Lignin Depolymerization Research Articles

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.

Efficient depolymerization of alkali lignin to monophenols using one-step synthesized Cu–Ni bimetallic catalysts inlaid in homologous biochar

The efficient depolymerization of lignin for the production of high-value chemicals is essential for the development of a sustainable and competitive biorefinery. In this study, we report the preparation of homologous biochar-supported inlaid Cu–Ni catalysts (CuxNiy/BC) using a one-step method, and their application for lignin depolymerization in mild water without the need for external hydrogen, additives, or co-catalysts. The Cu1Ni3/BC (0.05 g catalyst, 8 h, 280 °C, and 18 mL water) showed exceptional catalytic activity, with a monophenol yield of 94.35 ± 2.92 mg/g, bio-oil yield of 68.98 ± 1.42%, and a lignin conversion rate of 92.36 ± 1.99%, which outperformed both the biochar support and monometallic catalysts. The outstanding catalytic performance of Cu1Ni3/BC was attributed to its strong metal-support interaction, alloy sites, and well-dispersed nanoparticles. Reusability experiments demonstrated that the Cu1Ni3/BC exhibited relatively good stability for up to four reuses, with only a 2.32% reduction in catalytic activity per cycle. Mechanism investigation revealed that the Cu1Ni3/BC, with its abundant mesoporous structure and enhanced acid sites, displayed excellent deoxygenation ability, enabling effective cleavage of C–O/C–C bonds and the production of G-type phenols. This approach provides new insights into the large-scale utilization of lignin for the production of high-value chemicals.

Area-Selective Depolymerization of Hydroxycinnamates Visualized by Raman Imaging in Miscanthus sinensis cv. Cell Wall.

Lignin-carbohydrate complexes (LCCs) are regarded as a barrier for lignocellulosic biomass refinery. Here, confocal Raman microspectroscopy has been used to visualize the dissolution of hydroxycinnamates (HCMs) incorporated into LCCs by ether and ester bonds for energy crops Miscanthus sinensis cv. during successive NaOH (2.5% w/w) treatment. Raman spectral analysis indicated that mild NaOH treatment resulted in a higher proportion of HCM depolymerization in highly lignified middle lamella areas (>66.0%) than carbohydrate-abundant secondary walls. Furthermore, Raman imaging revealed preferential depolymerization of lignin from the sclerenchyma fiber (Sf) and parenchyma (Par) secondary wall with an increment of treatment time from 0 to 25 min, while middle lamella areas of Sf and Par were less affected where the depolymerization of HCMs was closely related with that of lignin (coefficient factors > 0.96). A better understanding of the depolymerization behavior for HCMs accompanied by lignin depolymerization was important to break LCC bonds in herbaceous biomass efficiently.

Conversion of lignin to high yields of aromatics over Ru–ZnO/SBA-15 bifunctional catalysts

The development of a highly active and robust catalyst for the conversion of lignin, the largest volume of renewable aromatic resource in nature, to value-added aromatics is of great importance in sustainable chemistry. However, the condensation of lignin depolymerization intermediates into recalcitrant macromolecules restricts high yields of desired products, and this remains a great challenge in lignin valorization. Here, a serious of bifunctional Ru–ZnO/SBA-15 catalysts were synthesized for lignin conversion to aromatics. Roles of Ru and ZnO in these catalysts for lignin conversion were in-depth analyzed. The effect of the ratio of Ru to ZnO was studied. Other reaction conditions including temperature and time were systematically optimized. More than 51.3% yields of aromatics were obtained over Ru–10ZnO/SBA-15 at 240 °C for 4 h. Structure-activity relationship research of the catalyst revealed that the high yield of product could be attributed to the following three reasons: 1) the Lewis acidic sites on Ru/SBA-15 are significantly improved after ZnO modification, and thus promotes the C–O bond cleavage; 2) the oxophilic ZnO is beneficial to absorb the substrate and reaction intermediates, and therefore enhances the hydrodeoxygenation activity of the adjacent Ru; 3). The interaction between Zn and Ru in Ru–ZnO/SBA-15 changes Ru electronic properties and leads to the formation of highly active catalytic sites with high electron density. These results indicated that the bifunctional Ru–ZnO/SBA-15 catalyst with high catalytic activity could be a good choice for lignin conversion to high-yield aromatics.

Selective Cleavage of Lignin Model Compounds via a Reverse Biosynthesis Mechanism.

Selective depolymerization of lignin remains a significant challenge in biomass conversion. The biosynthesis of lignin involves the polymerization of monolignol building blocks through oxidative radical coupling reactions. A strategy for lignin degradation leverages photoredox deoxygenative radical formation to trigger reverse biosynthesis, which cleaves model compounds of the β-O-4 and β-5-β-O-4 linkages to produce monolignols, precursors to flavoring compounds. This mild method preserves important oxygen functionality and serves as a platform for achieving selective lignin depolymerization.

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.

A solid acid-catalyzed depolymerization of pine lignin to obtain guaiacol using a hydrogen-free strategy

Guaiacol is an important raw material to produce many value-added chemicals. Here, we report a strategy for directly cleaving the Car-Cα and Cβ-O bonds in pine lignin over a solid acid to generate guaiacol. The guaiacol yield in 90% methanol aqueous solution is up to 18.2 wt%, which is 5.6 wt% higher than that in methanol. The reaction pathway is proposed by combining controlled experiments with density functional theory. Different from previous views, the Car-Cα bond cleavage was found to be related to deacetylation, while the formation of Cα=O bond was found to change the intramolecular charge distribution and promote the Car-Cα bond cleavage through validation experiments. Adding water promotes methanol decomposition to produce more active hydrogen species and thereby promote the Car-Cα bond cleavage. This work provided a feasible pathway for obtaining guaiacol by directly cleaving the Car-Cα and Cβ-O bonds in lignin.

Synergy of Single Atoms and Lewis Acid Sites for Efficient and Selective Lignin Disassembly into Monolignol Derivatives.

Lignin is the most abundant aromatic polymer from the natural and renewable lignocellulosic biomass resource. Developing highly efficient catalysts for lignin depolymerization to produce valuable monophenols with high yield and selectivity remains a desirable but challenging target in this field. Here, we design a synergistic catalyst combining atomically dispersed Mo centers and Al Lewis acid sites on a MgO substrate (Mo1Al/MgO) for the depolymerization of Eucalyptus lignin via the β-aryl ether bond cleavage. A near-theoretical monophenol yield of 46% with an ultrahigh selectivity of 92% for coniferyl and sinapyl methyl ether, as well as good cycling durability, was achieved simultaneously by Mo1Al/MgO in an inert N2 atmosphere. First-principles calculations and control catalytic experiments confirmed the synergistic catalysis mechanism between Mo1-O5 single-atom centers and the neighboring Al Lewis acid sites with the participation of a methanol solvent. This study validates the feasibility of designing better-performing catalysts with synergistic multiactive sites for the efficient and selective disassembly of complex renewable biopolymers into highly value-added products with lower cost and greater security.

Effect of solvent systems on the synergistic catalytic hydrogenolysis of alkaline lignin over Pd/C-NaOH

Solvents and catalysts are two of the most critical factors for the depolymerization of lignin. This paper investigated the effect of different pure solvents on the hydrogenolysis of the β-O-4 MC and one of the mixed solvents on the hydrogenolysis of alkaline lignin. It was found that MC had higher conversion (96.5% and 94.3%) and monophenol yield (83.4 wt% and 77.9 wt%) in ethanol and 1,4-dioxane. The effects of different ratios of mixed solvents (ethanol-H2O and 1,4-dioxane-H2O), reaction conditions (temperature, time, pressure), and the synergistic effect of alkali and Pd/C catalysts on the hydrogenolysis of alkaline lignin were investigated in this paper. The composition, structure, and distribution of the lignin degradation products were also analyzed by GC, GC–MS, GPC, FT-IR, and 2D-HSQC. The results showed that the optimum monomer yields were 37.5 wt% and 24.7 wt% for ethanol-H2O and 1,4-dioxane-H2O at 3:2 (v:v) and 2:3 (v:v), respectively. The optimum yields were obtained at 260 °C, 2 MPa H2, and 4 h. The properties of solvent (protic polar solvent, nonpolar aprotic solvent) could influence the hydrogenolysis greatly and selectively break bonds. The different ratios of solvents in the mixed solvent systems influenced the solubility of alkaline lignin and its hydrogenolysis results. In addition, the synergistic effect of Pd/C and NaOH could improve the monomer yield and inhibit the polymerization of intermediates in hydrogenolysis.

Bimetallic ruthenium- and zinc-doped beta zeolite for efficiently depolymerizing Kraft lignin

The efficient depolymerization of Kraft lignin is essential for comprehensive utilization of biomass. Herein, we prepared a bimetallic Ru/Zn@Beta zeolite catalyst using a co-impregnation synthesis approach that combined zeolite and metal. The bimetallic Ru/Zn@Beta catalyst efficiently depolymerized Kraft lignin, which has been credited as the electrostatic interaction of Zn Lewis acid sites and Ru hydrogen binding sites on Beta zeolite. Catalytic hydrodeoxygenation of Ru/Zn(1:2)@Beta catalyst for 9 h at 290 °C produced a maximum ethyl acetate soluble product yield of 96% with G-type selectivity reaching 74.7%. In these circumstances, compared to Kraft lignin, which produces a heating value of 26.0 MJ/kg, liquefied lignin possesses a much superior heating value of 30.39 MJ/kg. This research has established a paradigm for increasing lignin depolymerization efficiency by coupling Lewis acid and hydrogenation catalysts.

Molecular insights into the evolution of woody plant decay in the gut of termites.

Plant cell walls represent the most abundant pool of organic carbon in terrestrial ecosystems but are highly recalcitrant to utilization by microbes and herbivores owing to the physical and chemical barrier provided by lignin biopolymers. Termites are a paradigmatic example of an organism's having evolved the ability to substantially degrade lignified woody plants, yet atomic-scale characterization of lignin depolymerization by termites remains elusive. We report that the phylogenetically derived termite Nasutitermes sp. efficiently degrades lignin via substantial depletion of major interunit linkages and methoxyls by combining isotope-labeled feeding experiments and solution-state and solid-state nuclear magnetic resonance spectroscopy. Exploring the evolutionary origin of lignin depolymerization in termites, we reveal that the early-diverging woodroach Cryptocercus darwini has limited capability in degrading lignocellulose, leaving most polysaccharides intact. Conversely, the phylogenetically basal lineages of "lower" termites are able to disrupt the lignin-polysaccharide inter- and intramolecular bonding while leaving lignin largely intact. These findings advance knowledge on the elusive but efficient delignification in natural systems with implications for next-generation ligninolytic agents.

Selective photocatalytic aerobic oxidative cleavage of lignin C–O bonds over sodium lignosulfonate modified Fe3O4/TiO2

Lignocellulose shows significantly potential in sustainable conversion to high-quality fuel and value-added chemicals with the demands for realizing the rapid cycle of carbon resources and helping to reach carbon neutrality in nature. Selective tailoring of α-O-4, β-O-4, etc. linkages in lignin has always been viewed as “death blow” for its depolymerization. Herein, novel sodium lignosulfonate (SL) modified Fe3O4/TiO2 (SL-Fe3O4/TiO2) spherical particles have been developed and used as catalysts for selectively photocatalytic oxidative cleavage of organosolv lignin. As expected, 80% selective conversion of lignin in C2–C4 esters has been achieved, while C–O bonds in lignin model compounds can be effectively cleaved. Other than normal hydroxyl radical-mediated photocatalytic depolymerization of lignin over TiO2-based materials, in this contribution, mechanism studies indicate that photogenerated holes and superoxide anion radicals are main active species, which trigger the cleavage of α/β-O-4 bond, and the isotope-labeling study confirms the crucial factor of Cβ–H dehydrogenation in cleavage of β-O-4 bonds.

Fabrication and application of amphiphilic polyoxometalate catalyst (CTA)nH5-nPMo10V2O40 for transformation of lignin into aromatic chemicals

Conversion of renewable lignin into bio-aromatic chemicals offers a sustainable pathway to increase biorefinery profitability. However, the catalytic transformation of lignin into monomers remains a highly challenging task due to the complexity and stability of the lignin structure. In this study, a series of micellar molybdovanadophosphoric polyoxometalate (POM) catalysts, (CTA)nH5-nPMo10V2O40 (n = 1–5), were prepared by the ion exchange method and applied as oxidative catalysts for birch lignin depolymerization. These catalysts showed efficient cleavage of C-O/C-C bonds in lignin, and the introduction of an amphiphilic structure facilitated the generation of monomer products. The best catalytic activity was observed at 150 °C within 150 min under a 1.5 MPa oxygen atmosphere over (CTA)1H4PMo10V2O40, which yielded a maximum lignin oil yield of 48.7 % and lignin monomer yield of 13.5 %. We also employed phenolic and nonphenolic lignin dimer model compounds to explore the reaction pathway and demonstrated the selective cleavage of CC and/or CO lignin bonds. Moreover, these micellar catalysts have excellent recyclability and stability as heterogeneous catalysts, which can be used up to five times. The application of amphiphilic polyoxometalate catalysts facilitates the valorization of lignin, and we expect to develop a novel and practical strategy for harvesting aromatic compounds.

Oxidative Depolymerization of Kraft Lignin to Aromatics Over Bimetallic V–Cu/ZrO2 Catalysts

Zirconia-supported vanadium–copper catalysts (VCux:yZr) were used for the oxidative depolymerization of softwood LignoBoost Kraft lignin (LB). Various VCux:yZr catalysts were prepared (x:y = 0:1, 1:4, 1:2, 3:4, 1:1, and 1:0) by incipient wetness impregnation, and reactions were performed in alkaline water at 150 °C under an O2 pressure of 5 bar for 10 min. 1H–13C HSQC NMR spectroscopy was used for product identification and quantification. The most promising catalyst was VCu1:2Zr, giving a total monomer yield of 9 wt% and the highest selectivity for vanillin (59%). This catalyst was characterized before and after use by N2 physisorption, XRD, TGA, SEM-EDS, and XPS. Cleavage of the main interunit linkages in LB, including the β-O-4 bonds and recalcitrant C–C bonds, was also observed. The findings of this study demonstrate the potential of the V–Cu/ZrO2 catalyst system in the production of value-added aromatics from technical lignin under relatively mild conditions. This would contribute to the more sustainable use of an underutilized side-stream in forest-based industries, provided catalyst reuse can be successfully demonstrated.

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.

Solvent Screening for Separation of Lignin-Derived Molecules Using the NIST-UNIFAC Model

Liquid–liquid phase equilibria were calculated to investigate the potential separations of value-added components from products obtained by lignin depolymerization. In this study, the ability of the group-contribution model was evaluated in the prediction of mutual solubility and liquid–liquid phase equilibria of phenolic compounds. The phase equilibria behaviors of quaternary systems were evaluated by the NIST-UNIFAC model so that the predicted results were in good agreement with the available experimental data. In sequence, the partition coefficients of 29 lignin-derived molecules with complex and polar functional groups were predicted by the model with good accuracy (RMSE = 0.7424). The abilities of binary, ternary, and quaternary solvent systems were evaluated in the counter-current chromatography (CCC) separations of the products reported in the literature that are obtained through lignin depolymerization processes. Based on the empirical solvent selection criteria for CCC measurements, promising solvent systems were found for some of the lignin products. The difficult separations of some products in other cases can be attributed to the very similar chemical structures of the monomers. Finally, it was found that the NIST-UNIFAC model could qualitatively predict the solvent systems from the Arizona series, suitable for the separation of 4-hydroxybenzoic acid, vanillin, acetovanillone, syringaldehyde, acetosyringone, vanillic acid, and syringic acid.

Pyrolysis of LignoBoost lignin in ZnCl2-KCl-NaCl molten salt media: Insights into process-pyrolysis oil yield and composition relations

Depolymerization of lignin by pyrolysis has been identified as a viable route to produce renewable fuels and biobased platform chemicals. Herein we report the pyrolysis of LignoBoost lignin in a molten salt consisting of ZnCl2-KCl-NaCl (60:20:20 mol ratio) in a g-scale reactor set-up with a focus on the liquid phase yields and composition. The effects of relevant process parameters such as temperature (250–450 °C), reaction time (10–50 min), and N2 flow rate (10–30 mL min−1) on the product yields were elucidated using design of experiments. The highest bio-oil yield was 47.1 wt% (450 °C, 10 min) and the yield of organics in the bio-oil at this condition was 24 wt% (on lignin intake), the remainder being water. The latter is considerably higher than found for an experiment at similar conditions without salt (16 wt%). Temperature and reaction time were shown to have the largest effects on bio-oil yield. Prolonged reaction times resulted in higher amounts of gas phase components (H2, CO2) and water, and a reduced amount of solid products. Statistical analyses and validation experiments showed that the experimental product yields are in good agreement with the predicted values from the model. The properties and molecular composition of the liquid products were determined using various analytical techniques and reveal that the presence of a molten salt during pyrolysis has a positive influence on the composition of the liquid phase like a higher level of depolymerization and higher selectivity to aromatic and phenolic monomers compared to thermal pyrolysis.

Structure of aqueous solutions of lignin treated by sub- and supercritical water: Experiment and simulation

Hydrolysis lignin was treated with sub- and supercritical water (523–723 K and 10 MPa). As a result, depolymerization of lignin to mono- and oligolignols occurs. SEC data show water-soluble particles up to 4 nm after treatment of lignin by subcritical water (523 K and 10 MPa). After further treatment with supercritical water (723 K and 10 MPa), the concentration of water-soluble particles decreases by 2–3 times; and their size increased up to 10 nm. Molecular dynamics simulation of a model lignin particle in aqueous medium under standard (298 K, ρ = 0.997 g/cm3) and supercritical (673 K, ρ = 0.133 g/cm3) conditions was performed. Based on modeling, lignol monomers surrounded by solvate shells and water-soluble particles of lignin oligomers with a gel-like structure were identified as the structural units of the solutions. Under supercritical conditions, it causes the different ways of transformation of the system: further destruction of individual lignol molecules or condensation of lignols with the formation of insoluble precipitate.

Structural Characteristics-Reactivity Relationships for Catalytic Depolymerization of Lignin into Aromatic Compounds: A Review.

Developing renewable biomass resources is an urgent task to reduce climate change. Lignin, the only renewable aromatic feedstock present in nature, has attracted considerable global interest in its transformation and utilization. However, the complexity of lignin's structure, uncertain linkages, stability of side chain connection, and inevitable recondensation of reaction fragments make lignin depolymerization into biofuels or platform chemicals a daunting challenge. Therefore, understanding the structural characteristics and reactivity relationships is crucial for achieving high-value utilization of lignin. In this review, we summarize the key achievements in the field of lignin conversion with a focus on the effects of the β-O-4 content, S/G ratio, lignin sources, and an "ideal" lignin-catechyl lignin. We discuss how these characteristics influence the formation of lignin monomer products and provide an outlook on the future direction of lignin depolymerization.