionalized surfaces stabilize reaction intermediates, improving product selectivity.204 Both polymeric materials and COFs are also employed in pollutant degradation, generating ROS under visible light to break down organic contaminants, such as dyes and pharmaceuticals, thereby contributing to environmental remediation.
The catalytic performance of polymeric materials and COFs is closely linked to their structural features and surface properties. High porosity and large surface areas ensure efficient diffusion of reactants and access to active sites, enhancing reaction rates. The crystalline nature of COFs minimizes charge recombination and improves charge mobility, while the extended π-conjugated systems in polymers allow for efficient light harvesting and charge transport. Functional group tuning further enhances their catalytic performance by enabling selective interactions with reactants. For instance, hydrophilic groups such as –OH and –COOH improve interactions with aqueous reactants, while hydrophobic groups such as –CH3 and –CF3 tailor compatibility with organic substrates. Notwithstanding their impressive capabilities, challenges remain in scaling up these materials for industrial applications and optimizing their stability under real-world conditions. Scalability issues often arise from the high costs and complexity of synthesis methods, such as solvothermal and interfacial polymerization, which require precise temperature and pressure controls. Additionally, ensuring the uniformity and crystallinity of COFs at larger scales is a persistent hurdle. Ongoing research is addressing these challenges through innovative approaches, including solvent-free synthesis methods, microwave-assisted polymerization, and the development of recyclable catalytic systems. These efforts aim to reduce production costs and improve the feasibility of COFs for industrial deployment.
4. Metal-free thermal catalysis for biomass valorization
Thermal catalytic processes of biomass valorization offer a practical and efficient route for converting complex components such as lignin and cellulose into valuable chemicals and fuels. The thermal approach effectively breaks down robust chemical bonds in the lignin and cellulosic part, enabling the production of platform chemicals like aromatic monomers, hydroxymethylfurfural (HMF), furfural (FAL), and levulinic acid (LA), which are essential precursors in the chemical and fuel industries. Industrially, thermal pathways are favored for their versatility and compatibility with existing reactor technologies, such as fluidized beds and fixed-bed reactors, allowing seamless integration into bio-refinery operations. Furthermore, the thermal approach enables high-throughput processing and can utilize diverse feedstocks, including lignocellulosic residues.This section explores the role of metal-free catalysts in thermal catalytic conversions in two parts: (i) cellulosic/hemicellulosic, and, (ii) lignin. Emphasis is given to reaction mechanisms, catalyst design, and the potential to address challenges such as process scalability, thermal stability, and product separation.
4.1. Metal-free thermal catalysis for cellulosic and hemicellulosic biomass
Cellulose and hemicellulose are the two major polysaccharides in lignocellulosic biomass, accounting for over 65–70% of its composition. Cellulose is a linear polymer of β-1,4-linked glucose units (C6), and serves as a renewable source for hexose sugars, which can be thermally dehydrated to produce HMF, a key platform chemical. Hemicellulose, on the other hand, is a branched heteropolymer composed primarily of pentose sugars such as xylose and arabinose (C5). These sugars undergo acid-catalyzed dehydration to yield FAL, a versatile biomass-derived intermediate. Thus, cellulose and hemicellulose offer distinct chemical building blocks that can be valorized via metal-free thermal catalysis. To date, thermal metal-free catalysis has contributed significantly to the value-added conversion of HMF, FAL, carbohydrates, and glycerol, which are discussed in particular subsections below. | ||
| Fig. 12 Conversion of cellulose to HMF via hydrolysis and dehydration, followed by its selective oxidation to value-added products. | ||
Among various heteroatom-doped carbon catalysts, N-doped carbon materials have emerged as promising metal-free catalysts in the selective oxidation of HMF due to their ability to activate molecular O2. Several studies have demonstrated the effectiveness of N-doped carbon catalysts in HMF oxidation, highlighting the role of graphitic-N and surface defects in this transformation.
In pursuing sustainable catalytic oxidation, Nguyen et al. introduced an approach, developing N-doped nanoporous carbon (NNC-900) from ZIF-8 precursors through high-temperature calcination.206 Their work showcased the remarkable potential of the metal-free catalyst in oxidizing HMF to FDCA, a bio-based alternative to petrochemical-derived terephthalic acid. With the reaction operating at 80 °C, for 48 hours, using O2 as the oxidant and K2CO3 as an additive, NNC-900 achieved an impressive 80% FDCA yield, reinforcing its efficiency in biomass-derived oxidation pathways. Delving into the catalyst's structural evolution, elemental analysis, and N2 adsorption–desorption studies provided key insights into its porosity and N functionality. As calcination temperature increased from 600 °C to 900 °C, the N content decreased (from 29.4% to 15.3%), attributed to the decomposition of N-containing imidazole groups. Simultaneously, the specific surface area increased from 600 to 800 m2 g−1, maintaining a micropore size of 0.6 nm, ensuring efficient substrate diffusion. More importantly, the fraction of quaternary N (graphitic-N) increased from 4% to 25%, a transformation that directly correlated with FDCA yield (Fig. 13a). These graphitic-N (N–Q) sites played a pivotal role in catalysis, facilitating sp2 N–O2 adduct formation, which, in turn, generated ROS essential for aerobic oxidation. To further unravel the oxidation mechanism, the researchers conducted radical scavenging experiments. The addition of 1,4-benzoquinone, a known radical inhibitor, significantly suppressed FDCA formation, validating a radical-mediated oxidation pathway. Beyond efficiency, catalyst longevity remains a crucial factor in sustainable oxidation processes. Recyclability tests revealed that while NNC-900 retained activity over four cycles, a 10% decline in FDCA yield was observed, attributed to gradual N loss. This finding underscored the delicate balance of the total N content and specific N content required for the efficient oxidant activation and HMF oxidation to FDCA.
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| Fig. 13 (a) The effect of different catalysts on the HMF to FDCA conversion and the relationship between the yield of FDCA and the N–Q atomic% on the NNC-X samples. Reproduced with permission from ref. 206. Copyright 2016 Royal Society of Chemistry. (b) A plausible reaction mechanism for selective oxidation of 5-HMF to FDCA over NC-700 catalyst. Reproduced with permission from ref. 207. Copyright 2021 Elsevier. (c) Overall mechanism of the oxidation of HMF over the NC catalyst using HNO3. Reproduced with permission from ref. 208. Copyright 2018 Royal Society of Chemistry. (d) structure of bifunctional CC–SO3H–NH2 catalyst. Reproduced with permission from ref. 209. Copyright 2016 American Chemical Society. | ||
Building on sustainable catalytic systems, Rao et al. took an innovative approach by transforming bamboo sawdust and melamine into a highly active N-doped carbon catalyst (NC-700).207 It utilized a renewable carbon source and achieved an impressive 83% FDCA yield with complete HMF conversion in just 6 hours at 160 °C, a significant improvement over previous reports that required prolonged reaction times. The secret to NC-700's exceptional activity lay in the synergistic interplay between graphitic-N sites and surface defects. These active sites acted as triggers for molecular O2 activation, forming peroxide radicals that efficiently oxidized HMF through a well-defined sequence of transformations. As shown in Fig. 13b, the reaction took a distinctive mechanistic route, beginning with the hemiacetal formation of HMF (MHMFC) in methanol, which then progressed via oxidative esterification (MFFC, DMFDC) before hydrolyzing into FDCA. However, this cascade reaction was heavily influenced by a base, specifically K2CO3, facilitating smooth oxidation while preventing unwanted degradation. Interestingly, excess base led to a drastic drop in FDCA yield (61%), highlighting the delicate balance required for optimal conditions. The authors also investigated the pivotal role of the reaction medium. When carried out in pure water, the oxidation process paused, and FDCA yield fell to just 8%, with FFCA yield reaching 30%. It was attributed to water molecules competing for active N sites, effectively delaying the oxidation of HMF to FDCA. Despite its promising performance, NC-700 faced a familiar challenge of stability. Over multiple cycles, the FDCA yield dropped from 83% to 53%, underscoring the ongoing struggle to retain N content in doped carbon catalysts. This study demonstrated that along with the catalyst design, choosing a suitable solvent is equally important as it can greatly influence the activity.
In an effort to address the limitation of N leaching, Ren et al. developed NC-950, a catalyst synthesized from chitosan and urea pyrolysis at 950 °C.208 This innovative material exhibited remarkable efficiency, achieving 100% HMF conversion and an impressive 95.1% DFF yield at 100 °C under 10 bar O2, a significant advancement in the selective oxidation of HMF. The key to NC-950's success was attributed to its unique synergistic mechanism involving graphitic-N sites and HNO3. Unlike conventional oxidation pathways, HNO3 acted as an electron transfer mediator, undergoing a redox cycle where it was reduced to NO2 during HMF oxidation, then reoxidized by molecular O2 to regenerate HNO3, establishing a self-sustaining catalytic cycle (Fig. 13c). The catalyst's high surface area (1103 m2 g−1) and graphitic-N content (4.3%) further enhanced substrate interaction and oxidation efficiency. Beyond catalyst design, the reaction medium played a decisive role in performance. In nonpolar solvents like hexane, HMF conversion was severely limited due to the poor solubility of HMF and HNO3. Moderate-polarity solvents like ethanol, 1,4-dioxane, THF, and acetonitrile facilitated significantly higher conversions due to better catalyst dispersion and solubility. However, a curious irregularity was observed in highly polar solvents like DMF, DMSO, and water, where conversion dropped drastically to 11.3% in DMSO. This unexpected behaviour was attributed to poor O2 solubility, low dispersion, and the altered dissociation of HNO3, weakening its oxidizing ability. Notably, NC-950 demonstrated exceptional recyclability, maintaining its activity over six consecutive cycles without significant deactivation, a breakthrough in N-doped carbon stability.
As researchers sought to further enhance the performance of N-doped carbon catalysts, attention turned toward co-doping strategies to create synergistic effects. A breakthrough in this domain came from Yin et al., who developed a P,N-co-doped porous carbon (NPC-800) catalyst synthesized from peanut shells and phosphoric acid, embodying a sustainable waste-to-wealth approach.150 The study was originally conducted for oxidation of benzyl alcohol (BA), but when extended to HMF, NPC-800 demonstrated remarkable efficiency, achieving 92.6% HMF conversion with 100% selectivity to DFF at 120 °C under 1 bar O2 in 10 hours, outperforming many single-heteroatom-doped systems. The superior activity of NPC-800 originated from the interplay between graphitic-N and C3PO species, which facilitated oxygen activation and electron transfer, driving the efficient generation of ROS. Control experiments reinforced this dual-pathway hypothesis, when the reaction was conducted under an inert atmosphere, substrate conversion dropped drastically to 4.9%, highlighting the indispensable role of molecular O2. Moreover, the introduction of a radical scavenger led to a partial inhibition of activity (dropping BA conversion to 66.9%), confirming that both radical and non-radical mechanisms were at play. Delving deeper into these pathways, EPR analysis in methanol using TEMPO as a spin-trapping agent revealed characteristic sextet peaks, indicative of ˙OCH3 radical formation. When BA was introduced, peak intensity declined, suggesting radical consumption, evidence of a free radical-mediated oxidation process. Simultaneously, the non-radical pathway involved P(V) –C3PO species, which participated in a redox cycle. Initially, these species oxidized BA, generating an alcoholate intermediate, which then dehydrated into the final product, benzaldehyde. Meanwhile, P(V) was reduced to P(III), only to be reoxidized by dissolved O2, completing the catalytic cycle. FT-IR analysis confirmed this transformation, as the characteristic PO stretching vibration weakened post-reaction under an N2 atmosphere, substantiating the reduction of P(V) to P(III) during oxidation. Despite its exceptional catalytic efficiency, NPC-800 faced a gradual decline in activity over four cycles, primarily due to the oxidation of graphitic-N sites. To counteract this, researchers developed a granular form of NPC-800, which significantly improved catalyst stability in repeated reactions, marking a notable step toward practical and durable metal-free catalytic systems.
As research into metal-free catalytic systems evolved, synthesis strategies shifted to functionalization from hetero-atom doping. Contributing to this, Rathod et al. introduced an innovative approach by designing a bifunctional carbonaceous catalyst (CC–SO3H–NH2) that integrated acidic (–SO3H, –COOH) and basic (–OSiCH2CH2CH2NH2) sites (Fig. 13d).209 Structural characterization using 13C CP/MAS NMR spectroscopy validated the catalyst's design. Key chemical shifts at 128, 152, and 173 ppm confirmed the presence of polycyclic aromatic carbon atoms, phenolic –OH, and carboxyl (–COOH) groups, respectively. A broad peak at 139 ppm indicated aromatic carbons attached to –SO3H, while signals from 13 to 60 ppm revealed N-propyl carbon sites, confirming the integration of acid–base functionalities within the carbon framework. Unlike previous catalysts that relied exclusively on heteroatom doping, this multifunctional design enabled one-pot oxidation of HMF to DFF, expanding the scope to include direct carbohydrate transformation. Operating at 140 °C in DMSO under O2 flow, CC–SO3H–NH2 achieved an 85% DFF yield with 100% selectivity from HMF in just 9 hours. Remarkably, it also facilitated the multi-step conversion of glucose (51% yield) and fructose (69% yield) to DFF, highlighting its versatility. The reaction proceeded through a three-step mechanism: (i) base-catalyzed glucose isomerization to fructose, (ii) acid-catalyzed fructose dehydration to HMF, and (iii) selective oxidation of HMF to DFF using molecular O2. This unique acid–base synergy ensured high selectivity and conversion efficiency, setting it apart from conventional catalysts. Beyond reaction performance, stability and scalability were rigorously tested. The catalyst retained its activity over five cycles, demonstrating robust recyclability. Moreover, scalability studies with 5 g of HMF yielded an impressive 81% DFF, reinforcing its industrial potential for sustainable DFF production.
Thermal catalytic routes such as oxidation, hydrogenation, and reductive upgrading are the most explored pathways for FAL transformation, with the reaction outcome governed by catalyst properties, solvents, and reaction conditions. Oxidation produces intermediates like MA and furoic acid, while hydrogenation selectively converts FAL to FOL, an essential precursor for resins and biofuels (Fig. 14). To date, metal-free FAL conversion is limited to products like MA, SA, FOL, and furfurylidene malononitriles.
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| Fig. 14 Catalytic conversion pathways of FAL into high-value chemicals via hydrogenation, oxidation, amination, and cyclization. The products in green boxes are achieved through metal-free routes, whereas those in blue boxes remain unexplored. | ||
Among the various metal-free oxidation pathways for FAL conversion, the Baeyer–Villiger oxidation stands out due to its ability to selectively cleave and hydroxylate FAL, forming valuable carboxylated intermediates. A pioneering study by Zhang et al. showcased the conversion of FAL to MA using a P-doped carbon catalyst (P–C-600), derived from phytic acid pyrolyzed at 600 °C.211 This catalyst exhibited a high surface area (1038.6 m2 g−1), a lacunar graphene-like structure, and abundant Lewis acid sites, enabling efficient H2O2 activation and stabilization of reaction intermediates. To elucidate the role of P doping, 31P solid-state NMR spectroscopy was employed, revealing four distinct P–O moieties at 31, −11, −25, and −42 ppm. Interestingly, as pyrolysis temperature increased, the P–O bond content diminished, aligning with XPS findings. Notably, the catalytic activity correlated with Lewis acid site density rather than total acidity, which showed that P–C-600 had the highest proportion of Lewis acid sites among all tested catalysts. A comparative experiment using P–C-1200, which contained fewer P species, yielded only 18.5% MA, reinforcing the crucial role of Lewis acid sites in selective oxidation. Under optimized conditions (60 °C, H2O2 as the oxidant), P–C-600 delivered a remarkable 92.8% FAL conversion and 76.3% MA yield, significantly outperforming other synthesized catalysts. Baeyer–Villiger pathway governing this transformation involved the formation of a key intermediate, 5-hydroxy-furan-2-(5H)-one, which underwent selective oxidative cleavage to yield MA (Fig. 15a). The catalyst showed promising stability over 10 cycles (Fig. 15a), with consistent activity for the first four cycles before gradual deactivation due to humin deposition, representing a common challenge in biomass valorization. These findings underscore the importance of precise acid site engineering in metal-free oxidation catalysis, demonstrating that P–C-600 effectively balances surface acidity, active site density, and structural integrity to achieve high selectivity and conversion in FAL oxidation.
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| Fig. 15 (a) Catalytic pathway for FAL to MA conversion over P–C-600 catalyst and recyclability test over 10 consecutive cycles. Reproduced with permission from ref. 211. Copyright 2021 Royal Society of Chemistry. (b) Proposed reaction mechanism for production of SA from furan carbonyl compounds with Amberlyst-15 as solid acid catalyst in the presence of H2O2. Reproduced with permission from ref. 212. Copyright 2013 Elsevier. (c) FAL hydrogenation over MgO–C68 catalyst. (d) the preparation procedure of NC catalyst from the carbonization of COF synthesized by the polymerization of cyanuric chloride and piperazine. Reproduced with permission from ref. 214. Copyright 2019 Elsevier. | ||
While many studies have focused on the selective oxidation of FAL to MA, Choudhary et al. took oxidation one step further, successfully converting FAL into succinic acid (SA), a key platform chemical with diverse industrial applications.212 Using Amberlyst-15, a sulfonated polystyrene ion-exchange resin, in the presence of H2O2, they achieved a remarkable 99% FAL conversion and 74% SA selectivity under mild aqueous conditions (80 °C, 24 h). Unlike the Lewis acid-mediated partial oxidation to MA (as seen with P–C-600 by Zhang et al.), this Brønsted acid-driven system enabled deeper oxidation, making SA the dominant product. Notably, only minor byproducts, MA, and formic acid were detected (<10% selectivity), indicating a high degree of reaction control. Despite its high catalytic efficiency, Amberlyst-15 showed moderate stability, with SA yield decreasing to 70% after three cycles, primarily due to sulfur leaching. To further validate the scalability of the process, a 20 mmol-scale reaction was performed using 1 g Amberlyst-15, yielding 68% isolated SA with >99% FAL conversion after 36 hours, a promising indication of its industrial applicability. A deeper investigation into catalyst structure–activity relationships revealed that the combination of –SO3H groups and aromatic rings played a crucial role in enhancing SA selectivity. The π–π interaction between the tolyl ring in Amberlyst-15 and the furan ring of FAL likely stabilized a key five-membered furan-2(3H)-one intermediate, facilitating selective oxidation to SA (Fig. 15b). In contrast, solid acid catalysts lacking these structural features, such as Nafion NR50, Nafion SAC13, and H2SO4, exhibited lower SA yields. The study further showed that catalysts with moderate acidity, such as p-TsOH (H0 = −2.2) and Amberlyst-15, delivered higher SA selectivity (>70%), whereas highly acidic homogeneous catalysts (H2SO4, HCl) led to non-selective oxidation yielding more of FA and MA. This work demonstrates the power of tailored Brønsted acid catalysts in guiding selective oxidation pathways, offering a robust, scalable, and metal-free approach for converting furans into linear di-carbonyl compounds.
While the oxidation of FAL yields valuable intermediates such as MA and SA, crucial for fine chemicals and polymers, hydrogenation offers an alternative pathway, selectively reducing FAL's aldehyde group to FOL, a key biofuel precursor and resin component. This transformation can be efficiently carried out through catalytic transfer hydrogenation (CTH), utilizing alcohols as H2 donors, offering a sustainable, metal-free alternative to conventional hydrogenation methods.
In a notable breakthrough, Koppadi et al. developed a vapor-phase hydrogenation system for selective FAL-to-FOL conversion, employing a carbon–MgO composite (MgO–CX) catalyst.213 This metal-free catalyst, synthesized through an organic route using rice grains as a carbon source, was calcined with Mg salts, yielding a highly basic structure tailored for CTH applications. Among the tested variants, MgO–C68 (containing 68 wt% carbon) exhibited exceptional performance, achieving 100% FAL conversion with 98%FOL selectivity under optimized conditions (180 °C, 1:15 FAL-to-isopropanol (IPA) molar ratio). Interestingly, at higher temperatures, FAL conversion marginally increased, but selectivity dropped due to the formation of side products like furan-2-yl-but-3-ene-2-one and 2-(isopropoxymethyl) furan. A detailed CO2 temperature-programmed desorption analysis classified the MgO basic sites into three categories: weak, moderate, and strong. The MgO–CX catalysts exhibited all three types of sites, whereas pure carbon (C100) lacked basicity altogether, emphasizing the essential role of MgO in catalytic activity. The reaction proceeded via a Meerwein–Ponndorf–Verley (MPV) mechanism, wherein IPA donates hydrogen to FAL's carbonyl group, yielding FOL and acetone as a byproduct (Fig. 15c). The high density of basic sites in MgO played a pivotal role in stabilizing the transition state, while the nanoscale dispersion of MgO on graphitic carbon prevented over-reduction, ensuring high selectivity. Beyond its impressive performance, MgO–C68 demonstrated stability by retaining its activity over 12 hours of continuous operation.
Oxidation and hydrogenation are well-established pathways for transforming FAL, but condensation reaction also offers a route to high-value biopolymer precursors and functionalized chemicals. Among these, the Knoevenagel condensation is particularly notable, coupling FAL with activated methylene compounds to produce α,β-unsaturated carbonyl derivatives, valuable building blocks for bio-based materials. Hu et al. demonstrated the effectiveness of an N-doped carbon catalyst (NC-700) in catalyzing the Knoevenagel condensation of FAL and HMF.214 NC-700, derived from a COF synthesized via the polymerization of cyanuric chloride and piperazine, offered a highly porous, N-rich structure optimized for this transformation (Fig. 15d). Under mild conditions (40 °C, ethanol-water solvent, 1 hour), the catalyst achieved ∼99% FAL conversion with ∼99% selectivity for furfurylidenemalononitrile, a crucial precursor for bio-based polymers. The secret to NC-700's exceptional performance lay in its well-engineered active sites. Mechanistic studies revealed that graphitic-N (N–Q) sites played a pivotal role in activating the aldehydic group of FAL, lowering the activation barrier for the nucleophilic attack by malononitrile, the methylene donor. Meanwhile, defect sites within the carbon framework acted as selectivity promoters, suppressing side reactions such as aldol condensation or polymerization, which often outbreak similar reactions. This synergistic interplay between graphitic-N and defect sites ensured high conversion and remarkable selectivity. The catalyst's structural origins from COF precursors were critical to its performance. Cyanuric chloride and piperazine polymerization produced a highly ordered, porous architecture, which, upon pyrolysis at 700 °C, formed NC-700 with uniform N doping and abundant active sites. The ethanol–water solvent system further enhanced reactant solubility and diffusion, accelerating the reaction while maintaining eco-friendly processing conditions.
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| Fig. 16 Catalytic conversion pathways of carbohydrates into value-added chemicals via isomerization, oxidation, and dehydration. All these transformation routes have been explored using metal-free catalysts. | ||
The isomerization of glucose to fructose is a pivotal step in carbohydrate valorization, as fructose exhibits higher reactivity in dehydration reactions, ultimately leading to HMF production. Recognizing the importance of catalyst microenvironment and functional group tuning, Chen et al. explored N-functionalized fullerene (C60) catalysts for glucose isomerization, focusing on amine group variations and solvation effects. A series of C60-based catalysts were synthesized via amination reactions using ethanolamine (EA), diethylenetriamine (DETA), triethylenetetramine (TETA), and 1-pentylamine (PA), each designed to modulate catalytic activity through tailored N functionalities (Fig. 17a).216 Among these, C60–TETA emerged as the most effective catalyst, achieving 30% glucose conversion and 19% fructose yield. The study highlighted solvation-dependent catalytic effects, particularly in C60–TETA. Compared to C60–DETA, which had the same N loading, C60–TETA exhibited higher activity due to its distal amine groups, positioned farther from C60's hydrophobic surface. This structural orientation allowed for better solvation, generating higher local pH values, which enhanced catalytic efficiency. Furthermore, –OH groups played a crucial role in creating a polar microenvironment, improving amine solvation and substrate affinity. To enhance stability while maintaining activity, a hybrid catalyst (C60-mix2) was developed by combining EA and DETA in a 50:50 ratio. This modification improved recyclability, allowing the catalyst to retain its efficiency over three reaction cycles.
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| Fig. 17 (a) Molecular structures of the amine-grafted C60–N catalysts. Reproduced with permission from ref. 216. Copyright 2019 American Chemical Society. (b) General mechanism for the glucose isomerisation over basic sites. (c) Proposed mechanism for the dehydration of fructose using GO factionalized with O-containing groups. Reproduced with permission from ref. 218. Copyright 2018 Elsevier. | ||
Mechanistic insights revealed that OH− generated by amine groups acted as key catalytic species, while –OH groups stabilized the enediol transition state, enhancing solvation and hydrogen bonding with glucose. A generalized mechanism of the glucose isomerization to fructose over basic sites is provided in Fig. 17b. By leveraging molecular design principles, this study provides a compelling approach to optimizing glucose isomerization catalysts, emphasizing the synergistic effects of amines and hydroxyl groups in tailoring catalytic activity.
Once fructose is formed, it embarks on the next crucial step in biomass valorization, dehydration to HMF, a pivotal intermediate in biofuel and polymer synthesis. Recognizing the complexity of this transformation, Le et al. engineered an innovative dual-acid catalytic system, combining Brønsted (–SO3H) and Lewis (AlCl3) acid sites on g-C3N4.217 This hybrid catalyst, synthesized via sulfonation, featured a mesoporous structure, high thermal stability, and strong acidity, making it particularly proficient at directing carbohydrate conversion toward HMF. The reaction unfolded through a well-defined two-step process. First, glucose underwent isomerization to fructose, facilitated by Lewis acid sites, which composed a hydride shift mechanism, transforming the aldose sugar into a more reactive keto form. Then, Brønsted acid sites took over, catalyzing stepwise dehydration of fructose, sequentially eliminating three water molecules while stabilizing reactive carbocation intermediates. As a result, a remarkable HMF yield of 58% from glucose and 60% from fructose under mild conditions (120 °C, 3 hours in DMSO) was observed. Intriguingly, solvent polarity played a defining role in HMF formation. The study revealed a clear trend: DMSO > DMF > THF, where higher solvent polarity enhanced fructose dehydration efficiency. However, the presence of water proved disastrous, completely suppressing HMF formation. Two primary reasons explained this outcome were (i) water inhibits dehydration reactions, preventing efficient HMF formation, and, (ii) water molecules adsorb strongly onto the hydrophilic g-C3N4–SO3H surface, blocking catalytic active sites and disrupting reaction pathways. Beyond solvent effects, substrate structure dictated reaction efficiency. The catalyst was tested across a spectrum of carbohydrates, revealing a fascinating hierarchy: Fructose emerged as the ideal feedstock, delivering 60% HMF yield due to its natural tendency for dehydration. Sucrose (a glucose–fructose disaccharide) produced a moderate yield (∼28%), as glucose first required Lewis acid-mediated isomerization. Starch and cellulose posed significant challenges, primarily due to their complex glycosidic linkages. While starch's α-1,4-glycosidic bonds allowed some accessibility, cellulose's rigid β-1,4-glycosidic structure hindered effective conversion, leading to byproduct formation and low selectivity. Lewis acids (AlCl3) play an indispensable role in glucose-to-fructose isomerization, particularly when working with disaccharides and polysaccharides. Brønsted acids (–SO3H) are essential for fructose dehydration to HMF, but long-term stability remains a challenge. Though g-C3N4–SO3H remained active for four reaction cycles, a gradual decline in performance was observed due to the partial loss of –SO3H groups, exposing a key limitation of stability when processing complex biomass substrates. Solvent selection is critical, while polar aprotic solvents like DMSO enhance HMF yield, water severely inhibits reactivity. By carefully balancing catalyst functionality, reaction conditions, and solvent effects, this work paves the way for more efficient and scalable biomass valorization strategies.
For decades, researchers have relied on solvent-based systems to facilitate the dehydration of fructose into HMF. However, Shaikh et al. challenged this conventional approach, pioneering a solvent-free catalytic system using GO by conducting the reactions under solvent-free sonication conditions.218 Synthesized via a modified Hummers' method, GO offered a unique advantage: its oxygen-containing functional groups (carboxyl, epoxy, hydroxyl) and π-conjugated network could catalyze fructose dehydration without the need for external solvents. At 100 °C for 5 hours, this system achieved an impressive 90% fructose conversion with 87% HMF selectivity. Even at a reduced temperature of 80 °C, the catalyst maintained significant activity, though conversion and selectivity dropped to 81% and 69%, respectively. Interestingly, when the same reaction was performed in solvent-based systems like water or DMSO, the results were inferior, highlighting that GO's inherent catalytic properties, rather than solvent effects, played the dominant role in the reaction. The catalytic activity is attributed to GO's stepwise dehydration mechanism, where –COOH and –OH functional groups interacted with fructose, facilitating proton transfer and the removal of three water molecules (Fig. 17c). This process destabilized fructose, smoothly converting it into HMF. Simultaneously, π–π interactions between GO's graphitic layers and fructose molecules enhanced substrate adsorption, ensuring high selectivity and efficiency. Surprisingly, when the reaction was carried out in an N2 atmosphere, no HMF formation was observed, whether under solvent-free conditions or in water. This unexpected result revealed a crucial role of ambient oxygen in maintaining GO's catalytic activity, possibly by influencing the regeneration of active sites. However, once the reaction was complete, simply adding water helped separate the catalyst from the reaction mixture, allowing for easy recovery and reuse. GO also demonstrated remarkable recyclability, retaining its activity over three consecutive cycles with minimal deactivation. This breakthrough opens new doors for solvent-free biomass valorization, proving that engineered carbon materials like GO can drive reactions traditionally dependent on acids or solvents.
In pursuing metal-free oxidation strategies for carbohydrate valorization, researchers have explored catalysts that can efficiently transform fructose, glucose, and xylose into valuable oxygenated compounds by functionalizing carbon materials with distinct functional groups referring to a specific function. One such breakthrough came from Zhao et al., who developed a bifunctional carbon catalyst (GN-NS), engineered with a unique combination of sulfonic acid (–SO3H), carboxyl (–COOH), and nitro (–NO2) groups which were responsible for fructose dehydration to HMF, activated molecular O2, and enhancing electron transfer, respectively.219 This methodically designed catalyst, synthesized via a molten salt method followed by acid functionalization, demonstrated remarkable efficiency in oxidizing fructose into DFF. With the reaction operating at 150 °C, for 25 h, and using atmospheric O2, in DMSO system achieved a 70.3% yield of DFF. The success of this transformation lies in the synergistic roles of all the functional groups. The study highlighted a compelling one-pot approach, allowing for the direct transformation of fructose to DFF in a single step. For comparison, conducted under identical conditions GN-S catalyst (high in sulfonic acid content) yielded ∼55% DFF and GN-N (with abundant –COOH groups) yielded ∼40% DFF, reinforcing the distinct functional roles of –SO3H and –COOH groups in oxidation chemistry. Beyond its catalytic efficiency, GN-NS demonstrated excellent stability, retaining high activity over five consecutive cycles with minimal loss in performance.
Furthermore, Jorge et al. introduced a breakthrough catalyst, a sulfonated dendritic mesoporous silica nanospheres (DMSi-SA), designed to upgrade mono, di, and polysaccharides into FAL and ethyl levulinate with exceptional efficiency.220 This catalyst was synthesized using a biphasic oil–water system, creating a hierarchical porous structure, which was then functionalized with –SO3H groups via thiol oxidation. Unlike conventional silica-based catalysts, DMSi-SA featured a high surface area and a three-tiered pore system (3.5, 6.4, and 25.9 nm), allowing it to accommodate bulky carbohydrate molecules and facilitating superior catalytic performance.
The well-engineered pore structure and acidity balance made DMSi-SA remarkably effective in converting various sugars into value-added products. When tested under optimized conditions at 170 °C, 24 hours, and ethanol as the solvent, the catalyst demonstrated unparalleled efficiency, producing 99% FAL from D-xylose and 83% ethyl levulinate from D-fructose. Even more impressively, it converted sucrose with 89% efficiency, while glucose and cellulose, traditionally more resistant to transformation, yielded 62% and 27% ethyl levulinate, respectively. The success of these transformations originated from the synergistic interplay between Lewis and Brønsted acid sites, which enabled hydrolysis and dehydration reactions and ensured high selectivity toward the desired products. A closer look at the catalyst's design revealed the true source of its catalytic power. Unlike many mesoporous materials, DMSi-SA maintained its pore diameter even after functionalization, preventing diffusional restrictions that often outbreak silica catalysts. This preservation of pore structure allowed for unhindered molecular transport, ensuring that even bulky carbohydrate molecules could access the active sites with ease. Moreover, the abundance of thiol groups on its surface contributed not only to acid functionality but also acted as hydrogen bond donors, further enhancing catalytic activity. Yet, as with many promising catalysts, long-term stability presented a challenge. While DMSi-SA maintained its high efficiency over four reaction cycles in fructose conversion, its ethyl levulinate yield gradually declined from 83% to 54%, signaling partial deactivation. The performance drop was even more pronounced in glucose and xylose reactions, where sulfur leaching and polymerization byproducts reduced activity significantly. Despite this, the structural integrity of DMSi-SA remained largely intact, providing a foundation for further stability enhancements through improved surface modifications. By skillfully combining hierarchical porosity, tailored acidity, and molecular accessibility, this catalyst demonstrates the potential for scalable, eco-friendly biomass valorization strategies.
One more significant contribution of one-pot fructose to DFF transformation was made by Lv et al. using GO as a bifunctional catalyst. GO, synthesized via the Hummers' method, possessed –SO3H and O-containing functional groups, granting it both acidic and redox catalytic properties.221 This combination allowed GO to efficiently catalyze the dehydration of fructose to HMF and the subsequent oxidation of HMF to DFF in a single system. Operating at 140 °C in DMSO for 24 hours with molecular O2, the one-pot process achieved a 53% DFF yield. However, the research team found that a two-step method, initially conducting the reaction under N2 for 2 hours, followed by O2 for 22 hours, significantly improved the DFF yield to 72.5%. The advantage of this approach lies in minimizing undesired overoxidation, which often occurs when O2 is introduced too early in the process. The mechanistic insights behind GO's catalytic power were particularly intriguing. The –SO3H groups served as active sites for fructose dehydration, ensuring a high yield of HMF. Meanwhile, oxygen-containing groups, including –COOH functionalities and edge defects, played a crucial role in activating molecular O2, enabling a selective oxidation process that efficiently converted HMF to DFF. Characterization studies, including diffuse reflectance Fourier transform infrared spectroscopy (DRIFT) and thermogravimetric mass spectrometry (TG-MS), confirmed that the sulfate groups on GO remained stable even at 215 °C, reinforcing its robustness as a high-temperature acid catalyst. Further investigations sought to confirm the role of carboxyl groups in O2 activation. When 1-pyrene carboxylic acid was used as a molecular mimic for GO, it catalyzed the oxidation of HMF with 79.3% conversion and a 58.6% DFF yield, demonstrating that carboxyl groups were indeed responsible for oxygen activation, a finding consistent with previous studies on carbon-based catalysts. Additionally, GO facilitated substrate adsorption due to the synergistic interaction between –SO3H groups and oxygen functionalities, further enhancing catalytic efficiency. Beyond its effectiveness, GO exhibited remarkable recyclability, maintaining high activity over five consecutive cycles with only slight deactivation due to oxygen group reduction. This study established GO as an effective bifunctional catalyst for DFF production and highlighted the significance of controlling reaction conditions to optimize selectivity.
The oxidation of glucose into valuable organic acids represents a crucial pathway in biomass valorization, offering sustainable routes to platform chemicals such as succinic acid (SA). Among the most promising advancements in this field, Rizescu et al. developed a series of N-doped graphene catalysts to enhance the efficiency of catalytic wet oxidation (CWO) of glucose.222 Their research focused on two types of N-containing graphitic materials, N-doped graphene (N–G) derived from chitosan pyrolysis at 900 °C, and aminated reduced graphene oxide (NH2–rGO) synthesized via the hydrothermal treatment of GO with ammonia. Among the various catalysts tested, NH2–rGO(3.8) emerged as the most effective, achieving 100% glucose conversion with 68% selectivity to SA under 160 °C, 18 atm O2, and 20 hours. This exceptional performance was attributed to the optimized N-species distribution, where quaternary (graphitic-N) sites were identified as the primary active centers for O2 activation. These sites facilitated the generation of ROS, promoting glucose oxidation through tartaric and fumaric acid intermediates. Additionally, pyridinic N sites played a supportive role, accelerating early-stage oxidation steps and ensuring high selectivity toward SA. However, not all N-doped catalysts exhibited the same level of efficiency. Catalysts with higher N content, such as NH2–rGO(5.3) and NH2–rGO(8.5), displayed lower activity and selectivity, which the researchers linked to an increased proportion of pyrrolic N species. Unlike graphitic and pyridinic N, pyrrolic N was found to be less effective in promoting oxidation reactions, resulting in unwanted byproduct formation and reduced SA yields. These findings underscored the critical role of N speciation in tailoring catalytic performance, demonstrating that simply increasing N content does not necessarily translate to higher activity. Beyond laboratory-scale experiments, Rizescu et al. validated the catalyst's potential by performing a scale-up reaction with 5 mmol glucose, yielding 0.85 g of SA with 67% selectivity. The catalyst also retained high activity over four consecutive cycles, proving its reusability and stability under CWO conditions. This durability and efficiency suggest that NH2–rGO(3.8) could be a viable alternative to metal-based oxidation catalysts, offering a cost-effective and sustainable approach for large-scale SA production.
Expanding the horizons of carbohydrate oxidation, Li et al. turned their attention to the conversion of D-xylose to D-xylonic acid, a key building block for biodegradable polymers and pharmaceuticals.223 To achieve this transformation through a metal-free pathway, they developed a series of N-doped carbon (NC-X) catalysts, synthesized by pyrolyzing chitosan. Among the tested catalysts, NC-800, obtained through pyrolysis at 800 °C with 5 mmol HNO3, emerged as the top performer, achieving 100% D-xylose conversion and a 57.4% yield of D-xylonic acid under optimized conditions (100 °C, 1 MPa O2) in just 30 minutes. Such impressive efficiency was attributed to the dominant role of graphitic N sites, which provided superior reactant adsorption and catalytic activity compared to other N configurations. Mechanistic insights, supported by DFT calculations, revealed that graphitic-N sites were primary active centers, offering stronger adsorption energies for hydroxyl ions, D-xylose, and O2 than pyridinic-N sites, thereby accelerating the oxidation pathway. The reaction proceeded through a hydroxyl ion-mediated mechanism, where D-xylose was activated to form a geminal diol ion intermediate, a crucial transition state that facilitated C–H bond cleavage This step released electrons, which were subsequently scavenged by molecular O2, regenerating hydroxyl ions and ensuring the continuity of the catalytic cycle. The stability of NC-800 was another standout feature. Over six consecutive reaction cycles, the catalyst retained its high conversion efficiency. However, a gradual decline in D-xylonic acid yield was observed due to the accumulation of byproducts on the catalyst surface. Remarkably, this deactivation was fully reversible, and the thermal reactivation of the catalyst effectively restored its performance, underscoring the durability and recyclability of the N-doped carbon system.
 | ||
| Fig. 18 Catalytic valorization pathways of glycerol via acetalization, oxidation, deoxydehydration, and carboxylation. These transformations are explored through metal-free catalysis, enabling sustainable production of high-value chemicals from glycerol. | ||
Kundu et al. introduced a sulfonic acid-functionalized anthracene-derived porous organic polymer (AnPOP–SO3H), designed to selectively convert glycerol into cyclic acetals under mild, solvent-free conditions.225 Unlike traditional acid catalysts, AnPOP–SO3H offered a highly rigid, crosslinked framework, ensuring exceptional accessibility to active sites while repelling water molecules that typically hinder reaction efficiency. Synthesized via Friedel–Crafts alkylation of anthracene followed by sulfonation, the catalyst exhibited a high acidity of 1.726 mmol g−1, a large surface area of 485 m2 g−1, and remarkable hydrophobicity. The structural integrity and chemical stability of AnPOP–SO3H were further validated using solid-state 13C CP-MAS NMR spectroscopy, which revealed characteristic signals corresponding to quinonoid and anthracene ring carbons in the polymer network. A positive shift in the NMR spectrum after sulfonation, along with the emergence of new peaks at 139.7 ppm, confirmed the successful introduction of sulfonic acid groups, ensuring enhanced acidity without compromising the robustness of the material. These properties created an ideal catalytic environment, allowing for 99.3% glycerol conversion with 96.2% solketal selectivity at a low temperature range (25–40 °C) in just 1.5–3 hours. Unlike many conventional catalysts that struggle in aqueous conditions, AnPOP–SO3H's hydrophobic framework actively suppressed water adsorption, improving activity and selectivity. A key insight into the catalyst's electronic properties was obtained through frontier molecular orbital (FMO) analysis, which examined the highest occupied (HOMO) and lowest unoccupied (LUMO) molecular orbitals. The study revealed that the sulfonic acid group slightly reduced the HOMO–LUMO gap, indicating enhanced reactivity due to improved charge delocalization across the polymer network. Notably, the delocalization of π-electrons further reduced the HOMO–LUMO gap, demonstrating that the polymer structure significantly increased reactivity compared to its monomeric counterparts. Electrostatic potential (ESP) mapping also confirmed that sulfonic acid groups played a crucial role in polarizing the catalyst's framework, effectively attracting polar reactant molecules and improving catalytic efficiency. Perhaps the most impressive aspect of AnPOP–SO3H was its remarkable durability and recyclability. Even after ten consecutive reaction cycles, the catalyst retained 98% of its original activity, with no significant deactivation. This unparalleled stability, coupled with its high selectivity and solvent-free operation, positions AnPOP–SO3H as an industrial-grade catalyst for biodiesel upgrading and sustainable chemical transformations.
Among the many pathways for glycerol valorization, oxidation remains one of the most promising, leading to high-value chemicals such as dihydroxyacetone (DHA) and glycerol carbonate. However, achieving selective oxidation under metal-free conditions has remained a challenge. Addressing this, Gupta et al. developed an N-doped carbon nanotube (NCNT) catalyst, demonstrating a highly efficient system for the selective oxidation of glycerol to DHA using tert-butyl hydroperoxide (TBHP) as the oxidant.226 Their study revealed that NCNT-700, synthesized via chemical vapor deposition at 700 °C, exhibited the highest catalytic efficiency, achieving 36.5% glycerol conversion with 84.3% selectivity toward DHA under mild aqueous conditions (60 °C, 6 hours). A closer look at the catalyst's composition and active sites revealed that pyridinic N species played a critical role in activating TBHP, forming N–O species that selectively oxidized the secondary –OH group of glycerol into DHA (Fig. 19a). Unlike non-functionalized CNTs, which were inactive, NCNT catalysts displayed a direct correlation between pyridinic-N content and catalytic activity. Specifically, NCNT-700, with a pyridinic N concentration of 45.8%, outperformed NCNT-800 (22.6%) and other tested NCNT variants, confirming the pivotal role of N-functionalization in driving selective oxidation. To further elucidate the reaction mechanism, the researchers conducted quantum chemical calculations, modelling the oxidation pathway using propane-1,2-diol as a structural analogue of glycerol. The computational analysis revealed that oxidation proceeded via a two-step mechanism: first, the abstraction of the secondary C–H proton, followed by the abstraction of the alcoholic proton, with the latter being the rate-limiting step. Importantly, the study demonstrated that oxidized nitroxyl-like N–O species on NCNT surfaces drastically lowered the energy barrier for the first step, enabling efficient oxidation. In contrast, non-oxidized pyridinic-N species exhibited a much higher energy barrier (∼204 kJ mol−1), rendering them catalytically inactive. This explained why NCNT-700, with optimally oxidized N–O groups, displayed superior reactivity compared to other NCNTs. Beyond its high selectivity, NCNT-700 exhibited remarkable stability, retaining activity over eight consecutive reaction cycles without noticeable deactivation. The robustness of the catalyst underscored the stability of pyridinic-N sites, making NCNT-based catalysts a viable alternative to metal-based oxidation systems. The study further highlighted that the oxidation mechanism on NCNTs mirrored that of nitroxyl radicals, suggesting that the surface-bound N–O species played a role analogous to well-established radical-mediated oxidation pathways. By successfully merging computational insights with experimental validation, Gupta et al. provided a breakthrough in metal-free selective oxidation, proving that NCNTs, when properly functionalized, can be highly active, stable, and selective catalysts for glycerol valorization.
 | ||
| Fig. 19 (a) Proposed mechanism for the conversion of glycerol into DHA involving N-rich pyridine reactive sites. The calculated reaction energy barriers are given in parenthesis (in kJ mol−1), Reproduced with permission from ref. 226. Copyright 2016 John Wiley and Sons. (b) The potential energy profile for the production of the 2,3-dihydroxypropyl hydrogen carbonate by the carboxylation of glycerol with CO2 over the CN and PSCN surfaces based on (i) the relative electronic energy and (ii) the relative Gibbs free energy at 298.15 K, respectively. Reproduced with permission from ref. 227. Copyright 2023 Elsevier. | ||
As the push for sustainable CO2 utilization continues, researchers are exploring novel ways to convert CO2 into valuable carbonates and carboxylates. One such advancement came from Fao et al., who developed a P and S co-doped graphitic carbon nitride (PSCN) catalyst, designed to enhance the catalytic conversion of glycerol and CO2.227 Compared to pristine g-C3N4 (CN), the PSCN catalyst demonstrated significantly higher activity, achieving 85% glycerol conversion versus 60% for CN under 210 °C and 3 bar CO2, with acetonitrile as a dehydrating agent. The introduction of P and S functional groups not only enhanced Lewis acidic and basic sites but also promoted charge separation, dramatically reducing the activation energy barrier for the carboxylation reaction from 1.47 eV (CN) to 0.98 eV (PSCN), as confirmed by DFT calculations. DFT calculations revealed that glycerol preferentially adsorbed on the PSCN surface, with higher adsorption energy than CO2, driving its activation and subsequent carboxylation. However, the simulations also indicated that water exhibited stronger adsorption energy than CO2, suggesting that humidity could significantly impact reaction efficiency. To counteract this effect, a dehydrating agent was necessary to enhance CO2 adsorption and optimize the carboxylation process. This insight reinforced the importance of reaction conditions, emphasizing the need for strictly controlled environments or chemical additives to maintain high conversion rates. Beyond its adsorption properties, PSCN facilitated a more energetically favorable reaction pathway compared to CN. The formation of 2,3-dihydroxypropyl hydrogen carbonate, the key product, required a lower desorption barrier (0.81 eV) on PSCN than on CN, indicating that the rate-determining step proceeded with greater efficiency (Fig. 19b). The co-doping effect not only enhanced CO2 adsorption but also promoted charge separation, leading to a higher overall reaction rate. By optimizing electron transfer dynamics, PSCN effectively accelerated the carboxylation process, making it a superior alternative to conventional g-C3N4 catalysts. The co-doping strategy demonstrated here opens pathways for broader biomass utilization, offering new possibilities for sustainable CO2 conversion in diverse catalytic applications. With its enhanced adsorption properties, lower activation energy, and improved electron–hole separation, PSCN establishes a promising platform for next-generation carbon capture technologies, proving that tunable heteroatom doping can revolutionize metal-free catalysis. The catalytic activity reported in the literature for the valorization of cellulosic biomass via the thermal pathway is summarized in Table 3.
| S. no. | Catalyst | Reaction conditions | Reactant conversion (C) | Products yield (Y)/selectivity (S) | Ref. |
|---|---|---|---|---|---|
| HMF conversion | |||||
| 1 | NNC-900 | HMF (0.63 mmol), water (10 mL), catalyst (0.2 g), HMF/K2CO3 (1:3), O2 (100 mL min−1), time (48 h), temperature (80 °C) |
HMF ∼100% | FDCA 80% Y | 206 |
| 2 | NC-700 | HMF (35.5 mg), methanol (5 mL), HMF/catalyst (0.46 g), K2CO3 (5 mmol), O2 (2 MPa), time (6 h), temperature (160 °C) | HMF >99% | FDCA 83% Y | 207 |
| 3 | NC-950 | HMF (63 mg), ACN (10 mL), catalyst (20 mg), HNO3 (0.15 equiv.), O2 (10 bar), time (14 h), temperature (100 °C) | HMF >99% | DFF 95% Y | 208 |
| 4 | NPC-800 | HMF (0.5 mmol), toluene (2 mL), catalyst (100 mg), O2 (1 bar), time (12 h), temperature (100 °C) | HMF 92.6% | DFF 100% S | 150 |
| 5 | CC–SO3H–NH2 | HMF (1.5 mmol), DMSO (2 mL), catalyst (60 mg), O2 (20 mL min−1), time (9 h), temperature (140 °C) | HMF 85% | DFF 100% S | 209 |
| Furfural conversion | |||||
| 6 | PC-600 | FAL (2.5 mmol), water (5 mL), H2O2 (20 mmol) catalyst (0.15 g), time (6 h), temperature (60 °C) | FAL ∼92.8% | MA 76.3% S | 211 |
| 7 | Amberlyst-15 | FAL (1 mmol), water (3 mL), H2O2 (4 mmol) catalyst (50 g), time (24 h), temperature (80 °C) | FAL ∼99% | SA 74% S | 212 |
| 8 | MgO–C68 | FAL:IPA (1:15), H2O2 (20 mmol) catalyst (1 g), temperature (180 °C) |
FAL 100% | FOL 98% S | 213 |
| 9 | NC-700 | FAL (1 mmol), ethanol/water (4 mL (1:3)), malnonitrile (1.5 mmol) catalyst (20 mg), time (1 h), temperature (40 °C) |
FAL ∼99% | Furfurylidene malononitrile 99% Y | 214 |
| Carbohydrate conversion | |||||
| 10 | C60–TETA | 30 mol% N loading relative to glucose; 5 wt% glucose solution at 100 °C, 60 min | Glucose ∼24% | Fructose 12.5% Y | 216 |
| 11 | g-C3N4–SO3H | Glucose/fructose (1 mmol), DMSO (5 mL), catalyst (10 mg), time (3 h), temperature (120 °C) | Glucose/fructose NA | HMF 58% Y from glucose, 60% Y from fructose | 217 |
| 12 | GO | Fructose (2 mmol), catalyst (20 mg), time (5 h), temperature (100 °C) | Fructose 90% | HMF 87% Y | 218 |
| 13 | GN-NS | Fructose (200 mg), catalyst (10 mg), DMSO (5 mL), time (25 h), temperature (150 °C), atm. O2 | Fructose ∼100% C | HMF 70.3% Y | 219 |
| 14 | DMSi-SA | D-Xylose/D-fructose (1 mmol), catalyst (5 mol%), ethanol (3 mL), time (24 h), temperature (170 °C) | D-Xylose/D-fructose ∼100% | FAL 99% Y from D-xylose | 220 |
| Ethyl levulinate (EL) 83% Y from D-fructose | |||||
| 15 | GO | Fructose (2 mmol), DMSO (4 mL), catalyst (20 mg), time (2 h), temperature (140 °C), N2/O2 (20 mL min−1) | Fructose ∼100% C | DFF 72.5% S | 221 |
| 16 | NH2–rGO(3.8) | Glucose (0.5 mmol), catalyst (0.025 g), H2O (10 ml), temperature (160 °C), time (20 h), O2 (18 atm) | Glucose ∼100% C | SA 68% S | 222 |
| 17 | NC-800 | Xylose (1 mmol), catalyst (100 mg), H2O (25 ml), NaOH (5 equiv.) temperature (100 °C), time (30 min), O2 (1 MPa) | Xylose ∼100% C | Xylonic acid 57.4% Y | 223 |
| Glycerol conversion | |||||
| 18 | AnPOP–SO3H | Glycerol (1 mmol), acetone (4 mmol; 70% in water), catalyst (20 mg), temperature (40 °C), time (90 min) | Glycerol 99.3% | Solketal 96.2% S | 225 |
| 19 | NCNT | Glycerol (3 mmol), TBHP (6 mmol; 70% in water), glycerol/catalyst = 5:1 by mass, temperature (60 °C), time (6 h) |
Glycerol 36.5% | DHA 84.3% S | 226 |
| 20 | PSCN | Glycerol (15 mL), ACN (5 mL), CO2 (3 bar), temperature (210 °C), time (6 h) | Glycerol ∼85% C | Glycerol carboxylate NA | 227 |
4.2. Metal-free thermal catalysis for lignin valorisation
Lignin, the most abundant aromatic biopolymer in nature, remains one of the most underutilized components of lignocellulosic biomass despite constituting 15–30% of its total mass. Unlike carbohydrates, which consist of sugar-based monomers, lignin is a highly branched, heterogeneous polymer composed of three primary monolignols p-coumaryl, coniferyl, and sinapyl alcohols, which form a complex network of C–O and C–C linkages. Among these, the β-O-4 (β-aryl ether), α-O-4 (α-aryl ether), and 4-O-5 (diaryl ether) linkages are the most abundant and crucial for determining lignin's reactivity.228,229 However, selectively cleaving these bonds remains challenging due to their varying bond dissociation energies (BDEs) and chemical stability. The β-O-4 linkage accounts for nearly 50% of all linkages in lignin is more susceptible to cleave, and 4-O-5 bonds are more resistant, requiring harsher reaction conditions or specialized catalysts based on their BDEs as shown in Fig. 20.230 Furthermore, C–C linkages (e.g., β–β, β-5, and 5–5 bonds) in lignin are highly stable, making them difficult to break without excessive hydrogenation or high-energy inputs. These structural complexities result in low selectivity, unwanted side reactions, and catalyst deactivation, posing challenges for efficient lignin depolymerization. | ||
| Fig. 20 Schematic representation of major lignin ether linkages and their corresponding bond dissociation energies (BDEs, in kJ mol−1). | ||
Thermal catalytic approaches have emerged as promising strategies for selectively cleaving lignin's bonds and converting them into valuable chemicals. These approaches typically fall into two main categories. (i) Catalytic depolymerization – Breaking down lignin into phenolic monomers, benzyl alcohols, and small aromatic compounds by targeting labile ether bonds. (ii) Upgrading of lignin-derived intermediates – transforming depolymerized lignin products into functionalized aromatics, biofuels, and fine chemicals through oxidation, hydrodeoxygenation (HDO), or selective C–C bond cleavage. In this section, we explore key advancements in thermal catalytic lignin valorization, discussing recent studies that focus on selective bond cleavage, depolymerization pathways, and upgrading strategies to generate sustainable, high-value products from lignin.
Contributing to the thermal depolymerization of lignin, Totong et al. explored the catalytic depolymerization of alkaline lignin into valuable phenolic monomers using metal-free carbon-based catalysts, including GO, N-doped GO (N–GO), solvothermal carbon (STC), sulfonated STC (SO3–STC), and N-doped STC (N–STC).85 The catalysts were synthesized via chemical methods such as the Hummers' method for GO and solvothermal carbonization for STC, with additional functionalization using sulfuric acid or ammonia for sulfonation and nitrogen incorporation, respectively. Among the catalysts, N–STC demonstrated the highest catalytic performance with a lignin conversion of 55.2% and phenolic monomer yield of 7.52% under optimized conditions (250 °C, 3 hours). The group also provided a stepwise procedure for the product separation from the reaction mixture, presented in Fig. 21. Mechanistic investigations revealed that N doping enhanced basicity, hydrophilicity, and structural defects, facilitating effective lignin adsorption and ether bond cleavage via electron transfer. In contrast, excessive acidity in SO3–STC led to side reactions, reducing its efficiency. Recyclability and scalability were briefly mentioned, with N–STC showing potential due to its structural stability. The study highlights the promise of metal-free carbon catalysts, particularly N–STC, for sustainable lignin valorization through tailored active site engineering and functional group interactions.
 | ||
| Fig. 21 Schematic diagram of product separation process after depolymerization reactions. Reproduced with permission from ref. 85. Copyright 2019 American Chemical Society. | ||
The study by Gao et al., investigated the oxidative depolymerization of lignin model compounds and organosolv lignin using N-doped graphene (LCN) as a metal-free catalyst with tert-butyl hydroperoxide (TBHP) as the oxidant.231 Initially, 1-(benzyloxy)-2-methoxybenzene and BPE were selected as α-O-4 type model compounds whereas 2-phenoxy-1-phenylethanol (PPE–OL), 2-(2,6-dimethoxyphenoxy)-1-phenylethanol, and 2-(2,6-dimethoxyphenoxy)-1-phenylethanol were selected as β-O-4 type lignin model compounds and further the study focused on organosolv lignin using LCN. LCN, prepared via CVD of graphene oxide with acetonitrile, contains 6.2 wt% N, predominantly in graphitic form, facilitating TBHP activation. Mechanistic insights revealed that graphitic N sites generate tert-butoxyl radicals, initiating a free radical mechanism. It involves benzylic C–H activation to form benzylic radicals, followed by ROS mediated Cα–O and Cα–Cβ bond cleavage, producing phenoxy and benzylic radicals that yield aromatic monomers like benzaldehyde and phenolic acids as shown in Fig. 22a. Under mild conditions (80–140 °C, 12–24 hours), LCN achieved 98% conversion of α-O-4 and 91.8% conversion of β-O-4 linkages, with selective production of benzaldehyde and benzoic acid. For organosolv lignin, the catalyst facilitated 45.8% liquefaction, yielding low-molecular-weight, oxygen-enriched depolymerization products. The study also concluded that H2O2 is unsuitable in the LCN-catalysed oxidation of lignin model compounds, because of its fast decomposition under the applied conditions, and a more stable oxidant (TBHP) was required. Recyclability tests demonstrated >90% activity retention over four cycles, with minor N loss. This study highlights LCN as a sustainable, metal-free catalyst with robust activity, scalability, and high selectivity for lignin valorization via a radical-driven oxidation mechanism.
 | ||
| Fig. 22 (a) Proposed mechanism of oxidative depolymerisation of (i) 1-(Benzyloxy)-2-methoxybenzene, and (ii) 2-phenoxy-1-phenylethanol catalysed by LCN. Reproduced with permission from ref. 231. Copyright 2014 John Wiley and Sons. (b) Possible pathways for the oxidative depolymerization of α-guaiacylglycerol-β-guaiacol to guaiacol. Reproduced with permission from ref. 232. Copyright 2015 John Wiley and Sons. (c) P5CN-500 surface model with different available sites [N indicated with blue, P indicated with pink and C indicated by grey], and reaction pathway for oxidative cleavage of BPE to PhOH and BAlc on P5CN-500 and CN surface (Energies are in eV). The values in blue and red font are reaction energies and green inside curl brackets are activation barriers. The values provided for the P5NC-500 catalyst corresponds to the P–N sites. Reproduced with permission from ref. 135. Copyright 2024 John Wiley and Sons. | ||
Similarly, the study by Blandez et al. demonstrated the oxidative depolymerization of lignin model compounds using graphene-based materials as metal-free catalysts under aerobic conditions.232 Among the catalysts tested GO, synthesized via Hummers' method, exhibited the highest activity, achieving 100% conversion and 87% selectivity for guaiacol from the α-guaiacylglycerol-β-guaiacol ether substrate under optimized conditions operating at 140 °C, 5 bar O2, for 24 hours, in acetonitrile. The superior performance of GO was attributed to its oxygenated functional groups (epoxides, quinones), which facilitated molecular O2 activation to generate ROS, leading to efficient ether bond cleavage in the lignin model compound. Reduced graphene oxide (rGO), N-doped graphene [(N)G], and B-doped graphene [(B)G] showed significantly lower activity due to the absence or reduction of functional groups critical for oxygen activation. The authors also investigated the catalytic activity attributed to metal impurities in GO by conducting control experiments with Mn(OAc)2, which resulted in only 20% conversion in 24 hours. It suggests that while metal impurities contribute to some extent, the predominant catalytic activity arises from the intrinsic properties of GO. GO's ROS generation and oxygenated functional groups were key to catalytic activity, while control experiments established that aerobic conditions were essential for the reaction. The depolymerization route for the guaiacylglycerol-β-guaiacol ether is provided in Fig. 22b. GO demonstrated moderate recyclability, retaining activity over three cycles with slight decreases in selectivity due to partial reduction of oxygen functionalities. While scalability was not tested, the simplicity of GO synthesis and its metal-free nature suggest the potential for larger-scale applications in lignin valorization.
While thermal depolymerization targets ether bond cleavage, oxidation strategies further enhance selectivity by converting lignin-derived intermediates into benzoic acids, aldehydes, and ketones. Chauhan et al. introduced a metal-free catalytic system for the oxidative cleavage of lignin model compounds containing α-O-4, β-O-4, and 4-O-5 linkages, using a P, and N-co-doped carbon catalyst (P5NC-500) synthesized from phytic acid and chitosan.135 The catalyst, prepared via hydrothermal treatment and pyrolysis at 500 °C, exhibited optimized acidity and activity due to the presence of P–O–H and P–N sites. At 140 °C, 9–14 hours, using water as a solvent and H2O2 as the oxidant, the system achieved complete conversion of BPE (α-O-4 model), 2-phenoxy-1-phenylethan-1-ol (PPE–OL, β-O-4 model), and diphenyl ether (DPE, 4-O-5 model). The oxidative cleavage of BPE and PPE–OL proceeded through C–O and C–C cleavage, respectively, yielding 50% phenol, and the other fragment yielded benzyl alcohol, which was further oxidized to benzaldehyde, and benzoic acid. Whereas, DPE exclusively produced phenol, highlighting the catalyst's selective C–O cleavage ability. The catalyst structure was theoretically modelled (Fig. 22c), and mechanistic studies revealed that the P–O and P–N sites were pivotal for H2O2 activation, generating reactive –OH species. These species facilitated bond cleavage through exothermic H attack or OH-mediated substrate activation (Fig. 22c). Brønsted acidity from P–O–H sites and synergistic effects of P and N dopants were critical for activity. Theoretical insights showed that reaction barriers for BPE cleavage were significantly lower on P–N sites compared to nitrogen-only (N–C) sites, explaining the superior performance of P5NC-500 over N-doped controls. The catalyst demonstrated robust recyclability, retaining high activity and selectivity over five cycles with minimal degradation. Simulated lignin bio-oil, containing all three linkages, was converted to phenol (61%) and other aromatic monomers, showcasing the system's practical applicability. The catalytic activity reported in the literature for the valorization of lignin via the thermal pathway is summarized in Table 4.
| S. no. | Catalyst | Reaction conditions | Reactant conversion (C) | Products yield (Y)/Selectivity (S) | Ref. |
|---|---|---|---|---|---|
| 1 | N–GO | 0.1 g of alkaline lignin, catalyst (0.01 g), water (5 mL), time (3 h), temperature (250 °C) | Alkaline lignin 55.2% | Phenolic monomer 7.52% Y | 85 |
| 2 | LCN | α-O-4/β-O-4 (0.5 mmol), catalyst (0.01 g), TBHP (6 equiv., 70 wt% in water), H2O (3 mL) |
α-O-4 (80 °C, 12 h) 98% | α-O-4 71% Y | 231 |
| β-O-4 (120 °C, 24 h) 91.8% | β-O-4 97.4% | ||||
| 3 | GO | α-Guaiacylglycerol-β-guaiacyl ether (0.01 mmol), O2 (5 bars), catalysts (1.5 mg) time 24 h, temperature (140 °C) acetonitrile (1.5 mL) | α-Guaiacylglycerol-β-guaiacyl ether >99% | Guaiacol 95% Y | 232 |
| 4 | P5NC-500 | BPE/PPE–OL/DPE (0.5 mmol), temperature (140 °C), (30%) H2O2 (1 mL), catalyst (20 mg), water (3 mL) | BPE (9 h) 100% | 50% S for phenol | 135 |
| PPE–OL (12 h) 100% | |||||
| 50% S for other oxidized monomers | |||||
| DPE (14 h) 100% |
5. Photocatalytic biomass valorization over metal-free catalysts
Over the past decade, significant progress has been made in the thermal catalytic conversion of biomass using metal-free catalysts. As discussed above, various carbon-based materials, demonstrated excellent activity in thermocatalytic processes. Photocatalytic biomass valorization presents an energy-efficient alternative by utilizing sunlight to drive chemical transformations under milder conditions.233,234 Despite its potential, research in metal-free photocatalysis for biomass valorization remains limited, with g-C3N4 being the most extensively explored photocatalyst.235 To date, metal-free photocatalysis has been widely employed for valorizing biomass-derived compounds, including HMF, FAL, glycerol, carbohydrates, and lignin, mainly through the oxidative pathways. The formation of the ROS, including superoxide radicals (O2˙−), hydroxyl radicals (˙OH), and singlet oxygen (1O2), play a crucial role in oxidative depolymerization of lignin, cellulose, and hemicellulose into valuable platform chemicals. Understanding the reaction mechanism requires analytical techniques such as EPR spectroscopy with trapping agents like DMPO, along with various electron and hole scavengers. Table 5 highlights commonly used trapping agents and their applications in photocatalysis.| S. no. | Radical/ROS/scavenger type | Trapping agents & scavengers |
|---|---|---|
| 1 | Superoxide radical (O2−˙) trapping agents | 5,5-Dimethyl-1-pyrroline N-oxide DMPO → DMPO–O2− |
| 2,2,6,6-Tetramethylpiperidine TEMP → TEMP–1O2 | ||
| 2,2,6,6-Tetramethylpiperidine TEMPONE → alternative stable radical probe | ||
| Nitro blue tetrazolium (NBT), 1,4-benzoquinone (BQ), dihydroethidium (DHE), lucigenin | ||
| 2 | Hydroxyl radical (˙OH) trapping agents | DMPO → DMPO–OH |
| N-tert-Butyl-α-phenylnitrone PBN → Forms PBN–OH | ||
| TEMP → Indirect detection via TEMP–1O2 | ||
| Terephthalic acid (TPA), methyl orange, ethanol (EtOH), isopropanol (IPA) | ||
| 3 | Singlet oxygen (1O2) trapping agents | TEMP, furfuryl alcohol (FFA), 1,3-diphenylisobenzofuran (DPBF) |
| TEMP → TEMP–1O2 | ||
| TEMPONE → more stable than TEMP | ||
| 4 | General ROS Detection in EPR | DMPO & PBN → used for various radical adducts |
| TEMP & TEMPONE → general ROS monitoring | ||
| 5 | Photogenerated holes (h+) scavengers | Alcohols (methanol, ethanol, isopropanol) |
| Acids (formic acid, oxalic acid, citric acid) | ||
| Amines (EDTA, triethanolamine), inorganic compounds (Na2S, Na2SO3, KI) | ||
| 2,6-Di-tert-butyl-4-methylphenol BHT → traps photogenerated holes (h+) | ||
| 6 | Electron (e−) Scavengers | O2, H2O2, (K2S2O8), (K3[Fe(CN)6]) (FeCl3), (K3[Fe(CN)6]) |
| CCl4, NO3−, AgNO3, Fe3+ (Ferric ion solutions) | ||
| TEMPO & 4-OH-TEMPO → electron trapping |
The following sections provide a detailed discussion of recent literature, focusing on photocatalyst design, photocatalytic efficiency, characterization techniques, mechanistic insights, particularly radical scavenging studies, and catalyst recyclability.
5.1. Photocatalytic conversion of cellulosic/hemicellulosic part
The catalytic efficacy of exfoliated g-C3N4 was validated in the open system under controlled laboratory conditions, using UV and direct solar irradiation to assess its real-world applicability. Operating in aqueous suspension of HMF, MCN-520, and MCN-540 samples resulted in comparable and improved photocatalytic activity under artificial light irradiation, enhancing HMF conversion and selectivity toward DFF (∼42–45%). The catalytic performance increased when experiments were conducted under sunlight, achieving 50% selectivity for DFF formation at 40% HMF conversion. In comparison, bulk g-C3N4 exhibited significantly lower efficiency, with only 39% HMF conversion and 28% DFF selectivity. The difference highlighted the critical role of structural refinement in optimizing the photocatalyst's performance. The scavenging experiments underscored the necessity of photogenerated h+ in HMF conversion to DFF and e− in O2 reduction to O2˙−. In contrast, the scavenging experiment of ˙OH had no impact on the reaction, ruling out the involvement of ˙OH in the oxidation process, which also overruled the participation of H2O in the reaction. The catalyst demonstrated efficient recyclability, as MCN-520 retained nearly identical conversion and selectivity after four consecutive cycles for both the conversion of HMF (57–58%) and the selectivity versus DFF formation (38–41%), confirming its structural integrity. Despite the promising performance of exfoliated g-C3N4, the study also revealed challenges associated with its photostability. Prolonged exposure to light led to the undesired photolysis of DFF, resulting in the formation of 5-formyl-2-furoic acid as a side product. Therefore, it is essential to develop the photocatalytic process, which can provide desired products with good selectivity.
This problem can be addressed by optimizing the solvent polarity, as explored by Wu et al. in their report.237 As the polar solvents led to a decrease in conversion and selectivity, likely due to their competitive adsorption at catalytic sites and overoxidation. The group optimized the reaction medium by incorporating a PhCF3-acetonitrile solvent mixture, which enhanced O2 solubility and mitigated overoxidation likely due to its lower polarity. They adopted a post-synthetic water treatment of g-C3N4 followed by a secondary calcination strategy. This process led to significant refinements in the catalyst's textural and electronic properties, particularly favouring pore formation, which dramatically increased the surface area from 11.4 to 123.9 m2 g−1, creating more active sites for catalysis, while also narrowing the bandgap from 2.73 eV to 2.56 eV. This narrowing improved visible-light absorption and facilitated more efficient charge transfer, two essential factors in optimizing photocatalysis. Under visible light, the modified catalyst exhibited 85.6% selectivity toward DFF with 31.2% HMF conversion in the presence of O2 flow after a 6-hour reaction. This performance far exceeded that of bulk g-C3N4 and even surpassed metal oxide-based photocatalysts such as TiO2 (P25), which achieved only 17.7% selectivity under UV conditions. Given its moderate bandgap of 2.56 eV, the photocatalyst efficiently absorbed visible light, leading to the excitation of e− to the CB, where they subsequently reduced molecular O2 to O2˙−radicals. The authors acknowledged the basic character of g-C3N4 for the deprotonation of the –OH in HMF, forming an alkoxide anion, which could undergo electron transfer with h+ to generate a radical intermediate, ultimately yielding DFF. The O2˙− radicals combine with the H+ present in the system, generating H2O2, which could further over-oxidize DFF into FDCA as a side product of the reaction (Fig. 23A) Additionally, the modified catalyst exhibited remarkable stability and recyclability, maintaining its catalytic activity and selectivity for up to five reaction cycles without significant degradation. However, minor by-products such as FDCA were detected under prolonged irradiation, indicating the potential need for further optimization to suppress overoxidation.
 | ||
| Fig. 23 General photocatalytic reaction mechanism of HMF to DFF oxidation in (A) aprotic solvent, and (B) aqueous medium in presence of Pt cocatalyst. | ||
Further, to address the problem of DFF overoxidation because of the involvement of H2O2 in the reaction, there is a need to develop a process that can isolate the H2O2 from the reaction system. Song et al. took photocatalytic HMF oxidation to another level by integrating HMF oxidation with targeted H2O2 production, providing 99% selectivity for DFF.238 Their approach revolved around encapsulating g-C3N4 with conducting polymers, including polyaniline (PANI), polypyrrole (PPY), and poly(3,4-ethylenedioxythiophene) (PEDOT). The polymers were grafted onto g-C3N4 through in situ polymerization, forming a conductive shell around the semiconductor. This architecture fostered strong π–π interactions between the polymer and the g-C3N4 matrix, facilitating rapid charge transfer and reducing charge recombination. The photocatalytic performance of the modified catalysts followed the order: PANI@CN > PPY@CN > PEDOT@CN > CN, with PANI@CN exhibiting the highest catalytic activity. The production rates of DFF (2189 μmol g−1 h−1) and H2O2 (1751 μmol g−1 h−1) were 4.1 and 3.9 times higher, respectively, than those of pristine g-C3N4 demonstrating an optimal balance between bandgap narrowing (2.44 eV vs. 2.71 eV for pristine CN) and improved electron mobility, as confirmed by XPS and Mott–Schottky analysis. Under visible-light irradiation for six hours, PANI@CN exhibited an impressive 82.1% HMF conversion with over 99% selectivity to DFF. Beyond its high activity, PANI@CN displayed excellent stability, retaining its catalytic performance over five successive cycles without structural degradation, highlighting its potential for long-term application. Radical trapping experiments confirmed the involvement of ROS, with superoxide scavenging indicating H2O2 formation from O2˙− species in the presence of proton-coupled charge transfer mechanism (PCTM). ESR analysis with TEMPO confirmed the presence of e−/h+ pairs, as evidenced by a significant reduction in the TEMPO signal after light irradiation. The scavenging experiments revealed that the h+ facilitated HMF oxidation, while e− drove O2 reduction to H2O2. Theoretical calculations revealed enhanced catalytic efficiency in PANI@CN, with the activation energy for Cα–H cleavage decreasing from 18.12 eV (pristine g-C3N4) to 14.29 eV, demonstrating the conductive polymer shell's role in accelerating HMF oxidation. Unlike the mechanism proposed by Wu et al., where H+ abstraction was governed by basic sites, this system followed a different pathway: photogenerated holes oxidized HMF by α-H abstraction and OH dehydrogenation, leading to DFF formation, while photoexcited e− migrated to the polyaniline layer to reduce O2 to H2O2 (Fig. 23A). This well-coordinated charge transfer enabled efficient dual-functional photocatalysis, solving the problem of overoxidation and also providing H2O2 as a value-added byproduct.
To further increase the selectivity of the process, simultaneous H2 generation was targeted instead of ROS generation from molecular O2, which also addressed the challenge of green energy generation. For this purpose, the utilization of a Pt cocatalyst under metal-free conditions is a prerequisite, as it enables the direct reduction of H2O to H2 through photogenerated electrons at the Pt sites. This strategy was investigated by two research groups, discussed in detail below. In an innovative step toward dual-functional photocatalysis, Bao et al. introduced a highly efficient ultrathin g-C3N4 nanosheet (UCNT) system capable of simultaneously oxidizing HMF to DFF and producing H2 under visible light.239 The UCNT catalyst was derived from urea-based g-C3N4 (UCN) through a thermal etching process, which reduced the sheet thickness and significantly increased the surface area. This structural modification resulted in a downward shift in bandgap (2.85 eV) compared to bulk g-C3N4 (2.75 eV), attributed to the quantum confinement effect, which altered the redox potential and enhanced catalytic efficiency. The etching process introduced N–H functional sites, which played a crucial role in promoting HMF adsorption and facilitating charge transfer. Under visible light irradiation, UCNT provided an H2 evolution rate of 46.0 μmol and a DFF yield of 47.5 μmol with 95% selectivity, outperforming both MCN and UCN. In contrast, MCN produced only 4.1 μmol of H2 with 85% DFF selectivity, underscoring the significance of nanosheet morphology and surface functionalization. The reaction mechanism was elucidated through control experiments and EPR analysis, confirming a radical-mediated pathway. DFT calculations showed strong N–H interactions anchoring HMF to the catalyst surface, enabling α-C–H abstraction by photogenerated h+, forming a α-carbon radicals (confirmed by EPR) intermediate that oxidized to DFF (Fig. 23B). Electrons reduced H2O to H2 at Pt cocatalyst sites, as validated by deuterated experiments where D2O altered H2 evolution, confirming water as the H2 source. Beyond its high activity, UCNT demonstrated exceptional stability and recyclability, maintaining its performance over five consecutive reaction cycles without structural degradation or loss of active sites, as confirmed by XRD and FTIR analyses.
Shifting the focus away from g-C3N4, Yan et al. developed a series of conjugated microporous polymers (CMPs) with finely tuned donor–acceptor (D–A) structures, enabling simultaneous photocatalytic H2 production and selective oxidation of HMF to DFF.240 Using Stille cross-coupling, they synthesized CMPs by pairing 2,5-bis(trimethylstannyl)thiophene with electron-accepting aryl monomers of varying strengths: phenyl (CMP-Ph), pyridyl (CMP-Py), pyrimidyl (CMP-Pm), and triazine (CMP-Tz). As the electron-withdrawing ability increased, π-electron delocalization improved, progressively narrowing the bandgap from 2.82 eV (CMP-Ph) to 2.51 eV (CMP-Tz). The photocatalytic experiments were conducted under visible light irradiation, in a 20% acetonitrile aqueous solution, using in situ photoreduced Pt nanoparticles as a cocatalyst. Among these, CMP-Tz emerged as the most efficient photocatalyst, achieving an impressive H2 evolution rate of 3207 μmol g−1 h−1, significantly surpassing CMP-Pm (9.1 times lower), CMP-Py (2.6 times lower), and CMP-Ph (negligible activity). Simultaneously, CMP-Tz catalysed HMF oxidation with 95% selectivity to DFF, reaching near stoichiometric conversion in 10 hours. The CMP-Tz catalyst also displayed excellent stability, maintaining structural integrity over five reaction cycles. Control experiments using radical scavengers validated the radical-mediated mechanism, as HMF oxidation was significantly suppressed in the presence of DMPO. A further band alignment was proposed by DFT calculations, where LUMO energy levels shifted to more positive reduction potentials, confirming thermodynamic viability for H2 evolution. A detailed mechanistic study elucidated the electron flow during the reaction, which replicates the mechanism proposed by Bao and coworkers, undergoing the formation of an α-hydroxymethyl radical intermediate, which was subsequently oxidized to DFF. This study highlights the power of (D–A) tuning in CMPs, demonstrating how precise molecular engineering can unlock extraordinary photocatalytic properties.
After going through all the literature provided for photocatalytic HMF oxidation, the generalized mechanism is proposed. The photocatalytic oxidation of HMF is finely driven by the interplay between photogenerated e−/h+ pairs, playing a distinct yet complementary role in determining the reaction pathway. The fate of these charge carriers and, consequently, the selectivity of HMF oxidation is dictated by the reaction medium. In an aprotic environment, the photogenerated VB holes assist in the consecutive Cα and OH protons abstraction, which leads to the selective formation of DFF (Fig. 23A). The CB electrons play a pivotal role in directing the reaction by activating O2 (atmospheric/supplied), and facilitating a controlled oxidation pathway. CB electrons actively participate in O2 reduction, generating O2˙− radicals, which are reduced to H2O2 by taking up the H+ liberated by the action of holes on the DFF, resulting in the Cα radical intermediate. Here, conversely, in an aqueous medium, the photogenerated VB holes exhibit strong oxidative power, promoting the transformation of water into highly reactive hydroxyl ˙OH radicals (Fig. 23B). These radicals indiscriminately oxidize HMF, leading to DFF and, under prolonged exposure, further oxidation to FDCA. The dual role of electrons and holes becomes even more striking when a Pt cocatalyst is introduced, as it enhances charge separation and channels CB electrons toward the hydrogen evolution reaction (HER), enabling concurrent H2 production alongside HMF oxidation.
In alkaline conditions, along with O2 purging, CDHCN achieved 33.2% glycerol conversion, surpassing bare g-C3N4 and CDMCN, which provided only 17.3% and 23.6% conversion, respectively. In terms of selectivity, CDHCN produced 48% glyceraldehyde along with the formation of glyceric acid (8.9%) and glycolic acid (7.9%) as oxidized products. Scavenger studies identified ˙OH and O2˙− as the primary reactive species. The reaction's O2 dependence was confirmed, as O2 removal nearly suppressed oxidation. The drastic efficiency drops upon NaOH removal highlighted its role in promoting ˙OH radical generation under alkaline conditions. The generation of reactive radicals is dictated by a catalyst's band structure, which must have suitable CB and VB potentials to drive ROS formation, as illustrated in Fig. 24a. The CB at 0.33 eV efficiently reduced O2 to O2˙−, while the VB at +2.00 eV facilitated ˙OH radical generation, enabling glycerol oxidation. A strong heterojunction between CDs and g-C3N4 effectively generated ROS species and supported water oxidation (Fig. 24b).
 | ||
| Fig. 24 (a) Suitable band positions for radical generation, oxidation and reduction. (b) Band structure of CDHCN catalyst capable of superoxide, hydroxyl radical formation and water oxidation, and (c) Mechanistic pathway for glycerol oxidation on CDHCN catalyst. (b) and (c) Reproduced with permission from ref. 241. Copyright 2023 Elsevier. | ||
After all the required investigations, the authors concluded that the glycerol oxidation proceeds via two pathways: one forming glyceraldehyde and glyceric acid, and another yielding glycolic acid through C–C bond cleavage, with further oxidation producing oxalic acid (Fig. 24c). Light absorption in CDs generates e−/h+ pairs, where holes react with OH− to form ˙OH radicals, while electrons reduce O2 to O2˙−. Additionally, water oxidation at 0.82 eV provides protons (H+), which react with O2˙− to form H2O2. Under light, H2O2 further decomposes into ˙OH, contributing to enhanced oxidation. The heterojunction effect ensures charge stabilization, improving efficiency. CDHCN outperforms CDMCN due to its favorable band alignment, as CDMCN's VB at 1.93 eV is insufficient for ˙OH radical generation. In conclusion, CDHCN efficiently generates ROS (˙OH and O2˙−), achieves high glycerol oxidation rates, and utilizes water oxidation to supply protons for H2O2 formation, making it a highly effective photocatalyst. While CDHCN showed high catalytic performance, the recyclability of the catalyst remained untested, leaving an avenue for future exploration. Nevertheless, its green synthesis from biomass waste and strong oxidation efficiency emphasizes its sustainability and alignment with circular economy principles, paving the way for scalable and eco-friendly photocatalysis.
Liu et al. and Sun et al. independently investigated photocatalytic xylose oxidation, demonstrating how medium pH influences product distribution, yielding either xylonic acid or lactic acid as distinct oxidation products. In an innovative approach, Liu et al. developed a biochar-welded (D–A) system on g-C3N4 to enhance charge transport and π-electron delocalization, significantly improving the photocatalytic oxidation of D-xylose to xylonic acid.242 The catalyst was synthesized by integrating glucosamine-derived biochar into g-C3N4 (GM-CN), strengthening electron interactions and reducing charge recombination. The stepwise thermal polymerization of melamine and glucosamine, followed by controlled heating and exfoliation, facilitated biochar formation, disrupted interlayer stacking, and improved porosity. This integration reduced the bandgap from 2.71 eV to 2.48 eV, enhancing visible-light absorption and charge separation. GM-CN afforded an 87.52% xylonic acid yield under O2-saturated alkaline conditions, outperforming pristine g-C3N4 (65.5%). Continuous O2 bubbling activated the reaction, while the π-conjugated biochar network improved charge transport and selectivity over side products like formic acid (26.9%) and lactic acid (5.5%). The catalyst demonstrated stability, maintaining 86.4% efficiency over ten cycles without structural degradation. Radical trapping and EPR experiments confirmed O2˙− and 1O2 as the dominant reactive species, while scavenging tests ruled out ˙OH involvement in the reaction mechanism. The DFT calculations revealed that biochar doping lowered the work function and exciton binding energy (208.21 meV to 119.62 meV), facilitating charge separation, further validated by PL studies. Although the reaction was conducted in aqueous conditions, but the VB potential of the catalyst could not support the H+ abstraction from water; therefore, solvent was not involved in the reaction mechanism. Mechanistically, the authors concluded that the photoinduced electrons reduced oxygen to O2˙−, while holes activated oxygen to form 1O2, collectively driving xylose oxidation to xylonic acid (Fig. 25). GM-CN exemplifies a sustainable approach to biomass valorization by integrating renewable biochar into semiconductor photocatalysis. The donor–acceptor configuration was proved to be beneficial for enhanced light absorption, charge separation, and redox activity.
 | ||
| Fig. 25 Photocatalytic oxidation of xylose under neutral and basic conditions demonstrating the pH dependent product distribution. | ||
In a significant advancement to xylose oxidation to lactic acid (LA), Sun et al. designed a metal-free photocatalyst system that integrates a (D–A) fluorescent molecule, 1,2,3,5-tetrakis(carbazole-9-yl)-4,6-dicyanobenzene (4CzIPN) into a carboxymethylcellulose (CMC) hydrogel.243 The hydrogel-based photocatalyst, 4CzIPN@CMC-HG, was synthesized through a thermal polymerization process, forming a porous, highly absorbent matrix that immobilized 4CzIPN while preserving its photocatalytic properties. The successful encapsulation extended the visible-light absorption range and improved charge transfer efficiency. The system exhibited outstanding photocatalytic performance, achieving a 100% xylose conversion with an exceptionally high 90.7% yield of LA, outperforming both pure 4CzIPN and CMC hydrogel alone. Conducted under visible light at 50 °C in an alkaline KOH solution, the reaction selectively oxidized xylose, yielding only minor amounts of formic acid (∼5%) and acetic acid (∼3%), demonstrating high selectivity for LA formation. Furthermore, the photocatalyst maintained its efficiency over ten reaction cycles, with a retained LA yield of 94.8%, highlighting its exceptional stability. Even more remarkably, a 1000-fold scale-up experiment yielded 45.2% LA, proving its feasibility for large-scale industrial applications. Radical scavenger experiments and band structure analysis revealed that the photocatalytic oxidation of xylose by 4CzIPN@CMC-HG follows a O2˙−, and ˙OH assisted pathway, with O2 playing a minor role. Upon visible light excitation, 4CzIPN generates electron–hole pairs, leading to oxygen reduction and O2˙− formation, which drives selective xylose oxidation to LA. Owing to the basic conditions at 50 °C, the reaction begins with xylose isomerization to xylulose, followed by retro-aldol cleavage into glyceraldehyde and dihydroxyacetone (Fig. 25). These intermediates undergo dehydration and keto–enol tautomerization to yield pyruvaldehyde, which is converted to LA via the Cannizzaro reaction. Concurrently, the oxidation of xylose's C1 aldehyde produces xylonic acid, which undergoes α- and β-oxidation to intermediates that yield formic acid. Some LA is further oxidized, generating by-products like formic and acetic acid. Beyond its high activity and selectivity, 4CzIPN@CMC-HG's practical advantages make it a promising candidate for industrial adoption. The hydrogel matrix facilitated efficient catalyst recovery and prevented leaching, thereby ensuring long-term stability.
Further, Li et al. designed a layered g-C3N4 photocatalyst capable of actively transforming glucose into high-value chemicals under visible light irradiation.244 By employing a cyanamide-SiO2 templating method, they synthesized a nanostructured g-C3N4 material with enhanced charge separation and surface reactivity, unlocking superior catalytic performance in glucose oxidation. The synthesis began with the thermal polymerization of cyanamide and SiO2, followed by template removal using NH4HF, yielding a sheet-like layered structure. This process introduced an increase in the surface area and optimized bandgap of 2.78 eV, balancing electron mobility and light absorption for effective photocatalysis. The nanostructured g-C3N4 catalyst exhibited a remarkable 49.1% glucose conversion, far surpassing the benchmark TiO2 (P25), which achieved only 31.3% conversion. More notably, the reaction selectively produced gluconolactone (45.2%), alongside 5-HMF (14.6%), D-arabinose (12.6%), and lactose (1.8%). Like the above-discussed literature, the glucose oxidation followed a O2˙− mediated pathway. Scavenger experiments confirmed O2˙− as the primary oxidant, with ˙OH playing a secondary role, while the near complete inhibition of the reaction by an h+ scavenger highlighted the crucial role of photoinduced holes. Despite its good catalytic performance, one aspect remained unexplored, the reusability of the catalyst. The study provided no insights into whether g-C3N4 retained its efficiency over multiple cycles, leaving an open question for future research.
Integrating carbohydrate reforming with green H2 production, Speltini et al. developed an oxidized graphitic carbon nitride (o-g-C3N4) photocatalyst, significantly enhancing H2 evolution from glucose oxidation, which was used as a sacrificial agent under simulated and natural sunlight.245 The synthesis of o-g-C3N4 followed a two-step process: thermal condensation of dicyandiamide (DCD), followed by chemical oxidation in HNO3 under reflux. The resulting material exhibited a rougher, porous morphology and a higher surface area (from 6.1 to 12.3 m2 g−1), facilitating better light absorption. Structural characterization revealed the introduction of oxygen-rich functional groups such as CO, C–OH, and C–O–C, which played a pivotal role in enhancing charge separation and catalytic activity. The insertion of oxygen-containing functional groups, significantly improved material dispersion in aqueous media, strengthening interfacial coupling and redox activity. This chemical oxidation effectively introduced extra-electron redistribution among neighbouring C atoms, leading to enhanced π-electron delocalization, and reduction of charge recombination.
Under optimized conditions (0.25 g L−1 catalyst, 0.1 M glucose, 3 wt% Pt cocatalyst, and pH 11), o-g-C3N4 achieved a remarkable 26-fold enhancement in H2 production than bulk g-C3N4. In simulated sunlight, the material exhibited a hydrogen evolution rate of 870 μmol g−1 h−1, compared to 52 μmol g−1 h−1 for pristine g-C3N4. More impressively, under natural sunlight, o-g-C3N4 reached 2500 μmol g−1 h−1, far exceeding TiO2 (P25), which only achieved ∼577 μmol g−1 h−1. These findings highlight the practical viability of o-g-C3N4 for real-world solar H2 production. To understand the reaction mechanism, the researchers conducted control experiments that validated glucose oxidation as the primary electron source. When glucose was absent, H2 production dropped to negligible levels (∼2 μmol g−1 h−1), confirming that the process was not driven by direct water splitting but rather by glucose oxidation providing free electrons for H+ reduction to H2 on the Pt cocatalyst site (Fig. 26). Additionally, pH-dependent studies revealed that the highest H2 evolution occurred at pH-11, where the catalyst surface charge facilitated glucose adsorption and oxidation. A particularly promising aspect of this study was the demonstration of photocatalytic H2 production in different environmental water sources. While river water yielded comparable activity to distilled water (∼826 μmol g−1 h−1), seawater significantly enhanced H2 production to 2523 μmol g−1 h−1, likely due to chloride-assisted surface interactions. In terms of long-term stability, the catalyst retained high performance over multiple cycles, with structural integrity maintained even after extended use. By introducing oxygen functionalities to modulate electronic structure and charge transfer, o-g-C3N4 sets a new benchmark for metal-free photocatalysis, demonstrating how chemical oxidation can enhance photocatalytic efficiency in H2 evolution applications.
 | ||
| Fig. 26 Photocatalytic hydrogen (H2) evolution via carbohydrate (cellulose/glucose) oxidation in the presence of Pt co-catalyst. | ||
Applying solar-driven H2 evolution strategy to cellulose reforming, Hong et al. introduced ultrathin 2D g-C3N4 nanosheets (CNNs).246 Unlike conventional methods that require alkaline environments, this innovative approach harnessed visible light and nanostructured catalysts to enable efficient H2 evolution. The synthesis of CNNs involved thermal polymerization of melamine, followed by controlled oxidation, yielding wrinkled nanosheets (∼6 nm thickness) with a fourfold increase in surface area (93.1 m2 g−1) compared to bulk g-C3N4. This structural refinement not only improved reactant adsorption but also enhanced charge transport, significantly boosting photocatalytic activity. Additionally, a bandgap shift from 2.64 eV to 2.76 eV was observed, which was attributed to superior visible-light absorption while suppressing charge recombination. When tested under visible-light irradiation, CNNs exhibited an exceptional H2 evolution rate of 13.14 μmol g−1 h−1, nearly four times higher than bulk C3N4. The reaction proceeded in neutral water, using 0.5 g L−1 cellulose as an electron donor and 2 wt% Pt as a cocatalyst, marking a significant milestone in photocatalytic cellulose reforming without alkaline conditions. To unravel the underlying reaction mechanism, the researchers conducted radical scavenger experiments and charge transfer studies, revealing that CNNs exhibited a higher oxidation potential (VBM = 1.93 eV) compared to bulk catalyst, making it more effective in breaking glycosidic bonds and generating reactive intermediates. Similar to Speltini et al., the O2˙− and ˙OH played a crucial role in cellulose oxidation. Photoinduced holes oxidized cellulose, leading to the formation of glucose and small organic acids (R–CHO, R–COOH), while electrons migrated to Pt sites, facilitating H+ reduction to molecular H2 (Fig. 26). Beyond its high catalytic efficiency, CNNs exhibited remarkable stability, retaining consistent H2 production over five consecutive cycles, with no structural degradation. The enlarged valence band position and higher ˙OH radical formation capacity allowed CNNs to directly oxidize cellulose more effectively, dramatically improving photocatalytic reforming efficiency. The catalytic activity reported in the literature for the valorization of cellulosic biomass via the photocatalytic pathway is summarized in Table 6.
| S. no. | Catalyst | Light source | Reaction conditions | Reactant conversion (C) | Products yield (Y)/Selectivity (S) | Ref. |
|---|---|---|---|---|---|---|
| HMF conversion | ||||||
| 1 | MCN-540 | Solar light | HMF (0.5 mM), aq. sol. catalyst (50 mg), time (4 h), open atmosphere | HMF 69% | FDCA 43% S | 236 |
| 2 | g-C3N4 | 300 W xenon lamp | HMF (0.1 mmol, ACN/PhCF3 (5 mL), Catalyst (50 mg), O2 (10 mL min−1), time (6 h) | HMF 31.2% | DFF 85.6% Y | 237 |
| 3 | PANI@CN | Visible light conditions | HMF (0.8 mmol), aq. sol., catalyst (0.1 g), time (3 h) | HMF >82.1% | DFF 99% S, 2189 μmol g−1 h−1 | 238 |
| H2O2 1751 μmol g−1 h−1 | ||||||
| 4 | UCNT | 300 W Xe lamp (λ ≥ 420 nm) | 100 mL of HMF aqueous solution (10 mM), 3 wt % Pt, catalyst (100 mg), time (5 h) | HMF 47.5% | DFF 47.5% Y | 239 |
| H2 evolution rate = 46.0 μmol g−1 h−1 | ||||||
| 5 | CMP-Tz | 300 W Xe-lamp cutoff filter (λ > 420 nm) | 10 mL of 20% ACN aq. sol. with 20 mM HMF, catalyst (10 mg), wt% Pt | HMF 85% | DFF 95% S | 240 |
| H2 evolution rate = 3207 μmol g−1 h−1 | ||||||
| Glycerol conversion | ||||||
| 6 | CDHCN | 250 W high-pressure mercury vapor visible light lamp | 0.1 M (50 ml) aq. sol. of glycerol + 0.5 M NaOH (50 ml) sol. O2 (30 mL min−1) catalyst (50 mg), time (8 h) | Glycerol 33.2% | Glyceraldehyde 48% Y | 241 |
| Carbohydrate conversion | ||||||
| 7 | GM-CN | 300 W Xe lamp | 100 mg Xylose dispersed in 50 mL O2 saturated water, 50 mmol/L KOH, catalyst (100 mg), time (12 h) | Xylose ∼24% | Xylonic acid 87.52% | 242 |
| 8 | 4CzIPN@CMC-HG | 300 W xenon lamp | 0.1 g xylose dispersed in 10.0 mL of KOH, time (160 min), temperature (50 °C), catalyst (0.5 g) | Xylose 100% | Lactic acid 90.7% Y | 243 |
| 9 | g-C3N4 | 500 W Xe lamp | 10 mL of 0.1 M glucose aqueous solution, catalyst (10 mg), time (10 h) | Glucose 49.1% | Gluconolactone 45.2% S | 244 |
| 10 | o-g-C3N4 | Solar box 1500e | 0.25 g L−1 catalyst, 0.1 M glucose, 3 wt% Pt cocatalyst, and pH 11 | Glucose NA | H2 evolution rate 870 μmol g−1 h−1 | 245 |
| 11 | CNNs | 300W Xenon lamp UV (400nm < λ < 780nm) |
0.5 g L−1 Cellulose suspension, catalyst (50 mg), Pt cocatalyst (2 wt%), NaOH (3 mol L−1), time (4 h) | Cellulose NA | H2 evolution rate 13.14 μmol g−1 h−1 | 246 |
5.2. Photocatalytic lignin valorization
The photocatalytic lignin valorization can be conducted via the reductive or oxidative pathway,247 but the metal-free photocatalytic lignin valorisation has been explored through oxidative pathways using various strategies, which can be broadly categorized into different approaches. These include the oxidation of lignin monomers such as guaiacol, leading to the generation of H2O2, the oxidative cleavage of lignin dimers into value-added monomers, the utilization of lignin as a sacrificial agent for H2 evolution, and the oxidation of lignosulfonate to produce value-added acids while simultaneously liberating gases.Guaiacol, a key lignin-derived compound, holds immense potential for sustainable chemical valorization, but its selective oxidation remains challenging. Rojas et al. tackled this by designing g-C3N4 nanosheets, demonstrating their efficiency in photocatalytic guaiacol oxidation under mild, environmentally friendly conditions.248 The journey began with the thermal decomposition of urea, where varying pyrolysis times (1–5 hours) shaped the catalysts (PC1–PC5). As the heating time increased, the nanosheets became thinner, unlocking enhanced quantum confinement, increased surface area (up to 97 m2 g−1 for PC5), and improved charge transport. Optical studies revealed a gradual bandgap narrowing (2.80 eV to 2.76 eV), enhancing visible-light absorption, while PL spectra redshift pointed to better charge carrier separation, a crucial factor for efficient photocatalysis. Under visible light in an aqueous solution, these nanosheets catalyzed guaiacol oxidation, achieving an impressive 82% conversion with PC5. Product selectivity leaned towards p-benzoquinone (59%), pyrogallol (27%), and catechol (6%), showcasing the catalyst's precision in directing reaction pathways. The stability test confirmed its robustness, as the catalyst-maintained activity over three consecutive cycles with a minimal performance loss. Mechanistic studies unraveled, a ˙OH-mediated oxidation pathway, where ˙OH selectively attacked the methoxy (–OCH3) group rather than the hydroxyl (–OH) group due to its lower bond dissociation energy (Fig. 27a). This selective activation led to catechol, pyrogallol, and p-benzoquinone, with O2˙− primarily forming p-benzoquinone, while ˙OH radicals drove catechol and pyrogallol production. However, g-C3N4's band structure (VB = +1.40 eV, CB = −1.37 eV) posed a challenge, as it could not directly generate ˙OH from OH−. Instead, an alternative pathway emerged, O2 reduction to H2O2 (+0.38 eV), which further decomposed into ˙OH radicals, sustaining the oxidation process. This smart utilization of ROS ensured efficient and selective guaiacol conversion. This study positions g-C3N4 nanosheets as a powerful, metal-free photocatalyst for selective guaiacol oxidation. The ability to fine-tune product selectivity by controlling ROS generation paves the way for broader applications in green chemical transformations.
 | ||
| Fig. 27 (a) Proposed reaction pathway for photocatalytic oxidation of guaiacol over PC5 catalyst by ˙OH attack on OCH3 group. Reproduced with permission from ref. 248. Copyright 2021 Elsevier. (b) Schematic diagram of O2˙− and ˙OH mediated photocatalytic degradation mechanism for PPE–OL over N,P-CQDs/g-C3N4 heterojunction. Reproduced with permission from ref. 249. Copyright 2024 Elsevier. (c) The O2˙− mediated mechanism of photocatalytic cleavage of lignin C–C bonds using g-C3N4 photocatalyst in micellar aqueous media. Reproduced with permission from ref. 250. Copyright 2022 Elsevier. (d) The conversion of sodium lignosulfonate (SL) into high-value vanillic acid of HAPA–CN with simultaneous H2 evolution under visible light lighting. Reproduced with permission from ref. 249. Copyright 2024 Elsevier. | ||
While monomeric compounds like guaiacol provide valuable insights into lignin valorization, real lignin is a complex polymer interwoven with C–O and C–C linkages, primarily β-O-4, α-O-4, and β–β bonds. Among these, the β-O-4 linkage dominates, making its selective cleavage a critical step in efficient lignin depolymerization. Overcoming this barrier could unlock a scalable strategy for converting lignin into valuable chemical intermediates. Building on this challenge, Liu et al. introduced an advanced N,P co-doped carbon quantum dots (N, P-CQDs)/g-C3N4 heterojunction photocatalyst designed to efficiently target β-O-4 bonds.249 The catalyst was synthesized through a hydrothermal treatment of cellulose pulp fibers with NH4H2PO4, forming N,P-CQDs, which were then integrated into g-C3N4 derived from melamine. Such a carefully engineered heterojunction significantly enhanced charge separation and reaction selectivity, making it highly effective for lignin degradation. The photocatalytic activity of N,P-CQDs/g-C3N4 was tested using phenoxy(phenyl)ethanol (PPE–OL), a lignin model compound containing both β-O-4 and C–C bonds, under visible light in an aqueous medium. The optimized system exhibited an impressive 95.5% degradation efficiency, selectively yielding phenyl formate (11.8%), benzaldehyde (16.0%), and benzoic acid (6.1%), along with some phenol and p-tolyl benzoate. The heterojunction structure played a key role in this efficiency by reducing charge recombination, thus improving both degradation rates and selectivity. Notably, the catalyst demonstrated exceptional stability, maintaining over 85% activity after four cycles, reinforcing its practical viability. Mechanistic studies revealed that charge transfer within the N,P-CQDs/g-C3N4 heterojunction was central to its high photocatalytic activity. The CBM shifted positively, improving electron transfer efficiency, while the valence band maximum VBM was enhanced, facilitating oxidative reactions. Under visible light, N,P-CQDs absorbed photons, generating (e−/h+) pairs, which transferred to g-C3N4, reducing charge recombination. The photogenerated e− reacted with O2 adsorbed on the catalyst surface, generating O2˙−, while h+ oxidized OH− to form ˙OH. These ROS (O2˙− and ˙OH) selectively attacked β-O-4 and C–C linkages, breaking lignin into smaller, valuable oxidation products (Fig. 27b). Radical scavenging experiments confirmed their role, as the O2˙− scavenger significantly inhibited degradation, while the ˙OH scavenger reduced cleavage efficiency. This study establishes N,P-CQDs/g-C3N4 heterojunctions as a highly efficient and stable metal-free photocatalyst for lignin degradation. But, one of the persistent challenges in lignin depolymerization is achieving high selectivity in bond cleavage.
While previous studies, such as those by Liu et al., successfully demonstrated oxidative cleavage of β-O-4 linkages, the reaction suffered from poor product selectivity. Uncontrolled oxidation often leads to overoxidized or polymerized byproducts, limiting the efficiency of lignin valorization. Addressing this challenge, Xu et al. introduced an innovative approach, utilizing micellar aqueous media, which enhanced catalyst dispersion and provided a stable reaction environment that significantly improved reaction selectivity.250 Xu et al. hypothesized that traditional organic solvents like acetonitrile and acetone, commonly used in photocatalytic lignin depolymerization, limited reaction efficiency due to poor catalyst dispersion and unfavorable interactions with ROS. To overcome this, they developed a micellar aqueous system using sodium lauryl sulfonate (SDS) surfactants, which self-assemble in water, forming nano-micelles. To fine-tune the system, Xu et al. tested different SDS concentrations (SDS-2, SDS-5, SDS-8, SDS-10) and found that SDS-8 provided the most effective reaction environment. Interestingly, they observed that the addition of acetic acid further accelerated reaction kinetics, allowing full substrate conversion within just 2 hours, a significant improvement. The effectiveness of the micellar system was demonstrated using 1,2-diphenylethanol (Dpol), a lignin model compound containing β-O-4 and C–C bonds. Under visible light irradiation, the reaction showed only 23.4% conversion in acetonitrile with poor selectivity, whereas 56.2% conversion in SDS-8, yielding 40.7% benzaldehyde and 5.8% benzoic acid. Further improvement with acetic acid, boosting benzaldehyde yield to 52.6% and benzoic acid to 33.8%. The dioxane poplar lignin was also investigated in the optimized reaction conditions providing depolymerized products, confirmed by GC-MS, but was not quantified in the study. These results highlighted the critical role of the micellar medium in enhancing efficiency, optimizing selectivity, favoring better substrate dispersion, controlling oxidation, and preventing secondary side reactions. A major revelation in the study was the active role of water in the reaction mechanism. While previous research suggested that Cβ–H dissociation provided the necessary hydrogen for bond cleavage, Xu et al. conducted the reaction in D2O instead of H2O, achieving striking results. A significant (∼30%) decrease in conversion when D2O replaced H2O, proving that water itself actively participates in hydrogen transfer. Further, GC-MS detected deuterium-labeled benzaldehyde, confirming that water supplies hydrogen for C–C bond cleavage. To further understand how the micellar system enhanced selective lignin cleavage, Xu et al. investigated the reaction mechanism using EPR spectroscopy and radical scavenging experiments. The findings revealed that the reaction followed a stepwise O2˙− mediated pathway (Fig. 27c). Initially, Cβ–H abstraction by photogenerated holes generated a Cβ-centered radical, which was subsequently attacked by O2˙−, leading to the formation of a six-membered transition state. The hydrogen from Cβ–H and the hydroxyl group from water dissociation facilitated bond cleavage, forming benzaldehyde via dehydration. Finally, benzaldehyde underwent further oxidation to benzoic acid through the action of photogenerated holes and O2˙− radicals. Beyond selectivity and efficiency, the catalyst demonstrated excellent recyclability, maintaining high performance over three cycles without structural degradation. This stability underscores the practical applicability of the micellar system for long-term lignin valorization. This study marks a significant leap forward in lignin valorization strategies, demonstrating that micellar aqueous systems can effectively overcome selectivity challenges in the oxidative cleavage of models and real lignin.
While oxidative cleavage of lignin models has successfully enabled the selective breakdown of β-O-4 and C–C linkages, its true potential lies in simultaneous H2 evolution, offering a dual benefit of valuable chemical production and clean energy generation. Recognizing lignocellulose as a rich hydrogen source, researchers have sought efficient photocatalysts to drive biomass-to-H2 conversion. Rao and colleagues developed a monolayer g-C3N4 photocatalyst, demonstrating a highly efficient lignocellulose photoreforming strategy under visible-light irradiation.251 Through mechanical exfoliation and ball-milling in an N2 atmosphere, they transformed bulk g-C3N4 into ultrathin nanosheets (∼0.32 nm thick) with an expanded surface area (116.4 m2 g−1vs. 13.3 m2 g−1 for bulk) and an optimized bandgap shift (2.75 eV to 2.81 eV) for enhanced charge mobility. The H2 evolution performance of monolayer g-C3N4 was remarkable, achieving 872 μmol H2 gcat−1 from cellulose, 719 μmol H2 gcat−1 from hemicellulose, and 249 μmol H2 gcat−1 from lignin, nearly 10 times higher than bulk g-C3N4. More impressively, real-world substrates such as cellobiose, poplar wood, and even waste toilet paper demonstrated the catalyst's versatility, with waste lignocellulose yielding an exceptional 2167 μmol H2 gcat−1 after 12 hours of irradiation. Mechanistic studies revealed that monolayer g-C3N4's unique structure reduced exciton binding energy from 52.8 meV to 26.5 meV, drastically improving charge separation and electron mobility. Under visible-light illumination, photoexcited electrons migrated to Pt co-catalyst sites for proton reduction, while holes facilitated lignocellulose oxidation. The reaction was driven primarily by O2˙−, rather than ˙OH, as confirmed by radical scavenger experiments. Band structure analysis further underscored the high efficiency of monolayer g-C3N4, with an optimized CB (−1.58 eV) and VB (1.23 eV) facilitating charge transfer. Beyond its superior catalytic performance, the material demonstrated exceptional stability, maintaining high efficiency over five consecutive cycles without structural degradation. These findings establish monolayer g-C3N4 as a scalable, sustainable platform for biomass-to-H2 conversion.
Cheng's group demonstrated a groundbreaking approach for H2 evolution from lignocellulose using monolayer g-C3N4, a key limitation remained, no valuable lignin-derived products were obtained, which limits the broader applicability of this method in biomass conversion. Recognizing this gap, Jing et al. introduced a more holistic approach, O,P co-doped porous g-C3N4 (HAPA–CN), integrating persulfate (PS) activation to enable simultaneous lignin valorization, and H2 evolution.252 This advancement targeted H2 production and selectively converted lignin into vanillic acid, a high-value chemical intermediate. The HAPA–CN catalyst was synthesized through thermal polymerization of urea, hydroxyacetic acid (HA), and phytic acid (PA), followed by freeze-drying and calcination. This process resulted in a highly porous nanosheet structure, offering an increased surface area (61.59 m2 g−1vs. 45.14 m2 g−1 for pristine g-C3N4) and a lower bandgap (2.21 eV vs. 2.63 eV for pristine g-C3N4). These structural improvements enhanced light absorption, charge separation, and catalytic performance, making HAPA–CN an efficient platform for biomass valorization under visible light. To evaluate its performance, Jing et al. tested HAPA–CN for two key applications, organic pollutant degradation and lignin valorization. In pollutant degradation, HAPA–CN achieved an impressive 99.67% degradation of bisphenol A, and 2-mercaptobenzothiazole (MBT) degradation reached 99.80% with PS activation. Beyond pollutant degradation, HAPA–CN was highly effective in lignin valorization, converting sodium lignosulfonate (SL) into high-value vanillic acid with a record yield of 134.34 mg gSL−1 (Fig. 27d). This yield far exceeded those obtained with other catalysts, including HA–CN (93.51 mg gSL−1), PA–CN (55.65 mg gSL−1), and pristine g-C3N4 (27.32 mg gSL−1). Additionally, the HAPA–CN system facilitated the production of gaseous byproducts, including H2 (8.39 μL g−1 h−1), CO (34.78 μL g−1 h−1), CH4 (14.57 μL g−1 h−1), C2H4 (2.28 μL g−1 h−1), and C2H6 (0.23 μL g−1 h−1), proving its ability to enable simultaneous biomass valorization and clean energy generation. Infrared thermography revealed that under visible light irradiation, 0.05 HAPA–CN reached 81.3 °C in 30 s, significantly higher than g-C3N4 (41.3 °C in 30 s) due to enhanced near-red light absorption. This photothermal effect accelerates the overall photocatalytic rate. A deeper mechanistic analysis revealed that O,P co-doping significantly altered the band structure, improving charge separation and facilitating oxidation–reduction reactions. The CBM was −0.59 eV, while the VBM was 2.21 eV, making the catalyst highly effective for both oxidation and reduction processes. Photogenerated electrons reduced O2 to O2˙− radicals, which were further converted into ˙OH radicals and 1O2. Additionally, PS captured electrons to generate SO4˙− radicals, suppressing charge recombination and enhancing oxidation efficiency. The radical scavenging experiments confirmed that 1O2 scavenger significantly reduced lignin oxidation and gas evolution, while OH scavenger decreased lignin cleavage efficiency. O2˙− scavenger suppressed both pollutant degradation and lignin valorization, and the hole scavenger confirmed that photogenerated holes played a major role in oxidation. These findings established that O2˙−, SO4˙−, 1O2, and ˙OH were the dominant reactive species responsible for pollutant degradation and lignin valorization. Beyond its high catalytic efficiency, HAPA–CN exhibited remarkable stability and recyclability, maintaining its activity over five consecutive cycles. By incorporating persulfate-assisted charge separation and multi-ROS generation, Jing et al.'s work expands on previous lignin photoreforming studies, demonstrating how strategic catalyst engineering can unlock both hydrogen evolution and selective chemical production from biomass but still leaves room for improvement for achieving high and selective yield for H2 evolution. Table 7 summarises the catalytic activity of various reported metal-free catalysts for valorizing lignin through photocatalytic pathway.
| S. no. | Catalyst | Light source | Reaction conditions | Reactant conversion (C) | Products yield (Y)/Selectivity (S) | Ref. |
|---|---|---|---|---|---|---|
| 1 | PC5 | 150 W Xe lamp | 25 mL guaiacol solution (50 mg L−1), catalyst (25 mg), time (6 h) | Guaiacol 82% | p-Benzoquinone 59% S | 248 |
| 2 | N,P-CQDs)/g-C3N4 | 300 W Xe lamp | PPE–OL (5 mg), catalyst (10 g), ACN (5 ml), O2 atmosphere, time (1 h) | PPE–OL 95.5% | Phenyl formate (11.8%) S, benzaldehyde (16.0%), benzoic acid (6.1%) | 249 |
| 3 | g-C3N4 | 300 W Xe lamp | 1,2-Diphenylethanol (0.01 g), 3 mL of SDS-8 catalysts (0.01 g) time (4 h), temperature (15–40 °C) O2 atmosphere | 1,2-Diphenylethanol 56.2% | Benzaldehyde (40.7%) Y benzoic acid (5.8%) Y | 250 |
| 4 | g-C3N4 | 300 W Xe lamp | 50 mL of g-C3N4 dispersions (1.0 mg mL−1), 0.50 g of lignocellulose re-dispersed into 100 mL aq. sol., (pH = 10) Pt co-catalyst (1.0% wt), time (12 h) | Lignin NA | 249 μmol H2 gcat−1 | 251 |
| 5 | HAPA–CN | Sunlight | 10 mL of sodium lignosulfonate solution (2 g L−1), catalyst (10 mg), K2S2O8 (20 mg), time (30 min) | Sodium lignosulfonate | Vannilic acid 134.34 mg gSL−1 Y, H2 evolution rate 8.39 μL g−1 h−1 | 252 |
6. Electrocatalytic conversion of cellulosic/hemicellulosic part
In recent years, growing interest in metal-free electrocatalysis has been witnessed as a sustainable strategy for upgrading cellulosic and hemicellulosic biomass-derived platform molecules under mild, aqueous, and energy-efficient conditions. The literature highlighted in this section demonstrates that carbon-based electrodes, including sp2 and sp3-hybridized surfaces, heteroatom-doped porous carbons, and polymer-coated conductors can catalyze a diverse range of reactions such as the electrooxidation of HMF to FDCA, reductive dimerization of HMF and FAL into higher-carbon biofuel precursors, and cathodic/anodic transformation of LA into GVL, valeric acid, and C4–C8 oxygenates.Electrocatalytic processes generally proceed via inner-sphere or outer-sphere electron transfer mechanisms (Fig. 28). In inner-sphere pathways, the substrate is chemically adsorbed onto the electrode surface, and electron transfer occurs through orbital overlap and surface-bound intermediates, as commonly seen with metallic electrodes (e.g., Cu, Pd, Ni). In contrast, metal-free catalysts, due to their limited surface adsorption capability, absence of metallic d-orbitals, and inert π-structured carbon frameworks, typically operate via outer-sphere mechanisms, where electron transfer occurs through diffusion-controlled or mediator-assisted processes, and substrate transformation occurs in the solution phase without surface hybridization. As illustrated in Fig. 28, metal electrodes promote surface-coupled reactions through H* transfer, while metal-free catalysts facilitate electron tunneling or radical-based reactions at or beyond the Helmholtz layer, allowing access to selective and sustainable biomass conversions. The following section presents representative studies in detail, focusing on their catalyst design, electrochemical performance, mechanistic pathways, and catalyst recyclability.
 | ||
| Fig. 28 Schematic comparison of (a) inner-sphere and (b) outer-sphere electrocatalytic mechanisms on metal and metal-free electrodes. | ||
6.1. Electrocatalytic HMF oxidation to FDCA
Two groups, Wang et al., and Qin et al., both utilized heteroatom-doped carbon materials, N-doped and B,N-co-doped, respectively, each facilitating outer-sphere electron transfer due to their high surface area and active nitrogen species. Despite differences in synthesis and porosity, both systems achieve high FDCA yields under alkaline conditions. In contrast, Carli et al. adopted a redox-mediator approach using PEDOT films and TEMPO, where the electrode only regenerates TEMPO+, and the oxidation occurs homogeneously in solution, another form of outer-sphere mechanism but with distinct mediator dependence. Precisely, Wang et al. developed a highly efficient and robust metal-free electrocatalyst, GNPCH-900, for the oxidation of HMF to FDCA under alkaline conditions. The catalyst was synthesized by pyrolyzing HMF and urea at 900 °C, yielding ultrathin N-doped porous carbon sheets with a high BET surface area (533 m2 g−1), abundant microporosity, and a high concentration of pyridinic-N and graphitic-N.253 These features facilitated enhanced charge transport, active site accessibility, and substrate adsorption. A 400-hours continuous electrochemical test in 1 M KOH, GNPCH-900 showed outstanding performance, with FDCA yield and faradaic efficiency exceeding 90% at 1.446 V vs. RHE. GNPCH-900 outperformed benchmark metal catalysts like NiOOH and RuO2/Pt, including in a symmetric two-electrode configuration where 10 mA cm−2 was achieved at a cell voltage of 1.61 V. Mechanistic studies using phosphate poisoning, in situ ATR-IR, and DFT calculations revealed that pyridinic-N sites are the primary active centers, and the reaction predominantly proceeds via the HMFCA pathway. DFT simulations further confirmed lower adsorption energies and activation barriers for HMF and OH− on pyridinic-N compared to graphitic-N. The catalyst also demonstrated remarkable stability, maintaining performance over 400 hours without detectable degradation or metal contamination.Similarly, Qin et al. developed a multifunctional metal-free electrocatalyst, BNC-2, via pyrolysis of melamine, L-cysteine, and boric acid at 1000 °C, yielding a B,N co-doped porous carbon with abundant defects and hierarchical porosity.254 Among a series of materials (NC, BNC-1, BNC-2), BNC-2 showed superior performance in the electrooxidation of HMF to FDCA and N2 reduction, attributed to its frustrated Lewis pair (FLP)-like B–N sites and high surface area. For HMF oxidation, electrochemical tests conducted in 0.1 M NaOH at 1.9 V vs. RHE demonstrated that BNC-2 achieved 71% HMF conversion with a 57% yield of FDCA within 6 hours of operation. The material was applied for electrochemical nitrogen reduction reactions with a high FE of 15.2% and an NH3 production rate of 21.3 μg h−1 mg−1. In contrast, the B-free NC and the lower B-content BNC-1 displayed significantly lower activity, underlining the essential role of boron in enhancing catalytic efficiency. Cyclic voltammetry (CV) studies revealed a strong anodic current in the presence of HMF, while negligible activity was observed in blank electrolyte, indicating that the catalyst selectively oxidized HMF over water. Moreover, chronoamperometry measurements showed that upon successive additions of HMF, the current response quickly recovered without any notable decay, confirming the catalyst's high stability, excellent reusability, and resistance to passivation. Mechanistically, adjacent electron-deficient B and electron-rich N atoms at defects formed FLP-like sites that enabled outer-sphere HMF activation while suppressing HER. Raman and EPR revealed a disordered sp2/sp3 carbon framework with unpaired electrons, promoting charge transfer without strong chemisorption. Control experiments with bare carbon paper confirmed that activity originated from the BNC-2 catalyst. The integration of defect-rich carbon architecture, heteroatom-induced charge asymmetry, and hierarchical porosity rendered BNC-2 an ideal platform for sustainable metal-free catalysis. Notably, the catalyst design was scalable, involved no redox mediators, and functioned under mild alkaline conditions.
In a conceptually distinct but mechanistically aligned approach, Carli et al. utilized a redox-mediator-assisted outer-sphere pathway. They adopted a redox-mediator approach using PEDOT films and TEMPO, where the electrode only regenerates TEMPO+, and the oxidation occurs homogeneously in solution with distinct mediator dependence.14 Carli et al. investigated the performance of poly(3,4-ethylenedioxythiophene) (PEDOT) films, electrodeposited on graphite substrates with various counterions, perchlorate (PER), polystyrenesulfonate (PSS), Nafion (NAF), and aquivion (AQ), as metal-free anodes for the TEMPO-mediated electrooxidation of HMF to FDCA. Electrolysis was carried out in 0.5 M borate buffer (pH 9.2) containing 4 mM TEMPO and 12 mM HMF under a constant applied potential of +0.7 V vs. SCE. All PEDOT variants achieved >90% FDCA yield with faradaic efficiencies around 80%, but PEDOT/AQ stood out, delivering ∼30% faster HMF conversion than graphite and the most stable electrochemical response. It was attributed to low charge transfer resistance (∼1.2 Ω) and high electroactive surface area, enabled by the short-side-chain, Aquivion dopant that enhances both ionic and electronic conductivity. UV-vis spectroscopy and product analyses confirmed this pathway, with no direct adsorption or hybridization of HMF on the electrode surface eliminating inner-sphere contributions. Despite the homogeneous mechanism, the electronic structure and interfacial morphology of the PEDOT films played a crucial role in modulating mediator turnover and suppressing side reactions. Electrochemical impedance spectroscopy and Atomic force microscopy imaging demonstrated the structural integrity and recyclability of PEDOT/AQ over multiple redox cycles with no signs of degradation or delamination. In conclusion, the study establishes PEDOT-based films, particularly PEDOT/AQ, as robust, scalable, and tunable metal-free electrocatalyst platforms for efficient and selective HMF oxidation, highlighting the synergistic role of polymer backbone design and dopant engineering in mediator-based electrochemical biomass valorization. It shifts to mediator-assisted design, illustrating the versatility of metal-free systems.
6.2. Radical coupling pathways for C–C bond formation
Moving from oxidation to C–C coupling, Shang et al. and Kloth et al. highlighted how FAL and HMF can undergo reductive dimerization over carbon-based electrodes via outer-sphere radical pathways (Fig. 28b) Shang's work emphasizes suppression of hydrogen evolution on carbon paper, allowing selective formation of hydrofuroin, while Kloth et al. confirm outer-sphere coupling on boron-doped diamond (BDD) through mechanistic exclusion of adsorption pathways. These studies collectively demonstrate that when hydrogen adsorption is minimized either inherently (BDD) or structurally (carbon paper), the reaction pathway favors radical–radical recombination over hydrogenation.Kloth et al. explored the electroreductive dimerization of HMF to 5,5′-bis(hydroxymethyl)hydrofuroin (BHH), a C12 biofuel intermediate, using metal-free carbon electrodes such as BDD, glassy carbon (GC), and graphite foil in 0.1 M carbonate buffer (pH 9.2).255 Steady-state electrolysis was performed at −0.6 to −1.0 V vs. RHE. Among these, BDD yielded the highest BHH selectivity and faradaic efficiency (up to 55%) and 32% BHH selectivity at −0.1 V vs. RHE, due to its sp3-hybridized, inert surface that suppresses hydrogen adsorption and favors outer-sphere electron transfer. In contrast, GC and graphite promoted inner-sphere electrocatalytic hydrogenation to 2,5-di(hydroxymethyl)furan (DHMF) via surface-bound H*, leading to greater hydrogen evolution. Mechanistic analysis revealed that the difference stems from orbital hybridization: on sp2-carbon (GC, graphite), HMF π-orbitals interact with the surface, enhancing DHMF formation. On BDD, the lack of such interaction promotes outer-sphere coupling to BHH. Product selectivity was stable across triplicate runs, and BHH remained unaltered under electrolysis, confirming its stability. At high HMF concentrations, HER was suppressed and BHH selectivity increased, though some degradation products (likely humins) were observed. While extended recyclability was not assessed, the authors demonstrated operational reproducibility through triplicate runs using fresh and cleaned electrodes, which yielded consistent BHH selectivity and faradaic efficiency. These findings emphasize the importance of carbon hybridization and hydrogen adsorption behavior in guiding selective biomass electro-transformations on metal-free electrodes.
Shang et al. developed a highly selective electrochemical strategy for the dimerization of FAL into hydrofuroin, a valuable C10 jet fuel intermediate, using metal-free carbon paper electrodes under alkaline aqueous conditions.256 Electrolysis was carried out in a two-compartment batch cell with 0.1 M KOH electrolyte at an applied potential of −1.4 V vs. Ag/AgCl, achieving an impressive hydrofuroin yield of 94% and a faradaic efficiency of 93%. This performance was successfully translated to a flow-electrolyzer system, where under optimized conditions (−2.1 V), the process maintained a hydrofuroin yield of approximately 89% and a faradaic efficiency of 82%, while also significantly enhancing productivity. The mechanistic pathway involves electrohydrodimerization via a radical–radical coupling mechanism. FAL undergoes a one-electron reduction to generate a radical anion, which is rapidly protonated in the aqueous environment to form a neutral radical species. This intermediate can either couple to form hydrofuroin or be reduced further to FOL via hydrogen atom transfer. The use of carbon electrodes, which have weak hydrogen binding due to their sp2-hybridized surfaces, effectively suppresses surface hydrogenation and thus favors dimerization. In contrast, metal electrodes like copper promote hydrogen evolution and inner-sphere hydrogenation, leading predominantly to FOL. The sp2-hybridized carbon surface minimizes orbital overlap with FAL, making outer-sphere electron transfer the dominant route and avoiding strong chemisorption. Although extended recyclability studies were not explicitly performed, the system demonstrated excellent short-term operational stability in both batch and flow-cell modes, with consistent yields, faradaic efficiencies, and no observable electrode degradation or fouling, confirming the robustness and reusability of the metal-free carbon paper electrodes. Together, these studies establish a unified design principle by using electrodes with weak hydrogen-binding character either through sp3-hybridized surfaces like BDD or sp2-hybridized carbon structures with structural advantages, it is possible to suppress HER and drive selective radical coupling in electrochemical biomass valorization.
6.3. Levulinic acid conversion via reductive and oxidative routes
Santos et al. broaden the scope by exploring cathodic and anodic transformations of LA. Their study uniquely compares various electrode materials, showing that while Pb favors inner-sphere hydrogenation to valeric acid, carbon felt and graphite enable GVL formation via outer-sphere pathways (Fig. 28a and b).13 Oxidative cleavage on graphite further demonstrates the versatility of metal-free electrodes in enabling Kolbe and Hofer–Moest mechanisms, depending on the reaction conditions. The group used readily available, low-cost electrodes such as graphite plates, carbon felt, and iron foil under ambient aqueous conditions. Electrochemical hydrogenation of LA was performed at −1.8 V vs. Ag/AgCl in 0.5 M H2SO4 or 1 M NaOH. Among all tested cathodes, Pb exhibited the highest Coulombic efficiency (CE) for valeric acid (up to 60%), whereas metal-free carbon electrodes such as graphite and carbon felt favored the selective formation of GVL, achieving 18–21% FE and ∼70% selectivity. The use of carbon-based electrodes promoted direct single-step LA reduction into lactones, while metallic cathodes like Pb enabled hydrogenation of both ketone and carboxyl groups via a HER-suppressed environment.Oxidative reactions were also conducted at the graphite anode, converting LA into C8 products like 2,7-octanedione through Kolbe-type coupling and into C4 species (e.g., 4-hydroxy-2-butanone and 3-buten-2-one) via Hofer–Moest-type mechanisms. These transformations proceeded through radical and carbocationic intermediates, and their selectivity was tuned via electrolyte composition and operating conditions. The study further demonstrated the operational durability of the system, graphite and carbon felt electrodes were reused across multiple cycles with no significant drop in current density or selectivity, and the aqueous electrolyte was recycled without loss of efficiency. No pretreatment or synthesis was required for the carbon electrodes, they were used as received, highlighting the economic and practical simplicity of this approach. The approach readily adapts flow-cell operation, enhancing scalability and aligning with green chemistry principles for integrated biomass valorization.
6.4. Electrocatalytic lignin C–C bond cleavage
Together, the studies by Liu et al. and Zhang et al. mark a significant turning point in the development of sustainable electrocatalytic systems for lignin depolymerization, each leveraging nonmetallic carbon-based electrodes but navigating distinct mechanistic landscapes. While Liu et al. relied on radical pathways initiated by electrooxidation on defect-rich, hydrophilic carbon paper in ionic liquid media, Zhang et al. adopted an unconventional yet elegant route by harnessing the oxygen evolution reaction (OER) as a cooperative rather than a competing process. Liu et al. redefined how metal-free systems could operate under ambient conditions to cleave lignin's C–C bonds without harsh reagents.257 Rather than relying on conventional noble metal electrodes or alkaline environments, their system employed hydrophilic carbon paper (HLcp) and ionic liquid (IL)-based electrolytes in a three-electrode configuration. Among the carbon-based electrodes tested, HLcp delivered nearly complete conversion of PPE–OL, achieving 97% benzaldehyde and 96% quinone yields clearly outperforming even state-of-the-art RuO2–IrO2/Ti electrodes. This remarkable activity was attributed to HLcp's high defect density, microporosity, and oxygen-rich surface functionalities, all of which enhanced ROS generation and substrate activation. Wettability also proved that the critical hydrophilic surfaces facilitated stronger interfacial interactions with the IL-MeCN electrolyte, which in turn influenced reactivity. Notably, ILs with [NTf2]− outperformed [BF4]− and [OTf]− anions, revealing how electrolyte properties like conductivity and viscosity interplay with redox dynamics. The system demonstrated broad substrate tolerance, including α-O-4 and Cα–Cβ dimers, with >90% conversion. Mechanistic investigations, including isotope labeling and radical trapping, confirmed that carbon-surface electrooxidation initiates Cα–OH activation, followed by rearrangement and radical-mediated C–C bond cleavage, with water as the terminal oxygen source. This radical-driven mechanism, operated under mild, scalable, and metal-free conditions provides a valuable blueprint for sustainable lignin upgrading.While Liu et al. demonstrated the power of radical-mediated electrooxidation, Zhang et al. introduced a contrasting, non-radical approach that reimagined OER not as a liability but as an asset in electrocatalytic lignin conversion.258 In traditional aqueous electrocatalysis, OER competes for electrons and hampers selectivity. However, Zhang et al. exploited OER-derived ROS to promote selective C–C bond cleavage. Using defect-engineered carbon nanotubes (d-CNTs) treated by plasma to generate edge-rich sites, they created ROS-retentive surfaces ideal for anodic water splitting. Among the ROS generated, superoxide (O2−) was identified as the key species. Their study focused on 2-phenoxyacetophenone, a β-O-4 lignin model compound. Under mild alkaline conditions (0.5 V vs. Ag/AgCl, 1.0 M KOH), the system delivered nearly quantitative conversion, with phenol and benzoic acid yields of 62.3% and 43.4%, respectively. But the true impact lay in mechanistic resolution, using H218O labeling, kinetic isotope effect, in situ Raman, and DFT, the team elucidated a non-radical pathway where Cβ–H deprotonation forms an enolate intermediate, which then reacts with O2− to form a hydroperoxide that cleaves the Cα–Cβ bond. Though a minor radical route existed, the lower energy barrier (0.33 eV) of the non-radical path confirmed its dominance. The methodology's robustness was evident across substituted lignin models and even extended to DDQ/O2-preoxidized quasi-natural poplar lignin, where a 12.41 wt% monomer yield, including guaiacyl, syringyl, and p-hydroxyphenyl units was achieved. Structural validation via HSQC NMR, GPC, and GC-MS confirmed depolymerization efficacy.
Collectively, these studies present complementary strategies, Liu's system highlights electro-driven radical activation under IL mediation, while Zhang's route elevates the OER to a productive step via non-radical C–C scission. Both eliminate the need for precious metals or external oxidants and exemplify how voltage and water alone can enable green, selective lignin depolymerization. This convergence of design from defect engineering and electrolyte tailoring to mechanistic clarity opens new avenues for the scalable, metal-free, electrocatalytic valorization of lignin. The catalytic activity reported in the literature for the valorization of lignocellulosic biomass via electrocatalytic pathway is summarized in Table 8.
| S. No. | Catalyst | Reaction type (Duration) | Electrolyte/applied potential | Faradaic efficiency | Reactant conversion (C)/selectivity (S)/yield (Y) (%) | Ref. |
|---|---|---|---|---|---|---|
| 1 | GNPCH-900 | Oxidation of HMF to FDCA (400 h, continuous) | 1 M KOH/1.446 V vs. RHE | >90% FE | >90% C | 253 |
| 2 | BNC-2 | Oxidation of HMF to FDCA and N2 reduction (6 h) | 0.1 M NaOH/1.9 V vs. RHE | 15.2% for NH3 production | 71% HMF C | 254 |
| 57% FDCA Y | ||||||
| 3 | PEDOT films | Oxidation of HMF to FDCA using TEMPO (6 h) | 0.5 M borate buffer (pH 9.2)/+ 0.7 V vs. SCE | >80% FE | >90% FDCA Y | 14 |
| 4 | BDD | Reductive dimerization of HMF (2 h) | 0.1 M carbonate buffer (pH 9.2)/−1.0 V | 55% FE | 90% HMF C | 255 |
| 32% BHH S | ||||||
| 5 | Carbon paper | Reductive coupling of FAL (5 h) | 0.1 M KOH/1.4 V vs. Ag/AgCl | 93% FE | 94% hydrofuroin Y | 256 |
| 6 | Graphite/Carbon felt | LA hydrogenation (2–4 h) | 0.5 M H2SO4/1 M NaOH/−1.8 V vs. Ag/AgCl | 17–21% FE | 70% GVL S | 13 |
| 7 | HLcp | C–C cleavage of PPE–OL (8 h) | 0.5 M H2SO4/1.5 V vs. Ag/AgCl | — | 97% benzaldehyde S, 96% quinone | 257 |
| 8 | d-CNTs | C–C cleavage of 2-phenoxyaceto-phenone (7 h) | 1.0 M KOH/0.5 V vs. Ag/AgCl | — | 98.8% C, 62.3% phenol Y 43.4% benzoic acid Y | 258 |
7. Comparative analysis of thermocatalytic, photocatalytic, and electrocatalytic routes
The comparative analysis of thermocatalysis, photocatalysis, and electocatalysis for biomass valorization highlights distinct advantages and challenges associated with each approach which are discussed below.7.1. Mechanistic differences and efficiency considerations
Thermocatalysis harnesses external heat to drive bond cleavage, rearrangement, and deoxygenation in biomass conversion, offering high reaction rates and industrial viability. However, this process demands substantial energy input, often resulting in non-selective product formation, catalyst deactivation, and carbon deposition. For instance, Totong et al. reported a 55.2% lignin conversion using N-STC at 250 °C, but catalyst stability was compromised due to thermal degradation.85In contrast, photocatalysis operates under mild conditions, utilizing photoexcited charge carriers to drive selective oxidation and reduction. This approach is particularly effective for lignin depolymerization, HMF oxidation, and glycerol reforming, where ROS enables targeted bond cleavage. For example, Xu et al. achieved 52.6% benzaldehyde yield and 33.8% benzoic acid from lignin model, 1,2-diphenylethanol using g-C3N4 in a micellar aqueous system under visible light, where O2˙− radicals generated via photoexcited charge carriers facilitated selective C–C bond cleavage.250 However, its efficiency is constrained by poor charge separation and limited light absorption, limited reactor design, restricting large-scale applications.
Electrocatalysis offers a distinct mechanistic advantage by leveraging applied potential to generate reactive oxygen or hydrogen species directly at the electrode interface. It enables precise control over oxidative or reductive transformations under ambient temperature and pressure. However, its efficiency is constrained by catalyst fouling due to strong electrolytic environments. For instance, Liu et al. achieved nearly quantitative cleavage of the β-O-4 bond in lignin model compound (PPE–OL) using hydrophilic carbon paper and ionic liquid electrolytes, with 97% benzaldehyde and 96% quinone selectivity, outperforming even RuO2–IrO2 electrodes.258
Together, these approaches highlight mechanistic contrasts, thermal catalysis relies on heat-induced molecular activation, photocatalysis exploits photon-generated charge carriers, and electrocatalysis enables potential-controlled redox reactions. These distinctions fundamentally shape their energy efficiency, reaction selectivity, and environmental compatibility.
7.2. Selectivity and scalability
Selectivity is crucial in biomass valorization, determining the efficiency of producing high-value chemicals while minimizing byproduct formation. Metal-free thermal catalysis, while effective in cleaving robust C–O and C–C bonds, often leads to overoxidation, char formation, reducing selectivity. For example, Zhang et al. reported that P-C-600 catalyzed furfural oxidation to maleic acid with 92.8% conversion, but humin formation and reduced selectivity to 76.3%.211 In contrast, Rathod et al. demonstrated that a bifunctional carbonaceous catalyst (CC–SO3H–NH2) achieved 85% DFF yield with 100% selectivity from HMF, and the process was scalable, yielding 81% DFF from 5 g of HMF, reinforcing its industrial viability.Photocatalysis excels in selective oxidation and hydrogenation, enabling the production of valuable intermediates such as DFF, FDCA, and FOL with high specificity. However, limited light penetration and rapid charge recombination hinder scalability. Krivtsov et al. showed that exfoliated g-C3N4 achieved 50% DFF selectivity in an open solar-driven system, but low light penetration restricted further conversion.236
Electrocatalysis demonstrates high selectivity in biomass upgrading through tunable redox potentials and electrolyte engineering. For instance, Wang et al. employed a 3D nitrogen-doped amorphous carbon framework (GNPCH-900) to electrooxidize HMF to FDCA with >90% FDCA yield and showcased one of the highest faradaic efficiencies among metal-free systems.253 Moreover, scalable batch electrolysis systems and flow cells have enabled multigram conversion, supporting its potential for industrial translation.256 Nonetheless, challenges such as electrode fouling, ion diffusion limitations, and electrolyte recovery must be addressed.
In summary, thermal catalysis offers robustness but suffers from lower selectivity, photocatalysis provides superior specificity but limited scalability, while electrocatalysis balances tunable selectivity with promising scalability, though it requires further development in system engineering and long-term operation.
7.3. Catalyst lifetime and stability
Catalyst durability is pivotal in determining the feasibility of biomass valorization technologies. Thermal catalysts frequently degrade due to carbon deposition, heteroatom leaching, and sintering, limiting their reusability. Ren et al. observed a 10% decrease in FDCA yield after four cycles using an N-doped carbon catalyst, highlighting stability concerns.208Photocatalysts, particularly g-C3N4 and heteroatom-doped carbon materials, suffer from photo-corrosion, charge accumulation, and active site deactivation. Speltini et al. developed an oxidized g-C3N4 catalyst that exhibited stable H2 evolution over multiple cycles, demonstrating improved durability over conventional g-C3N4.245
Electrocatalytic systems offer operational stability under mild conditions but are susceptible to electrode passivation, active site oxidation, and structural degradation during prolonged electrolysis. Dos Santos et al. noted a performance decline over extended LA conversion runs on graphite due to surface fouling.13 However, stable performance over multiple cycles has been reported using carbon-felt or hydrophilic carbon paper electrodes, especially when combined with ionic liquids or optimized electrolyte interfaces. Enhancing electrode conductivity and designing anti-fouling surfaces are critical for improving durability. While each approach faces distinct deactivation pathways, thermal degradation, photodegradation, or electro-passivation advances in nano-structuring, surface engineering, and electrolyte design are critical for improving durability and recyclability.
7.4. Practical considerations and industrial viability
Thermal catalysis is widely utilized for high-throughput biomass processing, including pyrolysis, hydrothermal liquefaction, and hydrogenation reactions. However, its high energy demands and low selectivity pose challenges for sustainable applications. Choudhary et al. converted FAL into succinic acid (SA) using Amberlyst-15, demonstrating industrial feasibility at a 20 mmol scale, yielding 68% isolated SA with >99% FAL conversion.212 Similarly, Kundu et al. developed a sulfonic acid-functionalized anthracene-derived porous organic polymer (AnPOP–SO3H), which retained 98% of its original activity after ten consecutive reaction cycles, making it highly promising for biomass upgrading.212Photocatalysis, while highly selective and operational under mild conditions, faces scalability challenges due to limited light penetration and dependence on external light sources. Yan et al. optimized conjugated polymers to enhance charge separation, yet light penetration remains a significant barrier to industrial adoption.240
Electrocatalysis is emerging as a practical alternative owing to its compatibility with modular flow-cell configurations, electrochemical reactors, and renewable electricity inputs. Continuous electrolysis setups, such as those employing carbon felt or graphite plates, have demonstrated potential for upscaling under ambient pressure and temperature.13 Moreover, electrocatalysis allows reaction tuning via voltage control without needing external oxidants or reductants, offering significant sustainability benefits. Nevertheless, system cost, electrode degradation, and electrolyte compatibility remain challenges for widespread adoption.
Altogether, thermal catalysis dominates current industrial applications due to its maturity, while photocatalysis and electrocatalysis present complementary, sustainable pathways that require further optimization in reactor design, energy integration, and materials durability for commercialization.
Overall, while thermal catalysis remains the most established technology for large-scale biomass processing, it is inherently energy-intensive. Photocatalysis, though highly selective and sustainable, requires improved charge management and reactor designs for practical applications. Electrocatalysis introduces a highly tunable and energy-efficient platform that offers precise control over biomass redox conversions using electricity, but is currently limited by electrode stability and electrolyte design that require further optimization in reactor design, energy integration, and materials durability for commercialization.
Photothermal catalysis bridges the thermal and photonic domains, integrating light absorption with localized heating to improve charge carrier dynamics and catalytic rates. As shown by Jing et al., O,P-co-doped porous g-C3N4 enabled stable lignin valorization over five cycles due to synergistic photothermal effects. Similarly, Sun et al. demonstrated xylose oxidation to LA at 50 °C using 4CzIPN@CMC-HG in a 1000-fold scale-up. Going forward, hybrid systems such as photothermal and emerging photoelectrocatalytic configurations will play a pivotal role in overcoming individual limitations by combining the strengths of light-driven and voltage-driven platforms. These strategies hold promise for integrated biorefineries and green hydrogen production, provided materials optimization and reactor engineering advances continue to evolve.
8. Conclusion and future perspectives
Metal-free catalysis has emerged as a promising strategy for biomass valorization. Significant progress has been made in developing heteroatom-doped carbon catalysts, polymeric frameworks, and covalent organic frameworks (COFs) as efficient alternatives to metal-based catalysts. These advancements have deepened our understanding of catalytic depolymerization, oxidation, and hydrogenation of biomass-derived compounds. Additionally, photocatalytic strategies leveraging charge separation and ROS have enabled selective transformations of lignin, HMF, FAL, glycerol, and biomass-to-hydrogen systems. Integrating photothermal catalysis, which combines light-driven and thermally assisted processes, can further expanded the potential of metal-free catalytic systems. Electrocatalysis has emerged as a third pillar of sustainable biomass valorization. However, compared to thermal and photocatalytic routes, metal-free electrocatalysis is little explored, presenting a wide-open landscape for innovation in catalyst design, mechanistic investigation, and process integration. Furthermore, the convergence of photocatalysis and electrocatalysis into photoelectrocatalytic systems offers a powerful platform to combine photogenerated carrier generation with externally applied potential. Such metal-free photoelectrocatalytic configurations can lower onset potentials, suppress side reactions, and enhance charge separation, offering synergistic advantages in selective biomass oxidation and hydrogen evolution.In parallel, translating these catalytic advances into practical utility requires focusing on the downstream application of the derived products. FAL and HMF-based compounds such as FDCA, LA, and GVL are increasingly being explored as renewable monomers for bioplastics, green solvents, and pharmaceutical intermediates. Likewise, lignin-derived aromatics such as vanillin, syringaldehyde, and monophenols hold promise as precursors for resins, antioxidants, and fuel additives. The successful integration of these bio-derived molecules into industrial chemical frameworks could significantly advance decarbonization and circular economy objectives. Moreover, photocatalytic hydrogen evolution and carbon-dot-based systems are poised to support distributed green energy technologies and water remediation.
8.1. Challenges and research gaps
Despite these advancements, several challenges hinder widespread adoption and industrial feasibility. Thermal catalysis remains energy-intensive, increasing operational costs, while catalyst stability issues, particularly heteroatom leaching in nitrogen-doped carbon catalysts, limit long-term reusability. Many thermal catalytic systems also rely on acidic or basic activating reagents, leading to secondary waste generation and complicating product purification.Photocatalysis, although milder, suffers from poor charge separation, low quantum efficiency, and rapid charge recombination, restricting large-scale applications. The continued reliance on Pt-based cocatalysts for hydrogen evolution also poses sustainability concerns, necessitating the development of alternative metal-free cocatalysts. Furthermore, catalyst degradation remains a major hurdle, with carbon-based photocatalysts undergoing structural breakdown and thermal catalysts experiencing carbon deposition and sintering.
In electrocatalytic systems, key limitations include electrode passivation, limited electrolyte compatibility, and challenges in ensuring long-term operational stability under continuous flow. Using ionic liquids or expensive supporting electrolytes can raise sustainability concerns, while scalable reactor engineering and electrolyte recycling remain largely underdeveloped. Furthermore, a mechanistic understanding of electron transfer pathways and reactive intermediate evolution in electrocatalytic biomass conversion is still in the beginning, requiring advanced in situ/operando techniques for clarification. In particular, hybrid photoelectrocatalytic systems are nascent, and research on metal-free configurations remains scarce. Developing photoactive carbon-based electrodes capable of simultaneous light harvesting and bias-assisted redox transformations could open novel mechanistic pathways for selective and energy-efficient biomass processing.
While oxidative transformations have been extensively explored in metal-free catalysis, reductive valorization remains underdeveloped due to its reliance on metal-based catalysts. Additionally, mechanistic insights into photocatalytic biomass depolymerization are still lacking, particularly regarding electron–hole interactions and intermediate evolution. Expanding research beyond conventional N-doped carbon materials and g-C3N4 to non-carbon-based alternatives, such as boron nitrides, COFs, and polymeric catalysts could unlock new pathways. The impact of solvent effects and catalyst acidity/basicity tuning on product selectivity and reaction kinetics also requires further exploration.
8.2. Emerging strategies and future directions
The future of metal-free catalysis lies in integrating artificial intelligence (AI) and machine learning (ML) to accelerate catalyst discovery by predicting optimal doping configurations and reaction conditions. AI/ML tools are rapidly emerging as transformative enablers in the design and mechanistic understanding of metal-free catalysts for biomass valorization. ML models such as random forests, XGBoost, and neural networks have accurately predicted catalytic activity and product yields based on reaction parameters and material descriptors.259 For instance, in biodiesel synthesis, ML identified key fatty acid components and reaction conditions influencing yield, offering mechanistic insights through partial dependence plots.260 In photocatalytic systems, DFT-coupled ML approaches have clarified how conductive polymer shells and dopant sites modulate charge separation and activation barriers, guiding selectivity in biomass oxidation.261Advanced deep learning models, including convolutional neural networks and crystal graph networks, now predict adsorption energies and simulate strain effects with high accuracy.262 Generative models such as variational autoencoders (VAEs) and GANs are further enabling inverse catalyst design and the discovery of new reaction pathways.263 These data-driven approaches complement experimental workflows by reducing exhaustive screening and enhancing interpretability using tools like SHAP and feature importance analysis. As AI-integrated platforms evolve, they are expected to play a central role in guiding the discovery of robust, selective, and scalable metal-free catalysts for sustainable biomass conversion.
The coupling of metal-free catalysts with hybrid solar–thermal platforms has already demonstrated remarkable efficiencies, achieving over 90% conversion in biomass hydrolysis under mild conditions. Additionally, advanced characterization techniques such as in situ spectroscopy and operando XPS are providing critical insights into charge carrier dynamics and active site behavior, enabling the rational design of more selective and stable photocatalysts. Similarly, electrochemical in situ methods, such as operando FTIR, Raman spectroscopy, and differential electrochemical mass spectrometry (DEMS), are being deployed to map surface intermediates and monitor C–C and C–O bond scission during biomass electrooxidation. These tools will be pivotal in elucidating structure–activity relationships in next-generation metal-free electrocatalysts.
Further research should prioritize the development of photothermal nanomaterials and hybrid systems, including carbon-supported heterojunctions, and Z-scheme photocatalysts, to improve charge separation, light absorption, and substrate activation. The integration of solar-assisted catalytic systems with novel reactor architectures can enhance photon penetration and thermal homogeneity, improving both scalability and process efficiency. Electrocatalytic integration with renewable electricity sources (e.g., solar-powered electrolysis) and modular flow-cell designs represent another frontier in industrial biomass valorization. This approach enables decentralized chemical manufacturing using earth-abundant, metal-free electrodes under energy-efficient conditions. Moreover, hybrid photoelectrocatalytic systems may bridge the gap between light- and potential-driven pathways, particularly for reactions requiring sequential redox steps. Exploring the synergy between bandgap engineering and applied bias holds promise for expanding reaction scope and enhancing selectivity in multifunctional biomass conversion platforms.
The simultaneous valorization of biomass and green hydrogen production presents an exciting opportunity for sustainable catalysis. However, most current strategies compromise the generation of valuable biomass-derived intermediates. Developing integrated systems that maximize hydrogen evolution and selective biomass valorization will be essential. Transition-metal-free materials, such as carbon nanodots, exhibit great potential for efficient water splitting and hydrogen evolution and should be further explored to realize fully metal-free hydrogen production systems.
Beyond conventional catalysts, expanding research toward boron nitrides, COFs, and polymeric photocatalysts could unlock novel catalytic pathways and improve process efficiency. Tuning catalyst acidity and basicity through heteroatom functionalization and nanostructure engineering may further enhance product selectivity. A deeper understanding of solvent–catalyst interactions will also be crucial to optimize reaction conditions, minimize side reactions, and maximize carbon efficiency.
Metal-free catalytic systems represent a paradigm shift in sustainable biomass valorization, offering environmentally benign and scalable solutions for producing biofuels, fine chemicals, and green hydrogen. Including photo and electrocatalysis enrich this paradigm by introducing precise redox control, continuous operation, and compatibility with electrified chemical platforms. Moreover, the emergence of photoelectrocatalysis, particularly in metal-free configurations, offers a hybrid strategy with unique potential to overcome kinetic and thermodynamic limitations inherent in isolated photonic or electrochemical systems. Establishing techno-economic feasibility and regulatory frameworks will be key to the industrial adoption of metal-free electro- and photoelectrocatalytic platforms.
However, achieving industrial viability will require interdisciplinary collaboration between materials science, process engineering, and computational chemistry. The next decade will be critical in establishing metal-free catalysis as a cornerstone of green chemical manufacturing, driving the transition toward a circular bioeconomy and a sustainable energy future.
Conflicts of interest
The authors declare no conflict of interest.Data availability
It is a review article. All the data are published by other authors cited in this article. Permission will be taken from the publishers for the figures presented in this manuscript.Acknowledgements
AC thanks CSIR for fellowship (09/1005(0033)/2020-EMR-I). RS is grateful to IIT Ropar for the Faculty Research and Innovation Award.References
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