Lignin Depolymerization Research Articles

Lignin Depolymerization and Esterification with Carboxylic Acids to Produce Phenyl Esters

Lignin Depolymerization and Esterification with Carboxylic Acids to Produce Phenyl Esters

ELECTROCHEMICALLY TREATED LIGNIN IN PHENOLIC RESINS FOR PLYWOOD PANELS

In achieving the European Union's goal of making Europe the first climate-neutral continent by 2050, the scientific community is striving to develop processes and products that are technologically and economically sustainable to replace/substitute fossil-based feedstocks. In this effort, the European project LIBERATE has demonstrated the commercial opportunities of converting low-cost lignin feedstocks into high-value bio sustainable chemicals such as vanillin and mixed phenolic derivatives.Lignin is the largest regenerative source of aromatic organics. In current wood pulping industries, lignin is underutilized and widely considered a side stream primarily exploited for its energy content. A promising approach for the synthesis of biobased fine chemicals is a depolymerisation process for lignin using sodium peroxodicarbonate (PODIC®), an electrochemically produced oxidiser. Within the LIBERATE project, SINTEF has built a pilot scale plant for the electrochemical lignin depolymerisation. The plant provided CHIMAR with the phenol-rich reactor product solution from both kraft and organosolv lignin, which was used as phenol substitutes with up to 50wt% in the formulation of phenol-formaldehyde (PF) resins. These resins have been able to produce successful plywood panels on a laboratory scale. Although their quality is somewhat lower than that of the reference material with traditional PF resin, this work showed that the use of lignin fractions obtained by electrochemical treatment in such an application is possible and promising.

Conversion of highly polymerized lignin into monophenolic products via pyrolysis: A comparative study of acidic and alkaline depolymerization pretreatments using deep eutectic solvents

Converting industrial residual lignin into monophenolic compounds remains a formidable challenge within biorefinery processes, especially when dealing with highly polymerized lignin. This study focuses on the depolymerization of industrial softwood enzyme hydrolysis lignin (SEHL), a by-product of bioethanol production, which possesses a substantial molar mass (M̅w ˃ 10000 g/mol), The approach involves utilizing acidic or alkaline deep eutectic solvent (DES) for pre-depolymerization, followed by rapid pyrolysis, achieving the selective generation of phenolic monomers. Applying acidic or alkaline DES pretreatments initiates depolymerization by breaking the β–aryl ether bonds (β–O–4) in lignin, but consequently triggers distinct reaction pathways. It is found that this pre-depolymerization leads to higher yields of phenolic monomers dominated by 4–methylguaiacol during the subsequent pyrolysis, moreover, the yields resulted from acidic DES-lignin were much higher than those from alkaline DES-lignin. We have comprehensively elucidated the lignin depolymerization process during DES pretreatments through computational simulations and experimental investigations. These efforts have provided valuable insights into the mechanisms involved in the structural changes of DES-lignin. Furthermore, we have established a more profound comprehension of how both acidic and alkaline DES pretreatments enhance the performance of lignin pyrolysis. This study emphasizes the importance of thoroughly understanding the intricate interplay between lignin structure and its thermochemical properties. Such insights are crucial for efficiently valorizing highly polymerized lignin to produce valuable phenolic compounds using the rapid pyrolysis technique.

Depolymerisation of Kraft Lignin by Tailor-Made Alkaliphilic Fungal Laccases.

Lignins released in the black liquors of kraft pulp mills are an underutilised source of aromatics. Due to their phenol oxidase activity, laccases from ligninolytic fungi are suitable biocatalysts to depolymerise kraft lignins, which are characterised by their elevated phenolic content. However, the alkaline conditions necessary to solubilise kraft lignins make it difficult to use fungal laccases whose activity is inherently acidic. We recently developed through enzyme-directed evolution high-redox potential laccases active and stable at pH 10. Here, the ability of these tailor-made alkaliphilic fungal laccases to oxidise, demethylate, and depolymerise eucalyptus kraft lignin at pH 10 is evidenced by the increment in the content of phenolic hydroxyl and carbonyl groups, the methanol released, and the appearance of lower molecular weight moieties after laccase treatment. Nonetheless, in a second assay carried out with higher enzyme and lignin concentrations, these changes were accompanied by a strong increase in the molecular weight and content of β-O-4 and β-5 linkages of the main lignin fraction, indicating that repolymerisation of the oxidised products prevails in one-pot reactions. To prevent it, we finally conducted the enzymatic reaction in a bench-scale reactor coupled to a membrane separation system and were able to prove the depolymerisation of kraft lignin by high-redox alkaliphilic laccase.

Novel Mechanoenzymatic Strategy for Lignin Depolymerization

Novel Mechanoenzymatic Strategy for Lignin Depolymerization

Defect Engineering Promoted Photocatalysis for Lignin Depolymerization: Performance and Mechanism Insight

Defect Engineering Promoted Photocatalysis for Lignin Depolymerization: Performance and Mechanism Insight

Depolymerization of lignin model compounds by reactive oxygen species generated from peroxymonosulfate under mild conditions

Directional cleavage the linkages bonds between the lignin units is of great scientific interest although technically challenging. In this work, free radical depolymerization of lignin was investigated by using three model compounds. Peroxymonosulfate (PMS) was activated by N-doped carbon and thermal method, and could generate reactive oxygen species (ROS), including hydroxyl radical (OH∙), sulfate radical anion (SO4∙−), superoxide anion (O2∙−) and singlet oxygen(O12). pH of solution was used to regulate the types of ROS. The results showed that 1) pH value can tune the reaction pathway by regulating the type of ROS; 2) There should be an optimal temperature for maximum selectivity of monomer products in the depolymerization proces of lignin model compounds by ROS; 3) Hydroxyl group and Methoxy group can activate the reactivity of lignin model compounds, which is conductive to the directional generation of monomer products. In addition, the depolymerization mechanism of lignin model compound by PMS was proposed.This work illustrated the reaction characters and the influence factors of the lignin depolymerization by PMS, the results would facilitate the optimization of the lignin valorization process.

Selective electrocatalytic oxidation of C(OH)–C bond in lignin with Pt@CeO2 toward the synthesis of benzoic acid

The synthesis of carboxylic acid compounds by the selective electrooxidation cleavage of the C(OH)-C bond in lignin is significant to biomass conversion to produce high-value-added chemicals. However, it is challenging to selectively cleave the C(OH)-C bond due to its extremely high dissociation energy of 260–300 kJ mol−1. Herein, a simple and effective method of hydrothermal process and wet impregnation is proposed to synthesize Pt@CeO2 catalysts. The cleavage of the C(OH)-C bond by Pt@CeO2, a catalyst loaded with a small amount of noble metal, triggered the electrochemical oxidation of lignin, resulting in the highly selective synthesis of carboxylic acid compounds. Consequently, more reactive oxygen species (ROS) are produced in the electrocatalytic oxidation process, which enhances the cleavage of C(OH)-C bond and the oxidation of benzaldehyde. In addition, Pt@CeO2 can also be used for the electrochemical depolymerization of industrial alkali lignin to produce carboxylic acids. This research proposes an efficient way of producing carboxylic acids of high value by utilizing an electrocatalyst for the electrocatalytic oxidation of lignin.

Engineering multiple defect sites on ultrathin graphitic carbon nitride for efficiently photocatalytic conversion of lignin into monomeric aromatics via selective C–C bond scission

Lignin depolymerisation via photocatalytic cleavage of the selective interunit linkage in lignin could be a sustainable approach to produce monomeric aromatic chemicals. However, the insufficient investigation of interunit C–C bond fragmentation has obstructed the rational design of efficient photocatalytic system and further limit the yields of aromatic monomers from lignin depolymerisation. Herein, this work developed the ultrathin g-C3N4 with multiple defective sites by simple self-assembly process and in-situ thermal gas-shocking/etching process to catalyse the cleavage of lignin C–C bonds under visible light irradiation. Compared with the pristine g-C3N4, the developed g-C3N4 photocatalyst exhibited a superior catalytic activity (improved 102 %) and selectivity (∼90 %) in the cleavage of C–C bonds in lignin. This study demonstrated that the defects construction and ultrathin structure can optimise the electronic structures of g-C3N4 for better separation and transfer of photoinduced charges. And the control experiments and DFT calculation indicated that the created defect sites can promote the generation of essential reactive radicals (e.g., the activation of O2) and radical intermediates (C–H activation). The present work provides useful insights for the rational use of defect engineering in designing the efficient photocatalytic system for the conversion of lignin into aromatic monomers via the C–C bond cleavage.

Bimetallic polyoxometalates catalysts for efficient lignin depolymerization: Unlocking valuable aromatic compounds from renewable feedstock

Lignin, a complex and abundant polymer present in lignocellulosic biomass, holds immense potential as a renewable source for the production of valuable aromatic compounds. However, the efficient depolymerization of lignin into these compounds remains a formidable challenge. Here, we present a promising solution by harnessing polyoxometalates (POMs) catalysts, which exhibit improved catalytic performance and selectivity. We synthesized a series of NixCoy@POMs catalysts (POMs: CsPW or CsPMo) and explored their application in the depolymerization of pine lignin, aiming to investigate the influence of different metal species and doping ratios of POMs on catalytic performance. Through meticulous optimization of reaction conditions, we achieved significant yields of valuable aromatic compounds, including methyl vanillate, vanillin, and 4-hydroxy-3-methoxyacetophenone. Furthermore, the Ni0.75Co0.75@CsPMo catalyst demonstrated exceptional efficacy in catalyzing the cracking process of C-C and/or C-O bonds in a β-O-4 dimer model compound. Notably, our catalyst exhibited outstanding stability over five cycles, underscoring its suitability as an effective heterogeneous catalyst for cyclic lignin depolymerization. This study sheds light on the potential of POMs-based catalysts for advancing lignin valorization and offers new avenues for sustainable biomass conversion into valuable chemicals.

Cloning, expression, and biochemical characterization of β-etherase LigF from Altererythrobacter sp. B11

Lignin, a complex heteropolymer present in plant cell walls, is now recognized as a valuable renewable resource with potential applications in various industries. The lignin biorefinery concept, which aims to convert lignin into value-added products, has gained significant attention in recent years. β-etherases, enzymes that selectively cleave β-O-4 aryl ether bonds in lignin, have shown promise in lignin depolymerization. In this study, the β-etherase LigF from Altererythrobacter sp. B11 was cloned, expressed, purified, and biochemically characterized. The LigF-AB11 enzyme exhibited optimal activity at 32 °C and pH 8.5 when catalyzing the substrate PNP-AV. The enzyme displayed mesophilic behavior and demonstrated higher activity at moderate temperatures. Stability analysis revealed that LigF-AB11 was not thermostable, with a complete loss of activity at 60 °C within an hour. Moreover, LigF-AB11 exhibited excellent pH stability, retaining over 50 % of its activity after 1 h under pH conditions ranging from 3.0 to 11.0. Metal ions and surface impregnation agents were found to affect the enzyme's activity, highlighting the importance of considering these factors in enzymatic processes for lignin depolymerization. This study provides valuable insights into the biochemical properties of LigF-AB11 and contributes to the development of efficient enzymatic processes for lignin biorefineries. Further optimization and understanding of β-etherases will facilitate their practical application in the valorization of lignin.

Towards bioresource-based aggregation-induced emission luminogens from lignin β-O-4 motifs as renewable resources

One-pot synthesis of heterocyclic aromatics with good optical properties from phenolic β-O-4 lignin segments is of high importance to meet high value added biorefinery demands. However, executing this process remains a huge challenge due to the incompatible reaction conditions of the depolymerization of lignin β-O-4 segments containing γ-OH functionalities and bioresource-based aggregation-induced emission luminogens (BioAIEgens) formation with the desired properties. In this work, benzannulation reactions starting from lignin β-O-4 moieties with 3-alkenylated indoles catalyzed by vanadium-based complexes have been successfully developed, affording a wide range of functionalized carbazoles with up to 92% yield. Experiments and density functional theory calculations suggest that the reaction pathway involves the selective cleavage of double C-O bonds/Diels-Alder cycloaddition/dehydrogenative aromatization. Photophysical investigations show that these carbazole products represent a class of BioAIEgens with twisted intramolecular charge transfer. Distinctions of emission behavior were revealed based on unique acceptor-donor-acceptor-type molecular conformations as well as molecular packings. This work features lignin β-O-4 motifs with γ-OH functionalities as renewable substrates, without the need to apply external oxidant/reductant systems. Here, we show a concise and sustainable route to functional carbazoles with AIE properties, building a bridge between lignin and BioAIE materials.

Using guaiacol as a capping agent in the hydrothermal depolymerisation of kraft lignin

Abstract The depolymerisation of softwood kraft lignin was investigated, under hydrothermal conditions at 290 °C and 250 bar, with guaiacol in the reactor feed to evaluate its impact on the formation of char and on the molecular weights of the products. The effect of residence time was investigated in the time span 1–12 min. Lignin is depolymerised during the process and guaiacol is both formed and consumed during the reaction, with clearly noticeable changes as early as in the first minute of reaction. Although the addition of guaiacol in the reactor feed causes a reduction in the weight average molecular weight of the products, the yield of char increases. Longer residence times result in repolymerisation of the reaction products as well as a further increase in the yield of monoaromatic components and char.

Alkylammonium lead chloride perovskites: Synthesis, characterization and catalytic activity for conversion of lignin into value added products

Lignin is the utmost abundant source of renewable aromatics hence its depolymerization into functionalized aromatics is attractive but challenging as well. A lot of interest is diverting to photocatalysis as a promising sustainable approach to depolymerizing lignin. In the present studies, photocatalytic lignin depolymerization has been investigated under UV-light irradiation using i-propylamine lead chloride perovskites (AS1-AS3) as catalysts. These catalysts were characterized by Powder X-ray Diffraction, Scanning Electron Microscopy, Ultraviolet–Visible, Photoluminescence, and Fourier Transform Infra-Red Spectroscopic techniques to evaluate crystallite size, surface morphology, band gaps, and quantum yields. Depolymerization of lignin was monitored by taking UV spectra and catalytic pathways were studied through various kinetic models. The depolymerization extent was monitored for different catalyst doses (0.05g–0.1 g), lignin concentration (50ppm–200 ppm), and temperatures (25–100 °C). More than 50% depolymerization rate was obtained for each catalyst using 100 ppm lignin concentration. AS3 has shown maximum efficiency in depolymerizing lignin. Pseudo-first order and modified Freundlich were found to be best-fitted kinetic models for experimental data of lignin depolymerization. The activation energy (Ea) for the depolymerization reaction was calculated to be 6.8 kJ/mol which is extraordinarily less than conventional depolymerization of lignin exhibiting significant catalytic competencies of synthesized catalysts. One of the products (L-AS3) of lignin depolymerization activity carried out at room temperature was characterized through GCMS analysis which indicated the breakdown of the polymeric structure of lignin into small monomeric functionalities. 2-methoxy-4-methylphenol (9%), benzene (4%), cyclohexene (16%), phenol (6%), catechol (12%) and 2-methoxy-5-propenyl phenol (10%) were detected by GCMS analysis of lignin depolymerization product.

Green and sustainable production of high-purity lignin microparticles with well-preserved substructure and enhanced anti-UV/oxidant activity using peroxide-promoted alkaline deep eutectic solvent

Deep eutectic solvents (DESs) have emerged as promising and eco-friendly solvents for the efficient extraction of lignin from biomass due to their low cost and environmental benefits. Nevertheless, the prevalent use of acidic DESs in lignin extraction often results in excessive depolymerization and recondensation of lignin, thereby impeding its downstream applications. In this study, we developed a range of alkaline DESs (ADESs), both pure and peroxide-containing, for the extraction of high-quality lignin from bamboo. Moreover, carbon dioxide (CO2) was employed for the precipitation and regeneration of the extracted lignin. The obtained lignin fractions were comprehensively characterized in terms of yield, purity, morphology, solubility, structural features, and anti-UV/oxidant activity. The results showed that the monoethanolamine-based ADES demonstrated superior performance among the pure ADESs. Structural analysis confirmed the well-preserved substructures of lignin fractions obtained using ADESs, with β-O-4 bond retention ranging from 49.8 % to 68.4 %. The incorporation of a suitable amount of peroxide improved lignin yield, morphology, solubility, and anti-UV/oxidant activity. Additionally, the anti-UV/oxidant activity of lignin exhibited a positive correlation with its phenolic hydroxyl content. This study provides a valuable reference for the green and sustainable production and valorization of lignin within the existing biorefinery framework.

Nature‐Inspired Depolymerization of Soda Lignin: Light‐Induced Free Radical Promoted Cleavage of β‐O‐4 Bonds

AbstractThe development of sustainable valorization methods for lignin is a challenging task because the vast majority of the reported studies involve either harsh conditions or expensive transition metal catalysts. Inspired by the sunlight degradation of lignin compounds, known as lignin yellowing, the use of a commercially available cheap organic photoinitiator, namely, phenacyl bromide (PAB) is reported here, for the efficient cleavage of lignin model compound 2‐phenoxyacetophenone (2‐PAP) and for the depolymerization of soda pulped lignin (SL) under UV‐A irradiation and ambient conditions. Real‐time NMR investigations of the photoreaction between 2‐PAP and PAB shed light on the possible reaction mechanisms involving different radical species, HBr, and molecular oxygen. Interestingly, combined spectral, chromatographic, powder X‐ray diffraction, and thermal studies of the photoreaction between PAB and SL indicate the formation of guaiacyl alcohol as the main product. The unprecedented performance of PAB is attributed to the excess generation of phenacyl radicals, the generation of photolabile brominated species, and HBr playing key roles in the cleavage of β‐O‐4 linkages. This work represents a new edge for sustainable lignin valorization under mild reaction conditions and offers the opportunity for large‐scale production of valuable aromatics using technical lignins as feedstock.

Ru nanoparticles supported on various metal-modified (La, Zr, Cr, Ce and Ga) α-Al2O3 for the selective hydrogenolysis of lignin and its derived ethers

Controlling the C-O bond hydrogenolysis while suppressing the direct hydrogenation of aromatic rings in lignin-derived ethers is critical and challenging for the utilization of lignin. Herein, a series of metal-modified (La, Zr, Cr, Ce and Ga) α-Al2O3-supported Ru-based catalysts were synthesized for the conversion of diphenyl ether (DPE). The catalyst characterizations demonstrate that the metal modification cannot change the chemical valence of Ru but promotes the dispersion of Ru nanoparticles and adjusts the adsorption state of DPE. The results indicate that the metal modification inhibits the arenes hydrogenation at low temperatures but facilitates the C-O bond cleavage during heating (at low H2 pressures or an inert atmosphere). Ru-Cr/α-Al2O3 shows the highest reaction rate for DPE conversion at 1 MPa Ar and 200 °C. Moreover, Ru-Cr/α-Al2O3 not only efficiently converts different lignin model compounds by hydrogenolysis, but also achieves the controlled depolymerization of real lignin to generate guaiacols under mild conditions.

Efficient Photocatalytic Cleavage of Lignin Models by a Soluble Perylene Diimide/Carbon Nitride S-Scheme Heterojunction.

3,4,9,10-Perylenetetracarboxylic dianhydride (PDI) is one of the best n-type organic semiconductors and an ideal light-driven catalyst for lignin depolymerization. However, the charge localization effect and the excessively strong intermolecular aggregation trend in PDI result in rapid electron-hole (e- -h+ ) recombination, which limits photocatalytic performance. Herein, polymeric carbon nitride/polyhedral oligomeric silsesquioxane PDI (p-CN/P-PDI) S-scheme heterojunction photocatalyst was prepared by the solvent evaporation-deposition method for C-C bond selective cleavage of lignin β-O-4 model. Based on the material characterization results, the synergic role of polyhedral oligomeric silsesquioxane (POSS) and S-scheme heterojunction maintains appropriate aggregation domains, achieves better solar light utilization, faster charge-transfer efficiency, and greater redox capacity. Notably, the 3 % p-CN/P-PDI heterostructure exhibits a remarkable enhancement in cleavage conversion efficiency, achieving approximately 16.42 and 2.57 times higher conversion rates compared to polyhedral oligomeric silsesquioxane modified PDI (POSS-PDI) and polymeric carbon nitride (p-CN), respectively.

Efficient Photocatalytic Cleavage of Lignin Models by a Soluble Perylene Diimide/Carbon Nitride S‐Scheme Heterojunction

Abstract3,4,9,10‐Perylenetetracarboxylic dianhydride (PDI) is one of the best n‐type organic semiconductors and an ideal light‐driven catalyst for lignin depolymerization. However, the charge localization effect and the excessively strong intermolecular aggregation trend in PDI result in rapid electron‐hole (e−−h+) recombination, which limits photocatalytic performance. Herein, polymeric carbon nitride/polyhedral oligomeric silsesquioxane PDI (p‐CN/P‐PDI) S‐scheme heterojunction photocatalyst was prepared by the solvent evaporation‐deposition method for C−C bond selective cleavage of lignin β‐O‐4 model. Based on the material characterization results, the synergic role of polyhedral oligomeric silsesquioxane (POSS) and S‐scheme heterojunction maintains appropriate aggregation domains, achieves better solar light utilization, faster charge‐transfer efficiency, and greater redox capacity. Notably, the 3 % p‐CN/P‐PDI heterostructure exhibits a remarkable enhancement in cleavage conversion efficiency, achieving approximately 16.42 and 2.57 times higher conversion rates compared to polyhedral oligomeric silsesquioxane modified PDI (POSS‐PDI) and polymeric carbon nitride (p‐CN), respectively.

Continuous-Flow Microreactor Accelerates Molecular Collisions for Lignin Depolymerization to Phenolic Monomers and Oligomers.

Effective depolymerization of lignin is the most important step for its comprehensive utilization. So far, most of the studies on depolymerization of lignin focused on batch processing, whereas only a few studies relied on the microreactor. In this study, we developed a continuous-flow microreactor for depolymerization of lignin into monomeric and oligomeric compounds. The yields of monomers and oligomers can be adjusted by varying the temperature, pressure, residence time, NaOH dosage, and solvent. Under optimized conditions, the lignin conversion rate was 77.73 wt %, and the monomer yield was 13.26 wt %, with 77.81% being phenolic compounds. In addition, comparative characterizations on the raw lignin and products demonstrated that the oil products were mainly composed of phenolic tetramers and trimers, and the effective cleavage of the β-O-4 linkage of S-type lignin was responsible for the high yield of 2,6-dimethoxyphenol. It indicated that raw lignin could be effectively depolymerized continuously using the continuous-flow microreactor, and it will be a new strategy for comprehensive utilization of lignin to produce fine-chemical intermediates.