Sustainable Productions of Organic Acids and Their Derivatives from Biomass via Selective Oxidative Cleavage of C–C Bond

Visible-Light Promoted Iron-Catalyzed C-C Bond Cleavage of 1,2-Diols to Carbonyls

A simple visible-light-promoted iron-catalyzed aerobic oxidative C-C bond cleavage of vicinal diols was developed. This reaction avoids the use of noble metal catalysts or specialized oxidants, yielding aldehydes and ketones without overoxidation. The new method works under air and at room temperature and features mild conditions and simple operation. Notably, the protocol is applicable for complex natural products, achieving the bioinspired conversion of the natural abundant diterpene oridonin to the natural rare enmein-type diterpene.

Enhancement of Selective Catalytic Oxidation of Lignin β-O-4 Bond via Orbital Modulation and Surface Lattice Reconstruction

The orbital modulation and surface lattice reconstruction represent an effective strategy to regulate the interaction between catalyst interface sites and intermediates, thereby enhancing catalytic activity and selectivity. In this study, the crystal surface of Au-K/CeO2 catalyst can undergo reversible transformation by tuning the coordination environment of Ce, which enables the activation of the Cβ-H bond and the oxidative cleavage of the Cβ-O and Cα-Cβ bonds, leading to the cleavage of 2-phenoxy-1-phenylethanol. The t2g orbitals of Au 5d hybridize with the 2p orbitals of lattice oxygen in CeO2 via π-coordination, modulating the coordination environment of Ce 4 f and reconstructing the lattice oxygen in the CeO2 framework, as well as increasing the oxygen vacancies. The interface sites formed by the synergy between Au clusters in the reconstructed Ce-OL1-Au structure and doped K play dual roles. On the one hand, it activates the Cβ-H bond, facilitating the enolization of the pre-oxidized 2-phenoxy-1-phenylethanone. On the other hand, through single-electron transfer involving Ce3+ 4f1 and the adsorption by oxygen vacancies, it enhances the oxidative cleavage of the Cβ-O and Cα-Cβ bonds. This study elucidates the complex mechanistic roles of the structure and properties of Au-K/CeO2 catalyst in the selective catalytic oxidation of lignin β-O-4 bond.

Photothermal Catalysis of Cellulose to Prepare Levulinic Acid-Rich Bio-Oil

As a carbon-neutral and renewable raw material, cellulose can be transformed into biomass fuels to reduce the dependence on fossil fuels and carbon dioxide emissions. In view of harsh reaction conditions, low selectivity of product, and easy deactivation of the catalyst, this study studied the use of photothermal catalytic technology to convert cellulose into bio-oil rich in levulinic acid. It was discovered that a synergistic effect between heating and photocatalysis is present in cellulose degradation. Different metals were loaded on carbon nanotubes doped with titanium dioxide to prepare different photothermal catalysts, and their catalytic effects on cellulose were compared. It was found that TiO2-CNT loaded with platinum metal exhibited the highest catalytic performance. By adopting Pt/TiO2-CNT as the catalyst, the conversion rate of bio-oil reached 99.44%, and the selectivity of LA reached 44.41% at 220 °C for 3 h. As the photothermal catalysis increased the H/C ratio and decreased the O/C ratio of the liquid product, the calorific value reached 21.01 MJ/kg. This study can promote the further industrial application of lignocellulose to prepare fuel oil and decrease the environmental pollution caused by the massive consumption of fossil fuels.

Selective Transformation of Biomass and the Derivatives for Aryl Compounds and Hydrogen via Visible-Light-Induced Radicals

ConspectusFor sustainable development, exploring renewable resources is an urgent priority. Nonfood biomass, one of the largest renewable resources on Earth, primarily comprises three key components: lignin (ca. 15-30%), cellulose (ca. 35-50%), and hemicellulose (ca. 20-30%). Theoretically, nonfood biomass can be converted into green chemicals and energy. However, most studies have focused on the generation of chemicals and carbon-based energy under harsh conditions, often resulting in lower selectivities. Therefore, further efforts to explore efficient and selective methods for producing chemicals and hydrogen (H2) are essential to promoting the practical applications of renewable biomass. In this Account, we summarize our contributions to the efficient and selective transformation of biomass and its derivatives into aryl compounds and H2. These transformations were achieved using visible-light-induced photocatalytic systems that generate active radicals to selectively cleave C-C, C-O, C-H, and O-H bonds under mild conditions, without using noble metals. First, aryl compound production was achieved by chemoselective cleavage of C-C and C-O bonds using aryl carboxyl radicals and aryl ether radical cations. Specifically, the aryl carboxyl radical in the charge-transfer complex induced the chemoselective cleavage of C-C bonds of aryl carboxylic acids, which are platform molecules derived from lignin oxidation; the aryl carboxyl radical in free form facilitated the chemoselective cleavage of C-O bonds in the model of the 4-O-5 lignin linkage. Moreover, arenols and aryl alcohols were obtained via cooperation of the aryl ether radical cation and the vanadate-induced chemoselective cleavage of the C-O bonds of the models of various lignin linkages. Second, we developed a streamlined strategy for H2 production from biomass using a one-pot, two-step route with formic acid (HCO2H) as an intermediate for H2 storage by thermocatalysis. Using this strategy by photoredox catalysis, HCO2H was initially obtained via the alkoxy radical-induced gradual cleavage of C-C bonds in cellulose, hemicellulose, glucose, and their derivatives. Subsequently, efficient H2 production from biomass-based HCO2H was realized via hydroxyl radical (·OH)-induced C-H and the following cleavage of the O-H bonds, with cooperation of the nickel catalysis. Third, the highest H2 production capability from biomass was achieved via efficient water reforming. This process utilized alkoxy radicals followed by generated carbon cations via electrocatalysis, inducing a well-organized cleavage of C-C, O-H, and C-H bonds. We anticipate that these insights will inspire the development of more efficient, stable, and cost-effective catalytic systems, accelerating the utilization of biomass as a renewable resource and driving other related significant transformations.

Sustainable production of organic acids from chitin biomass catalyzed by Keggin-type heteropolyacid under hydrothermal condition

The "shell biorefinery," which valorizes the shell waste chitin into fine chemicals, has developed rapidly in recent years. Herein, we present a novel base-free heteropolyacid-catalyzed oxidation method for the transformation of chitin biomass into organic acid. After a series of optimization experiments, a 5.93 % yield of formic acid and 25.09 % yield of acetic acid were achieved in the presence of 0.5 equivalent of Mo-V-P heteropolyacids (H4PMo11VO40·2H2O) and air at 180 °C under hydrothermal conditions for 4 h. Meanwhile, we have demonstrated that the Keggin-type heteropolyacid catalysts are capable of efficiently converting microcrystalline chitin into organic acids. The synthesized heteropolyacids are well characterized with FT-IR, XRD, ICP-AES, and TGA. The possible reaction pathway was speculated accordingly. This method offers several advantages, including readily available raw materials, simple operation, and relatively higher yield.

Metal-Free Visible-Light Induced Oxidative Cleavage of C(sp3 )-C, and C(sp3 )-N Bonds of Nitriles, Alcohols, and Amines

Selective cleavage of unstrained (sp3 ) C-C/ C-N bonds under mild conditions is highly challenging due to the higher bond dissociation energy. A visible light mediated metal-free oxidative dehomologation of aryl acetonitriles, primary alcohols and diols to carboxylic acids via organophotocatalyzed C(sp3 )-CN, C(sp3 )-C(OH) bond cleavage is reported. Notably, this methodology was further extended towards selective synthesis of aldehydes via deamination of both primary as well as secondary amines. This mild protocol features wide array of substrate variation with excellent functional group tolerance, preparative-scale synthesis, and operational simplicity. Possible mechanisms for these transformations were demonstrated through a series of control experiments.

Low-Temperature Upcycling of Polypropylene Waste into H2 Fuel via a Novel Tandem Hydrothermal Process

Plastic waste is a promising and abundant resource for H2 production. However, upcycling plastic waste into H2 fuel via conventional thermochemical routes requires relatively considerable energy input and severe reaction conditions, particularly for polyolefin waste. Here, we report a tandem strategy for the selective upcycling of polypropylene (PP) waste into H2 fuel in a mild and clean manner. PP waste was first oxidized into small-molecule organic acids using pure O2 as oxidant at 190 °C, followed by the catalytic reforming of oxidation aqueous products over ZnO-modified Ru/NiAl2 O4 catalysts to produce H2 at 300 °C. A high H2 yield of 44.5 mol/kgPP and a H2 mole fraction of 60.5 % were obtained from this tandem process. The entire process operated with almost no solid residue remaining and equipment contamination, ensuring relative stability and cleanliness of the reaction system. This strategy provides a new route for low-temperature transforming PP and improving the sustainability of plastic waste disposal processes.

Catalytic Conversion of Lignin into Valuable Chemicals: Full Utilization of Aromatic Nuclei and Side Chains

ConspectusIn recent years, significant efforts have been directed toward achieving efficient and mild lignocellulosic biomass conversion into valuable chemicals and fuels, aiming to address energy and environmental concerns and realize the goal of carbon neutrality. Lignin is one of the three primary building blocks of lignocellulose and the only aromatic renewable feedstock. However, the complex and diverse nature of lignin feedstocks, characterized by their three-dimensional, highly branched polymeric structure and intricate C-O/C-C chemical bonds, results in substantial challenges. To tackle these challenges, we carried out extensive research on selectively activating and transforming chemical bonds in lignin for chemical synthesis. In this Account, we discuss our recent progress in catalytic lignin conversion.Our work is focused on two main objectives: (i) achieving precise and selective transformation of C-O/C-C bonds in lignin (and its model compounds) and (ii) fully utilizing the aromatic nuclei and side chains present in lignin to produce valuable chemicals. Lignin consists of interconnected phenylpropanoid subunits linked by interlaced C-C/C-O bonds. To unlock the full potential of lignin, we propose the concept of "the full utilization of lignin", which encompasses both the aromatic nuclei and the side chains (e.g., methoxyl and polyhydroxypropyl groups).For the conversion of aromatic nuclei, selective activation of C-O and/or C-C bonds is crucial in synthesizing targeted aromatic products. We begin with model compounds (such as anisole, phenol, guaiacol, etc.) and then transition to protolignin feedstocks. Various reaction routes are developed, including self-supported hydrogenolysis, direct Caryl-Csp3 cleavage, coupled Caryl-Csp3 cleavage and Caryl-O hydrogenolysis, and tandem selective hydrogenation and hydrolysis processes. These tailored pathways enable high-yield and sustainable production of a wide range of aromatic (and derived) products, including arenes (benzene, toluene, alkylbenzenes), phenols, ketones, and acids.In terms of side chain utilization, we have developed innovative strategies such as selective methyl transfer, coupling depolymerization-methyl shift, selective acetyl utilization, and new activation methods such as amine-assisted prefunctionalization. These strategies enable the direct synthesis of methyl-/alkyl-derived products, such as acetic acid, 4-ethyltoluene, dimethylethylamine, and amides. Additionally, aromatic residues can be transformed into chemicals or functionalized ingredients that can serve as catalysts or functional biopolymer materials. These findings highlight promising opportunities for harnessing both the aromatic nuclei and side chains of lignin in a creative manner, thereby improving the overall atom economy of lignin upgrading.Through innovative catalyst engineering and reaction route strategies, our work achieves the sustainable and efficient production of various valuable chemicals from lignin. By integrating side chains and aromatic rings, we have successfully synthesized methyl-/alkyl-derived and aromatic-derived products with high yields. The full utilization of lignin not only minimizes waste but also opens up new possibilities for generating chemical products from lignin. These novel approaches unlock the untapped potential of lignin, expand the boundaries of lignin upgrading, and enhance the efficiency and economic viability of lignin biorefining.

Photoinduced Biomimetic Room-Temperature C-O Bond Cleavage over Mn Doped CdS

Selective C-O bond cleavage is an efficient way for the biomass valorization to value-added chemicals, but is challenged to be operated at room temperature via conventional thermal catalysis. Herein, inspired from the DNA biosynthesis which involves a radical-mediated spin-center shift (SCS) C-O bond cleavage process, we report a biomimetic room-temperature C-O bond cleavage of vicinal diol (HOCHCH-OH). We construct a Mn doped CdS (Mn/CdS) as a photocatalyst to mimic the biologic SCS process. The Mn site plays pivotal role: (1) accelerates the photo-induced carrier separation, promoting the hole-mediated C-H bond cleavage to generate carbon-centered radicals, and (2) serves as the binding site for -OH groups, making it to be an easier leaving group. Mn/CdS achieves 0.28 mmol gcat -1  h-1 of hydroxyacetone (HA) from glycerol dehydration at room temperature under visible light irradiation, which is 3.5-fold that over pristine CdS and 40-fold that over bulk MnS/CdS. This study provides a new biomimetic room-temperature C-O bond cleavage process, which is promising for the biomass valorization.

Catalytic conversion of cellulosic biomass to harvest high-valued organic acids

Catalytic conversion of biomass provides an alternative way for the production of organic acids from renewable feedstocks. The emerging process contains complex reactions and strategies to cut down those complex biogenic materials into target molecules. Here, we review the catalytic conversion of cellulosic biomass toward high-valued organic acids. This work has summarized the key controlling reactions which lead toward formic acid, glycolic acid, or sugar acids in oxidative conditions and the main pathways for lactic acid or levulinic acid in the anaerobic environment from cellulosic biomass and its derivatives. We evaluate and compare different strategies and methods such as one-pot and two-step conversion. Additionally, the optimization of catalytic reactions has been discussed to realize the design of C-C coupling reactions, the development of multifunctional materials, and new efficient system. In all, this article gives an insight guide to precisely convert cellulosic biomass into target organic acids.

Solvent-Free Aerobic Oxidative Cleavage of Methyl Oleate to Biobased Aldehydes over Mechanochemically Synthesized Supported AgAu Nanoparticles

The performance of mechanochemically synthesized supported bimetallic AgAu nanoalloy catalysts was evaluated in the oxidative cleavage of methyl oleate, a commonly available unsaturated bio-derived raw material. An extensive screening of supports (SiO2 , C, ZrO2 , Al2 O3 ), metallic ratios (Ag : Au), reaction times, temperatures, and use of solvents was carried out. The performance was optimized towards productivity and selectivity for the primary cleavage products (aldehydes and oxoesters). The optimal conditions were achieved in the absence of solvent, using Ag8 Au92 /SiO2 as catalyst, at 80 °C, reaction time of 1 h, substrate to catalyst=555 and 10 bar of molecular oxygen. A strong support effect was observed: the selectivity to aldehydes was best with silica as support, and to esters was best using zirconia. This shows not only that mechanochemical preparation of bimetallic catalysts is a powerful tool to generate useful catalyst compositions, but also that a safe, green, solventless synthesis of bio-derived products can be achieved by aerobic oxidative cleavage.

Phase-dependent selectivity control over TiO2 in the photocatalytic oxidation of bio-polyols

Photocatalytic oxidation shows great potential in the valorization of biomass under mild conditions, while the selectivity control is particularly challenging for the complex and reactive bio-polyols. Herein, we report a selective photocatalytic process to convert bio-polyols into formic acid (FA) or carbon monoxide (CO) by controlling the phase of TiO2. The bio-polyols are facially oxidized to formic acid (FA) which is stable over rutile and could be dehydrated to CO over anatase TiO2. Through controlling the phase, FA or CO could be obtained from a wide range of bio-polyols with selectivity up to 63% or 52%. Our studies elucidate that the phase-dependent selectivity is essentially derived from the difference in the adsorption configuration of FA. In situ Fourier transform infrared spectroscopy (FTIR) and density functional (DFT) calculations were used to study the FA decomposition process on the surface of TiO2. The phase-dependent FA decomposition is mainly derived from the different surface geometry, which affects the configuration of FA adsorption. Molecular adsorbed FA on anatase favors the dehydration of FA to CO while bidentate dissociated adsorption of FA on the rutile phase is inert to be further converted. This work provides a new horizon to the design of photocatalytic systems for biomass conversion.

Recent progress of low-temperature plasma technology in biorefining process

In recent years, low-temperature plasma-assisted processes, featuring high reaction efficiency and wide application scope, have emerged as a promising alternative to conventional methods for biomass valorization. It is well established that charged species, chemically energetic molecules and radicals, and highly active photons playing key roles during processing. This review presents the major applications of low-temperature plasma for biomass conversion in terms of (i) pretreatment of biomass, (ii) chemo fractionation of biomass into value-added chemicals, and (iii) synthesis of heterogeneous catalyst for further chemo-catalytic conversion. The pretreatment of biomass is the first and foremost step for biomass upgrading to facilitate raw biomass transformation, which reduces the crystallinity, purification, and delignification. The chemo-catalytic conversion of biomass involves primary reactions to various kinds of target products, such as hydrolysis, hydrogenation, retro-aldol condensation and so on. Finally, recent researches on plasma-assisted chemo-catalysis as well as heterogeneous catalysts fabricated via low-temperature plasma at relatively mild condition were introduced. These catalysts were reported with comparable performance for biomass conversion to other state-of-the-art catalysts prepared using conventional methods.

Photocatalytic Production of Syngas from Biomass

ConspectusAs a renewable solar energy and carbon carrier, biomass exploration has received global attention. Photocatalytic valorization of biomass into fuels and chemicals is a promising and sustainable method for future chemical production. Photocatalysis has the potential to accomplish reactions under ambient conditions due to the unique reaction mechanisms involving photoinduced charge carriers and has recently been recognized as an efficient and feasible technology for biomass conversion. Biomass is widely used as sacrificial agent to scavenge holes in photocatalytic hydrogen evolution, and the carbon is eventually degraded to CO₂ with a minor amount of CO. The generation of CO instead of CO₂ is more economical and promising but also a challenge under photoreforming conditions.This is a new research direction, while until now there has still been the lack of a comprehensive review article to summarize and provide prospects for this topic. This Account will highlight our contributions in the research direction of the photocatalytic reforming of biomass into syngas (CO + H₂). In 2020, we first reported the photocatalytic conversion of biopolyols and sugars into syngas by employing a defect-rich Cu-TiO₂ nanorod photocatalyst and found that formic acid is a key intermediate to CO. Further study revealed that a facet-dependent electron-trapping state on anatase TiO₂ will affect the photocatalytic dehydration activity for formic acid intermediates by regulating the electron transfer process during the reaction, and the selective generation of FA or CO from photocatalytic biomass reforming was achieved via exposing the (100) or (101) facets, respectively. Visible light-driven syngas generation was further achieved over a CdS-based photocatalyst. Sulfate modification of CdS ([SO₄]/CdS) was constructed as the proton acceptor, thus efficiently facilitating the proton-coupled electron transfer process. Besides, we put forward an oxygen-controlled strategy to increase the CO generation rate without a significant decrease in CO selectivity via controlling the O₂/substrate ratio. Based on this system, a Z-scheme CdS@g-C₃N₄ core-shell structure and CdO-CdS semicoherent interface were created to facilitate charge transfer and enhance the O₂ activation, thus increasing the CO generation rate. Moreover, we also developed a photoelectrochemical approach to separately produce CO and H₂ from biomass. Nitrogen doping of a hexagonal WO₃ nanowire array was used to produce the photoanode. The built-in electric field generated via nitrogen doping promoted charge transfer, hence improving the efficiency of PEC reforming of biopolyols and sugars. This Account will systematically analyze the challenges in this research direction, the reaction route in the photocatalytic biomass reforming, and the factors affecting CO selectivity and give insight into the design of efficient photocatalytic systems.

Oxidative Cleavage of β-O-4 Linkage in Lignin via Co Nanoparticles Embedded in 3DNG as Catalyst

The cleavage of β-O-4 linkage in lignin is one of the key steps for oxidative conversion of lignin to low-molecular-weight aromatics. Herein, Co nanoparticles embedded in three-dimensional network of nitrogen-doped graphene (Co/NG@3DNG-X) were prepared through an immersion-pyrolysis procedure, in which X denotes the pyrolysis temperature. The detailed characterization of Co/NG@3DNG-X shows that the Co nanoparticles are coated with a few layers of nitrogen-doped graphene (NG) sheets that are further embedded in 3DNG matrix. The catalytic activities of the Co/NG@3DNG-X for the oxidative cleavage of β-O-4 linkage in lignin model compounds with O₂ as oxidant are explored. It is demonstrated that catalytic activities of as-prepared Co/NG@3DNG-X can be tuned by varying the pyrolysis condition, and the Co/NG@3DNG-900 shows the highest catalytic activity, which is attributed to the enriched Co-N_x species, the strong surface basicity, the high specific surface and the mesoporous motif of 3DNG network. More pronouncedly, the Co/NG@3DNG-900 can also effectively catalyze the oxidative cleavage of organosolv lignin, generating certain monomeric aromatics. Additionally, the intrinsic magnetic property of Co nanoparticles makes the Co/NG@3DNG-X be easily recovered from the reaction mixture, and the as-coated thin NG layer can protect Co nanoparticle from oxidation condition, which putting together afford the Co/NG@3DNG-X with good reusability and stability.

Preparation of Cellulose Nanofibers from Corn Stalks by Fenton Reaction: A New Insight into the Mechanism by an Experimental and Theoretical Study

Agricultural biomass wastes are an abundant feedstock for biorefineries. However, most of these wastes are not treated in the right way. Here, corn stalks (CSs) were assigned as the raw material to produce cellulose nanofibers (CNFs) via in situ Fenton oxidation treatment. In order to probe the formation mechanism of an in situ Fenton reactor, the bonding interaction of hydrated Fe²⁺ ions and fiber has been systemically studied based on adsorption experiments, IR spectroscopy, density functional theory (DFT) calculations, and Raman spectroscopy. The results indicate that the coordination of the hydrated Fe²⁺ ion to the fiber generates a quasi-octahedral-coordinated sphere around the Fe center. The Jahn-Teller distortion effect of the Fe center promotes the Fe-O₂H₂ bonding interaction via reduction of the energy gap of the d_z² orbital of the Fe center and π_2py/π_2pz orbitals of the H₂O₂ molecule. The oxidation treatment of the pretreated CS by the in situ Fenton process shows the formation of a new carboxyl group on the fiber surface. The scanning electron microscopy image shows that the Fenton-treated fiber was scattered into the nanosized CNFs with a diameter of up to 50 nm. Both experimental and theoretical studies show that the pseudo-first-order kinetic reaction could describe the in situ Fenton kinetics well. Moreover, the proposed catalytic cycle shows that the large thermodynamic barrier is the cleavage of the O-O bond of H₂O₂ to generate the ^•OH radical, and the whole catalytic cycle is found to be spontaneous at room temperature.

Simultaneous Generation of Methyl Esters and CO in Lignin Transformation

Lignin is an abundant renewable carbon source. Due to its complex structure, utilization of lignin is very challenging. Herein, we describe an efficient strategy for the simultaneous utilization of lignin, in which the methoxy groups in lignin react with carboxylic acids to generate methyl carboxylates and the other alkyl and phenyl carbons react with oxygen to predominantly form CO that can be used directly in carbonylation reactions. The method was applied to the methylation of various functionalized aryl and alkyl carboxylic acids, including natural compounds, to produce valuable chemicals, including pharmaceuticals. No solid or liquid residues remain after the reaction. Mechanistic studies demonstrate that a well-ordered C-C and C-O bond activation sequence takes place to realize total transformation of lignin. This work opens a way for transformation of the entire lignin polymer into valuable products, exemplified by the synthesis of the pharmaceutical, Ramipril, on a gram scale.

Waste Biomass Selective and Sustainable Photooxidation to High-Added-Value Products: A Review

Researchers worldwide seek to develop convenient, green, and ecological production processes to synthesize chemical products with high added value. In this sense, lignocellulosic biomass photocatalysis is an excellent process for obtaining various outcomes for the industry. One issue of biomass transformation via heterogeneous catalysis into valuable chemicals is the selection of an adequate catalyst that ensures high conversion and selectivity at low costs. Titanium oxide (TiO2), is widely used for several applications, including photocatalytic biomass degradation, depolymerization, and transformation. Graphite carbon nitride (g-C3N4) is a metal-free polymeric semiconductor with high oxidation and temperature resistance and there is a recent interest in developing this catalyst. Both catalysts are amenable to industrial production, relatively easy to dope, and suited for solar light absorption. Recent investigations also show the advantages of using heterojunctions, for biomass derivates production, due to their better solar spectrum absorption properties and, thus, higher efficiency, conversion, and selectivity over a broader spectrum. This work summarizes recent studies that maximize selectivity and conversion of biomass using photocatalysts based on TiO2 and g-C3N4 as supports, as well as the advantages of using metals, heterojunctions, and macromolecules in converting cellulose and lignin. The results presented show that heterogeneous photocatalysis is an interesting technology for obtaining several chemicals of industrial use, especially when using TiO2 and g-C3N4 doped with metals, heterojunctions, and macromolecules because these modified catalysts permit higher conversion and selectivity, milder reaction conditions, and reduced cost due to solar light utilization. In order to apply these technologies, it is essential to adopt government policies that promote the use of photocatalysts in the industry, in addition to encouraging active collaboration between photooxidation research groups and companies that process lignocellulosic biomass.

Single-Particle Insights into Plasmonic Hot Carrier Separation Augmenting Photoelectrochemical Ethanol Oxidation with Photocatalytically Synthesized Pd-Au Bimetallic Nanorods

Understanding the nature of hot carrier pathways following surface plasmon excitation of heterometallic nanostructures and their mechanistic prevalence during photoelectrochemical oxidation of complex hydrocarbons, such as ethanol, remains challenging. This work studies the fate of carriers from Au nanorods before and after the presence of reductively photodeposited Pd at the single-particle level using scattering and emission spectroscopy, along with ensemble photoelectrochemical methods. A sub-2 nm epitaxial Pd⁰ shell was reductively grown onto colloidal Au nanorods via hot carriers generated from surface plasmon resonance excitation in the presence of [PdCl₄]^2-. These bimetallic Pd-Au nanorod architectures exhibited 14% quenched emission quantum yields and 9% augmented plasmon damping determined from their scattering spectra compared to the bare Au nanorods, consistent with injection/separation of intraband hot carriers into the Pd. Absorbed photon-to-current efficiency in photoelectrochemical ethanol oxidation was enhanced 50× from 0.00034% to 0.017% due to the photodeposited Pd. Photocurrent during ethanol oxidation improved 13× under solar-simulated AM1.5G and 40× for surface plasmon resonance-targeted irradiation conditions after photodepositing Pd, consistent with enhanced participation of intraband-excited sp-band holes and desorption of ethanol oxidation reaction intermediates owing to photothermal effects.

Metal-Free Catalysis in C-C Single-Bond Cleavage: Achievements and Prospects

This review article emphasizes the C-C bond cleavage in organic synthesis via metal-free approach. Conventional organic synthesis mainly deals with the reactive π bonds and polar σ bonds. In contrast, the ubiquitous C-C single bonds are inherently stable and are less reactive, which poses a challenge to synthetic chemists. Although inert, such C-C single-bond cleavage reactions have gained attention amongst synthetic chemists, as they provide unique and more straightforward routes, with significantly fewer steps. Several review articles have been reported regarding the activation and cleavage of C-C bonds using different transition metals. However, given the high cost and toxicity of many of these metals, the development of strategies under metal-free conditions is of utmost importance. Though many research articles have been published in this area, no review article has been reported so far. Herein, we discuss the reactions in a more concise way from the year 2012 to today, with emphasis on important reactions. Mechanisms of all the reactions are also well addressed. We believe that this review will be beneficial for the readers who work in this field.

Highly efficient C(CO)-C(alkyl) bond cleavage in ketones to access esters over ultrathin N-doped carbon nanosheets

Selective oxidative cleavage of the C(CO)–C bond in ketones to access esters is a highly attractive strategy for upgrading ketones. However, it remains a great challenge to realize this important transformation over heterogeneous metal-free catalysts. Herein, we designed a series of porous and ultrathin N-doped carbon nanosheets (denoted as CN-X, where X represents the pyrolysis temperature) as heterogeneous metal-free catalysts. It was observed that the fabricated CN-800 could efficiently catalyze the oxidative cleavage of the C(CO)–C bond in various ketones to generate the corresponding methyl esters at 130 °C without using any additional base. Detailed investigations revealed that the higher content and electron density of the graphitic-N species contributed to the excellent performance of CN-800. Besides, the high surface area, affording active sites that are more easily accessed, could also enhance the catalytic activity. Notably, the catalysts have great potential for practical applications because of some obvious advantages, such as low cost, neutral reaction conditions, heterogeneous nature, high efficiency, and broad ketone scope. To the best of our knowledge, this is the first work on efficient synthesis of methyl esters via oxidative esterification of ketones over heterogeneous metal-free catalysts.

Ultrathin and metal-free N-doped carbon nanosheets showed high activity and selectivity for oxidative esterification of ketones via C(CO)–C bond cleavage to access methyl esters.

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

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

Spongy titanosilicate promotes the catalytic performance and reusability of WO3 in oxidative cleavage of methyl oleate

A tungsten containing catalyst catalyzed oxidative cleavage of methyl oleate (MO) by employing H2O2 as an oxidant and is known as an efficient approach for preparing high value-added chemicals, however, the tungsten leaching problem remains unresolved. In this work, a binary catalyst consisting of tungsten oxide (WO3) and spongy titanosilicate (STS) zeolite is proposed for MO oxidative cleavage. The function of STS in this catalyst is investigated. On the one hand, STS converts MO to 9,10-epoxystearate (MES), which further forms nonyl aldehyde (NA) and methyl azelaaldehydate (MAA) with the catalysis of WO3. In this way, MO oxidation and hydrolysis that generates unwanted diol product 9,10-dihydroxystearate (MDS) decreases obviously. On the other hand, STS decomposes peroxide and promotes the conversion of soluble peroxotungstate to insoluble polytungstate. Meanwhile, these tungsten species are allowed to precipitate on its surface instead of remaining in the liquid phase owing to its relative large specific area. Therefore, tungsten leaching can be reduced from 37.0% to 1.2%. Due to the cooperation of WO3 and STS, 94.4% MO conversion and oxidative cleavage product selectivity of 63.1% are achieved, and the WO3-STS binary catalyst maintains excellent catalytic performance for 8 recycling reactions.

Insight into the enhancing short-chain fatty acids (SCFAs) production from waste activated sludge via polyoxometalates pretreatment: Mechanisms and implications

Polyoxometalates (POMs), a versatile and environmentally-friendly inorganic material, have been extensively studied and applied in chemical catalytic oxidation and biological nutrients removal processes. However, little is known about effects of POMs pretreatment on anaerobic sludge fermentation. This study thereby filled such knowledge gap and provided insights into the underlying mechanisms. Results demonstrated the maximal short-chain fatty acids (SCFAs) production increased by 6.18 times with POMs rising from 0 to 0.05 g/g TSS. Mechanistic investigations revealed that the oxidation stress of POMs as well as reactive oxygen species (ROS) activated by POMs were responsible for the disintegration of waste activated sludge (WAS). More importantly, POMs pretreatment improved the biodegradability of organics released, providing more biodegradable substrates for SCFAs generation. Furthermore, the inhibition of POMs to SCFAs producers was less severe than that to SCFAs consumers, leading to SCFAs accumulation. Microbial community analysis exhibited that increased the population of hydrolysis (i.e., Longilinea) and SCFAs generation microbes (i.e., Acinetobacter and Fusibacter). Further evaluation showed that the POMs-based technology is economically and environmentally attractive for the pretreatment of WAS. Finally, a "closed-loop" concept of the reutilization of renewable POMs may provide an important implication of WAS management in the future.

Development of Sustainable Catalytic Pathways for Furan Derivatives

Biomass, the only globally available, renewable feedstock of organic carbon, is considered a viable alternative to fossil fuels. It can be efficiently utilized to produce various building blocks in accordance with green and sustainable chemistry principles. In this review, recent progress, such as the transformation of carbohydrates (C5 or C6 sugar, inulin, and cellulose) and their derivatives (furfural, hydroxymethylfurfural) into significant platform chemicals over polyoxometalates, zeolites, non-noble metals, and ionic liquids in single or multiphase, is evaluated.

Organic amine mediated cleavage of Caromatic-Cα bonds in lignin and its platform molecules

The activation and cleavage of C-C bonds remains a critical scientific issue in many organic reactions and is an unmet challenge due to their intrinsic inertness and ubiquity. Meanwhile, it is crucial for the valorization of lignin into high-value chemicals. Here, we proposed a novel strategy to enhance the Caromatic-Cα bond cleavage by pre-functionalization with amine sources, in which an active amine intermediate is first formed through Markovnikov hydroamination to reduce the dissociation energy of the Caromatic-Cα bond which is then cleaved to form target chemicals. More importantly, this strategy provides a method to achieve the maximum utilization of the aromatic nucleus and side chains in lignin or its platform molecules. Phenols and N,N-dimethylethylamine compounds with high yields were produced from herbaceous lignin or the p-coumaric acid monomer in the presence of industrially available dimethylamine (DMA).

Catalytic Reforming of the Aqueous Phase Derived from Diluted Hydrogen Peroxide Oxidation of Waste Polyethylene for Hydrogen Production

The thermal degradation and conversion of waste polyethylene (PE) using a two-step process including hydrothermal oxidation (HO) and aqueous phase reforming (APR) were investigated. The objective of this study was to achieve efficient disposal of waste PE and generate H₂ in a mild and green way. The effects of various HO conditions on both HO and APR processes were studied. A high H₂ O₂ concentration caused overoxidation of PE resulting in more CO₂ . Decreasing the H₂ O₂ concentration weakened the overoxidation. The process using diluted H₂ O₂ exhibited the highest selectivity for acetic acid among the produced carboxylic acids. When the HO temperature exceeded 200 °C, there was an increase in the CO₂ yield during the HO process and a decrease in the H₂ yield during the APR process. In addition, the effects of various monometallic and bimetallic catalysts on the reforming of the aqueous phase from the HO of PE were discussed. The highest H₂ mole fraction (51.52 %) in gaseous products from the APR process was obtained with Ru/mesoporous carbon. Nevertheless, Ru-Ni exhibited a higher stability than the monometallic Ru catalyst.

Alternative ball-milling synthesis of vanadium-substituted polyoxometalates as catalysts for the aerobic cleavage of C-C and C-O bonds

Vanadium-substituted phosphomolybdic acids (H_3+x[PMo_12-xV_xO₄₀], denoted as V_x) are well-known oxidation catalysts that are generally prepared by the hydrothermal treatment of MoO₃ and V₂O₅ in the presence of H₃PO₄. This synthesis procedure is highly energy consuming and the V_x yields are not always acceptable. In the present work, an alternative hybrid mechanochemical/hydrothermal synthesis of V_x is proposed, comprising the ball-milling of MoO₃ and V₂O₅, followed by a hydrothermal attack. The resulting materials, with 2 ≤ x ≤ 3, obtained from this new route were compared, in terms of yield, energy consumption and catalytic activity, with a reference V₃ sample prepared through a conventional hydrothermal treatment. The ball-milling step proved to lead not only to a shorter and far more energy-saving synthesis procedure, but also to high yields of V_x. Moreover, V_x from this alternative route proved to be generally more active than the conventionally prepared V₃ in the aerobic oxidative cleavage of C-O and C-C bonds in 2-phenoxyacetophenone, used herein as a lignin model compound.

Probing the interactions of GlcNH2 with boric acid via NMR spectroscopy

The essential role of boronic esters in controlling both the direction and selectivity of chemical reactions as well as their significant function in catalytic activity have been demonstrated for industrially important processes. The specific interaction analyses of the monosaccharide GlcNH2 with boric acid are of interest since monosaccharides serve as model systems for the more sophisticated carbohydrate molecules. The interaction of GlcNH2 with boric acid was systematically investigated by numerous NMR techniques. A 1 : 1 chelate boron complex coordinated at the cis-1,2 position of GlcNH2 was identified as the major species in DMSO-d6 solution via1H and 13C INEPT DOSY NMR spectroscopy. This specific boron nitrogen coordination mechanism was further supported by the 1H-15N HSQC spectra. Variations in the spin-lattice relaxation times (T1) of the 13C1 nucleus also provided quantitative data regarding this non-covalent interactions. This is an application of 1H, 13C INEPT DOSY, 1H-15N HSQC, and relaxation methods to study such aggregations in solutions. These methods have potential applications in the characterization of reactive intermediates in biomass conversions.

Molecular oxygen-mediated oxygenation reactions involving radicals

Molecular oxygen as a green, non-toxic and inexpensive oxidant has displayed lots of advantages compared with other oxidants towards more selective, sustainable, and environmentally benign organic transformations. The oxygenation reactions which employ molecular oxygen or ambient air as both an oxidant and an oxygen source provide an efficient route to the synthesis of oxygen-containing compounds, and have been demonstrated in practical applications such as pharmaceutical synthesis and late-stage functionalization of complex molecules. This review article introduces the recent advances of radical processes in molecular oxygen-mediated oxygenation reactions. Reaction scopes, limitations and mechanisms are discussed based on reaction types and catalytic systems. Conclusions and perspectives are also given in the end.

Succession of the microbial communities and function prediction during short-term peach sawdust-based composting

Short-term composting of raw materials for preparing oyster mushroom cultivation media is widely used in China, and its microbial mechanism needs to be further studied. 11-days' peach sawdust-based composting was performed to evaluate material conversion and microbial succession using physicochemical analysis and 16S rRNA and ITS sequencing. Composting bacteria demonstrated much higher abundance than fungi. Firmicutes, Actinobacteriota, and Proteobacteria were the dominant bacterial phyla, while most of fungal species belonged to Ascomycota. Moisture was the key factor at the beginning, while total nitrogen, temperature, and lignin became main influencing factors for composting maturity. Actinobacteriota, Firmicutes, and Proteobacteria of bacterial phyla, Eurotiomycetes and Sordariomycetes of fungal classes involved in lignocellulosic degradation. Bacterial function prediction analysis showed that carbohydrate metabolism and amino acid metabolism were the main metabolic pathways. These results confer a better understanding of material and microbial succession during short-term composting and also provide valuable utilization in mushroom industry.

Site-Specific Oxidation of (sp3)C-C(sp3)/H Bonds by NaNO2/HCl

A site-specific oxidation of (sp³)C-C(sp³) and (sp³)C-H bonds in aryl alkanes by the use of NaNO₂/HCl was explored. The method is chemical-oxidant-free, transition-metal-free, uses water as the solvent, and proceeds under mild conditions, making it valuable and attractive to synthetic organic chemistry.

Oxidative α-C-C Bond Cleavage of 2° and 3° Alcohols to Aromatic Acids with O2 at Room Temperature via Iron Photocatalysis

The selective α-C-C bond cleavage of unfunctionalized secondary (2°) and tertiary alcohols (3°) is essential for valorization of macromolecules and biopolymers. We developed a blue-light-driven iron catalysis for aerobic oxidation of 2° and 3° alcohols to acids via α-C-C bond cleavages at room temperature. The first example of oxygenation of the simple tertiary alcohols was reported. The iron catalyst and blue light play critical roles to enable the formation of highly reactive O radicals from alcohols and the consequent two α-C-C bond cleavages.