Highly Selective Photocatalytic Valorization of Lignin Model Compounds Using Ultrathin Metal/CdS

Construction of a hollow heterojunction interface to accelerate the photocatalytic cleavage of lignin Cβ-O bonds

Photocatalytic splitting of the Cβ-O bond is regarded as a prospective strategy for transforming lignin, and it is imperative to develop novel photocatalysts with effective photogenerated charges separation and solar absorption capacity. Herein, a novel hollow ZIF-8/CdS heterostructure photocatalyst was synthesized for the catalytic splitting of lignin Cβ-O bonds. The photocatalytic cleavage rate of Cβ-O bond of ligin β-O-4 reached 30.3∙mmol∙h-1∙g-1 within 20 min under visible light exposure. It is noteworthy that the utilization of intricate natural lignin molecules in this photocatalytic system has yielded successful depolymerization. The DFT and XPS results indicate a potential unidirectional electron migration from ZIF-8 to CdS in ZIF-8/CdS composites transfer. This electron transport path follows the direct Z-scheme heterostructure mechanism, resulting in the generation of an internal electric field between ZIF-8 and CdS. Impressively, the synergistic combination of the hollow structure and Z-scheme heterostructure effectively enhances the efficiency of charge carrier separation and maintains a robust redox potential, thereby facilitating Cα-radical generation. This study proposes a novel photocatalyst design strategy that integrates hollow structures and Z-scheme heterojunctions, with the aim of targeting the depolymerization of the Cβ-O bond in lignin.

Engineered Half-Unit-Cell MoS2/ZnIn2S4 Monolayer Photocatalysts and Adsorbed Hydroxyl Radicals-Assisted Activation of Cα-H Bond for Efficient Cβ-O Bond Cleavage in Lignin to Aromatic Monomers

Photocatalysis has high potential in the cleavage of Cβ-O bond in lignin into high-value aromatic monomers; however, the inefficient Cα-H bond activation in lignin and a low hydrogen transfer efficiency on the photocatalyst's surfaces have limited its application in photocatalytic lignin conversion. This study indicates that the cleavage of the Cβ-O bond can be improved by the generation of the Cα radical intermediate through Cα-H bond activation, and the formation of desirable aromatic products can be significantly improved by the enhanced hydrogen transfer efficiency from photocatalyst surfaces to aromatic monomeric radicals. We elaborately designed the half-unit-cell MoS2/ZnIn2S4 monolayer with a thickness of ∼1.7 nm to promote the hydrogen transfer efficiency on the photocatalyst surfaces. The ultrathin structure can shorten the diffusion distance of charge carriers from the interior to the surfaces and tight interface between MoS2 and ZnIn2S4 to facilitate the migration of photogenerated electrons from ZnIn2S4 to MoS2, therefore improving the selectivity of desirable products. The adsorbed hydroxyl radical (*OH) on the surfaces of MoS2/ZnIn2S4 from water oxidation can significantly reduce the bond dissociation energy (BDE) of Cα-H bond in PP-ol from 2.38 to 1.87 eV, therefore improving the Cα-H bond activation. The isotopic experiments of H2O/D2O indicate that the efficiency of *OH generation is an important step in Cα-H bond activation for PP-ol conversion to aromatic monomers. In summary, PP-ol can completely convert to 86.6% phenol and 82.3% acetophenone after 1 h of visible light irradiation by using 3% MoS2/ZnIn2S4 and the assistance of *OH, which shows the highest conversion rate compared to previous works.

Photorefinery of Biomass and Plastics to Renewable Chemicals using Heterogeneous Catalysts

The photocatalytic conversion of biomass and plastic waste provides opportunities for sustainable fuel and chemical production. Heterogeneous photocatalysts, typically composed of semiconductors with distinctive redox properties in their conduction band (CB) and valence band (VB), facilitate both the oxidative and reductive valorization of organic feedstocks. This article provides a comprehensive overview of recent advancements in the photorefinery of biomass and plastics from the perspective of the redox properties of photocatalysts. We explore the roles of the VB and CB in enhancing the value-added conversion of biomass and plastics via various pathways. Our aim is to bridge the gap between photocatalytic mechanisms and renewable carbon feedstock valorization, inspiring further development in photocatalytic refinery of biomass and plastics.

Improved Lignin Conversion to High-Value Aromatic Monomers through Phase Junction CdS with Coexposed Hexagonal (100) and Cubic (220) Facets

Photocatalysis has the potential for lignin valorization to generate functionalized aromatic monomers, but its application has been limited by the slow conversion rate and the low selectivity to desirable aromatic products. In this work, we designed the phase junction CdS with coexposed hexagonal (100) and cubic (220) facets to improve the photogenerated charge carriers' transfer efficiency from (100) facet to (220) facet and the hydrogen transfer efficiency for an enhanced conversion rate of lignin to aromatic monomers. Water is found as a sufficient external hydrogen supplier to increase the yields of aromatic monomers. These innovative designs in the reaction system promoted complete conversion of PP-ol to around 94% of aromatic monomers after 1 h of visible light irradiation, which shows the highest reaction rate and selectivity of target products in comparison with previous works. PP-one is a byproduct from the overoxidation of PP-ol and is usually difficult to be further cleaved to acetophenone and phenol as the desirable aromatic monomers. TEA was first identified in this study as a sacrificial electron donor, a hydrogen source, and a mediator to enhance the cleavage of the Cβ-O bonds in PP-one. With the assistance of TEA, PP-one can be completely cleaved to desirable aromatic monomer products, and the reaction time is reduced from several hours to 10 min of visible light irradiation.

Selective (sono)photocatalytic cleavage of lignin-inspired β-O-4 linkages to phenolics by ultrasound derived 1-D titania nanomaterials

Catalytic conversion of lignin to value-added aromatic compounds is still an open challenge, since the selective cleavage of the linkages interconnecting the aromatic molecules, especially the β-O-4 ones, is not efficiently achieved yet. Herein, novel titania-based nanostructured materials were synthesized using low-power-low-frequency ultrasound that demonstrated high efficiency for the selective cleavage of Cα-Cβ bond of β-O-4 linkages of lignin-inspired model compounds. Going a step ahead, experiments of sonophotocatalytic valorization of 2-phenoxy-1-phenylethanol were contacted for the first time, where the exposure to ultrasound leading to better conversion and selectivity towards the desired products in the case of the novel ultrasound-synthesized nano-photocatalyst. Mechanistic insights showcased that photogenerated holes are the main active species in the catalytic process. In general, this research work provides a green, effective, and cost-effective approach for the selective and efficient catalytic lignin valorization.

Robust Oxidase-Mimetic Supramolecular Nanocatalyst for Lignin Biodegradation

Enzymatic catalysis presents an eco-friendly, energy-efficient method for lignin degradation. However, challenges arise due to the inherent incompatibility between enzymes and native lignin. In this work, we introduce a supramolecular catalyst composed of fluorenyl-modified amino acids and Cu2+, designed based on the aromatic stacking of the fluorenyl group, which can operate in ionic liquid environments suitable for the dissolution of native lignin. Amino acids and halide anions of ionic liquids shape the copper site's coordination sphere, showcasing remarkable catechol oxidase-mimetic activity. The catalyst exhibits thermophilic property, and maintains oxidative activity up to 75 °C, which allows the catalyzed degradation of the as-dissolved native lignin with high efficiency even without assistance of the electron mediator. In contrast, at this condition, the native copper-dependent oxidase completely lost its activity. This catalyst with superior stability and activity offer promise for sustainable lignin valorization through biocatalytic routes compatible with ionic liquid pretreatment, addressing limitations in native enzymes for industrially relevant conditions.

Electrocatalytic Lignin Valorization into Aromatic Products via Oxidative Cleavage of Cα-Cβ Bonds

Lignin is the most promising candidate for producing aromatic compounds from biomass. However, the challenge lies in the cleavage of C-C bonds between lignin monomers under mild conditions, as these bonds have high dissociation energy. Electrochemical oxidation, which allows for mild cleavage of C-C bonds, is considered an attractive solution. To achieve low-energy consumption in the valorization of lignin, the use of highly efficient electrocatalysts is essential. In this study, a meticulously designed catalyst consisting of cobalt-doped nickel (oxy)hydroxide on molybdenum disulfide heterojunction was developed. The presence of molybdenum in a high valence state promoted the adsorption of tert-butyl hydroperoxide, leading to the formation of critical radical intermediates. In addition, the incorporation of cobalt doping regulated the electronic structure of nickel, resulting in a lower energy barrier. As a result, the heterojunction catalyst demonstrated a selectivity of 85.36% for cleaving the Cα-Cβ bond in lignin model compound, achieving a substrate conversion of 93.69% under ambient conditions. In addition, the electrocatalyst depolymerized 49.82 wt% of soluble fractions from organosolv lignin (OL), resulting in a yield of up to 13 wt% of aromatic monomers. Significantly, the effectiveness of the prepared electrocatalyst was also demonstrated using industrial Kraft lignin (KL). Therefore, this research offers a practical approach for implementing electrocatalytic oxidation in lignin refining.

Ni(OH)2 surface-modified hierarchical ZnIn2S4 nanosheets: dual photocatalytic application and mechanistic insights

The simultaneous utilization of electrons and holes to couple photocatalytic H2 production with selective biomass transformation has attracted immense interest toward achieving sustainability in the fields of energy and chemical industry. In this study, by assembling highly dispersed Ni(OH)2 onto ZnIn2S4 (ZIS), efficient H2 evolution along with highly selective photocatalytic oxidation of furfuryl alcohol (FA) to furfural (FF) in pure water was achieved under anaerobic conditions. The H2 production and FA conversion rates over the optimal Ni-ZIS sample reached about 686 and 583 μmol g-1 h-1, respectively, about 4.9 and 1.7 folds as those of pure ZIS. Moreover, the formation of by-products with C-C coupling was dramatically suppressed over Ni-ZIS, resulting in higher selectivity for FF (97%), which is about 2.7-fold that of pure ZIS (36%). Deep mechanistic studies were conducted to reveal the structural evolution and cocatalyst effects of Ni(OH)2. This study not only offers a feasible paradigm for modifying the surface of catalysts to tune the photoactivity and selectivity for product-oriented alcohol oxidation coupled with efficient H2 production in water but also reveals the working mechanism of the deposited Ni(OH)2 over ZIS toward coupling reactions.

Thermal, photonic, and electrocatalysis in lignin depolymerization research

In order to realize a sustainable bio-based future, it is essential to fully harness the potential of biomass, including lignin - a readily available biopolymer that ranks second in abundance and serves as a renewable source of aromatics. While lignin has traditionally been used for lower-value applications like fuel and power generation, unlocking its higher-value potential through diverse conversion and upgrading techniques is of paramount importance. This review focuses on the catalytic conversion of lignin, with a specific emphasis on selective depolymerization, a process that not only supports economically and environmentally sustainable biorefineries but also aligns with Green Chemistry principles, mitigating adverse environmental impacts. Furthermore, we provide a comprehensive discussion of reaction pathways and mechanisms, including C-O and C-C bond cleavage, among different catalysts. Lastly, we analyze and briefly discuss the prospects of rational catalyst design in biomass valorization.

Versatile synthesis of nano-icosapods via cation exchange for effective photocatalytic conversion of biomass-relevant alcohols

Branched metal chalcogenide nanostructures with well-defined composition and configuration are appealing photocatalysts for solar-driven organic transformations. However, precise design and controlled synthesis of such nanostructures still remain a great challenge. Herein, we report the construction of a variety of highly symmetrical metal sulfides and heterostructured icosapods based on them, in which twenty branches were radially grown in spatially ordered arrangement, with a high degree of structure homogeneity. Impressively, the as-obtained CdS-PdxS icosapods manifest a significantly improved photocatalytic activity for the selective oxidation of biomass-relevant alcohols into corresponding aldehydes coupled with H2 evolution under visible-light irradiation (>420 nm), and the apparent quantum yield of the benzyl alcohol reforming can be achieved as high as 31.4% at 420 nm. The photoreforming process over the CdS-PdxS icosapods is found to be directly triggered by the photogenerated electrons and holes without participation of radicals. The enhanced photocatalytic performance is attributed to the fast charge separation and abundant active sites originating from the well-defined configuration and spatial organization of the components in the branched heterostructures.

Radical-Mediated Photocatalysis for Lignocellulosic Biomass Conversion into Value-Added Chemicals and Hydrogen: Facts, Opportunities and Challenges

Photocatalytic biomass conversion into high-value chemicals and fuels is considered one of the hottest ongoing research and industrial topics toward sustainable development. In short, this process can cleave Cβ -O/Cα -Cβ bonds in lignin to aromatic platform chemicals, and further conversion of the polysaccharides to other platform chemicals and H2 . From the chemistry point of view, the optimization of the unique cooperative interplay of radical oxidation species (which are activated via molecular oxygen species, ROSs) and substrate-derived radical intermediates by appropriate control of their type and/or yield is key to the selective production of desired products. Technically, several challenges have been raised that face successful real-world applications. This review aims to discuss the recently reported mechanistic pathways toward selective biomass conversion through the optimization of ROSs behavior and materials/system design. On top of that, through a SWOT analysis, we critically discussed this technology from both chemistry and technological viewpoints to help the scientists and engineers bridge the gap between lab-scale and large-scale production.

Zinc Vacancy Promotes Photo-Reforming Lignin Model to H2 Evolution and Value-Added Chemicals Production

Lignin, rich in β-O-4 bonds and aromatic structure, is a renewable and potential resource for value-added chemicals and promoting H2 evolution. However, direct photo-reforming lignin remains a huge challenge due to its recalcitrant structure. Herein, a collaborative strategy is proposed by dispersing Pt on zinc-vacancy-riched ZnIn2 S4 (Pt/VZn -ZIS) for revealing the effect of lignin structure during photo-reforming process with lignin models. And a series of theoretical calculations and experimental results show that lignin model substances with more nucleophilic group structures will have a stronger tendency to occur the photo-reforming reactions. In addition, benefiting of Pt-S electronic channel is formed by occupying Pt atom onto zinc vacancies in ZnIn2 S4 , which can effectively reduce the energy barrier of H2 evolution and accompany the selective oxidation of lignin model from Cα-OH to Cα = O under simulated sunlight. The natural lignin is used to further demonstrate this selective oxidation mechanism. The presented work demonstrates the photo-reforming lignin model mechanism and the influence of lignin-structure during the process of photo-reforming.

Photocatalytic depolymerization of lignin via oxidizing cleavage of Cα-Cβ bonds in micellar aqueous media

Photocatalytic depolymerization of lignin to prepare high-value chemicals is a promising way to promote the valuable utilization of lignin. However, the complexity and stubbornness of lignin structure seriously decrease the photocatalytic efficiency and selectivity. Herein, the micellar aqueous media (SDS-8/HCl) consisting of sodium lauryl sulfonate and hydrochloric acid was successfully prepared. Photocatalyst TiO2 and SDS-8/HCl system can effectively depolymerize the typical β-1 lignin models and ethanol organosolv lignin to value-added chemicals by oxidizing cleavage of lignin Cα-Cβ bonds. The addition of hydrochloric acid solution (1 mol/L) improves the selectivity of photocatalytic breaking of lignin Cα-Cβ bonds. Chlorine ions are oxidized to chlorine radicals by photogenerated holes and hydroxyl radicals, dramatically increasing the photocatalytic efficiency. Electron paramagnetic resonance technique and Gas chromatography-mass spectrometry were used to demonstrate the presence of chlorine radicals. Under optimal conditions, the conversion of substrate Dpol is 98.4 %, and the obtained products are mainly benzaldehyde and benzoic acid. Isotope labeling experiments show that water is also involved in photocatalytic reactions and the oxygen needed to form the product benzaldehyde comes from water. Single-electron transfer processes are possible photocatalytic mechanisms that differ from the previous reports. Importantly, water and chlorine ions were found to be involved in photocatalytic reactions for the first time and promote the cleavage of lignin Cα-Cβ bonds. This work provides new ideas for photocatalytic cleavage of lignin Cα-Cβ bonds in heterogeneous photocatalytic systems using micellar aqueous media.

Photoreforming for Lignin Upgrading: A Critical Review

Photoreforming of lignocellulosic biomass to simultaneously produce gas fuels and value-added chemicals has gradually emerged as a promising strategy to alleviate the fossil fuels crisis. Compared to cellulose and hemicellulose, the exploitation and utilization of lignin via photoreforming are still at the early and more exciting stages. This Review systematically summarizes the latest progress on the photoreforming of lignin-derived model components and "real" lignin, aiming to provide insights for lignin photocatalytic valorization from fundamental to industrial applications. Considering the complexity of lignin physicochemical properties, related analytic methods are also introduced to characterize lignin photocatalytic conversion and product distribution. We finally put forward the challenges and perspective of lignin photoreforming, hoping to provide some guidance to valorize biomass into value-added chemicals and fuels via a mild photoreforming process in the future.

Review on the oxidative catalysis methods of converting lignin into vanillin

Vanillin plays an important role not only in food and flavouring, but also as a platform compound for the synthesis of other valuable products, mainly derived from the oxidative decarboxylation of petroleum-based guaiacol production. In order to alleviate the problem of collapsing oil resources, the preparation of vanillin from lignin has become a good option from the perspective of environmental sustainability, but it is still not optimistic in terms of vanillin production. Currently, catalytic oxidative depolymerization of lignin for the preparation of vanillin is the main development trend. This paper mainly reviews four ways of preparing vanillin from lignin base: alkaline (catalytic) oxidation, electrochemical (catalytic) oxidation, Fenton (catalytic) oxidation and photo (catalytic) oxidative degradation of lignin. In this work, the working principles, influencing factors, vanillin yields obtained, respective advantages and disadvantages and the development trends of the four methods are systematically summarized, and finally, several methods for the separation and purification of lignin-based vanillin are briefly reviewed.

Functionalized Regulation of Metal Defects in ln2S3 of p-n Homojunctions

The introduction of metal vacancies into n-type semiconductors could efficiently construct intimate contact interface p-n homojunctions to accelerate the separation of photogenerated carriers. In this work, a cationic surfactant occupancy method was developed to synthesize an indium-vacancy (V_In)-enriched p-n amorphous/crystal homojunction of indium sulfide (A/C-IS) for sodium lignosulfonate (SL) degradation. The amount of V_In in the A/C-IS could be regulated by varying the content of added cetyltrimethylammonium bromide (CTAB). Meanwhile, the steric hindrance of CTAB produced mesopores and macropores, providing transfer channels for SL. The degradation rates of A/C-IS to SL were 8.3 and 20.9 times higher than those of crystalline In₂S₃ and commercial photocatalyst (P25), respectively. The presence of unsaturated dangling bonds formed by V_In reduced the formation energy of superoxide radicals (^•O₂^-). In addition, the inner electric field between the intimate contact interface p-n A/C-IS promoted the migration of electron-hole pairs. A reasonable degradation pathway of SL by A/C-IS was proposed based on the above mechanism. Moreover, the proposed method could also be applicable for the preparation of p-n homojunctions with metal vacancies from other sulfides.

Photocatalytic reductive C-O bond scission promoted by low-work-function Cd single atoms and clusters

C-O bond scission via photocatalysis is an important step in biomass depolymerization. Here, we demonstrate the scission of strong ether C-O bonds promoted by low-work-function Cd single atoms and clusters. Their loading on ZnS benefits C-H bond scission, thus weakening the C-O bond for chemical bond breaking.

Lignin C-C bond cleavage induced by consecutive two-photon excitation of a metal-free photocatalyst

Photocatalytic lignin valorization has caught widespread attention; yet the reaction systems often employ noble metal complexes, hydrogen atom transfer (HAT) agents, and/or sacrificial electron donors/acceptors that do not comply with atom economy or environmental friendliness. Herein, we discovered that N-phenylphenothiazine (PTH) as a metal-free photocatalyst induced the cleavage of the lignin C_α-C_β bond under ambient conditions free of those additional agents with a high yield and selectivity toward benzoic acid. Transient spectroscopic investigations revealed that the energy-demanding C_α-C_β bond cleavage was induced by the potent oxidant, ²PTH˙⁺*, that was derived from consecutive two-photon excitation of PTH.

Combined conversion of lignocellulosic biomass into high-value products with ultrasonic cavitation and photocatalytic produced reactive oxygen species - A review

The production of high-value products from lignocellulosic biomass is carried out through the selective scission of crosslinked CC/CO bonds. Nowadays, several techniques are applied to optimize biomass conversion into desired products with high yields. Photocatalytic technology has been proven to be a valuable tool for valorizing biomass at mild conditions. The photoproduced reactive oxygen species (ROSs) can initiate the scission of crosslinked bonds and form radical intermediates. However, the low mass transfer of the photocatalytic process could limit the production of a high yield of products. The incorporation of ultrasonic cavitation in the photocatalytic system provides an exceptional condition to boost the fragmentation and transformation of biomass into the desired products within a lesser reaction time. This review critically discusses the main factors governing the application of photocatalysis for biomass valorization and tricks to boost the selectivity for enhancing the yield of desired products. Synergistic effects obtained through the combination of sonolysis and photocatalysis were discussed in depth. Under ultrasonic vibration, hot spots could be produced on the surface of the photocatalysts, improving the mass transfer through the jet phenomenon. In addition, shock waves can assist the dissolution and mixing of biomass particles.

Photocatalytic Reforming of Biomass: What Role Will the Technology Play in Future Energy Systems

Photocatalytic reforming of biomass has emerged as an area of significant interest within the last decade. The number of papers published in the literature has been steadily increasing with keywords such as 'hydrogen' and 'visible' becoming prominent research topics. There are likely two primary drivers behind this, the first of which is that biomass represents a more sustainable photocatalytic feedstock for reforming to value-added products and energy. The second is the transition towards achieving net zero emission targets, which has increased focus on the development of technologies that could play a role in future energy systems. Therefore, this review provides a perspective on not only the current state of the research but also a future outlook on the potential roadmap for photocatalytic reforming of biomass. Producing energy via photocatalytic biomass reforming is very desirable due to the ambient operating conditions and potential to utilise renewable energy (e.g., solar) with a wide variety of biomass resources. As both interest and development within this field continues to grow, however, there are challenges being identified that are paramount to further advancement. In reviewing both the literature and trajectory of the field, research priorities can be identified and utilised to facilitate fundamental research alongside whole systems evaluation. Moreover, this would underpin the enhancement of photocatalytic technology with a view towards improving the technology readiness level and promoting engagement between academia and industry.

Unbiased Photoelectrode Interfaces for Solar Coupling of Lignin Oxidation with Biocatalytic C═C Bond Hydrogenation

The pulp and paper manufacturers generate approximately 50 million metric tons of lignin per annum, most of which has been abandoned or incinerated because of lignin's recalcitrant nature. Here, we report bias-free photoelectrochemical (PEC) oxidation of lignin coupled with asymmetric hydrogenation of C═C bonds. The PEC platform consists of a hematite (α-Fe₂O₃) photoanode and a silicon photovoltaic-wired mesoporous indium tin oxide (Si/mesoITO) photocathode. We substantiate a new function of photoelectroactivated α-Fe₂O₃ to extract electrons from lignin. The extracted electrons are transferred to the Si/mesoITO photocathode for regenerating synthetic nicotinamide cofactor analogues (mNADHs). We demonstrate that the reduction kinetics of mNAD⁺s depend on their reduction peak potentials. The regenerated mNADHs activate ene-reductases from the old yellow enzyme (OYE) family, which catalyze enantioselective reduction of α,β-unsaturated hydrocarbons. This lignin-fueled biocatalytic PEC system exhibits an excellent OYE's turnover frequency and total turnover number for photobiocatalytic trans-hydrogenation through cofactor regeneration. This work presents the first example of PEC regeneration of mNADHs and opens up a sustainable route for bias-free chemical synthesis using renewable lignin waste as an electron feedstock.

Enhanced Photocatalytic Alcohol Oxidation at the Interface of RuC-Coated TiO2 Nanorod Arrays

Visible-light-driven organic oxidations carried out under mild conditions offer a sustainable approach to performing chemical transformations important to the chemical industry. This work reports an efficient photocatalytic benzyl alcohol oxidation process using one-dimensional (1D) TiO2 nanorod (NR)-based photoanodes with surface-adsorbed ruthenium polypyridyl photocatalysts at room temperature. The photocatalyst bis(2,2'-bipyridine)(4,4'-dicarboxy-2,2'-bipyridine)Ru(II) (RuC) was covalently anchored onto TiO2 nanorod arrays grown on fluorine-doped tin oxide (FTO) electrode surfaces (FTO|t-TiO2|RuC, t = the thickness of TiO2 NR). Under aerobic conditions, the photophysical and photocatalytic properties of FTO|t-TiO2|RuC (t = 1, 2, or 3.5 μm) photoanodes were investigated in a solution containing a hydrogen atom transfer mediator (4-acetamido-2,2,6,6-tetramethylpiperidine-N-oxyl, ACT) as cocatalyst. Dye-sensitized photoelectrochemical cells (DSPECs) using the FTO|t-TiO2|RuC (t = 1, 2, or 3.5 μm) photoanodes and ACT-containing electrolyte were investigated for carrying out photocatalytic oxidation of a lignin model compound containing a benzylic alcohol functional group. The best-performing anode surface, FTO|1-TiO2|RuC (shortest NR length), oxidized the 2° alcohol of the lignin model compound to the Cα-ketone form with a > 99% yield over a 4 h photocatalytic experiment with a Faradaic efficiency of 88%. The length of TiO2 NR arrays (TiO2 NRAs) on the FTO substrate influenced the photocatalytic performance with longer NRAs underperforming compared to the shorter arrays. The influence of the NR length is hypothesized to affect the homogeneity of the RuC coating and accessibility of the ACT mediator to the RuC-coated TiO2 surface. The efficient photocatalytic alcohol oxidation with visible light at room temperature as demonstrated in this study is important to the development of sustainable approaches for lignin depolymerization and biomass conversion.

Catalytic self-transfer hydrogenolysis of lignin with endogenous hydrogen: road to the carbon-neutral future

Due to the depletion of fossil sources, it is imperative to develop a sustainable and carbon-neutral biorefinery for supporting the fuel and chemical supply in modern society. Lignin, the only renewable aromatic source, is still an underutilized component in lignocellulose. Very recently, it has been found that hydrogenolysis is a promising technology for lignin valorization. However, high-pressure H₂ is necessary during lignin hydrogenolysis, resulting in safety problems. Furthermore, H₂ is mainly produced from steam reforming of fossil sources in industry, which makes the conversion of renewable lignin unsustainable and costly. Plentiful aliphatic hydroxyl and methoxy groups exist in native lignin and offer a renewable alternative to H₂, and can be hydrogen sources for the depolymerization and upgradation of lignin via the intramolecular catalytic transfer hydrogenation. The hydrogen source in situ generated from lignin is a type of green hydrogen, decreasing the carbon footprint. The purpose of this review is to provide a summary and perspective of lignin valorization via self-transfer hydrogenolysis, mainly focusing on a comprehensive understanding of the mechanism of catalytic self-transfer hydrogenolysis at the molecular level and developing highly effective catalytic systems. Moreover, some opportunities and challenges within this attractive field are given to discuss future research directions.

Radical generation and fate control for photocatalytic biomass conversion

Photocatalysis is an emerging approach for sustainable chemical production from renewable biomass under mild conditions. Active radicals are always generated as key intermediates, in which their high reactivity renders them versatile for various upgrading processes. However, controlling their reaction is a challenge, especially in highly functionalized biomass frameworks. In this Review, we summarize recent advanced photocatalytic systems for selective biomass valorization, with an emphasis on their distinct radical-mediated reaction patterns. The strategies for generating a specific radical intermediate and controlling its subsequent conversion towards desired chemicals are also highlighted, aiming to provide guidance for future studies. We believe that taking full advantage of the unique reactivity of radical intermediates would provide great opportunities to develop more efficient photocatalytic systems for biomass valorization.

Metal Sulfide Photocatalysts for Lignocellulose Valorization

Transition metal sulfides are an extraordinarily vital class of semiconductors with a wide range of applications in the photocatalytic field. A great number of recent advances in photocatalytic transformations of lignocellulosic biomass, the largest renewable carbon resource, into high-quality fuels and value-added chemicals has been achieved over metal sulfide semiconductors. Herein, the progress and breakthroughs in metal-sulfide-based photocatalytic systems for lignocellulose valorization with an emphasis on selective depolymerization of lignin and oxidative coupling of some important bioplatforms are highligted. The key issues that control reaction pathways and mechanisms are carefully examined. The functions of metal sulfides in the elementary reactions, including CO-bond cleavage, selective oxidations, CC coupling, and CH activation, are discussed to offer insights to guide the rational design of active and selective photocatalysts for sustainable chemistry. The prospects of sulfide photocatalysts in biomass valorization are also analyzed and briefly discussed.

Support-Activity Relationship in Heterogeneous Catalysis for Biomass Valorization and Fine-Chemicals Production

Heterogeneous catalysts are progressively expanding their field of application, from high-throughput reactions for traditional industrial chemistry with production volumes reaching millions of tons per year, a sector in which they are key players, to more niche applications for the production of fine chemicals. These novel applications require a progressive utilization reduction of fossil feedstocks, in favor of renewable ones. Biomasses are the most accessible source of organic precursors, having as advantage their low cost and even distribution across the globe. Unfortunately, they are intrinsically inhomogeneous in nature and their efficient exploitation requires novel catalysts. In this process, an accurate design of the active phase performing the reaction is important; nevertheless, we are often neglecting the importance of the support in guaranteeing stable performances and improving catalytic activity. This review has the goal of gathering and highlighting the cases in which the supports (either derived or not from biomass wastes) share the worth of performing the catalysis with the active phase, for those reactions involving the synthesis of fine chemicals starting from biomasses as feedstocks.

Visible Light-Driven Reforming of Lignocellulose into H2 by Intrinsic Monolayer Carbon Nitride

The photoreforming of lignocellulose is a novel method to produce clean and sustainable H₂ energy. However, the catalytic systems usually show low activity under ultraviolet light; thus, this reaction is very limited at present. Visible light-responsive metal-free two-dimensional graphite-phased carbon nitride (g-C₃N₄) is a good candidate for photocatalytic hydrogen production, but its activity is hindered by a bulky architecture. Although reported layered g-C₃N₄ modified with active functional groups prepared by the chemical exfoliation enhances the photocatalytic activity, it lost the intrinsic structure and thus is not conducive to understand the structure-activity relationship. Herein, we report an intrinsic monolayer g-C₃N₄ (∼0.32 nm thickness) prepared by nitrogen-protected ball milling in water, which shows good performance of photoreforming lignocellulose to H₂ driven by visible light. The exciton binding energy of g-C₃N₄ was estimated from the temperature-dependent photoluminescence spectra, which is a key factor for subsequent charge separation and energy transfer. It is found that monolayer g-C₃N₄ with smaller exciton binding energy increases the free exciton concentrations and promotes the separation efficiency of charge carriers, thereby effectively improving its performance of photocatalytic reforming of lignocellulose, even the virgin lignocellulose and waste lignocellulose. This result could lead to more active catalysts to photoreform the raw biomass, making it possible to provide clean energy directly from locally unused biomass.

Cooperative Coupling of Oxidative Organic Synthesis and Hydrogen Production over Semiconductor-Based Photocatalysts

Merging hydrogen (H₂) evolution with oxidative organic synthesis in a semiconductor-mediated photoredox reaction is extremely attractive because the clean H₂ fuel and high-value chemicals can be coproduced under mild conditions using light as the sole energy input. Following this dual-functional photocatalytic strategy, a dreamlike reaction pathway for constructing C-C/C-X (X = C, N, O, S) bonds from abundant and readily available X-H bond-containing compounds with concomitant release of H₂ can be readily fulfilled without the need of external chemical reagents, thus offering a green and fascinating organic synthetic strategy. In this review, we begin by presenting a concise overview on the general background of traditional photocatalytic H₂ production and then focus on the fundamental principles of cooperative photoredox coupling of selective organic synthesis and H₂ production by simultaneous utilization of photoexcited electrons and holes over semiconductor-based catalysts to meet the economic and sustainability goal. Thereafter, we put dedicated emphasis on recent key progress of cooperative photoredox coupling of H₂ production and various selective organic transformations, including selective alcohol oxidation, selective methane conversion, amines oxidative coupling, oxidative cross-coupling, cyclic alkanes dehydrogenation, reforming of lignocellulosic biomass, and so on. Finally, the remaining challenges and future perspectives in this flourishing area have been critically discussed. It is anticipated that this review will provide enlightening guidance on the rational design of such dual-functional photoredox reaction system, thereby stimulating the development of economical and environmentally benign solar fuel generation and organic synthesis of value-added fine chemicals.

Recent advances in the valorization of plant biomass

Plant biomass is a highly abundant renewable resource that can be converted into several types of high-value-added products, including chemicals, biofuels and advanced materials. In the last few decades, an increasing number of biomass species and processing techniques have been developed to enhance the application of plant biomass followed by the industrial application of some of the products, during which varied technologies have been successfully developed. In this review, we summarize the different sources of plant biomass, the evolving technologies for treating it, and the various products derived from plant biomass. Moreover, the challenges inherent in the valorization of plant biomass used in high-value-added products are also discussed. Overall, with the increased use of plant biomass, the development of treatment technologies, and the solution of the challenges raised during plant biomass valorization, the value-added products derived from plant biomass will become greater in number and more valuable.

Photocatalytic Conversion of Lignin into Chemicals and Fuels

Lignin, an underutilized component of lignocellulosic biomass, is regarded as a rich reservoir for the production of aromatic chemicals and fuels. Despite extensive research in recent years, lignin's potential is far from being fully unlocked. Photocatalysis that uses sustainable solar energy to drive lignin conversion under mild conditions has been identified as a promising strategy and received growing research interest. This review aims to present a critical introduction to the photocatalytic conversion of lignin, including a summary of lignin conversion pathways and mechanisms, as well as the latest cutting-edge innovations on photocatalyst design and reactor construction. Moreover, the screening of solvents and regulation of other key factors that are involved in photocatalytic lignin conversion are also elucidated and future perspectives and challenges for photocatalytic conversion of lignin into valuable products are discussed.

Photocatalytic Cleavage of β-O-4 Ether Bonds in Lignin over Ni/TiO2

It is of great importance to explore the selective hydrogenolysis of β-O-4 linkages, which account for 45–60% of all linkages in native lignin, to produce valued-added chemicals and fuels from biomass employing UV light as catalyst. TiO₂ exhibited satisfactory catalytic performances in various photochemical reactions, due to its versatile advantages involving high catalytic activity, low cost and non-toxicity. In this work, 20 wt.% Ni/TiO₂ and oxidant PCC (Pyridinium chlorochromate) were employed to promote the cleavage of β-O-4 alcohol to obtain high value chemicals under UV irradiation at room temperature. The Ni/TiO₂ photocatalyst can be magnetically recovered and efficiently reused in the following four consecutive recycling tests in the cleavage of β-O-4 ether bond in lignin. Mechanism studies suggested that the oxidation of β-O-4 alcohol to β-O-4 ketone by oxidant PCC first occurred during the reaction, and was followed by the photocatalysis of the obtained β-O-4 ketone to corresponding acetophenone and phenol derivates. Furthermore, the system was tested on a variety of lignin model substrates containing β-O-4 linkage for the generation of fragmentation products in good to excellent results.

Photocatalytic transformations of lignocellulosic biomass into chemicals

This review highlights recent advances in photocatalytic transformations of lignocellulosic biomass (polysaccharides and lignin) into chemicals (in particular organic oxygenates).