Selective Aerobic Benzylic Alcohol Oxidation of Lignin Model Compounds: Route to Aryl Ketones

A Metallomicellar Catalyst for Controlled Oxidation of Alcohols and Lignin Mimics in Water using Open Air as Oxidant

Alcohol groups and β-O-4 (C-C) linkages are widespread in biomass feedstock that are abundant renewable resource for value-added chemicals. The development of sustainable protocols for direct oxidation or oxidative cleavage of feedstock materials in a controlled fashion, using open air as an oxidant is an intellectually stimulating task to produce industrially important value-added carbonyls. Further, the oxidative depolymerization of lignin into fine chemicals has evoked interest in recent times. Herein, we report the first example of a catalyst system that could activate molecular oxygen from atmospheric air for controlled oxidation and oxidative cleavage/depolymerization of feedstock materials such as alcohols, β-O-4 (C-C) linkages and real lignin in water under open air conditions. The selectivity of carbonyl products is controlled by altering the pH between ~7.0 and ~12.0. The current strategy highlights the non-involvement of any external co-catalyst, oxidant, radical additives, and/or destructive organic solvents. The catalyst shows a wide substrate scope and eminent functional group tolerance. The upscaled multigram synthesis using an inexpensive catalyst and easily available oxidant evidences the practical utility of the developed protocol. A plausible mechanism has been proposed with the help of a few controlled experiments, and kinetic and computational studies.

Hydrodeoxygenation of Lignin-Based Compounds over Ruthenium Catalysts Based on Sulfonated Porous Aromatic Frameworks

Bifunctional catalysts are a major type of heterogeneous catalytic systems that have been widely investigated for biomass upgrading. In this work, Ru-catalysts based on sulfonated porous aromatic frameworks (PAFs) were used in the hydrodeoxygenation (HDO) of lignin-derived compounds: guaiacol, veratrole, and catechol. The relationship between the activity of metal nanoparticles and the content of acid sites in synthesized catalysts was studied. Herein, their synergy was demonstrated in the Ru-PAF-30-SO3H/5-COD catalyst. The results revealed that this catalytic system promoted partial hydrogenation of lignin-based compounds to ketones without any further transformations. The design of the Ru-PAF-30-SO3H/5-COD catalytic system opens a promising route to the selective conversion of lignin model compounds to cyclohexanone.

One-Pot Protolignin Extraction by Targeted Unlocking Lignin-Carbohydrate Esters via Nucleophilic Addition-Elimination Strategy

Protolignin extraction can facilitate structure elucidation and valorization of lignin in biorefinery, but is rather challenging due to the complex chemical bonds present. Here, we developed the in situ generated NH₃-reline (IGNR) system to realize one-pot protolignin extraction from lignocellulose. In the IGNR system, reline consisting of choline chloride and urea acted as both a solvent and a nucleophile generator, and the nucleophilic addition-elimination mechanism was verified by model compound studies. The in situ generated NH₃ could precisely cleave the lignin-carbohydrate esters in lignocellulose with a near-quantitative retention of carbohydrates. The extracted IGNR-Protolignin exhibited native lignin substructure with high molecular weight and high β-O-4' content (41.5 per 100 aromatic units). In addition, the up-scaled kilogram reaction demonstrated the feasibility of the IGNR system for potential industrial application in a green and sustainable pathway. This work represents a breakthrough toward protolignin extraction in practice with the future goal of achieving total biorefinery.

Cobalt catalysts (Co–N–C) for C–O bond cleavage in lignin-derived aryl ethers and lignin

The transformation of lignin into value-added chemicals represents one of the relevant approaches for sustainable development.

Effect of Benzyl Alcohol on Biomethanation from Lignite

Currently, biomethane obtained from coal resources, such as lignite and peat, serves as a sustainable biofuel urgently needed by the energy economy. To improve biomethane yield from lignite, the effects of different concentrations of benzyl alcohol, a degraded product of humic acid, on a biomethanation fermentation system were analyzed. The total biomethane yield, daily biomethane yield, coenzyme F₄₂₀, VFA (volatile fatty acids) concentration, alkalinity, and pH were used to determine the optimal benzyl alcohol concentration. The biomethanation fermentation system with 2000 mg/L benzyl alcohol produced up to 4.03 mL/g of biomethane, which was 1.15 times that produced from the control group. The coenzyme F₄₂₀, VFA, alkalinity, and pH of the system after adding 2000 mg/L benzyl alcohol were more preferable after adding other concentrations during the lignite biomethanation process. In summary, 2000 mg/L benzyl alcohol had a significantly positive effect on the lignite biomethanation fermentation system. When benzyl alcohol is added to the fermentation system, it accelerates the tricarboxylic acid cycle, which in turn produces more biomethane. Additionally, the self-supply of lignite microbial transformation nutrients from the perspective of chemical composition was explored as a novel approach in solving the common problem of low biomethane yield from a single lignite raw material. This also laid a foundation for subsequent steps through the adjustment of pretreatment conditions to ensure that the lignite pretreatment liquid contained increased benzyl alcohol, and a greater yield of biomethane can be produced after activated sludge addition.

Depolymerization of Lignin into Monophenolics by Ferrous/Persulfate Reagent under Mild Conditions

This study aimed to use a persulfate together with transition metal ions as the reagent to effectively depolymerize lignin into monophenolic compounds under mild conditions (ambient pressure, temperature <100 °C). The Box-Behnken experimental design in combination with the response surface methodology was applied to obtain optimized reaction conditions. The results showed that this reagent could depolymerize up to 99 % of lignin dimers to mainly veratraldehyde. This reaction also successfully depolymerized industrial lignins with a high yield of phenolic oils and monophenolic compounds. Quantum chemistry calculations using the density functional theory level indicated that the persulfate free radical attacks C_β to break the β-O-4 bond of lignin through a five-membered ring mechanism. This mechanism using persulfate free radicals has a lower activation barrier than that using hydroxyl radicals. Gel permeation chromatography and 2D-NMR spectroscopy demonstrated the effective cleavage of the β-O-4 bonds of lignin after depolymerization.

Copper-Catalyzed Oxidative Cleavage of the C-C Bonds of β-Alkoxy Alcohols and β-1 Compounds

Copper-catalyzed aerobic oxidation conditions were employed to promote the C-C bond cleavage of β-alkoxy alcohols and β-1 compounds (lignin model compounds). Besides these compounds, various 1,2 and 1,3-diols were successfully converted to aldehydes. We propose the Cu(I)-catalyzed mechanism explaining the C-C cleavage of these 1,2 and 1,3-dihydroxy compounds and β-alkoxy alcohols based on XPS data. Although our reaction conditions do not include large excess of bases and elaborated ligand-modified catalysts, copper salts with/without Me-TBD show good catalytic activities for C-C bond cleavage of various lignin model compounds.

Weak Bonds Joint Effects Catalyze the Cleavage of Strong C-C Bond of Lignin-Inspired Compounds and Lignin in Air by Ionic Liquids

Oxidation of lignin to value-added aromatics through selective C-C bond cleavage via metal-free and mild strategies is promising but challenging. It was discovered that the cations of ionic liquids (ILs) could effectively catalyze this kind of strong bond cleavage by forming multiple weak hydrogen bonds, enabling the reaction conducted in air at temperature lower than 373 K without metal-containing catalysts. The cation [CPMim]⁺ (1-propylronitrile-3-methylimidazolium) afforded the highest efficiency in C-C bond cleavage, in which high yields (>90 %) of oxidative products were achieved. [CPMim]⁺ could form three ipsilateral hydrogen bonds with the oxygen atom of C=O and ether bonds at both sides of the C-C bond. The weak bonds joint effects could promote adjacent C-H bond cleave to form free radicals and thereby catalyze the fragmentation of the strong C-C. This work opens up an eco-friendly and energy-efficient route for direct valorization of lignin by enhancing IL properties via tuning the cation.

Recent Advances in the Catalytic Depolymerization of Lignin towards Phenolic Chemicals: A Review

The efficient valorization of lignin could dictate the success of the 2nd generation biorefinery. Lignin, accounting for on average a third of the lignocellulosic biomass, is the most promising candidate for sustainable production of value-added phenolics. However, the structural alteration induced during lignin isolation is often depleting its potential for value-added chemicals. Recently, catalytic reductive depolymerization of lignin has appeared to be a promising and effective method for its valorization to obtain phenolic monomers. The present study systematically summarizes the far-reaching and state-of-the-art lignin valorization strategies during different stages, including conventional catalytic depolymerization of technical lignin, emerging reductive catalytic fractionation of protolignin, stabilization strategies to inhibit the undesired condensation reactions, and further catalytic upgrading of lignin-derived monomers. Finally, the potential challenges for the future researches on the efficient valorization of lignin and possible solutions are proposed.

Stabilization strategies in biomass depolymerization using chemical functionalization

A central feature of most lignocellulosic-biomass-valorization strategies is the depolymerization of all its three major constituents: cellulose and hemicellulose to simple sugars, and lignin to phenolic monomers. However, reactive intermediates, generally resulting from dehydration reactions, can participate in undesirable condensation pathways during biomass deconstruction, which have posed fundamental challenges to commercial biomass valorization. Thus, new strategies specifically aim to suppress condensations of reactive intermediates, either avoiding their formation by functionalizing the native structure or intermediates or selectively transforming these intermediates into stable derivatives. These strategies have provided unforeseen upgrading pathways, products and process solutions. In this Review, we outline the molecular driving forces that shape the deconstruction landscape and describe the strategies for chemical functionalization. We then offer an outlook on further developments and the potential of these strategies to sustainably produce renewable-platform chemicals.

Heterogeneous Catalyst Design Principles for the Conversion of Lignin into High-Value Commodity Fuels and Chemicals

Lignin valorization has risen as a promising pathway to supplant the use of petrochemicals for chemical commodities and fuels. However, the challenges of separating and breaking down lignin from lignocellulosic biomass are the primary barriers to success. Integrated biorefinery systems that incorporate both homo- and heterogeneous catalysis for the upgrading of lignin intermediates have emerged as a viable solution. Homogeneous catalysis can perform selected chemistries, such as the hydrolysis and dehydration of ester or ether bonds, that are more suitable for the pretreatment and fractionation of biomass. Heterogeneous catalysis, however, offers a tunable platform for the conversion of extracted lignin into chemicals, fuels, and materials. Tremendous effort has been invested in elucidating the necessary factors for the valorization of lignin by using heterogeneous catalysts, with efforts to explore more robust methods to drive down costs. Current progress in lignin conversion has fostered numerous advances, but understanding the key catalyst design principles is important for advancing the field. This Minireview aims to provide a summary on the fundamental design principles for the selective conversion of lignin by using heterogeneous catalysts, including the pairing of catalyst metals, supports, and solvents. The review puts a particular focus on the use of bimetallic catalysts on porous supports as a strategy for the selective conversion of lignin. Finally, future research on the valorization of lignin is proposed on the basis of recent progress.

Metal Catalyst-Free Oxidative C-C Bond Cleavage of a Lignin Model Compound by H2 O2 in Formic acid

Selective cleavage of the β-O-4 ether bond of lignin to produce aromatics is one of the most important topics for the sustainable production of chemicals from biomass. A simple system has been developed for C_α -C_β bond cleavage of a β-O-4 ketone-structured lignin model compound (LMC) by H₂ O₂ in formic acid under metal catalyst-free conditions. By using this simple system, with H₂ O₂ , formic acid, and mineral acid catalyst, over 90 % product yield is achieved in 6 h at room temperature. The reaction proceeds through the classic Baeyer-Villiger oxidation and in situ-generated performic acid serves as the key oxidant. The cleavage of alcoholic LMCs by using the presented method in a two-step process is also demonstrated.

Catalytic Scissoring of Lignin into Aryl Monomers

Lignin is an aromatic polymer, which is the biggest and most sustainable reservoir for aromatics. The selective conversion of lignin polymers into aryl monomers is a promising route to provide aromatics, but it is also a challenging task. Compared to cellulose, lignin remains the most poorly utilized biopolymer due to its complex structure. Although harsh conditions can degrade lignin, the aromatic rings are usually destroyed. This article comprehensively analyzes the challenges facing the scissoring of lignin into aryl monomers and summarizes the recent progress, focusing on the strategies and the catalysts to address the problems. Finally, emphasis is given to the outlook and future directions of this research.

Activation of lignin by selective oxidation: An emerging strategy for boosting lignin depolymerization to aromatics

Lignin is the most abundant, renewable aromatic resource on earth and holds great potential for the production of value-added chemicals. The efficient valorization of lignin requires to deal with several formidable challenges, especially to prevent it from re-condensation reactions during its depolymerization. Recently, a strategy involving the activation of lignin side chains by selective oxidation of the benzylic alcohol in β-O-4 linkages to facilitate lignin degradation to aromatic monomers has become very popular. This strategy provides great advantages for lignin selective degradation to high yields of aromatics under mild conditions, but requires an additional pre-oxidation step. The purpose of this review is to provide the latest cutting-edge innovations of this novel approach. Various catalytic systems, including those using chemo-catalytic methods, physio-chemo catalytic methods, and/or bio-catalytic methods, for the oxidative activation of lignin side chains are summarized. By analyzing the current situation of lignin depolymerization, certain promising directions are emphasized.

Selective Oxidation of Veratryl Alcohol over Au-Pd/Ce0.62Zr0.38O₂ Catalysts Synthesized by Sol-Immobilization: Effect of Au:Pd Molar Ratio

The selective oxidation of veratryl alcohol (VA), a model compound of lignin, with oxygen molecules to produce veratraldehyde (VAld) was studied over monometallic Au, Pd, and bimetallic Au:Pd nanoparticles supported on a Ce_0.62Zr_0.38O₂ mixed oxide for the first time. These bimetallic Au-Pd catalysts with Au:Pd molar ratios from 0.4 to 4.3 were synthesized by the sol-immobilization method. Furthermore, all the catalysts were characterized by inductively coupled plasma-atomic emission spectroscopy (ICP-AES), N₂ physisorption, X-ray photoelectron spectroscopy (XPS), scanning transmission electron microscopy-high angle annular dark field (STEM-HAADF) imaging, energy dispersive X-ray spectroscopy (EDXS), and temperature programmed reduction (TPR) techniques. A synergistic effect between gold and palladium was observed over all the bimetallic catalysts in a wide range of studied Au:Pd ratios. Remarkably, the optimum Au:Pd ratio for this reaction was 1.4 with a turnover frequency of almost six times larger than for the monometallic gold and palladium catalysts. Selectivity to veratraldehyde was higher than 99% for the monometallic Au, Pd, and all the bimetallic Au-Pd catalysts, and stayed constant during the reaction time.

Oxidation of Benzylic Alcohols and Lignin Model Compounds with Layered Double Hydroxide Catalysts

Alcohol oxidation to carbonyl compounds is one of the most commonly used reactions in synthetic chemistry. Herein, we report the use of base metal layered double hydroxide (LDH) catalysts for the oxidation of benzylic alcohols in polar solvents. These catalysts are ideal reagents for alcohol oxidations due to their ease of synthesis, tunability, and ease of separation from the reaction medium. LDHs synthesized in this study were fully characterized by means of X-ray diffraction, NH3-temperature programmed desorption (TPD), pulsed CO2 chemisorption, N2 physisorption, electron microscopy, and elemental analysis. LDHs were found to effectively oxidize benzylic alcohols to their corresponding carbonyl compounds in diphenyl ether, using O2 as the terminal oxidant. LDH catalysts were also applied to the oxidation of lignin β-O-4 model compounds. Typically, for all catalysts, only trace amounts of the ketone formed from benzylic alcohol oxidation were observed, the main products comprising benzoic acids and phenols arising from β-aryl ether cleavage. This observation is consistent with the higher reactivity of the ketones, resulting from weakening of the Cβ–O4 bond that was shown to be aerobically cleaved at 180 °C in the absence of a catalyst.

Cobalt-Catalyzed Oxidation of the β-O-4 Bond in Lignin and Lignin Model Compounds

In this work we demonstrate the use of Co(acac)₃ in combination with N-hydroxyphthalimide as an oxidant for the selective α-oxidation of the representative β-O-4 linkages in lignin model compounds. The oxidation reaction proceeds under mild conditions at 80 °C using 1,4-dioxane as the solvent and an oxygen atmosphere. The prior α-oxidation in the β-O-4 linkage of the lignin polymer is known to result in an easier cleavage of the adjacent C-O and C-C bonds because of a decrease in bond stability. Finally, the conditions were successfully transferred to kraft- and organosolv-lignin samples as proven by 2D-NMR (HSQC) experiments and gel permeation chromatography measurements.

Alternatives for Chemical and Biochemical Lignin Valorization: Hot Topics from a Bibliometric Analysis of the Research Published During the 2000–2016 Period

A complete bibliometric analysis of the Scopus database was performed to identify the research trends related to lignin valorization from 2000 to 2016. The results from this analysis revealed an exponentially increasing number of publications and a high relevance of interdisciplinary collaboration. The simultaneous valorization of the three main components of lignocellulosic biomass (cellulose, hemicellulose, and lignin) has been revealed as a key aspect and optimal pretreatment is required for the subsequent lignin valorization. Research covers the determination of the lignin structure, isolation, and characterization; depolymerization by thermal and thermochemical methods; chemical, biochemical and biological conversion of depolymerized lignin; and lignin applications. Most methods for lignin depolymerization are focused on the selective cleavage of the β-O-4 linkage. Although many depolymerization methods have been developed, depolymerization with sodium hydroxide is the dominant process at industrial scale. Oxidative conversion of lignin is the most used method for the chemical lignin upgrading. Lignin uses can be classified according to its structure into lignin-derived aromatic compounds, lignin-derived carbon materials and lignin-derived polymeric materials. There are many advances in all approaches, but lignin-derived polymeric materials appear as a promising option.

Perspective on Lignin Oxidation: Advances, Challenges, and Future Directions

Lignin valorization has gained increasing attention over the past decade. Being the world's largest source of renewable aromatics, its valorization could pave the way towards more profitable and more sustainable lignocellulose biorefineries. Many lignin valorization strategies focus on the disassembly of lignin into aromatic monomers, which can serve as platform molecules for the chemical industry. Within this framework, the oxidative conversion of lignin is of great interest because it enables the formation of highly functionalized, valuable compounds. This work provides a brief overview and critical discussion of lignin oxidation research. In the first part, oxidative conversion of lignin models and isolated lignin streams is reviewed. The second part highlights a number of challenges with respect to the substrate, catalyst, and operating conditions, and proposes some future directions regarding the oxidative conversion of lignin.

State-of-the-art catalytic hydrogenolysis of lignin for the production of aromatic chemicals

Catalytic hydrogenolysis of lignin is overviewed, concerning the cleavage of typical inter-unit linkages and the production of aromatic chemicals.

Organocatalytic Chemoselective Primary Alcohol Oxidation and Subsequent Cleavage of Lignin Model Compounds and Lignin

A one-pot two-step degradation of lignin β-O-4 model compounds initiated by preferred oxidation of the primary over the secondary hydroxyl groups with a TEMPO/DAIB system has been developed [TEMPO=2,2,6,6-tetramethylpiperidine-N-oxyl, DAIB=(diacetoxy)iodobenzene]. The oxidised products are then cleaved by proline-catalysed retro-aldol reactions. This degradation methodology produces simple aromatics in good yields from lignin model compounds at room temperature with an extension to organosolv beech-wood lignin (L1) resulting in known cleavage products.

Catalytic Oxidation of Lignin in Solvent Systems for Production of Renewable Chemicals: A Review

Lignin as the most abundant source of aromatic chemicals in nature has attracted a great deal of attention in both academia and industry. Solvolysis is one of the promising methods to convert lignin to a number of petroleum-based aromatic chemicals. The process involving the depolymerization of the lignin macromolecule and repolymerization of fragments is complicated influenced by heating methods, reaction conditions, presence of a catalyst and solvent systems. Recently, numerous investigations attempted unveiling the inherent mechanism of this process in order to promote the production of valuable aromatics. Oxidative solvolysis of lignin can produce a number of the functionalized monomeric or oligomeric chemicals. A number of research groups should be greatly appreciated with regard to their contributions on the following two concerns: (1) the cracking mechanism of inter-unit linkages during the oxidative solvolysis of lignin; and (2) the development of novel catalysts for oxidative solvolysis of lignin and their performance. Investigations on lignin oxidative solvolysis are extensively overviewed in this work, concerning the above issues and the way-forward for lignin refinery.

Lignin Hydrogenolysis: Improving Lignin Disassembly through Formaldehyde Stabilization

Lignocellulosic biomass is available in large quantities and constitutes an attractive feedstock for the sustainable production of bulk and fine chemicals. Although methods have been established for the conversion of its cellulosic fractions, valorization of lignin has proven to be challenging. The difficulty in disassembling lignin originates from its heterogeneous structure and its propensity to undergo skeletal rearrangements and condensation reactions during biorefinery fractionation or biomass pretreatment processes. A strategy for hindering the generation of these resistive interunit linkages during biomass pretreatment has now been devised using formaldehyde as a stabilizing agent. The developed method when combined with Ru/C-catalyzed hydrogenolysis allows for efficient disassembly of all three biomass fractions: (cellulose, hemicellulose, and lignin) and suggests that lignin upgrading can be integrated into prevailing biorefinery schemes.

Paving the Way for Lignin Valorisation: Recent Advances in Bioengineering, Biorefining and Catalysis

Lignin is an abundant biopolymer with a high carbon content and high aromaticity. Despite its potential as a raw material for the fuel and chemical industries, lignin remains the most poorly utilised of the lignocellulosic biopolymers. Effective valorisation of lignin requires careful fine-tuning of multiple "upstream" (i.e., lignin bioengineering, lignin isolation and "early-stage catalytic conversion of lignin") and "downstream" (i.e., lignin depolymerisation and upgrading) process stages, demanding input and understanding from a broad array of scientific disciplines. This review provides a "beginning-to-end" analysis of the recent advances reported in lignin valorisation. Particular emphasis is placed on the improved understanding of lignin's biosynthesis and structure, differences in structure and chemical bonding between native and technical lignins, emerging catalytic valorisation strategies, and the relationships between lignin structure and catalyst performance.

Green Diesel from Kraft Lignin in Three Steps

Precipitated kraft lignin from black liquor was converted into green diesel in three steps. A mild Ni-catalyzed transfer hydrogenation/hydrogenolysis using 2-propanol generated a lignin residue in which the ethers, carbonyls, and olefins were reduced. An organocatalyzed esterification of the lignin residue with an in situ prepared tall oil fatty acid anhydride gave an esterified lignin residue that was soluble in light gas oil. The esterified lignin residue was coprocessed with light gas oil in a continous hydrotreater to produce a green diesel. This approach will enable the development of new techniques to process commercial lignin in existing oil refinery infrastructures to standardized transportation fuels in the future.

Mild and Robust Redox-Neutral Pd/C-Catalyzed Lignol β-O-4' Bond Cleavage Through a Low-Energy-Barrier Pathway

A Pd/C catalyzed redox neutral C¢O bond cleavage of 2-aryloxy-1-arylethanols has been developed. The reactions are carried out at 80 °C, in air, using a green solvent system to yield the aryl ketones in near quantitative yields. Addition of catalytic amounts of a hydrogen source to the reaction mixture activates the catalyst to proceed through a low energy barrier pathway. Initial studies support a transfer hydrogenolysis reaction mechanism that proceeds through an initial dehydrogenation followed by an enol adsorption to Pd/C and a reductive C¢O bond cleavage.

Oxidation of lignin and lignin β-O-4 model compounds via activated dimethyl sulfoxide

Oxidation of lignin and β-O-4 models using activated DMSO compounds can give ketones or enol ethers.