The Biomass Energy Industry Branch of the China Industrial Development Promotion Association released a blue paper indicating that, as of 2020, the annual production of major biomass resources in China is approximately 3.494 billion tons. Biomass, as the most abundant renewable organic material on Earth, has advantages such as being green, low-carbon, and clean, and is considered an important development direction for future energy and chemical industries. For biomass conversion, traditional high-temperature gasification processes (usually > 700°C) consume a lot of energy and lead to additional CO₂ emissions. Photocatalytic technology, with its mild operating conditions and high conversion efficiency, has become an important tool in the field of biomass conversion. This article aims to introduce the latest research progress in photocatalytic biomass conversion and explore its application prospects and challenges.
Semi-conductors with appropriate band gaps are widely used as photocatalysts. Generally, semiconductors capture light and are excited to generate photoinduced charges, where photoinduced electrons transition from the valence band (VB) to the conduction band (CB), leaving positive and negative charges known as photoinduced holes and photoinduced electrons, respectively. The charges will transfer from the bulk to the surface and participate in surface redox reactions. For photocatalytic water-splitting reactions to produce hydrogen, thermodynamically, the CB minimum of the photocatalyst needs to have a more negative potential than the H⁺/H₂ energy level (-0.41 V vs. NHE), while the VB maximum should have a more positive potential than the water oxidation potential (0.82 V vs. NHE). Compared to the thermodynamic barrier of 1.23 V for water splitting, biomass reforming reactions are more likely to occur due to the lower redox potential of organic molecules. For example, the oxidation potential of glucose is only 0.05 V vs. NHE,[1] and the relatively low redox potential of biomass makes hole quenching more efficient, increasing the intrinsic driving force of the reaction and promoting photocatalytically excited electrons to reduce H⁺ to produce hydrogen. Additionally, photocatalytic hydrogen production from biomass can utilize narrow bandgap semiconductors to capture low-energy photons, such as visible and infrared light, which make up the vast majority of sunlight.
Schematic of photocatalytic water splitting and biomass conversion to hydrogen
However, meeting only the thermodynamic requirements does not guarantee the reaction will occur. The electron transfer at the semiconductor interface also has a significant impact on the photocatalytic reaction. Two main factors affecting interfacial electron transfer are the capture state of the photocarriers and the adsorption of organic molecules; the captured charges should transfer to the semiconductor surface to facilitate the photocatalytic reaction.[2] For proton-type biomass feedstocks, such as alcohols and acids, the proton-coupled electron transfer (PCET) process plays a crucial role in interfacial charge transfer. Compared to single proton transfer/electron transfer processes, the energy barrier for simultaneous proton and electron transfer in PCET is lower.
Kinetic advantages of the PCET process
Another factor is the adsorption state of molecules on the surface, which is influenced by the catalyst surface properties, molecular structure, and solvent conditions. For instance, under the interaction of water with acetic acid molecules, the coordination mode of acetic acid on the (101) surface changes from bridging bidentate to monodentate, reducing the adsorption energy by 1.17 eV,[3] thus, adjusting the adsorption configuration can effectively promote interfacial charge transfer and enhance photocatalytic activity.
Water gas (CO and H₂) is one of the major components in today’s chemical industry, with important applications in the synthesis of olefins or aromatics and in the production of liquid fuels through the Fischer-Tropsch synthesis method. Traditional biomass conversion to water gas is achieved through high-temperature gasification, which not only consumes a large amount of energy but also releases significant amounts of CO₂.[4]
Photocatalytic production of CO from biomass can break the C-C backbone, and photocatalytic reforming holds promise for C-C bond cleavage under mild conditions. However, the challenge with photocatalytic methods is to selectively break the C-C backbone while avoiding excessive oxidation to CO₂. For instance, from a thermodynamic perspective, CO₂ formation is more favorable than CO formation, with a lower reaction energy.
The cleavage of C-C bonds affects the CO to CO₂ ratio, with aldehyde intermediate decarbonylation reactions producing CO and decarboxylation reactions producing CO₂. Therefore, to selectively obtain CO, one must avoid excessive oxidation of aldehyde groups to carboxyl groups. Hydroxyl radicals (·OH) are a major cause of excessive oxidation of aldehydes to carboxyl groups. When using semiconductors with wider bandgaps (such as TiO₂ with 3.2 eV), photoexcited holes can oxidize water to generate ·OH, which has strong oxidizing ability. For example, the hole oxidation potential of TiO₂ is about 2.9 V vs. NHE, sufficient to oxidize water to ·OH (2.27 V vs. NHE).[5]
In 2020, Wang et al. used a water-acetonitrile solvent system to prevent excessive oxidation of bio-polyols to CO₂ during photocatalysis on Cu/TiO₂ by reducing the concentration of water, which significantly hindered the oxidation of water to ·OH, thus slowing CO₂ formation.[6] However, during photocatalysis, water is a necessary solvent for dissolving polyols and sugars. Bio-polyols and water will competitively adsorb on the semiconductor surface. Controlling the surface morphology and defects of the catalyst to promote the adsorption of biomass molecules on the semiconductor is also an effective way to improve CO selectivity.
Mechanism of glycerol hole oxidation reaction
Another factor affecting CO selectivity is the decomposition of formic acid (FA) intermediates. The decarbonylation (dehydration) reaction of FA produces CO, while the decarboxylation (dehydrogenation) reaction produces CO₂. Therefore, promoting FA dehydration theoretically increases CO selectivity. However, the dehydrogenation reaction (-48.8 kJ·mol⁻¹) is thermodynamically more favorable than the dehydration reaction (-28.5 kJ·mol⁻¹), making FA dehydration more difficult.[7] Experiments show that TiO₂ is selective for FA dehydration, but the currently reported TiO₂-based photocatalysts still tend to follow FA dehydrogenation.[8] This may be due to other components in TiO₂-based catalysts, such as metals and metal oxides, which promote the dehydrogenation reaction. In the case of Cu/TiO₂, the CO/CO₂ yield ratio is greatly influenced by the amount of copper loaded. High copper loading leads to the dominance of CuOₓ nanoparticles, which favor dehydrogenation reactions producing CO₂. At low copper loading, doping with Cu²⁺ is dominant, and FA dehydration occurs on the TiO₂ phase, generating CO. Therefore, reducing the copper content can enhance CO selectivity.
Using narrow bandgap semiconductors is another option for achieving high CO selectivity. Unlike metal oxides with larger bandgaps that are only active under UV light, metal sulfides have narrower bandgaps and milder oxidation abilities. Metal sulfides with mild oxidation abilities may not oxidize water to produce ·OH, thus preventing excessive oxidation of bio-polyols to CO₂. Under visible light irradiation, CdS has some activity in converting polyols to CO without producing CO₂, but its efficiency is relatively low.
Unlike fossil fuels, biomass contains a higher oxygen content and various oxygen-containing functional groups. This characteristic enables the combination of biomass conversion with hydrogen evolution reactions, promoting the simultaneous production of high-value chemicals and H₂. For example, lignocellulosic-derived polyhydroxy molecules, such as sugars or polyols, can be photocatalytically converted to lactic acid and H₂. To avoid excessive oxidation of biomass, photocatalysts should have moderately positioned VB, such as transition metal sulfides, phosphides, CuO, and g-C₃N₄, which are widely used in these systems due to their appropriate band positions. Noble metals are usually introduced as hydrogen evolution sites to facilitate the reaction.[9]
Photocatalytic Conversion of Lignocellulose-Derived Chemicals and Hydrogen Production Pathways
5-Hydroxymethylfurfural (HMF) is an important biomass derivative that can be further oxidized while simultaneously producing H₂. The choice of photocatalyst plays a crucial role in the selective oxidation of HMF to produce high-value chemicals and H₂. Transition metal sulfides or g-C₃N₄ are widely used in this process. In neutral water, Ni/CdS photocatalytically dehydrogenates HMF to produce 2,5-diformylfuran (DFF) and H₂. This reaction can be significantly accelerated in alkaline solutions but may be excessively oxidized to 2,5-furandicarboxylic acid (FDCA). Regulating the oxidative ability of photogenerated holes is an effective way to prevent the over-oxidation of DFF.[9]
Other biomass-derived alcohols, such as methanol, ethanol, benzyl alcohol, and furfuryl alcohol, can also be dehydrogenated to aldehydes and H₂. When the reaction occurs in pure alcohol, the resulting aldehyde can undergo an acetalization process, combining two alcohol molecules and forming an acetal on an acidic catalyst or at the acidic sites of a heterogeneous catalyst. For example, in the case of ethanol, the C=O group of the produced acetaldehyde can be further attacked by the O atom of another ethanol molecule, breaking the C=O π bond and forming a hemiacetal. This hemiacetal can then dehydrate with another ethanol molecule on an acidic catalyst to form 1,1-diethoxyethane (1,1- DEE).[10]
Traditional high-energy-density carbon-based fuels mainly consist of long carbon chains, with hydrocarbons typically containing more than 10 carbon atoms. However, most biomass resources contain basic units with fewer than 10 carbon atoms. For example, lignocellulose is the most abundant biomass resource, including cellulose, hemicellulose, and lignin. Glucose and xylose are the basic units of cellulose and hemicellulose, containing 6 and 5 carbon atoms, respectively. Lignin is derived from three basic units: p-coumaryl alcohol, coniferyl alcohol, and sinapyl alcohol. Lignin depolymerization typically produces C7-C9 monomers. To utilize biomass for producing high-energy liquid fuels, carbon chain length must be increased through C-C bond coupling.[11] Photocatalysis can achieve carbon-centered radical C-C bond coupling through homolytic cleavage of C-H bonds. In this process, the electrons of the C-H bond are transferred to quench the photoexcited holes, generating carbon-centered radicals and releasing protons. The coupling of carbon-centered radicals doubles the carbon chain, while the protons are reduced by photogenerated electrons to produce H₂. This process, known as photocatalytic dehydrogenative coupling, has been effectively used for the coupled production of H₂ and diesel or jet fuel precursors. Among the downstream products derived from lignocellulose, 2,5-DMF and 2-methylfuran (2-MF) are promising feedstocks for hydrogen production coupled with diesel fuel. These two compounds can be obtained through the deoxygenation of HMF and furfural, respectively.
Figure A. Photocatalytic Hydrogen Production Coupled with Methylfuran Production for Bio-Diesel Precursors; Figure B. Lignocellulose Composites; Figure C. Photocatalytic Hydrogen Production Coupled with Bio-Diesel Precursors from Lignin Oil
In addition to the co-production of fuel precursors, C-C bond coupling can also be used to synthesize value-added chemicals. Simple monohydric alcohols, such as methanol, ethanol, and butanol, can be extracted from biomass through chemical or biological methods. These simple monohydric alcohols can serve as raw materials for hydrogen production and the formation of diols. Diols are widely used as monomers for polyester synthesis. For example, ethylene glycol (EG) is a monomer for synthesizing polyethylene terephthalate (PET), the most widely used resin in our daily lives.
Photocatalytic technology shows great potential for extracting hydrogen from biomass and producing value-added chemicals under mild conditions. Although many new reactions and catalysts have been developed, their efficiency is still insufficient to support practical applications, necessitating the design of high-performance photocatalysts to improve efficiency. Traditional methods, such as improving light absorption and charge separation, remain effective, but the complexity of biomass molecules poses challenges for the selective production of target products. Modulating the structure of photocatalysts, such as introducing spatial effects and pore size effects, is expected to enhance selectivity. Simultaneously, controlling the oxidation depth to prevent deep degradation of biomass into CO₂ is crucial. Metal sulfides are favored for their better sunlight response and mild oxidation capabilities, but their valence band oxidation potentials need to be finely tuned to balance reaction rates and selectivity. Additionally, developing efficient photoreactors, promoting electron/proton transfer, and constructing semi-artificial photosynthesis systems are key paths to improving photocatalytic efficiency. Despite the significant advantages of photocatalytic technology, industrial-scale applications still face challenges such as poor solubility, low product selectivity, and lagging photoreactor technology. Future research should focus on optimizing catalyst design, developing efficient reactors, and exploring economically viable production processes.
Wang, M., Zhou, H.J., Wang, F. (2024). Photocatalytic biomass conversion for hydrogen and renewable carbon-based chemicals. Joule. 3, 604-621.
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