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Photocatalytic transformations of lignocellulosic biomass into chemicals
Abstract
As the largest renewable carbon resource, lignocellulosic biomass has great potential to replace fossil resources for the production of high-value chemicals, in particular organic oxygenates. Catalytic transformations of lignocellulosic biomass using solar energy have attracted much recent attention, because of unique reactive species and reaction patterns induced by photo-excited charge carriers or photo-generated reactive species as well as the mild reaction conditions, which may enable the precise cleavage of target chemical bonds or selective functionalisation of specific functional groups with other functional groups kept intact. Here, we present a critical review on recent advances in the photocatalytic transformation of lignocellulosic biomass with an emphasis on photocatalytic cleavage of C–O and C–C bonds in major components of lignocellulosic biomass, including polysaccharides and lignin, and the photocatalytic valorisation of some key platform molecules. The key issues that control the reaction paths and the reaction mechanism will be discussed to offer insights to guide the design of active and selective photocatalytic systems for biomass valorisation under mild conditions. The challenges and future opportunities in photocatalytic transformations of lignocellulosic biomass are also analysed.
... Photocatalytic reforming of renewable biomass has attracted much interest [1][2][3]. Compared to temperature-activated chemo-catalytic processes, photocatalytic reactions occur at ambient atmospheric pressure and temperature [4][5][6]. Hydrogen fuel can be produced from water decomposition when the biomass photoreforming is conducted in an aqueous system to produce valuable chemicals [7,8]. ...
... Based on the ratio of H 13 CO + (m/z = 30) to H 12 CO + (m/z = 29), the final formate product was 56% from HCO 3 − and 44% from xylose. With NaH 12 CO 3 − 12 C 5 -xylose to produce acetate, the lower part of Fig. 3b presents the acetate molecular ion ( 12 CH 3 12 COOH •+ ) with m/z = 60 and fragments of 12 COOH + (m/z = 45), 12 COO + (m/z = 44), and 12 CH 3 12 CO + (m/z = 43). With NaH 13 CO 3 − 12 C 5 -xylose, the acetate spectrum (the upper part of Fig. 3b) CO + (m/z = 45) and low intensity of 12 CH 3 12 CO + (m/z = 43) observed in the reaction with NaH 13 CO 3 indicate that HCO 3 − was the primary source for the acetate formation. ...
... With NaH 12 CO 3 − 12 C 5 -xylose to produce acetate, the lower part of Fig. 3b presents the acetate molecular ion ( 12 CH 3 12 COOH •+ ) with m/z = 60 and fragments of 12 COOH + (m/z = 45), 12 COO + (m/z = 44), and 12 CH 3 12 CO + (m/z = 43). With NaH 13 CO 3 − 12 C 5 -xylose, the acetate spectrum (the upper part of Fig. 3b) CO + (m/z = 45) and low intensity of 12 CH 3 12 CO + (m/z = 43) observed in the reaction with NaH 13 CO 3 indicate that HCO 3 − was the primary source for the acetate formation. As to the product ethylene glycol, Fig. 3c shows no mass shift for the product when replacing NaH 12 CO 3 with NaH 13 CO 3 to interact with xylose, indicating that xylose is the sole carbon source for ethylene glycol. ...
... It has been proposed that the photogenerated hole centers on oxide surfaces (O 2− + h + À! O •− ) or hydroxyl radicals (•OH) in the presence of H 2 O may be responsible for activating the C−H bond in CH 4 under mild conditions, forming •CH 3 for C−C coupling to C 2 H 6 as the major product 13 . However, these active oxygen species with strong oxidation ability are also known to cause uncontrollable oxidation reactions 12,13,22 , lowering the formation rate of C 2 compounds 12-14 . As a result, the C 2 formation rate can hardly exceed Nature Communications | (2024) 15:4453 1 1234567890():,; 1234567890():,; ...
... It has been proposed that the photogenerated hole centers on oxide surfaces (O 2− + h + À! O •− ) or hydroxyl radicals (•OH) in the presence of H 2 O may be responsible for activating the C−H bond in CH 4 under mild conditions, forming •CH 3 for C−C coupling to C 2 H 6 as the major product 13 . However, these active oxygen species with strong oxidation ability are also known to cause uncontrollable oxidation reactions 12,13,22 , lowering the formation rate of C 2 compounds [12][13][14] . As a result, the C 2 formation rate can hardly exceed 100 µmol g −1 h −1 over most of the photocatalysts reported to date 14 . ...
Photocatalytic coupling of methane to ethane and ethylene (C2 compounds) offers a promising approach to utilizing the abundant methane resource. However, the state-of-the-art photocatalysts usually suffer from very limited C2 formation rates. Here, we report our discovery that the anatase TiO2 nanocrystals mainly exposing {101} facets, which are generally considered less active in photocatalysis, demonstrate surprisingly better performances than those exposing the high-energy {001} facet. The palladium co-catalyst plays a pivotal role and the Pd²⁺ site on co-catalyst accounts for the selective C2 formation. We unveil that the anatase {101} facet favors the formation of hydroxyl radicals in aqueous phase near the surface, where they activate methane molecules into methyl radicals, and the Pd²⁺ site participates in facilitating the adsorption and coupling of methyl radicals. This work provides a strategy to design efficient nanocatalysts for selective photocatalytic methane coupling by reaction-space separation to optimize heterogeneous-homogeneous reactions at solid-liquid interfaces.
... Photocatalytic biorefining (PBR), which uses biomass instead of oil to produce energy and chemicals and is powered by photoenergy, is a potential sustainable route for providing carbon resources and green energy vectors 1 . Most PBR research focuses on treating biomass as a source of sacrificial hole reagents, and aims to use solar energy for H 2 generation from water or chemical fuel production from CO 2 (ref. ...
... There are many diverse photocatalysts for PBR, because biomass comprises a wide range of chemical structures. However, the structureactivity relationships of synthesized photocatalysts can be elucidated, as most photocatalysts for PBR can be classified in one of two categories: (1) photocatalysts that boost interfacial charge transfer (ICT) to accelerate PBR and (2) photocatalysts that control the reaction paths of radical intermediates for selective PBR. Biorefineries should aim to comprehensively utilize solid biomass, with the generation of one or two products 7,8 , and this requires reconstruction of the carbon-chain structure of the biomass. ...
... Photocatalysis is another approach that has emerged as a promising method for selective lignin upgrading under mild conditions [108]. Photoexcitation of catalysts produces charge carriers (electrons and holes) or excited chemicals species with high redox ability that can serve as reductants/oxidants and facilitate the selective transformation of lignin. ...
... Lignocellulose biomass is a bountiful and sustainable resource composed of cellulose, hemicellulose, and lignin, possesses substantial potential for the generation of hydrogen fuel through the utilization of photocatalytic methods . In comparison to the traditional means of hydrogen production involving fossil fuels or water splitting, this approach provides numerous advantages Wu et al., 2020;Mergbi et al., 2023). However, the recalcitrance of lignocellulose poses challenges to its efficient conversion. ...
C − H bond activation is a ubiquitous reaction that remains a major challenge in chemistry. Although semiconductor-based photocatalysis is promising, the C − H bond activation mechanism remains elusive. Herein, we report value-added coupling products from a wide variety of biomass and fossil-derived reagents, formed via C − H bond activation over zinc-indium-sulfides (Zn-In-S). Contrary to the commonly accepted stepwise electron-proton transfer pathway (PE-ET) for semiconductors, our experimental and theoretical studies evidence a concerted proton-coupled electron transfer (CPET) pathway. A pioneering microkinetic study, considering the relevant elementary steps of the surface chemistry, reveals a faster C − H activation with Zn-In-S because of circumventing formation of a charged radical, as it happens in PE-ET where it retards the catalysis due to strong site adsorption. For CPET over Zn-In-S, H abstraction, forming a neutral radical, is rate-limiting, but having lower energy barriers than that of PE-ET. The rate expressions derived from the microkinetics provide guidelines to rationally design semiconductor catalysis, e.g., for C − H activation, that is based on the CPET mechanism.
Photocatalytic carbon‐carbon (C─C) coupling of benzyl alcohol is a promising means to coproduce the value‐added chemicals with H2 but is generally subject to low efficiency in terms of photon utilization. Here, efficient benzyl alcohol C─C coupling is achieved over Zn2In2S5 containing a tunable content of Zn vacancies (VZn). The VZn tends to form shallow defect states below the conduction band that can expedite photocarrier separation by collecting the photo‐generated electrons. The VZn‐collected electrons are essential for a high selectivity of the C─C coupling reactions because they enable a fast elimination of the byproduct benzaldehyde by catalyzing its reduction back to the ketyl radicals. Under simulated sunlight, the VZn‐containing Zn2In2S5 accomplishes ≈100% conversion of benzyl alcohol for merely 1 h and attains ≈100% selectivity for the C─C coupling compounds for 2 h, delivering an apparent quantum yield as high as 7.7% at 420 ± 20 nm. The benefits of VZn have also been verified by the theoretical calculations that indicate reduced energy barriers for various surface reactions in the presence of VZn. This work brings fresh mechanistic insights into the role of VZn and can serve as a useful guideline in the design of efficient photocatalysts.
The hydrogenative rearrangement of bioderived furfural derivatives to various cyclopentanone derivatives is vital for synthesizing fine chemicals but challenging owing to the complex cascade reaction network. Herein, a noble metal–free...
Lignin is an underutilized lignocellulosic component of biomass and often called waste, is a rich reservoir of fuel and aromatic chemicals. Although there has been a lot of research in...
Lignin, a crucial component of lignocellulosic biomass, holds immense promise for advancing biorefineries and producing sustainable energy alternatives. This study engineered a CN/rGO/BMO(2:1) heterojunction photocatalyst with varying Pd NPs loading...
Lignocellulose as a form of biomass is inedible. It represents a renewable feedstock for the synthesis of chemicals and materials. Its utilization has become an area of growing interest. Of lignocellulose components, lignin is comparatively under-explored and under-utilized, despite its abundance. This Focus Review recognizes this missed opportunity and presents a concise overview on some of the most recent progress involving the generation and application of functional materials derived from lignin. Between the two commonly encountered forms of lignin, technical lignin is a by-product of the paper production industry and is highly processed under harsh conditions. As such, it has generally been used for filler and resin materials. By comparison, native lignin is rich in chemical functionalities and holds great promise for downstream chemical synthesis. In recognition of these potentials, “lignin-first” strategies have emerged to directly convert native lignin to building blocks rich in functional groups, such as alcohols and carbonyls, while maintaining the integrity of the aromatic structures in lignin. The lignin-first strategy complements the already well explored field of technical lignin utilization. These chemoselective, lignin-first methods promise routes to native lignin valorization into high-value building blocks while keeping cellulose and hemicellulose intact and, therefore, are particularly appealing. This Focus Review first recognizes the importance of the traditional strategies for technical lignin utilization and highlights some of the newest developments. It then puts an emphasis on these lignin-first approaches for improved native lignin utilizations.
This review comprehensively explores the integration of photocatalytic (PC) and photoelectrocatalytic (PEC) technologies for hydrogen production via water splitting, in synergy with the efficient conversion of biomass resources. This synergistic strategy significantly boosts hydrogen production rates in both PC and PEC systems and enhances solar energy conversion efficiency. Furthermore, it enables selective conversion of cost‐effective biomass via precise modulation of catalyst structures, paving a novel pathway for green hydrogen generation and synthesis of high‐value chemicals. Specifically, the manuscript highlights recent progress in the oxidation and coupling reactions of biomass derivatives, such as methanol, ethanol, glycerol, and 5‐hydroxymethylfurfural, in PC/PEC systems. It reveals their potential to enhance hydrogen production efficiency and the synthesis of high‐value organic compounds. Future research should focus on employing in‐situ characterization techniques and theoretical simulations to deeply understand reaction mechanisms, thus advancing the field of biomass conversion through photocatalytic/photoelectrocatalytic methods.
Furfural (mainly furfural and 5-hydroxymethylfurfural) and its derivatives stand out as an important platform for C5 and C6 compounds from lignocellulosic biomass. Catalytic valorization of these renewable carbohydrates not only...
Photocatalysis is an effective environment-friendly technique that has emerged as a viable option for the degradation of a wide range of contaminants. In most cases, natural sunlight has been used in photocatalysis which makes it cost-effective. Also, it has emerged as an efficient method for degrading harmful toxic chemicals which are hard to degrade by any other alternatives. Various metals and metal compounds like oxides, sulphides, and carbides are used as photocatalysts to increase the efficiency of degradation. The common metal compounds known as ceramics are ZnO, TiO2, MoS2, ZnS, Fe2O3, SiC, CdS, WO3, SrTiO3, SnO2, NiO, and ZrO2. These materials are also known as semiconductors and are used as effective photocatalysts to degrade harmful and toxic environmental pollutants. This review discusses various ceramic photocatalysts, their application, photocatalysis mechanism, and degradation of various organic and inorganic compounds in actual wastewater. The use of photocatalysts in the treatment of persistent organic and inorganic pollutants in wastewater, such as pharmaceutical compounds, pesticides, dyes, oil, microplastics, and heavy metals is also explored.
Controlling the number of oxygen vacancies ( V o s) in metal‐oxide nanoparticles (MO NPs; TiO 2 , SnO 2 , and CeO 2 ) using Li‐ions has been shown to endow MO NPs with improved photocatalytic and electrocatalytic properties. Increasing the number of V o s in nanostructures provides a foundation for efficient multi‐use catalyst designs that facilitate biomass oxidation reactions and improve electrocatalytic activity. Herein, the trends in the properties of MO‐based catalysts are investigated with varying amounts of V o s achieved by Li‐ion deposition. The controlled introduction of V o s into MO NPs tunes the band structures and surface chemistry, leading to enhanced activities of the NPs as photocatalysts for the selective oxidation of 2,5‐hydroxymethylfurfural and as electrocatalysts for acidic hydrogen evolution. It is believed that Li‐ion deposition in the MO matrix provides a novel approach for optimizing oxide‐based catalyst defect engineering, thereby enabling more efficient biomass conversion and water splitting. These findings contribute significantly to the field of multi‐purpose catalysis and are expected to inspire new catalyst designs for a more sustainable future. The results provide a pathway for the facile and efficient engineering of surface defect structures on catalytic metal oxide NPs toward designing high‐performance photocatalysts.
The reductive catalytic fractionation (RCF) of lignocellulose, considering lignin valorization at design time, has demonstrated the entire utilization of all lignocellulose components; however, such processes always require catalysts based on precious metals or high‐loaded nonprecious metals. Herein, the study develops an ultra‐low loaded, atomically dispersed cobalt catalyst, which displays an exceptional performance in the RCF of lignocellulose. An approximately theoretical maximum yield of phenolic monomers (48.3 wt.%) from lignin is realized, rivaling precious metal catalysts. High selectivity toward 4‐propyl‐substituted guaiacol/syringol facilitates their purification and follows syntheses of highly adhesive polyesters. Lignin nanoparticles (LNPs) are generated by simple treatment of the obtained phenolic dimers and oligomers. RCF‐resulted carbohydrate pulp are more obedient to enzymatic hydrolysis. Experimental studies on lignin model compounds reveal the concerted cleavage of Cα–O and Cβ–O pathway for the rupture of β‐O‐4 structure. Overall, the approach involves valorizing products derived from lignin biopolymer, providing the opportunity for the comprehensive utilization of all components within lignocellulose.
Lignin can be transformed into aromatic feedstock chemicals through photocatalysis, serving as an alternative to fossil resources, thereby meeting the growing demand for alternative energy resources. Herein, we introduce a...
Photocatalytic lignocellulose reforming for H 2 production presents a compelling solution to solve environmental and energy issues. However, achieving scalable conversion under benign conditions faces consistent challenges including insufficient active sites for H 2 evolution reaction (HER) and inefficient lignocellulose oxidation directly by photogenerated holes. Herein, it is found that Pt single atom‐loaded CdS nanosheet (Pt SA ‐CdS) would be an active photocatalyst for lignocellulose‐to‐H 2 conversion. Theoretical and experimental analyses confirm that the valence band of CdS shifts downward after depositing isolated Pt atoms, and the slope of valence band potential on pH for Pt SA ‐CdS is more positive than Nernstian equation. These characteristics allow Pt SA ‐CdS to generate large amounts of •OH radicals even at pH 14, while the capacity is lacking with CdS alone. The employment of •OH/OH ⁻ redox shuttle succeeds in relaying photoexcited holes from the surface of photocatalyst, and the •OH radicals can diffuse away to decompose lignocellulose efficiently. Simultaneously, surface Pt atoms, featured with a thermoneutral , would collect electrons to expedite HER. Consequently, Pt SA ‐CdS performs a H 2 evolution rate of 10.14 µmol h ⁻¹ in 1 m KOH aqueous solution, showcasing a remarkable 37.1‐fold enhancement compared to CdS. This work provides a feasible approach to transform waste biomass into valuable sources.
Eleven years ago, an important summary of the valorization of biomass (Tuck et al., Science 337:695–699, 2012, https://doi.org/10.1126/science.1218930) appeared. This milestone paper gave a new impulse to biomass research. The goal of the present work was to analyze by means of scientific literature statistics the main parameters of the evolution of thoughts, ideas, and results induced by this paper in a 10-year period following its publication (from August 2012 to August 2022).
The inclusion of colloidal particles into 3D porous constructs to realize heterostructure ensembles enhances surface interactions and energy transfer that can transform the catalytic properties of conventional catalysts. However, assembling hydrophobic colloidal nanoparticles within hydrophilic 3D porous matrices to create tailored heterostructures and catalytic properties is particularly challenging. Here, a modified ligand grafting method is presented to spontaneously assemble CdS inorganic colloidal quantum dots within a metal‐organic framework (MOF) derived porous framework for efficient photocatalytic CO2 reduction. The grafted long chain molecular ligands effectively suppress phase separation and endow a molecular‐recognition effect to form ordered assembly microstructures. The proof‐of‐concept assembly of CdS and Prussian blue analogues (PBA) derived Co3O4 facilitates a high density of heterojunctions formation, improving charge transfer and extended lifetimes of photo‐induced electrons. Consequently, CdS/Co3O4 assembly exhibits exceptionally active and stable photocatalytic CO2 reduction activity, with a CO production rate of 73.9 µmol g⁻¹ h⁻¹ and CO selectivity of 98.9%. Importantly, the ligand grafting method can be generalized to achieve spontaneous assembly of quantum dots within various metal organic framework derived porous scaffolds. This work provides a rational strategy for precise self‐assembly of colloidal nanoparticles within porous microstructures, allowing tailored heterointerfaces for functional materials and applications.
The conversion of the biomass into eco‐friendly fuels and chemicals has been extensively recognized as the essential pathway to achieve the sustainable economy and carbon neutral society. Lignin, as a kind of promising biomass energy, has been certified to produce the high‐valued chemicals and fuels. Numerous efforts have been made to develop various catalysts for lignin catalytic conversion. Both metal‐organic frameworks (MOFs) and covalent organic frameworks (COFs) belong to very important heterogeneous porous catalysts due to their regular porous structures, high specific surface area, and precisely tailored diversities. In the review, the first part focused on the catalytic conversion of lignin, lignin model compounds, and lignin derivatives using the pristine MOFs, functional MOF composites, and MOF‐derived materials. The second part summarized the catalytic conversion of lignin model compounds using pristine COFs and functional COF composites. The review here mainly concentrated on the design of the materials, screening of catalytic conditions, and explorations of the corresponded mechanisms. Specifically, (1) we summarized the MOF‐ and COF‐based materials for the effects on the catalytic transformation of lignin‐related substances; (2) we emphasized the catalytic mechanism of C–C and C–O bonds cleavage together with the structure–activity relationships; (3) we in‐depth realized the relationship between the chemical/electronic/structural properties of the MOF‐ and COF‐based catalysts and their catalytic performance for lignin‐related substances. Finally, the challenges and future perspectives were also discussed on the catalytic conversion of lignin‐related substances by MOF‐ and COF‐based catalysts.
The synthesis of valuable organic compounds from naturally available and renewable biomass is an open field of research towards adaptation in real-life applications. Photocatalytic valorization is assumed as a potential candidate, although the lower efficiency of the traditional batch photocatalytic reactor sets some drawbacks. Recently, photocatalytic microreactors revealed as a prosperous candidate for various photocatalytic reactions, especially for selective oxidation. This area of research is challenging due to the development of the proper photocatalytic microreactor for the targeted application. Deposition of the catalyst on the internal surface of the microreactor, the sufficient utilization of the irradiation, optimization of the reaction parameters are among the most vital parameters that should be considered upon the design. Although, to obtain the most active material and tune its crucial features to maximize its catalytic performance inside the microreactors is the uppermost important part. This work introduces ultrasound-assisted TiO2 deposition on the inner walls of a perfluoroalkoxyalkane microtube under mild conditions. The deposition experiments were carried out with commercial and sol-gel synthesized TiO2. The materials were characterized by XRD, UV–vis DRS, Scanning Electron Microscopy (SEM), and nitrogen sorption. The photocatalytic activities of the TiO2 nano-engineered fluoropolymer based microreactors were evaluated for the oxidation of benzyl alcohol in flow.
Methanol is a clean liquid energy carrier of sunshine and a key platform chemical for the synthesis of olefins and aromatics. Herein, we report the conversion of biomass-derived polyols and sugars into methanol and syngas (CO+H2) via UV light irradiation under room temperature, and the bio-syngas can be further used for the synthesis of methanol. The cellulose and even raw wood sawdust could be converted into methanol or syngas after hydrogenolysis or hydrolysis pretreatment. We find Cu dispersed on titanium oxide nanorod (TNR) rich in defects is effective for the selective C−C bond cleavage to methanol. Methanol is obtained from glycerol with a co-production of H2. A syngas with CO selectivity up to 90% in the gas phase is obtained via controlling the energy band structure of Cu/TNR.
Lignin is a major component of lignocellulosic biomass. Although it is highly recalcitrant to break down, it is a very abundant natural source of valuable aromatic carbons. Thus, the effective valorisation of lignin is crucial for realising a sustainable biorefinery chain. Here, we report a compartmented photo-electro-biochemical system for unassisted, selective, and stable lignin valorisation, in which a TiO2 photocatalyst, an atomically dispersed Co-based electrocatalyst, and a biocatalyst (lignin peroxidase isozyme H8, horseradish peroxidase) are integrated, such that each system is separated using Nafion and cellulose membranes. This cell design enables lignin valorisation upon irradiation with sunlight without the need for any additional bias or sacrificial agent and allows the protection of the biocatalyst from enzyme-damaging elements, such as reactive radicals, gas bubbles, and light. The photo-electro-biochemical system is able to catalyse lignin depolymerisation with a 98.7% selectivity and polymerisation with a 73.3% yield using coniferyl alcohol, a lignin monomer.
Lignin is the most abundant source of renewable aromatics. Catalytic valorization of lignin into functionalized aromatics is attractive but challenging. Photocatalysis is a promising sustainable approach. The strategies for designing well‐performing photocatalysts are desired but remain limited. Herein, a facile energy band engineering strategy for promoting the photocatalytic activity of zinc–indium–sulfide (ZnmIn2Sm+3) for cleavage of the lignol β‐O‐4 bond under mild conditions was developed. The energy band structure of ZnmIn2Sm+3 could be tuned by controlling the atomic ratio of Zn/In. It was found that Zn4In2S7 performed best for cleavage of the β‐O‐4 bond under visible‐light irradiation, owing to its appropriate energy band structure for offering adequate visible‐light absorption and suitable redox capability. Functionalized aromatic monomers with near 18.4 wt % yield could be obtained from organosolv birch lignin. Mechanistic studies revealed that the β‐O‐4 bond was efficiently cleaved mainly through a one‐step redox‐neutral pathway via a Cα radical intermediate. The thiol groups on the surface of Zn4In2S7 played a key role in cleavage of the β‐O‐4 bond.
The degradation of lignin into aromatic products is very important, but harsh conditions and metal‐based catalysts are commonly needed to cleave the inert bonds. Herein, an efficient self‐initiated radical photochemical degradation for lignin‐derived aryl ethers through ionic liquids (ILs) induction is demonstrated. The C−C/C−O bonds can be cleaved efficiently through free‐radical‐mediated reaction in the binary‐ILs system 1‐propenyl‐3‐methylimidazolium bis[(trifluoromethyl)sulfonyl] imide [PMim][NTf2] and the Brønsted acid 1‐propylsulfonic‐3‐methylimidazolium trifluoromethanesulfonate ([PrSO3HMim][OTf]) under ambient conditions. [PMim][NTf2] initiates the reaction by promoting the cleavage of the Cβ−H bond, and [PrSO3HMim][OTf] catalyzes the subsequent C−O−C bond fragmentation. Furthermore, alkyl, hydroxyl, and peroxy radicals are detected, which suggests degradation based on a photochemical free‐radical process. Additionally, alkali lignin could also be degraded in the IL system. This work sheds light on sustainable biomass utilization through a self‐initiated radical photochemical strategy under metal‐free and mild conditions.
The efficient utilization of lignocellulosic biomass has tremendous potential to reduce the excessive dependence on fossil fuels. Here, we provide an overview on the recent achievements in the catalytic production of value-added chemicals and fuels. When targeting chemicals, a key objective is to maximize the product selectivity to favor the subsequent separation. This can be achieved through the design of catalysts and optimization of catalytic systems based on the deep understanding of the catalytic mechanism. For production of fuels, attention should be paid to the establishment of an energy-efficient process for high-quality fuels. This can be realized through the design of C–C coupling reactions and the development of multifunctional catalysts to minimize the reaction steps from lignocellulose to fuels. In addition, the full utilization of lignocellulose into biofuels and chemicals in a single process is separately introduced. Finally, several personal perspectives on the opportunities and challenges within this promising field are discussed.
Photocatalytic hydrogen production from biomass is a promising alternative to water splitting thanks to the oxidation half-reaction being more facile and its ability to simultaneously produce solar fuels and value-added chemicals. Here, we demonstrate the coproduction of H2 and diesel fuel precursors from lignocellulose-derived methylfurans via acceptorless dehydrogenative C−C coupling, using a Ru-doped ZnIn2S4 catalyst and driven by visible light. With this chemistry, up to 1.04 g gcatalyst⁻¹ h⁻¹ of diesel fuel precursors (~41% of which are precursors of branched-chain alkanes) are produced with selectivity higher than 96%, together with 6.0 mmol gcatalyst⁻¹ h⁻¹ of H2. Subsequent hydrodeoxygenation reactions yield the desired diesel fuels comprising straight- and branched-chain alkanes. We suggest that Ru dopants, substituted in the position of indium ions in the ZnIn2S4 matrix, improve charge separation efficiency, thereby accelerating C−H activation for the coproduction of H2 and diesel fuel precursors.
The valorization of natural and renewable resources, like lignocellulosic biomass, towards value-added chemicals by low energy and economically viable processes still remains a global research and technological challenge. 5-hydroxymethylfurfural (HMF) is an important platform chemical that can be easily derived from biomass, and it can be further used as a feedstock for the production of building blocks for polymers or fuels. In this context, the partial oxidation of the hydroxyl group on the HMF molecule leads to the formation of the corresponding aldehyde, 2,5-diformylfuran (DFF), which may find multiple applications in bio-chemical industries. Herein, we present the synthesis and characterization of novel manganese (IV) oxide nanorods as catalyst for the HMF to DFF partial oxidation at ambient conditions. This 1D nanocatalyst operates at low energy light irradiation and without the addition of chemicals (bases or oxidants) as a highly selective photo-assisted catalyst. Under optimized experimental conditions, the HMF conversion was found to be above 99 %, while the DFF selectivity was almost 100 %. The presence of molecular O2 played a key role in triggering the selective oxidation, while the use of an aprotic and less polar organic solvent, such as acetonitrile, compared to water, further enhanced the reactivity of the catalyst.
Catalysis is an integral part of a majority of chemical operations focused on the generation of value‐added chemicals or fuels. Similarly, the extensive use of fossil‐derived fuels and chemicals has led to deterioration of the environment. Catalysis currently plays a key role in mitigating such effects. Thermal catalysis and photocatalysis are two well‐known catalytic approaches that were applied in both energy and environmental fields. Thermo‐photocatalysis can be understood as a synergistic effect of the two catalytic processes with key importance in the use of solar energy as thermal and light source. This Review provides an update on relevant contributions about thermo‐photocatalytic systems for environmental and energy applications. The reported activity data are compared with the conventional photocatalytic approach and the base of the photothermal effect is analyzed. Some of the systems based on the positive aspects of thermo‐ and photocatalysis could be the answer to the energy and environmental crisis when taking into account the outstanding results with regard to chemical efficiency and energy saving.
The lignin-first concept offers an opportunity to utilize the entire lignocellulosic biomass efficiently. However, current conversion strategies rely on high-temperature hydrogenolysis by supported metal catalysts, leading to low-functionalized products or difficulty in separation of solid catalyst from cellulose/hemicellulose. Here, we report the fractionation and valorization of lignocellulose via solar energy-driven conversion of native lignin at room temperature. We found that cadmium sulfide quantum dots not only catalyse the cleavage of β-O-4 bonds in lignin models quantitatively but also are efficient for the conversion of native lignin within biomass into functionalized aromatics under visible light, while cellulose/hemicellulose remain almost intact. Further, the colloidal character of quantum dots enables their facile separation and recycling by a reversible aggregation–colloidization strategy. The β-O-4 bond in lignin is cleaved by an electron–hole coupled photoredox mechanism based on a Cα radical intermediate, in which both photogenerated electrons and holes participate in the reaction. © 2018, The Author(s), under exclusive licence to Springer Nature Limited.
Efficient transformation of biomass to value-added chemicals and high-energy density fuels is pivotal for a more sustainable economy and carbon-neutral society. In this framework, developing potential cascade chemical processes using functionalised heterogeneous catalysts is essential because of their versatile roles towards viable biomass valorisation. Advances in materials science and catalysis have provided several innovative strategies for the design of new appealing catalytic materials with well-defined structures and special characteristics. Promising catalytic materials that have paved the way for exciting scientific breakthroughs in biomass upgrading are carbon materials, metal-organic frameworks, solid phase ionic liquids, and magnetic iron oxides. These fascinating catalysts offer unique possibilities to accommodate adequate amounts of acid-base and redox functional species, hence enabling various biomass conversion reactions in a one-pot way. This review therefore aims to provide a comprehensive account of the most significant advances in the development of functionalised heterogeneous catalysts for efficient biomass upgrading. In addition, this review highlights important progress ensued in tailoring the immobilisation of desirable functional groups on particular sites of the above-listed materials, while critically discussing the role of consequent properties on cascade reactions as well as on other vital processes within the bio-refinery. Current challenges and future opportunities towards a rational design of novel functionalised heterogeneous catalysts for sustainable biomass valorisation are also emphasized.
The development of new methods for the direct transformation of methanol into two or multi-carbon compounds via controlled carbon-carbon coupling is a highly attractive but challenging goal. Here, we report the first visible-light-driven dehydrogenative coupling of methanol into ethylene glycol, an important chemical currently produced from petroleum. Ethylene glycol is formed with 90% selectivity and high efficiency, together with hydrogen over a molybdenum disulfide nanofoam-modified cadmium sulfide nanorod catalyst. Mechanistic studies reveal a preferential activation of C-H bond instead of O-H bond in methanol by photoexcited holes on CdS via a concerted proton-electron transfer mechanism, forming a hydroxymethyl radical (⋅CH2OH) that can readily desorb from catalyst surfaces for subsequent coupling. This work not only offers an alternative nonpetroleum route for the synthesis of EG but also presents a unique visible-light-driven catalytic C-H activation with the hydroxyl group in the same molecule keeping intact.
Enantiopure vicinal amino alcohols and derivatives are essential structural motifs in natural products and pharmaceutically active molecules, and serve as main chiral sources in asymmetric synthesis. Currently known asymmetric catalytic protocols for this class of compounds are still rare and often suffer from limited scope of substrates, relatively low regio- or stereoselectivities, thus prompting the development of more effective methodologies. Herein we report a dual catalytic strategy for the convergent enantioselective synthesis of vicinal amino alcohols. The method features a radical-type Zimmerman-Traxler transition state formed from a rare earth metal with a nitrone and an aromatic ketyl radical in the presence of chiral N,N'-dioxide ligands. In addition to high level of enantio- and diastereoselectivities, our synthetic protocol affords advantages of simple operation, mild conditions, high-yielding, and a broad scope of substrates. Furthermore, this protocol has been successfully applied to the concise synthesis of pharmaceutically valuable compounds (e.g., ephedrine and selegiline).
A fast and efficient methodology for the selective oxidation of sugars into corresponding sodium aldonates is herein reported. Hydrogen peroxide was used as a cheap oxidant and electron scavenger, in the presence of only 0.003-0.006 mol % of gold in basic conditions. Three photocatalysts were studied, namely Au/Al2O3, Au/TiO2 and Au/CeO2, the latter being the most efficient (TOF > 750 000 h-1) and perfectly selective. Only 10 minutes exposition under standard incident sunlight irradiation (A.M.1.5G conditions - 100 mW/cm2) affords total conversion of glucose into the corresponding sodium gluconate. Demonstrating its versatility, this methodology was successfully applied to a variety of oligosaccharides leading to the corresponding aldonates in quantitative yield and high purity (>95%) without any purification step. The photocatalyst was recovered by simple filtration and re-used 5 times leading to the same conversion and selectivity after 10 min of illumination.
Lignocellulose, the main component of agricultural and forestry waste, harbours tremendous potential as a renewable starting
material for future biorefinery practices. However, this potential remains largely unexploited due to the lack of strategies that
derive substantial value from its main constituents. Here, we present a catalytic strategy that is able to transform lignocellulose
to a range of attractive products. At the centre of our approach is the flexible use of a non-precious metal catalyst in two distinct
stages of a lignocellulose conversion process that enables integrated catalyst recycling through full conversion of all process
residues. From the lignin, pharmaceutical and polymer building blocks are obtained. Notably, among these pathways are systematic
chemo-catalytic methodologies to yield amines from lignin. The (hemi)cellulose-derived aliphatic alcohols are transformed
to alkanes, achieving excellent total carbon utilization. This work will inspire the development of fully sustainable and
economically viable biorefineries.
Every twig and splinter used
Plant-based production of commodity chemicals faces steep competition from fossil resources, which are often cheaper and easier to partition. Sustainable use of renewable resources requires strategies for converting complex and recalcitrant biomolecules into streams of chemicals with extraordinary efficiency. Liao et al. developed a biorefinery concept in which wood is eventually fully converted into useful chemicals: phenol, propylene, pulp amenable to ethanol production, and phenolic oligomers that can be incorporated into ink production (see the Perspective by Zhang). A life-cycle assessment and techno-economic analysis highlight the efficiency of the process and reveal the potential for such biorefinery strategies to contribute to sustainable chemicals markets.
Science , this issue p. 1385 ; see also p. 1305
Herein, an environmentl-friendly CoP/Zn2In2S5 catalyst is reported as a visible-light photocatalyst for selective activation of the α-C−H bond in methanol to generate ethylene glycol with selectivity as high as 90%....
Photocatalytic H2 evolution from organic feedstocks with simultaneous utilization of photogenerated holes achieves solar energy storage and coproduces value-added chemicals. Here we show visible light H2 production from benzyl alcohol (BAL) with controllable generation of deoxybenzoin (DOB) or benzoin (BZ) through tandem redox reactions. Particularly, DOB synthesis circumvents the use of expensive feedstocks and environmentally unfriendly catalysts that are required previously. Under the irradiation of blue LEDs, the key of steering the major product to DOB rather than BZ is to decrease the conduction band bottom potentials of the ZnIn sulfide catalysts by increasing the Zn/In ratio, which results in the dehydration of intermediate hydrobenzoin (HB) to DOB proceeding in a redox-neutral mechanism and consuming an electron-hole pair. As a proof of concept, this method is used to synthesize DOB derivatives in gram scale.
Lignin is the largest carrier of aromatics on earth and its depolymerization can afford value-added aromatic products. Direct cleavage of the C−C bonds in lignin linkages is significant but challenging to obtain low-molecular-weight aromatic monomers. Herein, using vanadium catalysts under visible light, we selectively cleave the C–C bonds in β-1 and β-O-4 interlinkages occluded in lignin models and extracts by an oxidative protocol. Visible light irradiation triggered single electron transfer between the substrate and the catalyst, which further induced the selective Cα–Cβ bond cleavage and generated the final aromatic products through radical intermediates. Using this photocatalytic chemistry, the reactivity of lignin models and the selectivity of Cα–Cβ bond cleavage were significantly improved. More importantly, this protocol affords aromatic monomers through the fragmentation of organosolv lignins even at room temperature, indicating the potential of photocatalytic C–C bond cleavage of lignin linkages under ambient conditions.
Depolymerization of recalcitrant lignin is a crucial step in realizing the full potential of transforming biomass to value-added small organic molecules. Herein, we report a photocatalytic system consisting of ultrathin CdS nanosheets decorated with 1st-row transition metals (M/CdS) for the direct photocleavage of lignin model compounds to small aromatic products. In the meantime, the reducing power of M/CdS is utilized to simultaneously generate another valuable product H2, hence eliminating the necessity of sacrificial reagents and maximizing the energy conversion efficiency. We further demonstrate that by judiciously modulating the photocatalytic conditions, it is feasible to yield different products with very high selectivity using the same catalyst Ni/CdS. A series of control experiments were performed to investigate the mechanistic steps of each reaction and highlight the important roles played by both solvent and base in the photocleavage of the β-O-4 bond in lignin valorization.
A new composite photocatalyst ZnO/CoPzS8 was achieved by immobilizing cobalt tetra(2,3-bis(butylthio)maleonitrile) porphyrazine (CoPzS8) onto the surface of home prepared ZnO. The ZnO/CoPzS8 composite was well characterized by X-ray diffraction (XRD), UV-vis diffuse reflectance spectra (DRS), X-ray photoelectron spectroscopy (XPS) and Raman spectra. It turned out that CoPzS8 was successfully loaded onto the ZnO surface and there existed an interaction between ZnO and CoPzS8. By using ZnO/CoPzS8 composite photocatalyst, the photocatalytic oxidation of glucose to value-added chemicals in pure water was conducted under simulated sunlight irradiation without adding any acid or base. The ZnO/CoPzS8 composite exhibited a higher photocatalytic activity in comparison to pure ZnO and pure CoPzS8, owing to the synergistic effect between ZnO and CoPzS8. Gluconic acid, arabinose, glycerol and formic acid were all observed as oxidation products when pure ZnO or ZnO/CoPzS8 composite used. More importantly, glucaric acid was also obtained in the ZnO/CoPzS8 photocatalytic system, indicating that the presence of CoPzS8 changed the glucose reaction pathway. In addition, the active species generated in the photocatalytic process were further identified by electron spin resonance (ESR) technology and the scavenger experiment. The possible photocatalytic conversion pathway of glucose has been proposed according to the experimental results.
A homogeneous, redox-neutral photo fragmentation of diol derivatives was developed. Under Photo/HAT dual catalysis, diol derivatives such as lignin model compounds and diol monoesters undergo selective β C(sp3)—O bond cleavage...
It remains challenging to achieve the selective cleavage of C–C bonds in lignin or lignin model compounds to produce aromatic products in high yield and selectivity. We have developed a redox-neutral photocatalytic strategy to accomplish this goal in both β-O-4 and β-1 lignin models at room temperature (RT) via proton-coupled electron transfer (PCET) process without any pretreatments of substrate, by adjusting the alkalinity of base to obtain a lignin models/base PCET pair with a bond dissociation free energy close to 102 kcal/mol. Without breaking down Cβ–Cγ bond and any C–O bonds, this PCET method is 100% atom economy and produces exclusive Cα–Cβ bond cleavage products, such as benzaldehydes (up to 97%) and phenyl ethers (up to 96%), in high to excellent yields and selectivities. Preliminary studies indicated that the PCET strategy is also effective for the depolymerization of native lignin at RT, thus providing significantly important foundation to the depolymerization of lignin.
Plasmonic catalysis has drawn significant interest recently, as the catalysis can be driven by visible light. Here, we show a new tactic to apply low-flux visible-light irradiation on plasmonic metal nanoparticles (NPs) to initiate catalysis with surface-bound transition-metal complexes under mild conditions. Ni²⁺ complexes (as catalytic reaction sites) and Au or Ag NPs were immobilized on γ-Al2O3 nanofibers to produce plasmonic-antenna-promoted catalysts. The light irradiation on Au or Ag NPs enhanced photocatalytic activity of the Ni²⁺ complexes for reductive cleavage of C–O bond by 18-fold or 17-fold, respectively. The intense electromagnetic near-fields of the plasmonic metal NPs significantly increased the chemisorption of the reactant to the Ni²⁺ active sites. The light-excited hot electrons transfer via a molecular bridge of the aromatic ring of the reactants. The light-enhanced chemisorption plays a key role in this photocatalyst’s structure that comprises a plasmonic antenna and catalytically active metal complex sites.
The selective conversion of glucose into value-added chemicals in the presence of only water is a challenging topic. In this work, selective photocatalytic oxidation of glucose in water was studied using iron thioporphyrazine modified SnO2 (SnO2/FePz(SBu)8) as the catalyst and atmospheric air as the oxidant under simulated sunlight irradiation. It was found that value-added organic acids including glucaric acid, gluconic acid and formic acid could be obtained from the oxidation of glucose under such conditions. The effects of the FePz(SBu)8 content, glucose concentration and additional addition on the conversion of glucose and the selectivity of organic acid were further explored. Under the optimized conditions, the total selectivity for organic acids on SnO2/FePz(SBu)8 photocatalyst reached up to 52.2% at 34.2% glucose conversion. More importantly, it has been demonstrated that the presence of FePz(SBu)8 on the surface of SnO2 can keep the selectivity of organic acids unchanged under conditions of increasing the glucose conversion. To illustrate the synergistic effect for the enhanced photocatalytic activity between FePz(SBu)8 and SnO2, surface photocurrent, electron spin resonance (ESR) spectra and adsorption behavior were carried out on the pure SnO2 and SnO2/FePz(SBu)8. It was found that the introduction of FePz(SBu)8 could enhance the separation of photogenerated charge, promote the generation of active species for photocatalysis and improve the adsorption capacity of glucose, which are beneficial to the enhancement of photocatalytic activity. Additionally, a possible pathway of glucose oxidation was proposed through both detailed analysis of the oxidation intermediate of glucose and comparative experiments with different organic acids as the substrates, indicating that the formation of organic acid were fulfilled by two parallel and subsequent reactions at the beginning of reaction.
Manipulating surface organic ligands has emerged as a useful strategy to enhance the efficiency of heterogeneous catalysts. However, ligand-controlled catalysis for biomass valorization remains unexplored due to the limited knowledge of the ligand functions in heteroge-neous catalytic systems involving macromolecules. Here, we report a strong dependence of photocatalytic conversion of native lignin into high-value functionalized aromatics on the ligand attached on cadmium sulfide quantum dots (QDs). The formation of QD colloi-dal solution by tuning the hydrophilicity/hydrophobicity of ligands is essential to intimate contact between QDs and lignin, leading to high catalytic activity. The surface engineering by manipulating the anchor group and the length of ligands demonstrates that the ligand participates in the electron transfer process, a crucial step of photocatalysis. The electron-decay kinetic study reveals a ligand-mediated electron-tunneling mechanism. These insights offer useful guidance for the design of efficient QD photocatalysts for biomass valorization through ligand modification.
Biomass is a naturally abundant, sustainable and clean resource, which has great potential to replace a portion of the finite petroleum and fossil feedstock for sustainable production of value-added chemicals and fuels. However, an efficient conversion process is still difficult to be achieved due to the complex nature of biomass. Recently, the simple, mild, and environmentally benign photocatalytic process appears to be a new research avenue for lignocellulosic biomass transformation. This review provides insights into the state-of-the-art accomplishments in photocatalytic conversion of lignocellulosic biomass and its derivatives, including selective cleavage of dominant bonds of lignin, valorization of processed and native lignin, photo-reforming reactions of cellulose and its intermediates, and the depolymerization of robust native lignocellulose under visible or UVA light irradiations. In addition, electricity production from photocatalytic conversion of biomass is also discussed as an innovative lignocellulosic biomass transformation process. We then put forward perspectives for photocatalytic conversion of native lignocellulose and future challenges to increase the profitability and sustainability of photocatalysis biorefinery system.
A new iron tetra(2,3-bis(butylthio)maleonitrile)porphyrazine (FePz(SBu)8) has been synthesized, then it was loaded on H-ZSM-5 zeolite to obtain a supported biomimetic photocatalyst H-ZSM-5/FePz(SBu)8. Using H2O2 as oxidant, the photocatalytic selective oxidation of glucose in water under visible light (λ ≥ 420 nm) irradiation was carried out in presence of H-ZSM-5/FePz(SBu)8. Under such conditions, the glucose can be efficiently converted into value-added chemicals such as glucaric acid, gluconic acid, arabinose, glycerol and formic acid. More importantly, in comparison with pure FePz(SBu)8 and pure H-ZSM-5 zeolite, the H-ZSM-5/FePz(SBu)8 exhibited a higher photocatalytic activity for glucose oxidation and the formation of glucaric acid was observed only when H-ZSM-5/FePz(SBu)8 was used, deriving from the synergistic effect between FePz(SBu)8 and H-ZSM-5 zeolite. Some reaction parameters of glucose oxidation catalyzed by the H-ZSM-5/FePz(SBu)8 were discussed, such as loading amount of FePz(SBu)8, H2O2:glucose ratio, glucose concentration, and so on. It was demonstrated that the Soret-band of FePz(SBu)8 contributed more to the visible light photocatalytic activity than the Q-band during the photocatalytic process. The stability of H-ZSM-5/FePz(SBu)8 during the photocatalytic process was further evaluated by the reusability test. In addition, the generation of reactive oxygen species was determined by electron spin resonance (ESR) technology and scavenger experiments. A possible reaction pathway of glucose oxidation was also discussed.
Traditionally, conventional heat has been required for a large proportion of the oxidation and degradation process to utilise lignin from biomass. A photocatalysis system which is considered as a novel and green strategy for chemical reactions has been applied and photo-MnO2 catalytic lignin oxidation method has been developed. In this study, we investigated a promising photocatalytic heterogeneous system for lignin oxidation. Recyclable MnO2 which is readily available, and blue light which is harzardless light are used in this system. 1-Phenylethanol was used as a model compound to study the suitable conditions for this system. After optimizing the reaction conditions, organosolv lignin, kraft lignin, and alkali lignin were applied to this system and showed the successful oxidation of lignins and further degradation.
A new approach to synthesize valuable 3,4-dialkoxyanilines and alkyl propionates from lignin-derived 4-propylguaiacol and -catechol with overall isolated yields up to 65% has been described. The strategy is based on the introduction of nitrogen via a Beckmann rearrangement. Amino introduction therefore coincides with a C-defunctionalisation reaction; overall a replacement of the propyl chain by an amino group is obtained. The process only requires cheap bulk chemicals as reagents/reactants and does not require column chromatography to purify the reaction products. Furthermore, all carbon atoms from the biorenewable lignin-derived monomers are transformed into valuable compounds. Greenness was assessed by performing a Green Metrics analysis on two dialkoxyanilines. A comparison was made with literature routes for these compounds starting from a petrochemical substrate.
Lignin, which is a highly cross-linked and irregular biopolymer, is nature's most abundant source of aromatic compounds and constitutes an attractive renewable resource for the production of aromatic commodity chemicals. Herein, we demonstrate a practical and operationally simple two-step degradation approach involving Pd-catalyzed aerobic oxidation and visible-light photoredox-catalyzed reductive fragmentation for the chemoselective cleavage of the β-O-4 linkage - the predominant linkage in lignin - for the generation of lower-molecular-weight aromatic building blocks. The developed strategy affords the β-O-4 bond cleaved products with high chemoselectivity and in high yields, is amenable to continuous flow processing, operates at ambient temperature and pressure, and is moisture- and oxygen-tolerant.
Heterogenous photocatalysis in microflow system for generation of value added chemicals is a novel green chemistry approach requiring the understanding of photocatalysis, microfluidics and reactor design. Research in the development of low energy and environmental friendly based photo-microreactor system for photocatalysis is yet to be explored. Commercial ZnO nanoparticles were deposited in the inner wall of the fluoropolymer using low energy ultrasound bath at mild conditions and later used for selective conversion of aromatic alcohol to aldehyde. The deposition of the nanoparticles in the fluoropolymer is mainly attributed to the physical changes taking place inside the microtubes under the effect of ultrasound
WO3/g-C3N4 (graphitic carbon nitride) composite was prepared via a directly calcination method with ammonium tungstate hydrate and melamine as raw materials, and applied in the photocatalytic oxidation of 5-hydroxymethylfurfural (5-HMF) to 2,5-diformylfuran (DFF) for the first time. Its physicochemical properties were characterized by HR(TEM), XRD, FT-IR, XP, UV-vis, PL, PC and EIS methods. The results revealed that the doping of WO3 into g-C3N4 can reduce the possibility of charge recombination and thus enhancing the photocatalytic activity. The impact factor, such as incident wavelength, reaction time, and solvent were also studied on the photocatalytic behavior. 4.7%WO3/g-C3N4 composite catalyst exhibits the highest 5-HMF conversion of 27.4% with DFF selectivity of 87.2% under the irradiation of visible-light (>400 nm). As the active species, [rad]O2⁻ and h⁺ plays an important role to convert 5-HMF to DFF. Finally, the Z-scheme mechanism and reaction route were proposed by combining the photocatalytic performance with the active oxygen species research of the WO3/g-C3N4 catalyst.
Lignin model compounds are degraded through aerobic oxidation upon treatment with light and copper, which operate in relay achieving considerably better efficiency and different product distribution in comparison to thermal copper‐catalyzed aerobic degradation.
The cleavage of C-O bond in lignin β-O-4 model compounds to form aromatics has been achieved via a two-step process, comprising visible-light photocatalytic oxidation and in situ carbonic acid-facilitated hydrogenolysis. In the first step, with readily available persulfate as radical initiator and cheap copper as catalyst, the secondary alcohol in the β-O-4 alkyl-aryl ether linkage is selectively oxidized to the corresponding ketone in up to 99% yield under visible light irradiation. The second step features the C-O bond cleavage of lignin β-O-4 ketones promoted by in situ acidic EtOH/H2O/CO2 system in the presence of zinc powder, producing acetophenones and phenols in high yield. This protocol provides a novel alternative to selective fragmentation of β-O-4 linkage to aromatic monomers under mild reaction conditions.
One prominent goal of the 21st century is to develop a sustainable carbon‐neutral biorefinery. Lignin is an important component of lignocellulosic biomass; however, it is currently underutilized due to its highly crosslinked, complex and randomly polymerized composition that cause a significant challenge to its depolymerization and valorization. Chemical catalytic approaches based on transition metals represent the primary research area to drive the degradation reactions. Recently, alternative photocatalytic strategies that employ the sustainable solar energy to initiate the transformation of lignin have started to emerge. This Concept article examines the new development of photocatalyzed reactions and the insights on the C‐O and C‐C bond cleavage reactions of lignin models in both homogeneous and heterogeneous systems.
Environmental-friendly utilization and conversion of abundant lignocellulosic biomass waste into chemical fuels is considered as one of the efficient approaches developing renewable energy. Herein, we merge photocatalysis and acid hydrolysis to solve problems in biomass conversion and hydrogen production. Specifically, photocatalytic reforming cellulose to hydrogen has been accomplished by hydrolysis of cellulose in 0.6 M sulfuric acid solution at 403 K in the presence of a photocatalyst (e.g. platinized TiO2) under UV-light irradiation. Carbohydrates as the sacrificial electron donors are in situ generated via acid hydrolysis of cellulose. An unexpected production of 5-hydroxyl methyl furfural (HMF) has also been found during the combined process. In addition, efficient and repeatable hydrogen production has been achieved from photoreforming of raw biomass waste (paper pulp) as continuous supply of electron donors. Compared to traditional thermochemical gasification or pyrolysis of cellulose in high temperature and pressure, this work provides an alternative approach for producing hydrogen and valuable chemicals from lignocellulosic biomass.
Cellulose represents the major component of the abundant and inedible lignocellulosic biomass on earth. The valorization of cellulose into liquid biofuels and high value-added bio-based chemicals has drawn intensive attentions in recent years. However, because of the rigid structure of crystalline cellulose, the breakage of β-1,4-glycosidic bonds, the first step of cellulosic biomass utilization is still a critical challenge under mild conditions. Herein, we report the cleavage of β-1,4-glycosidic bonds of cellobiose on Ir/HY catalyst with high activity and high selectivity (>99%) under visible light illumination at temperature not exceeding 100 °C. We found that the hydrolysis of cellobiose under mild condition is mainly owing to a cooperation effect between the Ir nanoparticles as the plasmonic photothermal source and acid catalysis of HY zeolite. This work provides a distinctive, sustainable pathway to efficiently convert cellulose to chemicals driven by solar energy under mild conditions.
A novel Ir(ppy)2(bpy) complex-containing mesoporous cellular silica foams (Ir(ppy)2(bpy)-MCFs) was prepared by a facile thiol-ene click reaction between the prefabricated thiol-functionalized mesoporous cellular silica foams and vinyl-tagged [Ir(ppy)2(bpy)]PF6 complex. The elaborate Ir(ppy)2(bpy)-MCFs material possessed the large surface area (355 cm²/g), open foam-like mesoporous structure with 30 nm cell pore size and 3.0 nm window pore size. Importantly, it had the well-defined molecular configuration of Ir(ppy)2(bpy) active species. As expected, it exhibited excellent catalytic reactivity and selectivity in the visible-light-driven reductive depolymerization of oxidized lignin β-O-4 model compounds including p-hydroxyphenyl (H)-, guaiacyl (G)- and syringyl (S)-type units under the mild conditions. Meanwhile, it showed the comparable catalytic efficiency with the corresponding homogeneous [Ir(ppy)2(bpy)]PF6 catalyst. These high catalytic performances could be attributed to aerogel-like three-dimensional pore structure and high visible-light transparency, which efficiently decreased the mass transfer resistance and the photon propagation hindrance. Furthermore, it could be easily recycled and reused at least six times without the remarkable loss of catalytic activity.
In the presence of a palladium-loaded TiO2 photocatalyst, cleavage of benzyl phenyl ether in low-molecular-weight alcohol solvents under de-aerated conditions afforded toluene and phenol simultaneously in a 1:1 molar ratio.
Cellulose is a promising renewable and abundant resource for the production of high‐value chemicals, in particular organic oxygenates, because of its high O/C ratio. The sustainable production of hydroxyl carboxylic acids and dicarboxylic acids, such as gluconic/glucaric acid, lactic acid, 2,5‐furandicarboxylic acid, adipic acid and terephthalic acid, most of which are monomers of key polymers, have attracted much attention in recent years. The synthesis of these organic acids from cellulose generally involves several tandem reaction steps, and thus multifunctional catalysts that can catalyze selective activation of specific C‐O or C‐C bonds hold the key. This review highlights recent advances in the development of efficient catalytic systems and new strategies for the selective conversion of cellulose or its derived carbohydrates into functionalized organic acids. The reaction mechanism is discussed to offer deep insights into the regioselective cleavage of C‐O or C‐C bonds.
Photocatalysis is a potentially promising approach to harvest aromatic compounds from lignin. However, the development of an active and selective solid photocatalyst is still challenging for lignin transformation under ambient conditions. We herein report a mild photocatalytic oxidative strategy for C−C bond cleavage of lignin β-O-4 and β-1 linkages using a mesoporous graphitic carbon nitride catalyst. Identifications by solid-state NMR techniques and DFT calculations indicate that π-π stacking interactions are most likely present between the flexible carbon nitride surface and lignin model molecule. Besides, low charge recombination efficiency and high specific surface area (206.5 m2 g-1) of the catalyst also contribute to its high catalytic activity. Mechanistic investigations reveal that photogenerated holes, as the main active species, trigger the oxidation and C–C bond cleavage of lignin models. This study sheds light on the interaction between complex lignin structures and the catalyst surface, and provides a new strategy of photocatalytic cleavage of lignin models with heterogeneous photocatalysts.
The innovative combination of photocatalysis and biomass utilization represents a new and promising approach to achieve a higher grade of sustainability in chemical processes. A growing number of publications deal with topics like biomass conversion to solar fuels and the selective production of fine chemicals from waste. Despite the recalcitrant structure of several biological waste streams, which hampers the technical processing, huge progress has been achieved by the use of photocatalytic systems. This review analyzes recent examples of this promising field and investigates their potential for large-scale applications. Overall, the major critic is the lack of mechanistic investigations hampering the development of photocatalytic systems for biomass conversion. Therefore, this review represents a guideline, emphasizing the strategy and mechanistic considerations for the technical application of sustainable photocatalytic and photochemical reactions.
Today, there is a general consensus in academic literature that second generation biomass is the only valuable carbohydrate resource for non-food applications. In the meantime, fermentation of sugars towards bio-ethanol, mainly form first generation biomass, is becoming widespread at an industrial level. This paper tries to investigate if there can be a valuable role for edible resources (here: sugars) in the chemical industry without affecting the food security. Moreover, a connection is proposed between a broad range of multiple technologies and sustainable resources, with main attention to the native C skeleton and functionality of biomass. Going deeper in sustainability, this paper selected four criteria, taking into account the entire valorisation route, to apply to the most promising carbohydrate-derived molecules. By doing this, the most promising chemicals and working points from a sustainable point of view are highlighted.
Cellulosic biomass is the largest source of renewable organic carbon on our planet. Cellulose accounts for 40–50 wt % of this lignocellulose, and it is a feedstock for industrially important chemicals and fuels. The first step in cellulose conversion involves its depolymerization to glucose or to its hydrogenated product sorbitol. The hydrolysis of cellulose to glucose by homogeneous mineral acids was the subject of research for almost a century. However, homogeneous acids have significant drawbacks and are neither economical nor environmentally friendly. In 2006, our group reported for the first time the ability of heterogeneous catalysts to depolymerize cellulose through hydrolytic hydrogenation to produce sorbitol. Later, we reported the hydrolysis of cellulose to glucose using carbon catalyst containing weakly acidic functional groups. Understanding the reaction between cellulose and heterogeneous catalyst is a challenge as the reaction occurs between a solid substrate and a solid catalyst.
Biomass-based aldehydes, especially the sugar-derived furfural (FF), 5-hydroxylmethylfurfural (HMF), and lignin-derived vanillin, are abundant and renewable, but their downstream applications often require further catalytic upgrading into chain-extended intermediates for the production of useful biofuels and renewable materials. This review article focuses on the catalytic upgrading processes that involve C–C bond forming, chain-extension coupling reactions of FF, HMF, and vanillin, as well as subsequent transformations of the resulting intermediates into value-added biofuels and polymeric materials. The topics are organized according to the three major types of the catalysts commonly employed for such upgrading and transformations: organic catalysts, metal-based catalysts, and recyclable supported catalysts.
Lignin, a major component of lignocellulose, is the largest source of aromatic building blocks on the planet and harbors great potential to serve as starting material for the production of biobased products. Despite the initial challenges associated with the robust and irregular structure of lignin, the valorization of this intriguing aromatic biopolymer has come a long way: recently, many creative strategies emerged that deliver defined products via catalytic or biocatalytic depolymerization in good yields. The purpose of this review is to provide insight into these novel approaches and the potential application of such emerging new structures for the synthesis of biobased polymers or pharmacologically active molecules. Existing strategies for functionalization or defunctionalization of lignin-based compounds are also summarized. Following the whole value chain from raw lignocellulose through depolymerization to application whenever possible, specific lignin-based compounds emerge that could be in the future considered as potential lignin-derived platform chemicals.
In pursuit of more sustainable and competitive biorefineries, the effective valorisation of lignin is key. An alluring opportunity is the exploitation of lignin as a resource for chemicals. Three technological biorefinery aspects will determine the realisation of a successful lignin-to-chemicals valorisation chain, namely (i) lignocellulose fractionation, (ii) lignin depolymerisation, and (iii) upgrading towards targeted chemicals. This review provides a summary and perspective of the extensive research that has been devoted to each of these three interconnected biorefinery aspects, ranging from industrially well-established techniques to the latest cutting edge innovations. To navigate the reader through the overwhelming collection of literature on each topic, distinct strategies/topics were delineated and summarised in comprehensive overview figures. Upon closer inspection, conceptual principles arise that rationalise the success of certain methodologies, and more importantly, can guide future research to further expand the portfolio of promising technologies. When targeting chemicals, a key objective during the fractionation and depolymerisation stage is to minimise lignin condensation (i.e. formation of resistive carbon–carbon linkages). During fractionation, this can be achieved by either (i) preserving the (native) lignin structure or (ii) by tolerating depolymerisation of the lignin polymer but preventing condensation through chemical quenching or physical removal of reactive intermediates. The latter strategy is also commonly applied in the lignin depolymerisation stage, while an alternative approach is to augment the relative rate of depolymerisation vs. condensation by enhancing the reactivity of the lignin structure towards depolymerisation. Finally, because depolymerised lignins often consist of a complex mixture of various compounds, upgrading of the raw product mixture through convergent transformations embodies a promising approach to decrease the complexity. This particular upgrading approach is termed funneling, and includes both chemocatalytic and biological strategies.
http://pubs.rsc.org/en/content/articlelanding/2018/cs/c7cs00566k#!divAbstract
Efficient cleavage of C–O bonds in lignin and its models is of great importance for the production of value-added aromatic compounds. A simple TiO2 photochemical approach was applied for β-O-4 ketone hydrogenolysis by an electron transfer photocatalytic reaction. The conversion of the substrate 2-(2-methoxyphenoxy)-1-(4-methoxyphenyl) ethanone was 89.8% in 30 min, with the yield of products 4-methoxy acetophenone of 75.2% and o-methoxyphenol of 98.9%. Compared to the selective hydrogenolysis role of TiO2, Pd/TiO2 catalyst prefers to simultaneously catalyzing the hydrogenative cleavage of C–O ether bond and the hydrogenation of C[dbnd]O to CH–OH.