Catalytic Transformation of Lignocellulose into Chemicals and Fuel Products in Ionic Liquids

Valorisation of Corncob Residue towards the Sustainable Production of Glucuronic Acid

The production of glucuronic acid (GA) directly from actual biomass via chemocatalysis is of great significance to the effective valorisation of biomass for a sustainable future. Herein, we have developed a one-step strategy for the conversion of cellulose in corncob residue into GA with the cooperation of Au/CeO2 and maleic acid, achieving a 60.3% yield. Experimental and density functional theory (DFT) results show that maleic acid is effective in the fractionation of cellulose from corncob residue and the depolymerisation of cellulose fragments to glucose, on account of the good capacity for proton migration. Au/CeO2 is responsible for the selective oxidation of glucose to GA, in which the formation of glucaric acid is restrained, due to the weak capacity of Au/CeO2 on the proton transfer without the occurrence of the ring-opening reaction of glucose. Therefore, the relay catalysis of Au/CeO2 and maleic acid enables the production of GA via the complex cascade reactions. This work may provide insight regarding the conversion of actual biomass to targeted products.

Mechanistic insights into reductive deamination with hydrosilanes catalyzed by B(C6F5)3: A DFT study

Selective defunctionalization of synthetic intermediates is a valuable approach in organic synthesis. Here, we present a theoretical study on the recently developed B(C6F5)3/hydrosilane-mediated reductive deamination reaction of primary amines. Our computational results provide important insights into the reaction mechanism, including the active intermediate, the competing reactions of the active intermediate, the role of excess hydrosilane, and the origin of chemoselectivity. Moreover, the study on the substituent effect of hydrosilane indicated a potential way to improve the efficiency of the reductive deamination reaction.

Successive Cleavage and Reconstruction of Lignin β-O-4 Models and Polymer to Access Quinoxalines

The construction of N-heterocyclic compounds from lignin remains a great challenge due to the complex lignin structure and the involvement of multiple steps, including the cleavage of lignin C-O linkages and the formation of heterocyclic aromatic rings. Herein, the first example of KOH mediated sustainable synthesis of quinoxaline derivatives from lignin β-O-4 model compounds in a one-pot fashion under transition-metal-free conditions has been achieved. Mechanistic studies suggested that this transformation includes highly coupled cascade steps of cleavage of C-O bonds, dehydrative condensation, sp³ C-H bond oxidative activation, and intramolecular dehydrative coupling reaction. With this protocol, a wide range of functionalized quinoxalines, including an important drug compound AG1295, were synthesized from lignin β-O-4 model compounds and β-O-4 polymer, showcasing the application potential of lignin in pharmaceutical synthesis.

Efficient Reaction Systems for Lignocellulosic Biomass Conversion to Furan Derivatives: A Minireview

Lignocellulosic biomass as abundant, renewable, and sustainable carbon feedstock is an alternative to relieve the dependence on fossil fuels and satisfy the demands of chemicals and materials. Conversions of lignocellulosic biomass to high-value-added chemicals have drawn much attention recently due to the high availability of sustainable ways. This minireview surveys the recent trends in lignocellulosic biomass conversion into furan derivatives based on the following systems: (1) ionic liquids, (2) deep eutectic solvents, and (3) biphasic systems. Moreover, the current challenges and future perspectives in the development of efficient routes for lignocellulosic biomass conversion are provided.

Comparative investigation of the structural characteristics of tobacco stalk lignin during the DES and alkaline deconstruction toward sustainable materials

Lignin polymer as a natural aromatic macromolecule presents significant prospects in producing functional and sustainable materials, and achieving a comprehensive characterization will facilitate their target valorization. In the present study, deep eutectic solvent (DES) and alkaline delignification were adopted to deconstruct tobacco stalk before and after hydrothermal pretreatment, obtaining diverse lignin fractions with fascinating characteristics. DES lignin exhibited a higher yield and homogenous molecular structure than MWL. A severe cleavage of the inter-unit linkages in lignin was also observed. This result mostly originated from the efficient delignification of the DES deconstruction system adopted. Moreover, all the recovered lignin fractions exhibited good micro-nanoparticle size that can enhance the valorization of lignin in nanomaterial production, in which the hydrothermal-assisted DES deconstruction promoted the formation of the smaller lignin nanoparticle size. Next, all the recovered lignin presented an excellent UV absorption and structure-related absorption performance or thermal properties. Overall, this work provides an important foundation for further exploiting DES/alkaline delignification lignin that can be applied as an ideal feedstock for producing sustainable functional or micro/nanomaterials.

Metal-Free, Adjustable, and Recyclable Catalytic Systems for the Construction of C-C Bonds by Activating Propargylic Alcohols

Pyrano[3,2-c]coumarin derivatives and C3-substituted 4-hydroxycoumarins as important skeletal structures of active natural products and pharmaceutically relevant molecules have received increasing attention. However, developing an adjustable system for selectively synthesizing them is still a challenging task. Herein, sulfonic acid-functionalized ionic liquid was successfully used as the catalyst for the alkylation of 4-hydroxycoumarin derivatives with secondary aromatic propargylic alcohols using dimethyl carbonate as the green solvent, giving up to 98% yield. On the other hand, protonated imidazole-based ionic liquid-catalyzed cyclization was also selectively achieved with a nearly quantitative yield. Developed metal-free catalytic systems exhibited well adjustable and recyclable properties, avoiding the contamination of metal and halogen, reducing the neutralization after the reaction, and benefiting the separation between the catalyst and the product. New strategies were applied for performing the gram-scale reaction smoothly. The adjustable systems might occur through two different mechanisms involving propargylic or allenic carbocation and hydrogen bonding effects between the catalysts and the substrates.

A unique Co@CoO catalyst for hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran

The development of precious-metal-free catalysts to promote the sustainable production of fuels and chemicals from biomass remains an important and challenging target. Here, we report the efficient hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran over a unique core-shell structured catalyst, Co@CoO that affords the highest productivity among all catalysts, including noble-metal-based catalysts, reported to date. Surprisingly, we find that the catalytically active sites reside on the shell of CoO with oxygen vacancies rather than the metallic Co. The combination of various spectroscopic experiments and computational modelling reveals that the CoO shell incorporating oxygen vacancies not only drives the heterolytic cleavage, but also the homolytic cleavage of H2 to yield more active Hδ- species, resulting in the exceptional catalytic activity. Co@CoO also exhibits excellent activity toward the direct hydrodeoxygenation of lignin model compounds. This study unlocks, for the first time, the potential of simple metal-oxide-based catalysts for the hydrodeoxygenation of renewable biomass to chemical feedstocks.

Progress in Catalytic Conversion of Renewable Chitin Biomass to Furan-Derived Platform Compounds

Chitin is one of the most abundant biopolymers on Earth but under-utilized. The effective conversion of chitin biomass to useful chemicals is a promising strategy to make full use of chitin. Among chitin-derived compounds, some furan derivatives, typically 5-hydroxymethylfurfural and 3-acetamido-5-acetylfuran, have shown great potential as platform compounds in future industries. In this review, different catalytic systems for the synthesis of nitrogen-free 5-hydroxymethylfurfural and nitrogen-containing 3-acetamido-5-acetylfuran from chitin or its derivatives are summarized comparatively. Some efficient technologies for enhancing chitin biomass conversion have been introduced. Last but not least, future challenges are discussed to enable the production of valuable compounds from chitin biomass via greener processes.

Electroreductive CO coupling of benzaldehyde over SACs Au-NiMn2O4 spinel synergetic composites

Electroreductive CO coupling provides a prospective strategy for biomass derivative upgrading via reducing the number of oxygen-containing functional groups and increasing their molecular weight. However, exploring superior electrocatalysts with effective reactivity and high selectivity for target products are still a challenge. In this work, single atom Au surface derived NiMn₂O₄ (SACs Au-NiMn₂O₄) spinel synergetic composites were fabricated by a versatile adsorption-deposition method and applied in electroreductive self-coupling of benzaldehyde to dibenzyl ether. The SACs Au-NiMn₂O₄ spinel synergetic composites enhanced electroreductive coupling of benzaldehyde, significantly improved the yield and selectivity of dibenzyl ether. Systematic characterizations and density functional theory calculation revealed that atomically dispersed Au occupied surface Ni²⁺ vacancies, which played a dominated role in CO coupling of benzaldehyde. Detailed calculation results showed that benzaldehyde preferred to adsorb on Ni octa-hedral sites of NiMn₂O₄ spinel synergetic structure, single atom Au surficial derivation over NiMn₂O₄ further reduced the adsorption energy (E_ads) of benzaldehyde on SACs Au-NiMn₂O₄, thus the CO coupling of benzaldehyde to dibenzyl ether was promoted. Moreover, single atom Au surficial derivation lowered the energy barrier of rate-determining step, facilitated the formation of dibenzyl ether species. Our work also paves an avenue for rational design single atom materials using spinel as support.

A comparative evaluation of antibacterial activities of imidazolium-, pyridinium-, and phosphonium-based ionic liquids containing octyl side chains

Antibacterial activity is an essential property of ionic liquids. In this work, a comprehensive study has been performed on the antibacterial activity of ionic liquids to be utilized for further research and applications. Eighteen ionic liquids viz. Octyl Imidazolium, octyl Pyridinium, quaternary phosphonium-based cations containing bromide, sodium methane sulphonates, bis(trifluoromethane sulfonyl) imide, dichloroacetate, tetrafluoroborate, hydrogen sulfate were prepared and characterized with the help of different spectroscopic techniques. All these samples of ionic liquids were tested for their antibacterial activity against the most commonly occurring bacteria in the environment, i.e., Enterobacter aerogenes (E. aerogenes), Proteus vulgaris (P. vulgaris), Klebsiella pneumoniae (K. pneumoniae), Pseudomonas aeruginosa (P. aeruginosa), Escherichia coli (E. coli), and Streptococcus pyogenes (S. pyogenes). Most of the ionic liquids show good antibacterial properties, and imidazolium-based ionic liquids were even more antibacterial as compared to positive control. It was observed that a unique combination of cation and anion is essential to achieve desired antibacterial properties. The mechanism of antibacterial activity was further investigated using density functional theory calculations. A good correlation was found between experimental and theoretical studies.

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.

Mechanochemical Transformations of Biomass into Functional Materials

Biomass is one of the promising alternatives to petroleum-derived materials and plays a major role in our fight against climate change by providing renewable sources of chemicals and materials. Owing to its chemical and structural complexity, the transformation of biomass into value-added products requires a profound understanding of its composition at different scales and innovative methods such as combining physical and chemical processes. In this context, the use of mechanochemistry in biomass valorization is currently growing owing to its potentials as an efficient, sustainable, and environmentally friendly approach. This review highlights the latest advances in the transformation of biomass (i. e., chitin, cellulose, hemicellulose, lignin, and starch) to functional materials using mechanochemical-assisted methods. We focused here on the methodology of biomass processing, influencing factors, and resulting properties with an emphasis on achieving functional materials rather than breaking down the biopolymer chains into smaller molecules. Opportunities and limitations associated this methodology were discussed accordingly for future directions.

Amberlyst-15 supported zirconium sulfonate as an efficient catalyst for Meerwein-Ponndorf-Verley reductions

The Meerwein-Ponndorf-Verley (MPV) reaction is an important chemoselective route for carbonyl group hydrogenation, and thus designing new and effective catalysts for this transformation remains important and challenging. In this work, a new sulfonate coordinated Zr(IV) catalyst was prepared by the coordination of Zr(IV) onto the sulfonate groups of Amberlyst-15, which can effectively catalyze the MPV reaction and quantitatively convert carbonyl compounds to the corresponding alcohols with high reactivity and stability. Detailed mechanistic investigations reveal that the catalytic performance of Zr-AIER can be attributed to the synergetic effect between Zr⁴⁺ and the sulfonate group, and the porous structure with high surface area.

Identification of Crucial Intermediates in the Formation of Humins from Cellulose-Derived Platform Chemicals Under Brønsted Acid Catalyzed Reaction Conditions

Humins are one of the undesirable products formed during the dehydration of sugars as well as the conversion of 5-hydroxymethylfurfural (HMF) to value-added products. Thus, reducing the formation of humins is an important strategy for improving the yield of the aforementioned reactions. Even after a plethora of studies, the mechanism of formation and the structure of humins are still elusive. In this regard, we have employed density functional theory-based mechanistic studies and microkinetic analysis to identify crucial intermediates formed from glucose, fructose, and HMF that can initiate the polymerization reactions resulting in humins under Brønsted acid-catalyzed reaction conditions. This study brings light into crucial elementary reaction steps that can be targeted for controlling humins formation. Moreover, this work provides a rationale for the experimentally observed aliphatic chains and HMF condensation products in the humins structure. Different possible polymerization routes that could contribute to the structure of humins are also suggested based on the results. Importantly, the findings of this work indicate that increasing the rate of isomerization of glucose to fructose and reducing the rate of reaction between HMF molecules could be an efficient strategy for reducing humins formation.

Theoretical Mechanism on the Cellulose Regeneration from a Cellulose/EmimOAc Mixture in Anti-Solvents

The experiments on cellulose dissolution/regeneration have made some achievements to some extent, but the mechanism of cellulose regeneration in ionic liquids (ILs) and anti-solvent mixtures remains elusive. In this work, the cellulose regeneration mechanism in different anti-solvents, and at different temperatures and concentrations, has been studied with molecular dynamics (MD) simulations. The IL considered is 1-ethyl-3-methylimidazolium acetate (EmimOAc). In addition, to investigate the microcosmic effects of ILs and anti-solvents, EmimOAc-nH₂O (n = 0–6) clusters have been optimized by Density Functional Theory (DFT) calculations. It can be found that water is beneficial to the regeneration of cellulose due to its strong polarity. The interactions between ILs and cellulose will become strong with the increase in temperature. The H-bonds of cellulose chains would increase with the rising concentrations of anti-solvents. The interaction energies between cellulose and the anions of ILs are stronger than that of cations. Furthermore, the anti-solvents possess a strong affinity for ILs, cation–anion pairs are dissociated to form H-bonds with anti-solvents, and the H-bonds between cellulose and ILs are destroyed to promote cellulose regeneration.

Highly Effective and Chemoselective Hydrodeoxygenation of Aromatic Alcohols

Effective hydrodeoxygenation (HDO) of aromatic alcohols is very attractive in both conventional organic synthesis and upgrading of biomass-derived molecules, but the selectivity of this reaction is usually low because of...

Evolution of catalytically active species in paired PdCl2-CuCl2/[BMim]Cl for hydrolysis of β-1,4-glycosidic bonds

In this work, we identified the prevailing catalytically active species evolved from paired PdCl2-CuCl2 in 1-butyl-3-methylimidazolium chloride ([BMim]Cl) that displayed dramatic synergism between the two metal chlorides in catalyzing the...

Selective Conversion of Acetone to Mesitylene over Tantalum Phosphate Catalysts

We report the synthesis of tantalum phosphate catalyst with abundant strong acid sites, which is very active for the catalytic conversion of acetone into mesitylene. Characterizations, kinetic studies, and theoretical...