Enhancing Photocatalytic β-O-4 Bond Cleavage in Lignin Model Compounds by Silver-Exchanged Cadmium Sulfide

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.

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.

Ultrasonic-enhanced photocatalysis through piezoelectric and cavitation effects for lignin depolymerization

Although a promising method for lignin depolymerization, photocatalysis faces the challenge of low efficiency. In this study, MoS2/ZnO heterojunction catalysts, endowed with piezocatalysis and photocatalytic capabilities, were crafted through Zn ion intercalation for the depolymerization of phenoxyphenylethanol (PP-ol) and alkali lignin. Then, the synergistic interplay between ultrasonic-induced piezoelectric fields and heterojunctions was analyzed. The amalgamation of the piezoelectric field and heterojunction in MoS2/ZnO catalysts resulted in a diminished photogenerated hole/electron recombination efficiency, thereby fostering the generation of ·OH during the reaction. This pivotal role of ·OH emerged as a crucial reactive substance, converting 95.8 % of PP-ol through β-O-4 bond breaking within a 3-h treatment. By incorporating ultrasonic, the contact probability of PP-ol with the catalyst was significantly improved, resulting in efficient conversion even with a reduced amount of acetonitrile in the solvent system (20 %). Furthermore, ultrasonic-light methods show high efficiency for depolymerizing Alkali lignin (AL), with 33.2 % of lignin undergoing depolymerization in a 4-h treatment. This treatment simultaneously reduces the molecular weight of AL and cleaves numerous chemical bonds within it. Overall, this work presents a green approach to lignin depolymerization, providing insights into the synergistic action of ultrasonic and photocatalysis.

Inorganic-Organic Dual-Ligand-Regulated Photocatalysis of CdS@ZnxCd1-xS@ZnS Quantum Dots for Lignin Valorization

In a dual-functional lignin valorization system, a harmonious oxidation and reduction rate is a prerequisite for high photocatalytic performance. Herein, an efficient and facile ligand manipulating strategy to balance the redox reaction process is exploited via decorating the surface of the CdS@ZnxCd1-xS@ZnS gradient-alloyed quantum dots with both inorganic ligands of hexafluorophosphate (PF6-) and organic ligands of mercaptopropionic acid (MPA). Inorganic ion ligands in this system provide a promotion for intermediator reduction reactions. By optimizing the ligand composition on the quantum dot surface, we achieve precise control over the extent of oxidation and reduction, enabling selective modification of reaction products; that is, the conversion rate of 2-phenoxy-1-phenylethanol reached 99%. Surface engineering by regulating the ligand type demonstrates that PF6- and thiocyanate (SCN-) inorganic ion ligands contribute significantly toward electron transfer, while MPA ligands have beneficial effects on the hole-transfer procedure, which is predominantly dependent on their steric hindrance, electrostatic action, and passivation effect. The present study offers insights into the development of efficient quantum dot photocatalysts for dual-functional biomass valorization through ligand design.

Progress on quantum dot photocatalysts for biomass valorization

Biomass with abundant reproducible carbon resource holds great promise as an intriguing substitute for fossil fuels in the manufacture of high-value-added chemicals and fuels. Photocatalytic biomass valorization using inexhaustible solar energy enables to accurately break desired chemical bonds or selectively functionalize particular groups, thus emerging as an extremely creative and low carbon cost strategy for relieving the dilemma of the global energy. Quantum dots (QDs) are an outstandingly dynamic class of semiconductor photocatalysts because of their unique properties, which have achieved significant successes in various photocatalytic applications including biomass valorization. In this review, the current development rational design for QDs photocatalytic biomass valorization effectively is highlighted, focusing on the principles of tuning their particle size, structure, and surface properties, with special emphasis on the effect of the ligands for selectively broken chemical bonds (C─O, C─C) of biomass. Finally, the present issues and possibilities within that exciting field are described.

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.

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.

Enhancing oxidation ability of graphitic carbon nitride photocatalysts to promote lignin Cα-Cβ bond cleavage in micellar aqueous media

As the most abundant aromatic resource, lignin is an appreciated biomass material to obtain aromatic high-value chemicals. However, the selective cleavage of lignin C_α-C_β bonds under mild conditions constitutes a challenge. Herein, a photocatalyst having high oxidation capacity was successfully synthesized by codoping S and Cl atoms into graphite carbon nitride (g-C₃N₄). The resulting S,Cl/CN-1.5 photocatalyst exhibits enhanced photogenerated electron-hole separation ability and higher valence band potential than g-C₃N₄. S,Cl/CN-1.5 can efficiently break lignin C_α-C_β bonds in micellar aqueous medium to produce benzaldehyde and benzyl alcohol as the main products. Mechanism studies show that the photocatalytic cleavage of lignin C_α-C_β bonds proceeds via single-electron transfer and C_β radical mechanisms in which hydroxyl radicals and photogenerated holes play an important role. Isotopic experiments show that the O atoms required for the photocatalytic cleavage of lignin C_α-C_β bonds come from water in the micellar aqueous medium based on the full contact between water and substrate. Although O₂ atmosphere is beneficial for the photocatalytic efficiency, O₂ is not necessary for the photocatalytic cleavage of lignin C_α-C_β bonds. This research provides a useful guide for designing efficient photocatalysts to depolymerize lignin into high-value chemicals.

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.

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.

High Triplet Energy Iridium(III) Isocyanoborato Complex for Photochemical Upconversion, Photoredox and Energy Transfer Catalysis

Cyclometalated Ir(III) complexes are often chosen as catalysts for challenging photoredox and triplet-triplet-energy-transfer (TTET) catalyzed reactions, and they are of interest for upconversion into the ultraviolet spectral range. However, the triplet energies of commonly employed Ir(III) photosensitizers are typically limited to values around 2.5-2.75 eV. Here, we report on a new Ir(III) luminophore, with an unusually high triplet energy near 3.0 eV owing to the modification of a previously reported Ir(III) complex with isocyanoborato ligands. Compared to a nonborylated cyanido precursor complex, the introduction of B(C₆F₅)₃ units in the second coordination sphere results in substantially improved photophysical properties, in particular a high luminescence quantum yield (0.87) and a long excited-state lifetime (13.0 μs), in addition to the high triplet energy. These favorable properties (including good long-term photostability) facilitate exceptionally challenging organic triplet photoreactions and (sensitized) triplet-triplet annihilation upconversion to a fluorescent singlet excited state beyond 4 eV, unusually deep in the ultraviolet region. The new Ir(III) complex photocatalyzes a sigmatropic shift and [2 + 2] cycloaddition reactions that are unattainable with common transition metal-based photosensitizers. In the presence of a sacrificial electron donor, it furthermore is applicable to demanding photoreductions, including dehalogenations, detosylations, and the degradation of a lignin model substrate. Our study demonstrates how rational ligand design of transition-metal complexes (including underexplored second coordination sphere effects) can be used to enhance their photophysical properties and thereby broaden their application potential in solar energy conversion and synthetic photochemistry.

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.

Highly-toughened PVA/nanocellulose hydrogels with anti-oxidative and antibacterial properties triggered by lignin-Ag nanoparticles

In this study, Ag nanoparticles were firstly reduced on the surface of lignin nanoparticles (LNP_Ag) by direct reaction of silver nitrate without the use of a catalyst. Thermogravimetric analysis, Zeta potential and transmission electron microscopy measurements were performed to give evidence of the effectiveness of the reaction. After that, glutaraldehyde crosslinked PVA hydrogels, were produced by addition of unmodified LNP and Ag loaded LNP (LNP_Ag) in presence of cellulose nanocrystals (CNC). Thermal, mechanical, rheological, microstructural and biological anti-oxidative and anti-bacterial properties of the resulted hydrogels were investigated. It was proved that all the three nanofillers were homogeneously dispersed in PVA, and the pore diameter of the hydrogels was in the range of 0.5-2.0 μm. Nevertheless, the hydrogels showed high toughness, long-term and repeatable adhesiveness to a variety of substrates. In particular, composite hydrogels containing LNP_Ag nanoparticles showed excellent radical scavenging and antibacterial activities. Consequently, the effects of PVA-2CNC-2LNP_Ag on wound healing in mice model of full-thickness skin resection were evaluated by hematoxylin and eosin staining, taking as a reference the PVA-2CNC-2LNP system. The results showed that the wound healing time of PVA-2CNC-2LNP_Ag group was faster than that of neat PVA and PVA-2CNC, highlighting the role of LNP_Ag in enhancing the contact-active anti-oxidative and antibacterial activities mechanism in composite hydrogels. We expected that PVA hydrogels incorporating LNP_Ag could be used as green and efficient biomedical wound dressing materials.

Fine tuning of CdxZn1-xS for photo-depolymerization of alkaline lignin into vanillin

Lignin is abundant and contains a large number of aromatic groups. Herein, Cd_xZn_1-xS photocatalyst with tunable band gap energy was successfully synthesized by using 3-mercaptopropionic acid as a structure tuning additive. Cd_xZn_1-xS can depolymerize alkaline lignin to vanillin by the photocatalytic process. Each gram of alkaline lignin can produce 46.5 mg of vanillin. 2-Phenoxy-1-phenylethanol (PP-ol) and other model compounds were used to understand the depolymerizing process of lignin. Fine tuned Cd_xZn_1-xS can effectively cleave the C_β-O-4 bond existed in PP-ol under simulated sunlight. The highest conversion of PP-ol was 89.5% with phenol and acetophenone yields of 66.2% and 33.5%, respectively. The mechanism studies confirm that the C_α-H in PP-ol and lignin is firstly dehydrogenated to form C_α radical intermediates, and then the photogenerated electrons break the adjacent C_β-O bond. This research provides a new strategy to prepare valuable chemicals by virtue of renewable biomass and simulated sunlight.

Highly Efficient Ag3PO4/g-C3N4 Z-Scheme Photocatalyst for Its Enhanced Photocatalytic Performance in Degradation of Rhodamine B and Phenol

Ag3PO4/g-C3N4 heterojunctions, with different g-C3N4 dosages, were synthesized using an in situ deposition method, and the photocatalytic performance of g-C3N4/Ag3PO4 heterojunctions was studied under simulated sunlight conditions. The results revealed that Ag3PO4/g-C3N4 exhibited excellent photocatalytic degradation activity for rhodamine B (Rh B) and phenol under the same light conditions. When the dosage of g-C3N4 was 30%, the degradation rate of Rh B at 9 min and phenol at 30 min was found to be 99.4% and 97.3%, respectively. After five cycles of the degradation experiment for Rh B, g-C3N4/Ag3PO4 still demonstrated stable photodegradation characteristics. The significant improvement in the photocatalytic activity and stability of g-C3N4/Ag3PO4 was attributed to the rapid charge separation between g-C3N4 and Ag3PO4 during the Z-scheme charge transfer and recombination process.

Photocatalytic depolymerization of organosolv lignin into valuable chemicals

Catalytic conversion of lignin to certain aromatic compounds has been extensively studied but still has great challenges. Photocatalytic depolymerizing lignin is a very promising method to obtain valuable chemicals. Herein, Zn₄In₂S₇ (ZIS)-based photocatalyst was successfully synthesized by simply combining ZIS and graphene oxide (GO). Photocatalyst ZIS-100 can efficiently depolymerize organosolv lignin into phenols and ketones. The relative content of valuable compounds in the depolymerized product was increased by 2.5 times as compared that without photocatalyst. The photocatalyst can effectively break C_β-O bonds in 2-phenoxy-1-phenylethanol (PP-ol, a model compound) and the conversion of PP-ol is 93.27%. Mechanism studies show that the thiol groups on the surface of ZIS-100 play an important role in the formation of C_α radical intermediates. Photocatalytic cleavage of C_β-O bond mainly follows a one-step reaction mechanism through a self‑hydrogen transfer process. This study provides a new strategy for selectively breaking C_β-O bond in lignin to form valuable chemicals.

Strategy for Encapsulation of CdS Quantum Dots into Zeolitic Imidazole Frameworks for Photocatalytic Activity

Encapsulating CdS quantum dots (QDs) into zeolitic imidazole framework-8 (ZIF-8) can offer several advantages for photocatalysis. Various types of capping agents have been used to encapsulate QDs into ZIF-8 nanopores. An effective method for encapsulating CdS QDs into ZIF-8 is to use 2-mercaptoimidazole as the capping agent. This is because 2-mercaptoimidazole is similar to the imidazolate ligands of ZIFs and can used for capping active species with simultaneous encapsulation during the crystal growth of ZIF-8. Compared to other widely used capping agents such as polyvinylpyrrolidone (PVP), using 2-mercaptoimidazole for encapsulating CdS QDs into ZIF-8 revealed photocatalytic effects along with the molecular sieving effect when using differently sized molecular redox mediators such as methyl viologen (MV²⁺) and diquat (DQ²⁺).