Photocatalytic Hydrogenation of Biomass-Derived Model Molecules to Value-Added Chemicals

Keywords

Biomass - Selective hydrogenation - Photocatalysis - Hydrodeoxygenation - Metal-semiconductor catalyst

Significance

This review focuses on the photocatalytic hydrogenation of biomass model molecules into high-value-added chemicals, providing a viable strategy and theoretical basis for finding sustainable alternatives to traditional petrochemical processes. This approach may contribute to the development of a low-carbon sustainable chemical industry and promote the effective utilization of renewable carbon resources and environmental protection.

1

Introduction

In response to reducing the heavy dependence on fossil fuels and meeting global needs for sustainable development, the quest for abundant and easily accessible resources has been the cornerstone of achieving the ambitious goal of carbon neutrality [1]. In light of this, numerous renewable resources have been explored, such as solar energy, biomass energy, wind power, and hydrogen energy, based on the unique advantages of diversified energy supplies and energy security [2]. Among them, biomass has gained significant attention due to its abundant availability and renewability [3]. As the only renewable carbon source in nature, biomass utilization offers a viable pathway to reduce dependence on fossil resources and mitigate carbon emissions, which can effectively convert agricultural residues, forestry by-products, and industrial organic waste into value-added chemicals, thus improving resource efficiency and reducing environmental pressures [4].

From the viewpoint of composition and resources, lignocellulose is the most important component of biomass, comprising three key fractions: cellulose, hemicellulose, and lignin [5]. Cellulose is a linear polysaccharide composed of β-1,4-glucose units, characterized by its high crystallinity and chemical stability, which makes it a potential precursor for the production of platform molecules (e.g., polyols and 5-hydroxymethylfurfural). Hemicellulose, a complex branched polysaccharide composed of pentoses (e.g., xylose) and hexoses (e.g., mannose and galactose), is readily hydrolyzed into fermentable sugars and extensively utilized in the production of biofuels and chemicals [6] [7] [8]. Lignin is an aromatic polymer with a highly intricate structure and is the only renewable source of aromatic compounds and can be transformed into a variety of value-added aromatic chemicals via catalytic cracking, oxidation, or hydrogenation [9] [10] [11] [12]. In addition to lignocellulose, biomass-derived oils, such as inedible vegetable oils or wasted cooking oils, serve as valuable carbon sources that can be processed into fatty acids and their derivatives [13] [14] [15] [16]. The diverse composition of biomass, coupled with its rich oxygenated functional groups (such as hydroxyl, carboxyl, and aldehyde groups), offers a versatile reaction platform for selective catalytic transformations. In recent years, several typical promising platform molecules from biomass, including 5-hydroxymethylfurfural (HMF), furfural (FAL), and ethylene glycol, have been found to have significant potential in the production of fuels, polymers, and fine chemicals [17] [18] [19] [20] [21]. Additionally, biomass-derived fatty acids can be efficiently decarboxylated or hydrodeoxygenated to produce long-chain hydrocarbon fuels, providing a sustainable alternative to diesel and jet fuel [22], [23].

Despite the great potential and promise of biomass for the production of value-added chemicals, its complex structure and high oxygen content feature impose significant challenges for depolymerization and removal of oxygen components, as the selective cleavage of various C–O and C–C bonds requires overcoming relatively high activation energy barriers [24]. Over the past few decades, various catalytic technologies have been developed for biomass valorization, such as hydrolysis, hydrogenolysis, HDO, etc., among which HDO is widely accepted as one of the most rational upgrading strategies [25] [26] [27]. Typically, most HDO reactions require high temperatures (120–500 °C) and high-pressure hydrogen (3–20 MPa) to ensure efficient removal of oxygen from biomass molecules, followed by producing hydrocarbons and other value-added products. Although it is effective, this process is characterized by high energy consumption and operational costs [28] [29] [30] [31]. In addition, competing side reactions such as over-reduction and polymerization tend to affect product selectivity and generate unwanted by-products in high-temperature reaction systems. Catalyst deactivation due to sintering, poisoning, and structural degradation under harsh conditions further limits its applicability [24], [32], [33]. These bottlenecks highlight the urgent need for more reliable catalytic technologies capable of achieving efficient and sustainable biomass conversion under milder conditions.

Solar energy is considered a renewable and clean energy source on the earth. Photocatalytic hydrogenation strategy provides a sustainable alternative to biomass appreciation by using solar energy to drive redox reactions that may allow chemical conversion to occur at ambient temperatures and pressures [34]. Some photocatalytic systems have been verified highly selective and efficient in activating specific functional groups (e.g., C=O and C=C) while minimizing side reactions and over-reduction [35], [36]. Moreover, the photocatalysis process may provide the opportunity to reduce the energy barrier of rate-limiting steps in some catalytic systems, or change the adsorption and desorption behavior of reaction intermediates on the catalyst surface, thus changing the reaction activity and selectivity [37]. Recent studies have demonstrated the ability of photocatalytic hydrogenation to selectively convert HMF and FAL into value-added products such as 2,5-dimethylfuran (DMF) and furfuryl alcohol (FOL), which have broad applications in energy and polymer industries [38], [39]. Similarly, photocatalytic decarboxylation has been shown to efficiently convert fatty acids into long-chain hydrocarbons under mild conditions, providing a sustainable pathway for biofuel production [22]. Therefore, photocatalysis has unique advantages in terms of energy efficiency, selectivity, and environmental compatibility, making it a promising transformative approach for biomass upgrading.

In recent years, photocatalytic conversion of biomass model molecules has attracted extensive attention, and some useful knowledge has been accumulated. The mechanisms and strategies of biomass conversion into value-added products via oxidation, the exploration of photocatalytic biomass oxidation coupled with hydrogen evolution reactions, and the design of high-performance catalysts have been carefully studied and reviewed [40] [41] [42] [43]. In the field of lignin photocatalytic depolymerization, some important insights have been gained into the catalyst design, reaction pathways, and reaction mechanisms, and the transformation of C–H/O–H/C–C/C–O bonds [43] [44] [45]. For photocatalytic reduction of biomass, the catalyst preparation and characterization, catalytic performance, and reduction pathways and mechanisms have been summarized [18], [20], [46]. This review concerns the recent achievements in photocatalytic hydrogenation for biomass-derived model molecules to value-added chemicals. The model reactions of photocatalytic hydrogenation in biomass upgrading, the design of photocatalysts, and the photocatalytic hydrogenation mechanisms (especially metal–support synergies) are reviewed. In addition, the influences of solvents and hydrogen sources are also discussed. Furthermore, current challenges and perspectives for biomass catalytic hydrogenation are also present. This review aims to provide some useful knowledge on photocatalytic hydrogenation over heterogeneous catalysts for biomass utilization.

2

Overview of photocatalytic hydrogenation

2.1

Photocatalytic technologies

Photocatalysis is a sustainable and versatile technology that utilizes photocatalysts to activate chemical transformations under light irradiation. Specifically, when illuminated, a semiconductor absorbs photons with energy (hν) greater than its band gap energy (E _g) (hν ≥ E _g), which excites the material to generate photo-induced electrons (e ⁻) and holes (h ⁺). These e ⁻ and h ⁺ then participate in the reduction and oxidation half-reactions, respectively, realizing chemical bond transformations. The discovery of photocatalysis dates back to 1972, when Fujishima and Honda found that a TiO₂ electrode could decompose water to produce hydrogen under sunlight, leading to the proposal of the “Honda-Fujishima effect” and marking the beginning of photocatalysis research [47]. Based on this foundational discovery, the field of photocatalysis has rapidly developed over the past five decades, resulting in a large number of advanced organic and inorganic semiconductor catalysts, including metal oxides [48], metal sulfides [49], graphitic carbon nitrides [50], metal–organic frameworks (MOFs) [51], and covalent organic frameworks (COFs) [52]. Moreover, their applications extend to a variety of fields such as environmental remediation, energy conversion, and resource recovery. In environmental treatment, photocatalysis has been extensively explored in the fields of organic pollutants degradation, atmospheric contaminants depletion, and greenhouse gas elimination [53]. In energy conversion, photocatalysis is widely studied for solar energy conversion, water splitting for hydrogen production, and the reduction of carbon dioxide into multicarbon and value-added fuels [54] [55] [56]. In terms of resource recovery, photocatalytic technology also offers new opportunities for the recovery of precious metals from electroplating wastewater [57]. However, the photocatalytic reactions are often affected by the recombination rates of photogenerated electron–hole pairs, which are regarded as a key challenge limiting their practical application [58]. To address this problem, researchers have put great efforts into the design of photocatalysts to optimize charge carrier dynamics through various strategies (e.g., doping, heterostructure building, defect engineering, and coupling with plasmonic nanoparticles) and to extend the light absorption range to the visible region [58] [59] [60]. For instance, the Pt/TiO₂ photocatalyst showed superior light absorption capabilities and efficient charge transfer than TiO₂, thus significantly boosting photocatalytic performance under visible light conditions [61]. Xie et al. demonstrated that introducing disordered pore structures on the surface of KTaO₃ ultrathin sheets enhanced the coupling between charge carriers and phonons, facilitating the formation of hole polarons [62]. This strategy effectively suppresses the recombination of photogenerated electron–hole pairs, thereby extending the lifetime of charge carriers and significantly improving the photocatalytic performance, particularly in nitrogen fixation reactions. Therefore, the rational design of efficient catalytic systems with well-controlled active sites and reaction kinetics based on careful considerations of the exact reaction characteristics and target products is the basis for the development of photocatalytic technology.

2.2

Photocatalytic hydrogenation process

Photocatalytic hydrogenation is considered a green and efficient technology that utilizes light energy to drive hydrogenation reactions with the presence of H₂ or hydrogen transfer reagents. In photocatalytic hydrogenation reactions ([Figure 1]), photogenerated e ⁻ are responsible for reducing protons or hydrogen donors, while photogenerated h ⁺ oxidize electron donors to maintain charge neutrality. The choice of electron donors, such as alcohols, water, or formic acid, plays a critical role in improving reaction efficiency and aligning with green chemistry principles. By balancing charge transfer and providing protons for reduction reactions, these agents prevent recombination and enhance overall photocatalytic activity. Specifically, the whole reaction involves three main steps: (1) the generation of charge carriers, (2) the activation of hydrogen sources, and (3) the selective hydrogenation of target functional groups of the reactants.

Figure 1 Schematic diagram of the carbon dioxide photocatalytic hydrogenation reaction process over a semiconductor-supported photocatalyst. Adapted with permission from Ref. [65] Copyright 2020 The Royal Society of Chemistry. [rerif]

The process begins with the photocatalyst absorbing photons with energy exceeding its bandgap under irradiation, which excites e ⁻ from the valence band (VB) to the conduction band (CB), leaving positively charged h ⁺ in the VB, thus generating photogenerated electron–hole pairs (e ⁻/h ⁺):

$$\text{Catalyst}+\mathit{h\nu }\to e^{-}\left(\mathrm{CB}\right)+h^{+}\left(\mathrm{VB}\right)$$

The photogenerated e ⁻ in the CB possesses strong reducing ability, while the h ⁺ in the VB has strong oxidation capacity. The photogenerated carriers participate in distinct reactions: acceptors (hydrogen sources or substrates) accept the photogenerated e ⁻ from the catalyst to undergo a reductive half-reaction, while h ⁺ then take away electrons from donors (alcohols, water, or formic acid) to undergo an oxidation half-reaction. For instance, water oxidation driven by h ⁺ releases oxygen and generates protons (H⁺):

$${\text{2H}}_2\mathrm{O}+4h^{+}\to {\mathrm{O}}_2+{\text{4H}}^{+}$$

Simultaneously, photogenerated e ⁻ reduce protons or other hydrogen sources into reactive hydrogen species (H·):

$${\text{4H}}^{+}+4e^{-}\to \text{4H}·$$

These H· subsequently react with substrate molecules to complete the hydrogenation process.

Hydrogen source activation is crucial to photocatalytic hydrogenation. In the presence of H₂, molecular hydrogen is dissociated on the catalyst surface into reactive hydrogen atoms via photogenerated electrons:

$${\mathrm{H}}_2+2e^{-}\to \text{2H}·$$

Alternatively, chemical hydrogen storage materials such as formic acid (HCOOH) or ammonia borane (NH₃BH₃) can release H₂ under photocatalytic conditions, providing flexibility in hydrogen supply. Photocatalytic water splitting also offers a sustainable hydrogen source, where water oxidation produces H⁺ that are reduced by e ⁻ into H·.

Substrate activation is equally critical, particularly for molecules with multiple reactive functional groups. Functional groups like carbonyl (C=O) are selectively adsorbed on the catalyst surface and activated through charge transfer or polarization, followed by selective hydrogenation. The reaction can be described as:

$$\mathrm{R}–\mathrm{C}=\mathrm{O}+\text{2H}·\to \mathrm{R}–\mathrm{CH}–\mathrm{OH}$$

The efficiency and selectivity of this process depend on the photocatalyst’s surface structures and electronic states, such as metal active sites and oxygen vacancies, which promote adsorption and facilitate the activation of adsorbed functional groups. For instance, PtNi/TiO₂ enhances the hydrogenation of HMF by strengthening the PtNi⋯O interactions and weakening the PtNi⋯C interactions, facilitating the adsorption of HMF and lowering the activation energy for the hydrogenation process [63]. Abundant surface oxygen vacancies in TiO₂ enhance hydrogenation reactions by improving C=O adsorption [64].

In view of the fact that active H· is also required and plays a crucial role in determining the reactivity and selectivity, photocatalytic decarboxylation of fatty acids is also summarized herein. Photogenerated h ⁺ promote the decarboxylation of fatty acids, generating alkyl radicals (C _n−1), while photogenerated e ⁻ contribute to the formation of H· on the catalyst surface. These H· rapidly combine with the alkyl radicals, inhibiting radical coupling and oxidation reactions, thereby preventing unnecessary oligomerization, which effectively enhances the selectivity for the desired product—C _n−1 alkanes.

$${\mathrm{RCH}}_2{\mathrm{COO}}^{-}+h^{+}\to {\mathrm{RCH}}_2·+{\mathrm{CO}}_2$$

$${\mathrm{RCH}}_2·+\mathrm{H}·\to {\mathrm{RCH}}_3$$

[Figure 1] briefly depicts the photocatalytic hydrogenation process of CO₂ over a semiconductor-supported catalyst involving charge generation and transfer, substrate adsorption and activation, and surface reactions [65]. The studies on CO₂ photocatalytic hydrogenation have provided helpful knowledge for screening and optimizing photocatalysts, controlling reaction pathways, and tuning selectivity [66]. Although the photocatalytic hydrogenation reaction of biomass-derived molecules undergoes a very similar process as that of CO₂, it has greater complexity not only because of their varying structures and functional groups but also because of complex side reactions. For instance, in a biomass molecule containing both C=C and C=O bonds, the selective hydrogenation can only be achieved by precisely tuning the surface structure and electronic properties of catalysts to selectively activate target functional groups and suppress undesired side reactions. In addition, multiple components or active sites need to be incorporated into the catalyst design to modulate the adsorption conformation of those biomass molecules with greater steric hindrance effects.

3

Photocatalytic hydrogenation reaction of biomass-derived model molecules

Photocatalytic hydrogenation provides a promising green route for converting biomass-derived molecules into bulk chemicals, value-added chemicals, and biofuels. This section demonstrates the current progress of photocatalytic hydrogenation of some lignocellulose-derived model molecules and other biomass molecules.

3.1

Photocatalytic hydrogenation of lignocellulose-derived molecules

3.1.1

Photocatalytic hydrogenation of HMF

HMF, derived from the dehydration of hexose sugars, is one of the most widely studied platform molecules in biomass valorization. Its hydrogenation or HDO produces two main products: 2,5-bis(hydroxymethyl)furan (BHMF) and DMF. BHMF is regarded as a versatile building block for biodegradable plastics, while DMF, with its high energy density, is regarded as a promising biofuel with a high octane number [67], [68]. Achieving selective and efficient HMF hydrogenation requires precise control of the aldehyde and hydroxyl groups’ activation at the molecular level, as well as the suppression of side reactions such as polymerization. Although the catalytic reactions and mechanisms have been extensively studied in thermocatalytic systems, the situation may be different under photocatalytic conditions due to the presence of photogenerated electrons and photothermal effects.

Among the investigations on photocatalytic HMF hydrogenation, catalyst design is considered to be the most important issue [39], [63], [69] [70] [71] [72]. Chen et al. developed a Pt/g-C₃N₄ photocatalyst for the selective hydrogenation of HMF to BHMF by the H₂ produced by photo-induced water splitting, achieving a 6.5% BHMF yield with a turnover frequency (TOF) of 0.457 h⁻¹ [69]. The g-C₃N₄ semiconductor had excellent light absorption and efficient charge separation capability, but little hydrogenation activity, whereas platinum nanoparticles provided active sites for the selective hydrogenation of the aldehyde group. This work highlights the significance of synergies between noble metals (hydrogenation active sites) and semiconductor materials for the design of efficient photocatalysts for HMF transformation.

Metal-organic frameworks (MOFs) were also studied for HMF photocatalytic hydrogenation, and the reaction was catalyzed by Pd nanoparticles immobilized on an amine-functionalized MOF (MIL-101(Fe)-NH₂) with the presence of formic acid-triethylamine [70]. This photocatalyst achieves high selectivity and efficiency for the hydrogenation of HMF and a series of aromatic aldehydes. A yield of 27% of BHMF is obtained on the Pd/MIL-101(Fe)-NH₂ catalyst via visible-light-induced transfer hydrogenation. The amine group not only stabilizes the Pd nanoparticles but also adjusts the Pd electronic states.

Amorphous TiO₂ was also found to be effective for HMF hydrogenation in the solution of alcohol (ethanol, methanol, or isopropanol). The hydrogenation can be achieved by a cascade reaction on TiO₂, where the photogenerated electrons and protons are stored in amorphous TiO₂, and then they contribute to HMF hydrogenation to BHMF in the dark via proton-coupled electron transfer [71]. This catalytic system achieves a remarkable photocatalytic performance with HMF conversion of >99% and BHMF selectivity of 99%. As displayed in [Figure 2], under UV irradiation, photoinduced electrons and protons generated using alcohol as hydrogen donors are stored on the surface of the amorphous TiO₂. Subsequently, even without light irradiation, HMF can also react with the stored H⁺/e ⁻ pairs via proton-coupled electron transfer, yielding BHMF. The excellent selectivity of BHMF is due to the oriented adsorption of aldehyde groups and the rapid desorption of BHMF with the assistance of acetaldehyde.

Figure 2 Hypothetical mechanism of HMF hydrogenation to BHMF over A-TiO₂. Adapted with permission from Ref. [71]. Copyright 2019 The Royal Society of Chemistry [rerif]

HMF hydrogenation was also studied over the P25 TiO₂ catalyst with p-methoxybenzyl alcohol (MeOBA) as hydrogen donors [39]. This system achieves a 30% BHMF yield at 24 hours without external H₂. The TiO₂ catalyst catalyzes the photogenerated electrons for the selective reduction of HMF, while the holes are consumed in the oxidation of MeOBA to p-methoxybenzaldehyde, improving overall reaction efficiency. Mechanistic studies reveal that the oxidation of MeOBA acted as a hole scavenger, enhancing electron availability for HMF hydrogenation.

Nagaraja et al. developed a NiTiO₃/ZnIn₂S₄ heterojunction photocatalyst, achieving complete HMF conversion with 100% selectivity for BHMF under visible–light irradiation in triethanolamine (10 v/v %) aqueous solution [72]. As shown in [Figure 3], the formed Z-scheme heterojunction between NiTiO₃ and ZnIn₂S₄ facilitates efficient charge separation and interfacial electron transfer, significantly improving catalytic activity. The system utilizes water as the sole hydrogen source and triethanolamine as a sacrificial e ⁻ donor, and it shows enhanced photostability and recyclability. This study highlights the effectiveness of heterojunction engineering in optimizing non-noble metal catalysts for HMF hydrogenation.

Figure 3 Schematic representation of the mechanism for photocatalytic H₂ generation and subsequent HMF reduction by NiTiO₃/ZnIn₂S₄. Adapted with permission from Ref. [72]. Copyright 2022 Elsevier Inc. All rights reserved. [rerif]

The PtNi/TiO₂ catalyst identified the importance of interfacial electronic modulation, which significantly improved the photocatalytic performance by modulating the interfacial electronic coupling between PtNi and TiO₂ compared to Pt/TiO₂ and Ni/TiO₂ [63]. The optimized interfacial properties not only effectively promote charge migration but also improve adsorption-desorption dynamics of intermediates while suppressing side reactions, resulting in superior activity with the highest yield of BHMF (100%) and selectivity (99.7%).

There are a number of catalysts that also show good performance for the hydrogenation of HMF, although these catalysts mainly focus on other model reactions in the reports [73] [74] [75] [76] [77] [78]. Pd/g-C₃N₄ achieved a conversion rate of 42% for HMF [73]. The Pt/NiInO _x , Au/SiC, and Pd/g-C₃N₄/ZnO catalysts exhibited good catalytic performance for the hydrogenation of HMF, with both conversion rate and selectivity exceeding 90% [74] [75] [76]. For Pd/g-C₃N₄/ZnO, different Pd loadings lead to diverse products: 1% Pd@g-C₃N_4(0.73)/ZnO_(0.27) favored the formation of BHMF, while 3% Pd@g-C₃N_4(0.73)/ZnO_(0.27) produced bis-hydroxymethyl tetrahydrofurfuryl alcohol [74]. Pt/NiMg-MOF achieved 99.9% BHMF selectivity at 76.5% conversion [77]. The Pd/WO_3−x catalyst obtained 65.7% BHMF selectivity at 84.4% HMF conversion [78].

In the photocatalytic hydrogenation of HMF, the choice of solvents such as methanol and ethanol plays a dual role, which not only act as electron donors, being oxidized by holes to generate the corresponding aldehyde, but also serve as hydrogen sources, providing protons for further hydrogenation reactions. Triethanolamine, when used as an electron donor, helps extend electron lifetime. In addition, water is always selected as the hydrogen source to supply protons, thereby enabling high-efficiency HMF hydrogenation. Under H₂ atmosphere, it is also essential to construct catalysts with metal active sites that can facilitate H₂ dissociation, providing sufficient active hydrogen species to achieve more efficient catalytic performance. These studies collectively demonstrate that the reaction pathway and selectivity for HMF hydrogenation are highly dependent on the electronic properties and surface structure of the catalyst, as well as the reaction medium.

3.1.2

Photocatalytic hydrogenation of FAL

FAL is a key platform molecule derived from hemicellulose, and it finds widespread applications in the production of resins, polymers, solvents, etc. Photocatalytic hydrogenation of FAL to FOL and tetrahydrofurfuryl alcohol (THFA) offers a sustainable and green chemical route with significant industrial relevance [79]. To achieve high FOL selectivity, the reaction needs to catalyze the hydrogenation of the C=O bond in FAL, without hydrogenation of the C=C bond and without involving unwanted side reactions such as polymerization or coupling. In recent years, research has primarily focused on catalyst design, optimization of reaction conditions, and mechanistic exploration, and revealed the characteristics and unique advantages of different catalytic systems.

Metal-semiconductor composite catalysts were verified to be efficient for FAL photocatalytic hydrogenation [75], [80], [81]. The catalyst with Cu nanoparticles encapsulated in carbon carrier prepared by the pyrolysis of the metal-organic framework HKUST-1 in H₂/Ar, exhibited high catalytic performance and stability with 99% yield of FOL in isopropanol under visible light irradiation, at 100 °C and 1 atm of H₂ [82]. The Cu nanoparticles could absorb visible light and generate photoexcited electrons due to the localized surface plasmon resonance (LSPR) effect, and these hot electrons transferred the H₂ absorbed on the Cu nanoparticle surface, forming Cu–H species. Finally, two active H∙ attacked the O and C atom of the carbonyl group of FAL separately to produce FOL. Selectively photocatalytic hydrogenation of FAL to FOL was also achieved on the ultrathin SnNb₂O₆ nanosheets supported Pt catalyst at room temperature in methanol [83]. This composite photocatalyst showed almost a 100% yield of FOL. The Pt clusters/bimetallic NiMg-MOF-74 (MNM) ultrathin nanosheets were reported for the photocatalytic precise hydrogenation of FAL to FOL [77]. Pt/MNM-0.25 achieves 99.9% conversion and 99.9% FOL selectivity in methanol under visible light irradiation with 1 atm H₂. It is proposed that the surface Ni^II sites with low electron density contribute to the activation of the C=O bond, while the photogenerated electrons promote the activation of H₂ on surface Pt clusters ([Figure 4]). The synergistic effect of surface-optimized Ni^II sites and Pt clusters is emphasized. The Pt/NiInO_x catalyst effectively facilitated the photocatalytic hydrogenation of FAL to FOL in methanol under LED light irradiation and 1 atm of H₂, achieving 99.9% conversion and 99.9% selectivity [75]. The synergy between Pt nanoparticles and the p-n heterojunction in the NiInO_x support enhances electron–hole separation and promotes efficient hydrogen dissociation and activation of the C=O bond in FAL. A Pd/g-C₃N₄ photocatalyst was also reported for the hydrogenation of FAL in acetonitrile solution containing formic acid-triethylamine under visible light. However, only a 27% yield of FOL was obtained [73].

Figure 4 Schematic mechanism of FAL photocatalytic hydrogenation to FOL over Pt/MNM-0.25. Adapted with permission from Ref. [77]. Copyright 2023 Elsevier Inc. All rights reserved. [rerif]

Except for using H₂, the photocatalytic hydrogenation of FAL has been studied by using organic solvents as hydrogen transfer reagents. The selective hydrogenation of FAL to FOL by transfer hydrogenation has been explored on a series of TiO₂-based catalysts at room temperature in 2-pentanol [64], [84], [85]. Kominami et al. studied the transfer hydrogenation of FAL in a 2-pentanol suspension of TiO₂, and found that the high conversion and selectivity were ascribed to the stoichiometric reaction of FAL and 2-pentanol to FOL and 2-pentanone under UV light [84]. The alteration of TiO₂ surface electronic and coordination structures has a significant influence on their photocatalytic performance. Wang et al. reported the switch of C=O hydrogenation and C–C coupling in photocatalytic hydrogenation of FAL in methanol by modulating the exposed facets of crystal plane of TiO₂ ([Figure 5]) [64]. The surface oxygen vacancy determines the substrate adsorption and charge transfer, and the exposed facet with abundant oxygen vacancies (i.e., TiO₂ (001)) improves the photocatalytic hydrogenation FAL to FOL (~90% FOL selectivity). TiO₂ (P25)-supported metal catalysts (Cu, Ni, Pt, and Pd) were also investigated for photocatalytic hydrogenation of FAL in methanol [85]. Interestingly, the TiO₂ with an ultralow metal loading promotes the formation of vacancies on the catalyst surface, which is crucial for the UV light absorption and FAL activation. In addition, the tiny amount of water is also influential to the performance due to its contribution to the intermediates desorption and the inhibition of coupling side reactions ([Figure 6]). Pd/TiO₂ photocatalyst was also adopted for hydrogenation of FAL to FOL under UV-visible light irradiation in an N₂ atmosphere at room temperature, achieving remarkable activity with 100% yield [86]. In this reaction system, ethanol plays a dual role as a hydrogen source and solvent, not only providing hydrogen atoms to hydrogenate FAL but also activating coupling by-products through proto-coupled electron transfer reaction to promote their conversion to the target product, FOL. Interestingly, the same Pd/TiO₂ photocatalyst was also extended to the selective hydrogenation of diols (such as hydrobenzoin) to monoalcohols under mild conditions. Ethanol facilitates proton-coupled electron transfer for efficient conversion of diols into monoalcohols with yields ranging from 81% to 99%.

Figure 5 Photocatalytic conversion of FAL over different TiO₂ nanocrystals. (A) Reaction pathway of FAL hydrogenation and coupling. Yield, selectivity, and time curves over (B) A-bipyramid, (C) A-sheet, and (D) R-rod. Adapted with permission from Ref. [64]. Copyright 2020 Elsevier Inc. All rights reserved. [rerif]

Figure 6 (a) Photocatalytic hydrogenation of FAL on different 0.01 wt% M/P25 catalysts (the right set of columns represents systems adding 50 μL of water); (b) influence of water amounts on catalytic performance of 0.01% Pt/P25 catalyst. Adapted with permission from Ref. [85]. Copyright 2022 Elsevier Inc. All rights reserved. [rerif]

Cu-based catalysts are widely used for the hydrogenation of the C=O bond in the fields of both thermocatalysis and photocatalysis owing to the superior C=O adsorption capability. The Cu/Cu₂O-MC catalyst, consisting of Cu and Cu₂O nanoparticles deposited on mesoporous carbon, was studied for FAL hydrogenation under visible light in isopropanol [80]. The synergistic effect between Cu₂O and Cu results in the superior performance for selective transfer hydrogenation of FAL to produce FOL under visible light, with a 90.9% yield of FOL. It is found that both Cu and Cu₂O are excited when irradiated, and Cu acts as a mediator to promote electron transfer from the excited Cu₂O to the carbon support.

The photoactive covalent triazine polymer was also used for the hydrogenation of FAL to FOL in aqueous solution under visible light with ascorbic acid as a sacrificial electron donor [87]. The cooperation of alternating thiophene and triazine units results in a unique electron donor (D) and receptor (A) structure in the conjugated skeletons. The D–A structure promotes the delocalization of π electrons, enhances charge separation, and then facilitates the hydrogenation. Ag–In–Zn–S nanocrystals were designed and examined for the photocatalytic reduction of FAL [38]. Red-emitting nanocrystals (R, E _g = 2.0 eV) selectively reduced FAL to FOL, while green-emitting nanocrystals (G, E _g = 3.2 eV) favored pinacol coupling, producing deoxyfuroin in the presence of triethylamine/p-toluenethiol and potassium tert-butoxide/isopropyl alcohol couples. The PtNi/TiO₂, Au/SiC, and Co₃O₄/LDH-350 catalysts also exhibit high efficiency for FAL hydrogenation, although these reported works focus mainly on the other model reactions [63], [76], [88].

FAL hydrogenation to THFA was also reported by the Pd/g-C₃N₄ photocatalyst in isopropanol at ambient temperature with 2 bar H₂ [89]. And 3 wt % Pd/g-C₃N₄ exhibits nearly 100% FAL conversion and 100% THFA selectivity at a reaction time of 5.5 h with irradiation under a 150 W white LED. The Pd@g-C₃N₄/ZnO catalyst achieves 90% selectivity for FOL with 1% Pd loading in 3 h, and produces THFA as the sole product with 3% Pd loading in 4 h [74]. Pd/CeZrO _x catalysts were also prepared to explore the photocatalytic hydrogenation of FAL [81]. The 1Pd/CeZrO _x (1:0.5) catalyst gives rise to 98.5% THFA yield under mild conditions (0.2 MPa H₂, 15 W blue LED).

In summary, during the hydrogenation of FAL, although Pt, Pd, and Cu all show good activity, their catalytic mechanisms differ. Noble metals like Pd and Pt, due to their superior electronic properties and hydrogen activation capabilities, possess sufficient surface active sites for transferring photogenerated electrons and dissociating H₂. In contrast, the non-noble metal Cu absorbs visible light by the LSPR effect, generating hot electrons that activate hydrogen dissociation, significantly promoting the hydrogenation of the C=O bond. The efforts in this field have provided some useful knowledge for the photocatalyst structures, reaction pathways, and mechanisms. Although high FOL selectivity can be obtained in photocatalytic hydrogenation of FAL, some issues still need to be addressed, such as how to regulate the hydrogenation selectivity of C=C and C=O bonds, whether the reaction can be realized in water without sacrifices agents, and whether the reaction can be performed at a high FAL/catalyst mass ratio, and so on.

3.1.3

Photocatalytic hydrogenation of vanillin

Vanillin, as an important model molecule derived from lignin, has garnered significant attention due to its possible applications in food, pharmaceuticals, and chemical industries. The photocatalytic hydrogenation of vanillin into value-added products such as vanillyl alcohol (VOL) and 2-methoxy-4-methylphenol (MMP) provides a green and sustainable chemical approach to meet critical demands in these fields. VOL, widely used as a food additive, is valued for its excellent antioxidant properties, while MMP serves as an essential chemical intermediate in the fragrance and pharmaceutical industries [78], [90], [91]. Conversion of vanillin to VOL requires selective hydrogenation of the aldehyde group while prohibiting aromatic ring hydrogenation and undesired side reactions. Moreover, the switch of the main product from VOL to MMP by adjusting the catalyst is also desired.

Varma et al. developed a PdAg@g-C₃N₄ composite catalyst that achieved efficient vanillin-to-MMP conversion through the synergistic interaction between the precious metal and the semiconductor [90]. This catalyst achieves a vanillin conversion rate of 100% and a MMP selectivity of >99% in water under visible light with formic acid as a source of hydrogen. Although Pd or AgPd supported on other carbon supports can also give rise to a high MMP selectivity, the vanillin conversion rates over them are not satisfactory.

By optimizing the electronic structure of g-C₃N₄ through nitrogen doping, the Pd@N₀.₇₅-C₃N₄ photocatalyst broadens the material's visible-light absorption range and improves the separation and migration efficiency of photogenerated charges [91]. The incorporation of Pd nanoparticles further enhanced active sites, and by adjusting the Pd loading, the main product can be switched from VOL to MMP. The 1% Pd@N_0.75CN catalyst gives a 94.4% VOL selectivity at 79.6% vanillin conversion in isopropanol. For 2% Pd@N_0.75CN, 99% MMP selectivity is achieved in 4.5 h. The photogenerated electrons are more likely to transfer to Pd nanoparticles due to their higher work function, which facilitates desorption of active H species (Pd–H). The vanillin adsorbs on the catalyst surface via its carbonyl group on Pd and the aromatic ring interacting with N-doped CN. The higher Pd loading promotes the VOL adsorption on the catalyst and then improves the MMP selectivity.

Under simulated sunlight irradiation, Pd/WO_3-x gives a maximum vanillin conversion up to 86.8 % with nearly 100% MMP selectivity in n-butanol with polymethylhydrosiloxane as hydrogen donor [78]. Oxygen vacancies play a crucial role in this system by promoting vanillin adsorption and activation while reducing the recombination rate of photogenerated electron–hole pairs, thereby significantly enhancing catalytic efficiency. The incorporation of Pd nanoparticles enhances light absorption, while the LSPR effect of WO_3−x further accelerates the reaction rate.

The Pd/CeZrO _x catalyst shows excellent performance for the hydrogenation of vanillin, achieving >99% selectivity at 89.1% conversion for MMP in isopropanol under blue LED illumination at 6h and 0.2 atm of H₂ [81]. The Pd/g-C₃N₄/ZnO nanocomposite catalyst exhibits high efficiency for the hydrogenation of vanillin, similar to its performance with other biomass-derived substrates [74]. The reaction selectivity was influenced by the Pd loading. The 1% Pd@gC₃N_4(0.73)/ZnO_(0.27) selectively reduced the aldehyde group, producing VOL with high conversion (92%) and selectivity (86%) at 8 h. 3% Pd@gC₃N_4(0.73)/ZnO_(0.27) catalyzed HDO reaction to produce MMP, achieving 97% conversion rate and 100% selectivity at 10 h.

In addition, surface-engineered TiO₂ catalysts play an important role in the product distribution for the hydrogenation of vanillin [64]. The anatase nanosheet catalyst, rich in oxygen vacancies, primarily produces VOL through the selective reduction of the aldehyde group. In contrast, the rod-shaped rutile catalyst with fewer oxygen vacancies favors the formation of coupling products.

In summary, for the hydrogenation of vanillin, the series of Pd-based catalysts, including PdAg@g-C₃N₄, Pd/CeZrO _x , Pd/g-C₃N₄/ZnO, and Pd/WO_3−x , follow a similar catalytic mechanism, which fundamentally relies on the synergy between metal and semiconductor to promote the hydrogenation of vanillin. The metal-semiconductor interface is the primary active site, where photogenerated electrons are transferred to the metal particles, preventing electron–hole recombination and improving light absorption. However, the differences among these catalysts lie in the properties of the supports and the interactions between the metal and the semiconductor partners.

3.1.4

Photocatalytic hydrogenation of other biomass-derived model molecules

In addition to the above-mentioned biomass model molecules, some other biomass-derived molecules, such as phenol [92], guaiacol [93], maleic acid [94], cinnamaldehyde (CAL) [74], and ethyl levulinate [95], have also been investigated for photocatalytic hydrogenation. For instance, the photocatalytic hydrogenation of CAL focuses on achieving selective hydrogenation of the carbonyl group while avoiding excessive hydrogenation of the conjugated C=C double bond. Au/SiC with an LSPR effect significantly enhances the activation of the carbonyl group while suppressing undesired reactions at the double bond. Under mild conditions with isopropanol as the sacrificial agent, 100% C=O hydrogenation selectivity with a TOF of 487 h⁻¹ was achieved on Au/SiC [76]. The hydrogenation cascade reaction pathway of CAL can be regulated by changing Pd loading in the Pd@g-C₃N₄/ZnO heterojunction catalysts [74]. Low Pd loading favored the formation of cinnamyl alcohol, whereas high Pd loading resulted in fully hydrogenated products. The Co₃O₄/LDH with abundant oxygen vacancy exhibits high efficiency for the selective hydrogenation of CAL under visible light at 100 °C [88]. The catalyst achieves 99% conversion with 98% selectivity for cinnamyl alcohol within 3h. The oxygen vacancies on the MgAl-LDH support facilitate selective activation of the C=O group, while preventing C=C hydrogenation.

Furthermore, guaiacol and phenol, as representative lignin-derived aromatic compounds, are ideal model molecules for studying photocatalytic C–O bond cleavage and C=C hydrogenation. Guaiacol HDO to cyclohexanol with 98% yield is obtained on AgPd/Fe@CNX in water under visible light conditions, with formic acid as a hydrogen source [93]. The synergy between noble metals and the carbon-nitrogen-doped support enables highly efficient C–O bond cleavage and C=C hydrogenation. The selective hydrogenation of phenol to cyclohexanone on the Pd/titanate catalyst achieves nearly 100% conversion and selectivity under mild conditions, with water as the solvent and simulated sunlight irradiation [92]. The monolayer titanate nanosheets provide abundant acid and basic sites, which form strong interactions with phenol via hydrogen bonding. This interaction facilitates the adsorption and activation of phenol molecules while suppressing further hydrogenation of cyclohexanone. Meanwhile, Pd clusters act as active sites for the dissociation of hydrogen molecules, providing atomic hydrogen for the selective hydrogenation process. This synergistic interaction between the Pd clusters and titanate nanosheets is responsible for the selectivity of cyclohexanone.

3.2

Hydrogen-assisted photocatalytic decarboxylation of fatty acids

Photocatalytic decarboxylation has emerged recently as an efficient and sustainable strategy for converting biomass-derived long-chain fatty acids into diesel-range alkanes [96] [97] [98]. The conversion of fatty acids to C _n−1 alkane involves a radical-mediated pathway, and the high reactivity of the free radical intermediates tends to induce the formation of coupled long-chain products or oxidation. The decarboxylation product selectivity is related to the reactivity of radical intermediates, which are rapidly terminated by the surface H species on Pt/TiO₂ ([Figure 7]) [99]. This feature facilitates the rapid termination of photogenerated radical intermediates with hydrogen, effectively suppressing undesirable side reactions such as radical dimerization and oxidation. About 90% yield for C _n−1 alkanes can be obtained from C12 to C18 fatty acids under UV irradiation (365  nm) at 30 °C and low hydrogen pressure (≤0.2 MPa).

Figure 7 (a) The route of a photocatalytic decarboxylation process to produce alkanes from bio-derived fatty acids; (b) proposed mechanisms for the solar-driven decarboxylation of fatty acids over Pt/TiO₂ under H₂ atmosphere. Adapted with permission from Ref. [99]. Copyright 2022 Elsevier Inc. All rights reserved. [rerif]

Based on this foundation, subsequent research has focused on optimizing the performance of Pt/TiO₂ catalysts and elucidating the underlying mechanisms [100], [101]. Oxygen vacancies on the TiO₂ surface have been identified as critical in the adsorption and activation of fatty acids [102]. The introduction of oxygen vacancies through chemical reduction (e.g., via NaBH₄) significantly enhances the electron density of TiO₂, improving its interaction with photogenerated radicals and thereby increasing decarboxylation efficiency. Furthermore, bimetallic core-shell structures, such as Au–Pd/TiO₂, have enhanced charge separation efficiency and reduced electron-hole recombination, resulting in superior catalytic activity and selectivity [103].

Efforts to extend the applicability of photocatalytic decarboxylation systems have also focused on the development of non-noble metal catalysts. For instance, TiO₂-supported Ni or Cu catalysts have been optimized to achieve a good conversion of fatty acids under UV-visible light irradiation, offering a cost-effective alternative to noble-metal-based systems. Furthermore, a range of semiconductor photocatalysts (α-Fe₂O₃, Fe₃O₄, Ag/V₂O₅, Bi₂O₃, Sb₂S₃, and SnS) with photothermal effect have been verified to significantly accelerate the photocatalytic decarboxylation of long-chain fatty acids to C _n−1 n-alkanes [96] [97] [98], [104] [105] [106]. However, Pt-containing catalysts remain at the forefront among the reported catalysts due to their exceptional ability to activate hydrogen and trap photogenerated electrons, which are crucial for the rapid hydrogenation of radical intermediates. A proposed “photo-oxidation-reduction cycle” mechanism is widely used for interpreting photocatalytic decarboxylation, wherein photogenerated holes oxidize the fatty acids to alkyl radicals, and photogenerated electrons mediate proton reduction to hydrogenate the radicals into alkanes.

4

Development of photocatalysts for photocatalytic hydrogenation systems

The development of efficient photocatalysts is critical for the photocatalytic hydrogenation of biomass-derived molecules [34], [107]. Photocatalysts should not only efficiently harvest light and separate photogenerated carriers but also enable selective adsorption and activation of the target functional groups, such as aldehydes, ketones, and carboxylic acids, while minimizing undesired side reactions. These requirements present challenges in balancing catalytic activity, selectivity, and stability, especially for structurally complex biomass substrates. This section discusses the catalyst design principles and recent advances in plasmonic and semiconductor-based photocatalysts, active site engineering, solvent optimization, and hydrogen source selection, providing insights into future directions for applicable and sustainable photocatalytic hydrogenation systems.

4.1

Plasmonic catalysts

Plasmonic catalysts, with their unique LSPR effect, have demonstrated tremendous potential in photocatalytic reactions. Noble metal nanoparticles (e.g., Au, Ag, and Cu) can significantly enhance light absorption through the LSPR effect, generating high-energy hot electrons as well as intense localized electromagnetic fields [108]. These characteristics may facilitate the activation of reactants and could improve catalytic efficiency and selectivity. In the field of photocatalytic biomass hydrogenation, the LSPR effect offers a promising pathway to address issues such as the high energy barrier and the moderate selectivity of C–O bond cleavage on traditional catalysts.

The effect of hot electrons on catalytic reactions varies by the catalyst and reaction pathway. In the Pd concave nanostructures, the photothermal effect on the active sites may affect this surface reaction process [109]. Guo et al. investigated the role of LSPR effects in Au/SiC catalysts during hydrogenation reactions [76]. Hot electrons generated by the LSPR of Au nanoparticles under visible light effectively activate the hydrogen atoms generated from 2-propanol oxidation, promoting the hydrogenation of CAL with high selectivity and efficiency. The Au/SiC catalyst is effective not only for CAL hydrogenation but also for other α,β-unsaturated aldehydes. The silver nanocubes were explored for the hydrogenation of carbonyl compounds under visible light [110]. Hot electrons generated by LSPR are found to effectively activate hydrogen molecules, facilitating their dissociation and catalyzing the hydrogenation of ketones and aldehydes with high selectivity for the C=O bond. It is also found that the polarity of the solvent interacts synergistically with the plasmonically excited hot electrons, further enhancing the reaction. A catalyst combining Ag nanowires (AgNWs) with ZIF-8, which supported Pd nanoparticles (AgNWs/ZIF-8/Pd), was developed to significantly enhance the rate and selectivity of the 2(5H)-furanone hydrogenation reaction through the LSPR effect [35]. The LSPR effect of Ag nanowires promotes the generation of hot electrons, which are then directly injected into the Pd nanoparticles to form electron-rich Pd nanoparticles, thus promoting the adsorption of hydrogen and the conversion of reactants.

The LSPR effect affects the catalytic performance, which depends on the catalyst itself and the reducing agent used [111]. When H₂ was used as the reducing agent, Au/SiO₂ and Au/TiO₂ catalysts showed enhanced catalytic activity. In contrast, when NaBH₄ was used, the catalytic activity of Au/TiO₂ decreases under LSPR excitation due to the transfer of hot electrons to TiO₂. This finding suggests that the impact of LSPR effects depends on the reaction pathway and choice of reducing agent.

At present, the earth-abundant, inexpensive Cu catalysts have received increasing attention. For example, a Cu/Cu₂O-MC catalyst achieves efficient hydrogenation of FAL under visible light by utilizing the synergistic effect between metallic Cu and Cu₂O [80]. Cu₂O absorbs light to generate photoexcited electrons, which are efficiently transferred to mesoporous carbon (MC) through Cu's low work function. Furthermore, plasmonic Cu nanoparticles can also be directly excited under light to form hot electrons, which can transfer to MC, thus accelerating the reaction and promoting efficient electron utilization. Similarly, Cu@C was studied for FAL hydrogenation [82].

4.2

Semiconductor catalysts

Semiconductor catalysts with unique electronic structure and optical properties are the cornerstone of photocatalysis technology, which drives the activation and selective conversion of reactant molecules through photogenerated electron–hole pairs. Designing efficient semiconductor catalysts with regulated electron-hole properties may provide precise solutions for the photocatalytic C–O and C–C bonds activation of biomass molecules with low energy consumption.

TiO₂ is one of the most widely studied semiconductor materials in photocatalysis. Its wide bandgap (~3.2 eV) makes it highly active under UV light, but limits its response to visible light. Recent advancements in material modification, such as doping, defect engineering, and heterojunction design, have significantly extended their light absorption range [112] [113] [114]. For instance, the defect-engineered TiO₂ catalyst with high oxygen vacancy density exhibits outstanding performance in the photocatalytic hydrogenation of FAL [64]. This catalyst not only enhances the separation efficiency of photogenerated carriers but also improves the interaction between reactants and the catalyst surface, resulting in high selectivity for FOL. The TiO₂ defects and active sites engineering in terms of the target functional groups of biomass molecules are essential for improving the catalytic performance.

g-C₃N₄, a metal-free semiconductor, has garnered significant attention due to its intrinsic visible-light activity. Compared to TiO₂, g-C₃N₄ has a narrower bandgap (~2.7  eV), making it more effective under visible light [115], [116]. When combined with noble or non-noble metals, g-C₃N₄ has the potential to be used as an ideal carrier for biomass molecule conversion catalysts. For instance, Pt supported on g-C₃N₄ efficiently catalyzes the hydrogenation of HMF to BHMF under visible light, while significantly suppressing side reactions [69]. Furthermore, the combination of g-C₃N₄ with other semiconductor materials such as ZnO has further enhanced its charge separation efficiency and light harvesting capabilities, broadening its application prospects in biomass hydrogenation [74], [117].

In recent years, emerging narrow-bandgap semiconductors, such as NiO, In₂O₃, ZnIn₂S₄, and NiTiO₃, have shown great promise in photocatalytic biomass hydrogenation. For example, NiO, when combined with In₂O₃ to form a p–n heterojunction, not only extends light absorption but also improves the separation of photogenerated electron–hole pairs [75]. In view of that, the reduced Ni species have good catalytic activity for biomass C–O activation, and the Ni-based heterojunction may find potential usage for biomass photocatalytic hydrogenation. For instance, a Z-scheme heterojunction catalyst based on NiTiO₃/ZnIn₂S₄ displays high efficiency in the conversion of HMF to BHMF, effectively regulating charge transfer pathways to maximize photocatalytic performance [72].

4.3

Synergistic in classic M/TiO₂ catalysts

TiO₂-supported metal catalysts (M/TiO₂) represent one of the most extensively studied systems in photocatalytic biomass conversion due to their high activity, tunable selectivity, and structural stability. The unique merit of this system lies in the synergistic interaction between the metal nanoparticles and the TiO₂ support. Metal nanoparticles/clusters commonly serve as active sites, facilitating efficient hydrogen activation and capturing photogenerated electrons, while the TiO₂ substrate provides a platform for light absorption, charge carrier separation, and reactant adsorption. This metal-support synergistic enables M/TiO₂ catalysts to catalyze a number of complex biomass conversion reactions, such as selective hydrogenation and decarboxylation, with remarkable efficiency and product specificity.

Recently, more studies have focused on the exploration of interfacial interactions of M/TiO₂, which are believed to play a pivotal role in catalytic performance. The metal–TiO₂ interface serves as the primary active center, where photogenerated electrons are transferred to the metal particles, preventing electron-hole recombination, while oxygen vacancies on the TiO₂ surface enhance charge separation and stabilize intermediates. These interfacial effects not only amplify the intrinsic activity of the catalyst but also facilitate precise control of reaction pathways, particularly in the selective conversion of biomass-derived molecules with multiple functional groups.

Noble metals, such as Pt and Pd, are widely employed in M/TiO₂ systems due to their superior electronic properties and hydrogen activation capabilities. The high selectivity in the decarboxylation of fatty acids to alkanes over Pt/TiO₂ was attributed to the rapid recombination of photogenerated alkyl radicals and surface hydrogen to form C _n−1 alkanes [99]. The hydrogen spillover from Pt to TiO₂ resulted in H-rich surfaces on both the Pt and TiO₂, which facilitated the rapid radical termination and inhibited the unfavorable oligomerization. For the PtNi/TiO₂ catalyst, the electronic coupling between PtNi and TiO₂ enhances photogenerated charge separation and promotes selective hydrogenation of HMF [63]. The dual-metal configuration optimized hydrogen activation and product selectivity.

The Pd/TiO₂ catalyst efficiently converts FAL to FOL, with Pd nanoparticles providing active hydrogen and the TiO₂ support suppressing over-reduction reactions through its strong interfacial electronic effects [85]. Further advancements were performed by incorporating bimetallic structures, such as Pd-Cu or Pd-Ni, which exploit metal-metal synergistic effects to enhance charge transfer and catalytic stability.

The high cost and limited availability of noble metals have driven interest in non-noble metal-based M/TiO₂ catalysts, such as Ni/TiO₂. These materials offer a cost-effective alternative while maintaining high catalytic activity in specific reactions. Ni/TiO₂ enriched by oxygen vacancies performed well in the photocatalytic decarboxylation of octanoic acid [102]. Ni nanoparticles not only provide moderate hydrogen activation capability but also facilitate charge separation at the Ni/TiO₂ interface, improving overall catalytic efficiency.

The rational design of M/TiO₂ catalysts is better performed based on a comprehensive consideration of substrate activation, surface competition adsorption, charge transfer, solvent effect, and so on. Developing bimetallic or multimetallic catalysts may optimize the synergistic effects between different metals, improving charge transfer and catalytic activity. Additionally, modulating the TiO₂ support to generate defects, additional active sites, or interfacial interactions may be helpful.

4.4

Solvent selection

For photocatalytic biomass hydrogenation, the solvent significantly influences the adsorption and diffusion of reactants, the behavior of active catalytic sites, and the reaction pathways. Therefore, solvent selection plays a pivotal role in regulating both the efficiency of photocatalytic reactions and the selectivity of target products. Recent studies have revealed that solvent properties, including polarity, hydrogen donor capacity, and diffusion limitation, have profound impacts on biomass hydrogenation reactions.

For the photocatalytic hydrogenation of FAL on Pt/TiO₂ in methanol, the presence of a tiny amount of water as a co-solvent significantly enhances the FOL selectivity. Water enhances desorption of intermediates on the catalyst surface and reduces the formation of coupling byproducts, thereby increasing the selectivity for FOL. However, the presence of water in methanol solution also causes a slight decrease in the reaction rate, which may be due to the weakened adsorption of FAL on the catalyst and the altered electron transfer property [85]. For the photocatalytic hydrogenation of HMF over amorphous TiO₂, the influences of solvent on reaction rate and selectivity were studied [71]. The A-TiO₂ shows good catalytic performance in methanol, ethanol, or isopropanol, but it gives poor reactivity in water or water–alcohol mixed solution. Besides the low hydrogen donor capability of water, the surface of A-TiO₂, the adsorption of HMF on the catalyst is also altered. In addition, the HMF decomposition even takes place when pure water is used as a solvent.

In most cases, alcohol-based solvents (e.g., isopropanol, methanol, and ethanol) exhibit unique advantages in photocatalytic reactions due to their dual role as solvents and hydrogen donors [89]. For instance, isopropanol dehydrogenation on Pd-based catalysts generates active hydrogen species, which significantly enhance the conversion efficiency of CAL to hydrocinnamyl alcohol (with a nearly 100% selectivity) [74]. Methanol demonstrates a high hydrogen release rate and chemical stability, making it suitable for reactions requiring rapid hydrogenation [63], [64]. In addition, a number of other solvents, such as acetonitrile [70], [73], benzyl alcohol [118], and dioxane [110], have shown potential in specific reactions.

4.5

Hydrogen source selection

The choice of hydrogen source is crucial for photocatalytic biomass hydrogenation, as it not only determines reaction activity and selectivity but also directly influences the economic viability of the catalytic process. H₂, alcohols, formic acid, and other emerging hydrogen donors exhibit distinct characteristics under various catalytic systems.

H₂ is the most commonly used hydrogen source due to its high hydrogen-donating capacity and strong affinity for catalytically active sites (especially for the transition metals). However, under mild conditions (e.g., ambient temperature and pressure), the activation efficiency of H₂ is often limited by the active sites on the catalyst surface. For instance, when using H₂ as the hydrogen source, HMF hydrogenation on PtNi/TiO₂ catalysts fails to produce BHMF, while it gives a 99.7% BHMF selectivity by using methanol as the hydrogen source [63]. Therefore, alcohols (e.g., isopropanol, methanol, and ethanol) and formic acid have gained significant attention in recent years due to their ability to readily generate active hydrogen under photocatalytic conditions [71], [76], [90], [93]. Although the photocatalytic hydrogenation reaction using H₂ as a hydrogen source under mild conditions seems to be inferior to the use of alcohols, the situation may be different for the reaction carried out at high temperatures, where the activation efficiency of H₂ at the active sites is significantly increased.

Water splitting, as the most sustainable hydrogen source, holds great potential in photocatalytic systems. For instance, HMF hydrogenation to BHMF was performed over ZnIn₂S₄/NiTiO₃ heterojunction catalyst using active hydrogen generated from water splitting [72]. However, the efficiency of water splitting is often constrained by the rapid recombination of electron-hole pairs, which necessitates further optimization of catalyst structures to address this challenge.

Therefore, to obtain a high catalytic efficiency, the reaction pathway, catalyst properties, hydrogen activation energy barrier, and active hydrogen migration behavior should be carefully considered when selecting the hydrogen source.

5

Conclusions and outlooks

This review summarizes the recent advances on photocatalytic hydrogenation of biomass-derived model molecules (HMF, FAL, vanillin, fatty acids, etc.) and the relative catalytic mechanisms. In addition, the influences of solvent and hydrogen source on the reaction pathway and catalytic performance are also discussed. Therefore, the selection of a suitable reaction system is conducive to improving the reaction efficiency and selectivity, thus promoting the application of photocatalysis in the conversion of biomass-derived molecules into high-value-added products.

Although some meaningful progress has been made and useful knowledge has been gained, the use of photocatalytic hydrogenation for the practical application of biomass utilization is still faced with several scientific problems.

1. Photocatalyst: Although many novel photocatalysts have been designed and studied to improve catalytic performance by modulating the light absorption, charge separation and transfer, substrate adsorption, and hydrogen spillover, the active sites on the photocatalysts have not been fully understood. Especially for the semiconductor-supported metal catalysts, the exact structure and the role of the interfacial area are still ambiguous. In addition, under light conditions, the dynamic reconfiguration of active sites should be considered carefully, which directly affects the exposure of active sites, adsorption of reactants, and migration efficiency of photogenerated electrons and holes. In addition, deactivation of photocatalysts deserves more attention, i.e., deactivation triggered by photothermal-induced active site changes or the carbon deposition in the complex reaction systems containing various reactants and sacrificial agents.

2. Reaction type expansion: Currently, the reported studies focus on the photocatalytic hydrogenation of biomass model molecules with aldehyde groups, such as HMF and FAL, which are well suited for transfer hydrogenation reactions catalyzed by photocatalysts. However, these aldehyde hydrogenation reactions can also be easily achieved by traditional thermocatalysis with a high yield. Compared to hydrogenation, biomass hydrohygenolysis or HDO reactions are more significant and challenging in terms of potential utilization, but they always face the disadvantages of harsh reaction conditions and high-energy consumption in conventional thermocatalysis. Photocatalysis is expected to provide new solutions to these problems if the activation of H species and cleavage of C-O bonds with photogenerated electrons or holes can be rationally combined in the reaction, and if the photothermal effect can positively influence the reaction kinetics and adsorption thermodynamics.

3. Efficiency and economy: Most photocatalytic hydrogenation reactions are regarded as green pathways due to the mild reaction conditions and the absence of H₂. However, further research is needed to reduce the use of various hydrogen- and electron-donor agents and improve reactant-to-catalyst mass ratios to improve catalytic efficiency and reduce process costs. Therefore, the apparent quantum efficiency should also be considered to evaluate photocatalyst performance, though it is frequently neglected in many studies. Furthermore, uniform performance and economic evolution criteria are essential for reliable comparisons across various photocatalytic systems.

4. Industrial potential: Although laboratory research has achieved remarkable results in improving catalyst performance and optimizing reaction conditions, photocatalysis technology still faces multiple obstacles in industrial applications. First, most catalysts contain precious metal components, and their high cost and scarcity significantly limit the economics of industrialization. Secondly, the design and optimization of photocatalytic reactors are difficult to balance the efficiency of light energy capture with the stability of large-scale continuous operation. In the future, it is necessary to explore photo-thermal coupled reactors and large-scale solar energy utilization technologies together with the development of non-precious metal catalysts to achieve low-cost and high-efficiency industrial applications.

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