Oxidative Depolymerization Of Lignin Research Articles

O2-permeable membrane reactor for continuous oxidative depolymerization of lignin

O2-permeable membrane reactor for continuous oxidative depolymerization of lignin

Lignin oxidative depolymerization to phenolic compounds in a continuous-flow manufacturing platform

Effective oxidative depolymerization of lignin to phenolic monomers and oligomers is a crucial step in the economic feasibility of biorefinery. To date, most studies on the processing technical of lignin have focused on intermittent treatment, with little attention paid to microreactor. In this research, a novel and efficient process technical of oxidative depolymerization lignin via a continuous manufacturing platform was developed, in which the lignin alkaline solution is the disperse phase and the hydrogen peroxide is the continuous phase (oxidants, 8 wt%). The results show that different products can be obtained through varying the reaction conditions of lignin depolymerization. Under the optimized condition, 93.03 wt% conversion of raw lignin and 22.37 wt% yield of monomer was obtained. Besides, comparative characterizations on the samples of raw lignin and depolymerization products by GC-MS-FID and HSQC NMR analysis illustrated that 84.26 wt% of monomers (the sum of yield of 18.85 wt%) is phenolic compounds, with syringaldehyde (yield of 4.22 wt% and selectivity of 18.86 wt%) and vanillin (yield of 3.49 wt% and selectivity of 15.60 wt%) are the main products and the high yields attributed to the efficient cleavage of C-O linkages (mainly β-O-4 linkages) of lignin. Moreover, GPC and elemental analyses show that the light oil product with an oxygen content of 41.28% was mainly composed of dimers and trimers. This work demonstrating that continuous-flow oxidative depolymerization was an effective strategy on lignin processing technical and will be an interesting field of research on lignin comprehensive utilization.

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.

Controllable oxidative depolymerization of lignin to produce aromatic aldehydes and generate electricity under mild conditions with direct biomass fuel cells as flexible reactors

Lignin was oxidized and depolymerized by a direct biomass fuel cell (DBFC) for electricity generation and production of aromatic aldehydes. The yield of aromatic aldehydes could be easily controlled by changing the anodic catalyst, cathodic oxidants and external loading. Cobalt and (VO2)2SO4 were screened as the most suitable anodic catalyst and cathodic oxidant, respectively. A maximum power density of 199.8 mW/cm2 was obtained under the optimized conditions for electricity generation; however, the yield of the aromatic aldehydes was greatly dependent on the type of lignin used, reaction conditions and the external loading (output voltage) of the DBFC. At a lignin concentration of 0.5 g/L, aromatic aldehydes yield of 7.8 % was obtained at an output voltage of 0.9 V. The formation of aromatic aldehydes was mainly caused by cleavage of β-O-4 linkage and oxidation of side chains by the hydroxyl radicals formed via oxidation of OH– by Co3+ on the anode.

Insights into co-solvent roles in oxidative depolymerization of lignin to vanillin and lactic acid

Efficient low-boiling-point co-solvent systems were developed for the oxidative depolymerization of lignin to high-value platform chemicals, mainly vanillin and lactic acid, using molybdenum (Mo)-based solid oxide catalysts and hydrogen peroxide (H2O2) oxidant. The co-solvent, designed with the matching Hansen solubility parameter (HSP) values to lignin, facilitated efficient solvation of lignin. Various factors, including co-solvents, catalyst supports, H2O2 concentrations, and active metals, were detailly investigated. Optimal yields of total liquid products (EA oils: 19.8 %) were obtained in water-tetrahydrofuran (H2O-THF) co-solvent systems, increased by 61.0 % compared to that of the sole aqueous solution, employing a MoO3/Al2O3 catalyst. Additionally, the oxidized lignin (O-lignin) or EA oils was analyzed using UV–vis, CHN, GPC, and FT-IR techniques. Results indicated that the co-solvent system remarkably inhibited the repolymerization of lignin fragments, and enhanced the oxidative cleavage of lignin side chains. Lastly, a hypothetical mechanism explained the reaction pathway of lignin depolymerization in co-solvent systems.

Selective production of p-hydroxybenzaldehyde in the oxidative depolymerization of alkali lignin catalyzed by copper-containing imidazolium-based ionic liquids

The oxidative depolymerization of alkali lignin (AL) catalyzed by copper-containing imidazolium-based ionic liquids ([C4C1im]CuCl3) was investigated with CH3OH-H2O (9/1, v/v) as solvent under an initial oxygen pressure of 1.5 MPa at 150 °C for 3 h. Compared with other transition-metal chlorides and metal-containing ionic liquids (ILs), [C4C1im]CuCl3 presented a relatively remarkable catalytic capacity for AL oxidative depolymerization due to the cooperative effects of [C4C1im]+ and CuCl3−. [C4C1im]CuCl3 molar concentration (xMcIL) greatly influenced AL conversion and product selectivity. The highest AL conversion of 70.4% was achieved at xMcIL = 0.18 mol.%. For the obtained small-molecule products, the highest selectivity of p-hydroxybenzaldehyde reached 51.3% at xMcIL = 0.72mol.% (corresponding yield of 103.6 mg·g−1). The inter-unit linkages of lignin were effectively cleaved in the oxidative depolymerization catalyzed by [C4C1im]CuCl3, and the H unit in lignin was preferentially destroyed. The molecular interactions and microenvironment of [C4C1im]CuCl3–solvent (CH3OH-H2O) system were investigated with xMcIL varied from 0.05 mol.% to 5.12 mol.% by using attenuated total reflectance-infrared spectroscopy and molecular-dynamics method, respectively. The catalytic properties of [C4C1im]CuCl3 strongly depended on the interactions of [C4C1im]CuCl3 and solvent molecules with changes in microstructure organization. The optimum catalytic capacity of [C4C1im]CuCl3 was obtained when the ion pair formed through hydrogen bonding at xMcIL = 0.18–0.36 mol.%. The solvation of solvent molecules on CuCl3− and the formation of ionic cluster with the hydrogen-bonding self-association of [C4C1im]CuCl3 were unfavorable for the oxidative depolymerization of lignin.

MgFe2O4Nanoparticle/Peracetic Acid Hybrids for Catalytic Oxidative Depolymerization of Lignin

Lignin is a promising feedstock for renewable fuels and chemicals due to its aromatic skeleton and natural abundance. Lignin can be converted to diverse aromatic monomers as well as dicarboxylic acids depending on the applied conversion technologies. Despite its great potential, its native and processing-induced heterogeneity and complexity limit the conversion efficiency and product selectivity. In this study, magnesium ferrite (MgFe2O4) nanoparticle–peracetic acid (PAA) has been investigated as an efficient catalyst–oxidant incorporation for catalytic oxidative depolymerization of lignin under mild conditions. Typically, the increase in processing severity can enhance the lignin conversion while it results in the further decomposition of aromatic compounds to dicarboxylic acids. However, in this study, the incorporation of MgFe2O4 nanoparticles and PAA not only enhanced the total product yield but also improved the selectivity of aromatic monomers. The oxidative depolymerization system using the catalyst–oxidant combination resulted in 46 wt % of total oil product with a 61% selectivity of aromatic monomers under mild temperature (90 °C). In addition, this combination catalyst showed relatively good cycling stability based on the total product yield after recycling five times via magnetic separation. Overall, MgFe2O4 nanoparticles play an important role as a co-catalyst with a PAA oxidant in the oxidative conversion of lignin with an enhanced conversion efficiency and recyclability, and it will facilitate the valorization of lignin in future bio-based fuels and chemicals.

Cover Feature: Synthesis and Computational Investigation of Antioxidants Prepared by Oxidative Depolymerization of Lignin and Aldol Condensation of Aromatic Aldehydes (ChemSusChem 12/2023)

The Cover Feature shows novel antioxidants synthesized by oxidative depolymerization of lignin to form aromatic aldehydes followed by aldol condensation with methyl ethyl ketone (MEK). Aldol condensation was found to greatly improve the antioxidation ability of lignin depolymerized products. p-Hydroxybenzaldehyde, vanillin, and syringaldehyde as representative aromatic aldehydes were further employed for aldol condensation with MEK, resulting in successful synthesis of new antioxidants. The newly synthesized antioxidants could be used as a green additive of biodiesel to increase its oxidation stability. Density functional theoretical calculations confirmed that electron-donating groups such as methoxyl and conjugated side chains could effectively improve the antioxidation ability of the obtained antioxidants. More information can be found in the Research Article by D. Gao et al.

Synthesis and computational investigation of novel antioxidants prepared by oxidative depolymerization of lignin and aldol condensation of aromatic aldehydes.

Novel antioxidants were synthesized by CuSO4 catalyzed oxidative depolymerization of lignin to form aromatic aldehydes followed by aldol condensation with methyl ethyl ketone (MEK). Aldol condensation was found to greatly improve the antioxidation ability of lignin depolymerized products. Three lignin monomeric aromatic aldehydes, i.e. p-hydroxybenzaldehyde, vanillin, and syringaldehyde were further employed for aldol condensation with MEK, resulting in successful synthesis of new antioxidants, 1-(4-hydroxyphenyl)pent-1-en-3-one (HPPEO), 1-(4-hydroxy-3-methoxyphenyl)pent-1-en-3-one (HMPPEO) and 1-(4-hydroxy-3,5-dimethoxyphenyl)pent-1-en-3-one (HDMPPEO), respectively. Kinetic modeling illustrated that p-hydroxybenzaldehyde had the largest rate to react with MEK, followed by vanillin and then syringaldehyde, which was probably affected by the presence of methoxyl groups. Syringaldehyde derived product (HDMPPEO) displayed the best antioxidation ability. As revealed by density functional theoretical calculation, electron-donating group such as methoxyl group and conjugated side chain could effectively improve the antioxidation ability. Hydrogen atom transfer (HAT) mechanism was prone to occur in non-polar solvents while the sequential proton loss electron transfer (SPLET) mechanism was favored in polar solvents. This work thus can inspire new pathways for valorization of lignin to produce high value-added products.

Oxidative depolymerization of lignin to vanillin and lactic acid in an aqueous solution

A heterogeneous molybdenum (Mo)-based oxide catalyst system in combination with hydrogen peroxide as an oxidant was developed for oxidative depolymerization of lignin to yield high-value platform chemicals in a nearly neutral aqueous solution. The impact of various reaction conditions, including the ratios of H2O2 and/or MoO3/Al2O3 catalyst to lignin, reaction temperature and duration, and lignin concentration, were investigated in a batch system or a continuous flow fixed-bed system. The structural changes between dealkaline lignin and the recovered lignin (O-lignin and M−lignin) were analyzed using UV–vis, FT-IR, and GPC techniques. The combination of MoO3 and H2O2 resulted in a superior peroxo complex, promoting the formation of vanillin and lactic acid. The medium and weak acid feature of MoO3/Al2O3 catalyst induced the highest vanillin yield (5.6%) and the selectivity (86.9%) of main platform products. However, the highest yield of lactic acid (3.4%) was observed on MoO3/SiO2 with more weak acid sites. Hypothetical formation mechanisms for vanillin and lactic acid were proposed based on the depolymerization of lignin model compounds.

The Laccase-Lig Multienzymatic Multistep System in Lignin Valorization.

Invited for this month's cover is the work by Claudia Crestini and collaborators at Ca'Foscari University of Venice, Italy, and University of Insubria, Italy. The image shows the formation of low-molecular-weight compounds by the oxidative depolymerization of lignin by the laccase-Lig multienzymatic multistep system. The Research Article itself is available at 10.1002/cssc.202201147.

Front Cover: The Laccase‐Lig Multienzymatic Multistep System in Lignin Valorization (ChemSusChem 20/2022)

The Front Cover shows the formation of low-molecular-weight compounds by the oxidative depolymerization of lignin by the laccase-Lig multienzymatic multistep system. More information can be found in the Research Article by E. Vignali, M. Gigli, et al.

On the Oxidative Valorization of Lignin to High-Value Chemicals: A Critical Review of Opportunities and Challenges.

The efficient valorization of lignin is crucial if we are to replace current petroleum-based feedstock and establish more sustainable and competitive lignocellulosic biorefineries. Pulp and paper mills and second-generation biorefineries produce large quantities of low-value technical lignin as a by-product, which is often combusted on-site for energy recovery. This Review focuses on the conversion of technical lignins by oxidative depolymerization employing heterogeneous catalysts. It scrutinizes the current literature describing the use of various heterogeneous catalysts in the oxidative depolymerization of lignin and includes a comparison of the methods, catalyst loadings, reaction media, and types of catalyst applied, as well as the reaction products and yields. Furthermore, current techniques for the determination of product yields and product recovery are discussed. Finally, challenges and suggestions for future approaches are outlined.

Enhanced oxidative depolymerization of lignin in cooperative imidazolium-based ionic liquid binary mixtures

The aerobic oxidation of lignin model 2-phenoxyacetophenone (2-PAP) in cooperative ionic liquid mixtures (CoILs) with 1-ethyl-3-methylimidazolium acetate ([C2C1im]OAc) and 1-benzyl-3-methylimidazolium bis(trifluoromethylsulfonyl)imide ([BZC1im]NTf2) was investigated. Complete degradation of 2-PAP was achieved with [C2C1im]OAc/[BZC1im]NTf2 molar ratio (RIL) of 1/1 and 1/2 at 100 °C for 2 h. The conversion and product yields from CoILs were higher than those in pure ILs, indicating the cooperative effects of [C2C1im]OAc/[BZC1im]NTf2 on cleaving aryl–ether bonds. [C2C1im]OAc promoted the catalytic cleavage of aryl–ether bonds and solvation, and [BZC1im]NTf2 induced the formation of alkyl radicals and enhanced the product selectivity. Accordingly, the highest conversion of alkali lignin (79.8%) was obtained with RIL of 5/1 at 100 °C for 2 h, and phenol monomers (306 mg/g) were selectively produced. The CoILs exhibited good catalytic capacities for oxidative depolymerization of lignin, which strongly depends on the changes in intermolecular interactions and structural organization with varying RIL.

New Insights into Green Protocols for Oxidative Depolymerization of Lignin and Lignin Model Compounds.

Oxidative depolymerization of lignin is a hot topic in the field of biomass valorization. The most recent and green procedures have been herein detailed. Photochemical and electrochemical approaches are reviewed highlighting the pros and cons of each method. Mechanochemistry activated strategies are able to combine oxidation and depolymerization in the deconstruction of lignin. Homogenous and heterogeneous catalytic systems are exemplified stressing the green aspects associated with both the procedures. Solvent-free approaches as well as those carried out in alternative media are listed. Finally, the few examples of selenium catalyzed lignin valorization reported so far are cited.

Catalytic Oxidation of Veratryl Alcohol Derivatives Using RuCo/rGO Composites.

Chemoselectively oxidizing Cα -OH to C=O has been considered as a key step for the oxidative depolymerization of lignin. In this work, we design and prepare a series of composites of RuCo alloy nanoparticles and reduced graphene oxide (RuCo/rGO) with different Ru to Co ratios and explore their catalytic activities in the oxidation of veratryl alcohol derivatives, which usually serve as the model compounds for studying lignin oxidation. It is illustrated that the Ru to Co ratio determines the morphology and average size of the RuCo alloy nanoparticles on rGO, and the overall catalytic activities of the composites. The RuCo alloy nanoparticles on rGO with Ru to Co ratios of 1 : 0 to 1.2 : 1 show a unique flower-shaped morphology that increases the exposure of the active sites and thus promotes their contact with the substrates. The RuCo/rGO composites exhibit high catalytic activities for the oxidation of Cα -OH to aldehydes at 100 °C for 2 h. Additionally, the Co component affords the RuCo/rGO composites with magnetic properties that make the separation and recovery of the catalyst simple. Given the high catalytic performances and easy recovery, the RuCo/rGO composites would be potentially useful for the depolymerization of lignin.

Laccase-mediated synthesis of bioactive natural products and their analogues.

Laccases are a class of multicopper oxidases that catalyse the one-electron oxidation of four equivalents of a reducing substrate, with the concomitant four-electron reduction of dioxygen to water. Typically, they catalyse many anabolic reactions, in which mostly phenolic metabolites were subjected to oxidative coupling. Alternatively, laccases catalyse the degradation or modification of biopolymers like lignin in catabolic processes. In recent years, laccases have proved valuable and green biocatalysts for synthesising compounds with therapeutic value, including antitumor, antibiotic, antimicrobial, and antioxidant agents. Further up to date applications include oxidative depolymerisation of lignin to gain new biomaterials and bioremediation processes of industrial waste. This review summarizes selected examples from the last decade's literature about the laccase-mediated synthesis of biologically active natural products and their analogues; these will include lignans and neolignans, dimeric stilbenoids, biflavonoids, biaryls and other compounds of potential interest for the pharmaceutical industry. In addition, a short section about applications of laccases in natural polymer modification has been included.

Nano WO3‐Catalyzed One‐Pot Process for Mild Oxidative Depolymerization of Lignin and its Model Compounds

AbstractDespite challenges related to the robust and irregular structure of lignin, the valorization of this aromatic biopolymer has aroused great interest. However, the current methods exhibit problems such as harsh reaction conditions, complicated operation, and difficult recovery of catalyst. Herein we present a one‐pot process for the mild oxidative depolymerization of lignin and lignin model compounds catalyzed by nano WO3, along with tert‐butyl hydrogen peroxide (TBHP) as the oxidant and NaOH as the additive, which exhibits advantages of both homogeneous and heterogeneous catalysis. Under the optimized condition, it yielded 80.4 wt % of liquid oil from organosolv lignin with 7.6 wt % of vanillic acid as the main monomer product, accounting for 91.6 wt % monomeric selectivity. Mechanism studies on the model substrate suggest that the reaction proceeds via an oxidation of Cα−OH to C=O followed by C−O bond cleavage to afford phenol and ketone products which may undergo further oxidation to produce aromatic carboxylic acids. We have developed an operationally simple procedure for mild fragmentation of lignin and lignin model compounds with excellent yields, which provides the potential to expand the existing lignin usage from energy source to value‐added commodity chemicals.

Oxidative depolymerization of lignin using nitric acid under ambient conditions

In this work, we demonstrated an innovative and cost-effective strategy to produce high- value depolymerized kraft lignin (DKL) from softwood kraft lignin (SKL) and hardwood kraft lignin (HKL). The depolymerization of kraft lignin was successfully achieved by an oxidative approach using nitric acid under ambient conditions (room temperature and pressure). This novel approach has the following features: ambient process conditions, no heat required, absence of catalysts or organic solvents, and no further purification process being required. The obtained DKL products were characterized extensively by gel permeation chromatography with ultraviolet detector (GPC-UV), Fourier transform infrared (FTIR) spectroscopy, thermogravimetric analysis (TGA), pyrolysis–gas chromatography–mass spectrometry (Py-GC/MS), potentiometric titration and elemental analysis. Treatment of SKL or HKL at a KL/nitric acid (HNO3) mass ratio of 1:4 (w/w) at room temperature for 84 h resulted in DKL products with a weight-average molecular weight (Mw) of 1350 Da, much lower than that of the original lignins. In addition, the DKL products displayed a significant increase in carboxylic acid group content, a high solubility in water at low pH and, significantly lower decomposition temperatures compared to the original lignins. It was confirmed that nitration of the lignin structure occurred during nitric acid-oxidative depolymerization, as evidenced by the presence of 5% nitrogen (N) content in the DKL products.

Green and sustainable route for oxidative depolymerization of lignin: New platform for fine chemicals and fuels.

Depolymerization of lignin biomass to its value-added chemicals and fuels is pivotal for achieving the goals for sustainable society, and therefore has acquired key interest among the researchers worldwide. A number of distinct approaches have evolved in literature for the deconstruction of lignin framework to its mixture of complex constituents in recent decades. Among the existing practices, special attention has been devoted for robust site selective chemical transformation in the complex structural frameworks of lignin. Despite the initial challenges over a period of time, oxidation and oxidative cleavage process of aromatic building blocks of lignin biomass toward the fine chemical synthesis and fuel generation has improved substantially. The development has improved in terms of cost effectiveness, milder reaction conditions, and purity of compound individuals. These aforementioned oxidative protocols mainly involve the breaking of C-C and C-O bonds of complex lignin frameworks. More precisely in the line with environmentally friendly greener approach, the catalytic oxidation/oxidative cleavage reactions have received wide spread interest for their mild and selective nature toward the lignin depolymerization. This mini-review aims to provide an overview of recent developments in the field of oxidative depolymerization of lignin under greener and environmentally benign conditions. Also, these oxidation protocols have been discussed in terms of scalability and recyclability as catalysts for different fields of applications.