Lignin Depolymerization Process Research Articles

Hydrolysis-dominated catalytic system: Hydrogen-free hydrogenolysis of lignin from Pd-MoOx/TiO2

Lignin is continuously investigated by various techniques for valorization due to its high content of oxygen-containing functional groups. Catalytic systems employing hydrolysis‑hydrogenolysis, leveraging the synergistic effect of redox metal sites and acid sites, exhibit efficient degradation of lignin. The predominance of either hydrolysis or hydrogenolysis reactions hinges upon the relative activity of acid and metal sites, as well as the intensity of the reductive atmosphere. In this study, the Pd-MoOx/TiO2 catalyst was found to primarily catalyze hydrolysis in the lignin depolymerization process, attributed to the abundance of moderate acidic sites on Pd and the redox-assisted catalysis of MoOx under inert conditions. After subjecting the reaction to 240 °C for 30 h, a yield of 48.22 wt% of total phenolic monomers, with 5.90 wt% consisting of diphenols, was achieved. Investigation into the conversion of 4-propylguaiacol (4-PG), a major depolymerized monomer of corncob lignin, revealed the production of ketone intermediates, a phenomenon closely linked to the unique properties of MoOx. Dehydrogenation of the propyl is a key step in initiating the reaction, and 4-PG could be almost completely transformed, accompanied by an over 97 % of 4-propylcatechol selectivity. This distinctive system lays a new theoretical groundwork for the eco-friendly valorization of lignin.

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.

Exoproteomic Study and Transcriptional Responses of Laccase and Ligninolytic Peroxidase Genes of White-Rot Fungus Trametes hirsuta LE-BIN 072 Grown in the Presence of Monolignol-Related Phenolic Compounds.

Being an abundant renewable source of aromatic compounds, lignin is an important component of future bio-based economy. Currently, biotechnological processing of lignin through low molecular weight compounds is one of the conceptually promising ways for its valorization. To obtain lignin fragments suitable for further inclusion into microbial metabolism, it is proposed to use a ligninolytic system of white-rot fungi, which mainly comprises laccases and peroxidases. However, laccase and peroxidase genes are almost always represented by many non-allelic copies that form multigene families within the genome of white-rot fungi, and the contributions of exact family members to the overall process of lignin degradation has not yet been determined. In this article, the response of the Trametes hirsuta LE-BIN 072 ligninolytic system to the presence of various monolignol-related phenolic compounds (veratryl alcohol, p-coumaric acid, vanillic acid, and syringic acid) in culture media was monitored at the level of gene transcription and protein secretion. By showing which isozymes contribute to the overall functioning of the ligninolytic system of the T. hirsuta LE-BIN 072, the data obtained in this study will greatly contribute to the possible application of this fungus and its ligninolytic enzymes in lignin depolymerization processes.

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.

A Review on Catalytic Depolymerization of Lignin towards High-Value Chemicals: Solvent and Catalyst

Lignin is a type of natural aromatic material with potential application prospects obtained from lignocellulosic biomass. Recently, the valorization of lignin has received increasing attention from both industry and academia. However, there is still a challenge in the efficient valorization of lignin due to the complexity and stability of the lignin structure. Recent work has been focused on the catalytic depolymerization of lignin to explore a promising and efficient way to valorize lignin into chemicals with high value and biofuels. Considerable research has focused on catalysts, solvents, and reaction parameters during the lignin depolymerization process, which significantly affects product distribution and productivity. Thus, in a catalytic depolymerization process, both catalysts and solvents have a significant influence on the depolymerization effect. This review article assesses the current status of the catalytic hydrogenolysis of lignin, mainly focusing on the solvents and catalysts during the reaction. First, various solvents applied in the lignin depolymerization reactions are extensively overviewed. Second, the recent progress of metal catalysts as well as their supports is summarized. Furthermore, a discussion of the challenges and prospects in this area is included.

Efficient Lignin Depolymerization Process for Phenolic Products with Lignin-Based Catalysts and Mixed Solvents

Due to their diverse functional groups and structural composition, carbon-based catalysts exhibit superior performance in various catalytic reaction systems, such as hydrogenolysis, hydrogenation, and oxidation. Herein, we propose an effective strategy of lignin liquid-phase depolymerization by applying the designed lignin-based carbon catalysts to produce aromatic chemicals. A series of Ni–Mo bimetallic carbon-based catalysts were synthesized to explore the effect of different preparation methods on the catalytic activity. The results demonstrate that the embedded Ni–Mo/C-WMO catalyst with multi-active sites of Ni, MoO3, and Mo2C results in a prominent catalytic effect on lignin depolymerization. The Ni–Mo/C-WMO catalyst can achieve a lignin conversion of 87.62% with a 42.25% monophenol yield in the methanol and water system. With the synergistic contribution of multiple active sites of the catalyst and the mixed-solvent system (water, methanol, and 1, 4-dioxane), a remarkably high yield of 62.95% of mono-phenolic compound was achieved at 260 °C and 3 MPa N2 with a reaction time of 4 h. The selectivity of 2-methoxy-4-methylphenol and 2-methoxy-4-ethylphenol in liquid products was 40.85 and 36.42%, respectively. Vanillin, a typical product from lignin depolymerization reported in the literature, was further degraded to monophenol in this system. The outcomes also confirmed that the in situ hydrogen production system of methanol and water coupled with 1,4-dioxane facilitated the lignin depolymerization significantly.

Biochemical and structural characterization of a sphingomonad diarylpropane lyase for cofactorless deformylation

Lignin valorization is being intensely pursued via tandem catalytic depolymerization and biological funneling to produce single products. In many lignin depolymerization processes, aromatic dimers and oligomers linked by carbon-carbon bonds remain intact, necessitating the development of enzymes capable of cleaving these compounds to monomers. Recently, the catabolism of erythro-1,2-diguaiacylpropane-1,3-diol (erythro-DGPD), a ring-opened lignin-derived β-1 dimer, was reported in Novosphingobium aromaticivorans. The first enzyme in this pathway, LdpA (formerly LsdE), is a member of the nuclear transport factor 2 (NTF-2)-like structural superfamily that converts erythro-DGPD to lignostilbene through a heretofore unknown mechanism. In this study, we performed biochemical, structural, and mechanistic characterization of the N. aromaticivorans LdpA and another homolog identified in Sphingobium sp. SYK-6, for which activity was confirmed invivo. For both enzymes, we first demonstrated that formaldehyde is the C1 reaction product, and we further demonstrated that both enantiomers of erythro-DGPD were transformed simultaneously, suggesting that LdpA, while diastereomerically specific, lacks enantioselectivity. We also show that LdpA is subject to a severe competitive product inhibition by lignostilbene. Three-dimensional structures of LdpA were determined using X-ray crystallography, including substrate-bound complexes, revealing several residues that were shown to be catalytically essential. We used density functional theory to validate a proposed mechanism that proceeds via dehydroxylation and formation of a quinone methide intermediate that serves as an electron sink for the ensuing deformylation. Overall, this study expands the range of chemistry catalyzed by the NTF-2-like protein family to a prevalent lignin dimer through a cofactorless deformylation reaction.

Perspective on oligomeric products from lignin depolymerization: their generation, identification, and further valorization

A systematic summary on the current state-of-art of the formation, identification, and further valorization of oligomeric products during various lignin depolymerization processes.

Molecular simulation of lignin-related aromatic compound permeation through gram-negative bacterial outer membranes

Lignin, an abundant aromatic heteropolymer in secondary plant cell walls, is the single largest source of renewable aromatics in the biosphere. Leveraging this resource for renewable bioproducts through targeted microbial action depends on lignin fragment uptake by microbial hosts and subsequent enzymatic action to obtain the desired product. Recent computational work has emphasized that bacterial inner membranes are permeable to many aromatic compounds expected from lignin depolymerization processes. In this study, we expand on these findings through simulations for 42 lignin-related compounds across a gram-negative bacterial outer membrane model. Unbiased simulation trajectories indicate that spontaneous crossing for the full outer membrane is relatively rare at molecular simulation timescales, primarily due to preferential membrane partitioning and slow diffusion within the lipopolysaccharide layer within the outer membrane. Membrane partitioning and permeability coefficients were determined through replica exchange umbrella sampling simulations to overcome sampling limitations. We find that the glycosylated lipopolysaccharides found in the outer membrane increase the permeation barrier to many lignin-related compounds, particularly the most hydrophobic compounds. However, the effect is relatively modest; at industrially relevant concentrations, uncharged lignin-related compounds will readily diffuse across the outer membrane without the need for specific porins. Together, our results provide insight into the permeability of the bacterial outer membrane for assessing lignin fragment uptake and the future production of renewable bioproducts.

Effect of Solvent Quality on Structure and Dynamics of Lignin in Solution.

Due to its significant aromatic content, lignin is an attractive source of valuable organic chemicals. As most of the proposed lignin depolymerization processes are expected to be liquid-phase, it is necessary to understand the effect of solvent quality on the structure and dynamics of lignin. Here we use all-atom molecular dynamics simulations to understand the evolution of lignin structure as a function of methanol concentration in methanol/water solution at different temperatures. We utilize two different lignin models: softwood consisting of guaiacyl (G) monomer and hardwood consisting of heteropolymer containing guaiacyl/syringyl (S) with a 1.35:1 ratio. The presence of additional methoxy groups in the hardwood lignin leads to a more extended configuration than softwood lignin with increasing methanol concentration. Structural features (radius of gyration and solvent accessible surface area) of lignin correlate with the strength of intermolecular forces quantified using cohesive energy density. We find that methanol preferentially solvates the nonpolar segments of the lignin polymer while water molecules solvate the polar functional groups. Thus, as the methanol concentration increases, methanol can better solvate lignin polymer, leading to a more extended configuration suitable for catalytic transformation to value-added chemicals.

Depolymerization of Kraft lignin into liquid fuels over a WO3 modified acid-base coupled hydrogenation catalyst

Lignin depolymerization to prepare valuable liquid fuels and phenolics is an important route to improve its efficient utilization, whereas some intractable challenges remain on account of the recalcitrant three-dimensional structure and the depolymerized intermediates are apt to condensation. Although some catalysts can work on lignin model compounds, the depolymerization rate is unable to compete effectively with the condensation reaction in the actual lignin depolymerization process. Based on this, the selective adsorption and activation of the C-O and C–C bonds, as well as the stabilization of depolymerization intermediates are essential to achieving efficient lignin depolymerization. Herein, a novel W-modified acid-base coupled hydrogenation catalyst was reported for the effective lignin conversion to generate liquid fuels. The Lewis acid sites WOx and TiO2 as well as the base sites MgO were favorable for adsorption and polarization of C–C and C-O bonds, promoting the cleavage of C–C and C-O bonds, while the hydrogenation site NiO activated the H2 molecular and inhibits the condensation reaction of reactive intermediates. Thereby, the cleavage of C-O and C–C bonds was facilitated by selective hydrodeoxygenation, leading to enhanced yields in petroleum ether solubilities and monomers. After reacting at 300 °C for 24 h, 63.03% petroleum ether solubles yield (mainly monomers and dimers) and 21.56% monophenol yield obtained with the HHV increased from 25.41 MJKg−1 to 33.90 MJKg−1, and the reusability was accredited.

Online investigation of lignin depolymerization via reactor-integrated electrospray ionization high-resolution mass spectrometry

Depolymerization of lignin can yield phenolic monomeric platform molecules, but depolymerization and repolymerization are proposed as concurrent processes, which greatly limit the yield of monomeric products. Molecular-level characterization of heavy intermediates during depolymerization process is crucial for developing efficient methods to depolymerize raw lignin into monomers. In this study, a reactor-integrated negative-ion electrospray ionization high-resolution mass spectrometry was used to real-time analyze the lignin depolymerization process under operando conditions. Elemental composition, structural information as well as time-resolved evolution of a series of dimers and oligomers were obtained, which is hard to achieve with traditional GC-MS methods. The real-time evolution profiles of monomers, dimers, and oligomers reveals a possible repolymerization process during lignin depolymerization. These results provide insight into the complex lignin depolymerization process.

An Efficient Lignin Depolymerization Process for Phenolic Products Via Synergistic Effects between Lignin-Based Multi-Active Site Catalysts and Mixed-Solvents

An Efficient Lignin Depolymerization Process for Phenolic Products Via Synergistic Effects between Lignin-Based Multi-Active Site Catalysts and Mixed-Solvents

Monitoring Molecular Weight Changes during Technical Lignin Depolymerization by Operando Attenuated Total Reflectance Infrared Spectroscopy and Chemometrics.

Technical lignins are increasingly available at industrial scale, offering opportunities for valorization, such as by (partial) depolymerization. Any downstream lignin application requires careful tailoring of structural properties, such as molecular weight or functional group density, properties that are difficult to control or predict given the structure variability and recalcitrance of technical lignins. Online insight into changes in molecular weight (M w), to gauge the extent of lignin depolymerization and repolymerization, would be highly desired to improve such control, but cannot be readily provided by the standard ex‐situ techniques, such as size exclusion chromatography (SEC). Herein, operando attenuated total reflectance infrared (ATR‐IR) spectroscopy combined with chemometrics provided temporal changes in M w during lignin depolymerization with high resolution. More specifically, ex‐situ SEC‐derived M w and polydispersity data of kraft lignin subjected to aqueous phase reforming conditions could be well correlated with ATR‐IR spectra of the reaction mixture as a function of time. The developed method showed excellent regression results and relative error, comparable to the standard SEC method. The method developed has the potential to be translated to other lignin depolymerization processes.

Effect of Various Microwave Absorbents on the Microwave-Assisted Lignin Depolymerization Process

Microwave absorbents play a vital role in governing the monomeric products from lignin during the microwave-assisted depolymerization (MAD) process. The influence of various microwave absorbents on...

Electrochemical oxidative valorization of lignin by the nanostructured PbO2/MWNTs electrocatalyst in a low-energy depolymerization process

In this study, we have investigated the application of the non-precious β-PbO2/MWNTs nanocomposites, which were developed via a simple wet-chemistry procedure, toward electrochemical oxidation of lignin. Structural characterization of the synthesized electrocatalysts was performed through X-ray diffraction (XRD), transmission electron microscopy (TEM), scanning electron microscopy (SEM), and BET surface area analysis. Cyclic voltammetry (CV), linear sweep voltammetry (LSV), and chronoamperometry (ChA) measurements were carried out to evaluate and compare the performance of different electrocatalysts toward lignin oxidation. The 33.3 wt% β-PbO2 nanocomposite possessed the highest electro-catalytic activity and stability for depolymerization of lignin. Several oxidation products such as vanillin and methyl salicylate were also identified by gas chromatography-mass spectroscopy (GS-MS) analysis. The enhanced electrochemical surfaces areas of the low-cost β-PbO2/MWNTs electrocatalyst combined with its good stabilities offer a promising route for developing an energy-efficient lignin depolymerization process. Electrochemical conversion of lignin-rich waste streams to functionalized compounds is appropriate for production of targeted biofuel and platform chemicals.

A novel environmentally friendly process for depolymerization of hydrolysis lignin using Kraft cooking liquor: chemicals recoverable by the Kraft recovery cycle

AbstractHydrolysis lignin (HL) refers to a lignin‐rich residue obtained after the enzymatic hydrolysis of biomass. It is recalcitrant, heterogeneous, insoluble in most common solvents, and less reactive than other lignins. To enhance the reactivity of HL, a novel environmentally friendly depolymerization approach was demonstrated to produce depolymerized hydrolysis lignin (DHL) using Kraft cooking liquor, white liquor (WL) – recoverable by the Kraft recovery cycle. The effects of various process parameters such as reaction time, WL/HL ratio, and reaction temperature on lignin depolymerization were investigated using a 2 L Parr reactor under N2. The DHLs obtained were then characterized by Gel permeation chromatography (GPC)‐Ultraviolet Detector (GPC‐UV), 31P Nuclear Magnetic Resonance (NMR) spectroscopy, and ultraviolet visible (UV–visible) spectroscopy, while the filtrates were characterized by high‐performance liquid chromatography (HPLC) for saccharinic acids. The DHL yield reached 45–70% from the treatments at 150–190 °C for 1 h at a WL/HL mass ratio of 1:4 ~ 2:1. The weight‐average molecular weight (Mw) value of the DHL obtained at 190 °C after treatment for 1 h at a WL/HL ratio of 2:1 (w/w) was 2600 Da. Moreover, a significant increase in non‐condensed phenolic hydroxyl and carboxylic acid group content was observed with decreasing Mw. Compared with various existing lignin modification approaches, the approach reported here is less expensive and more environmentally friendly if integrated into Kraft pulp mill operations with the residual WL from the lignin depolymerization process being recycled to the mill chemical recovery cycle. Process scale‐up was also demonstrated using a 20 L circulating reactor. In this case, the Mw of the DHL produced after treatment at 170 °C for 2 h was 2400 Da. © 2020 Society of Chemical Industry and John Wiley & Sons, Ltd.

Topology of Pretreated Wood Fibers Using Dynamic Nuclear Polarization

In the continuously developing field of lignocellulosic biomass, high-yield lignin depolymerization processes are sought to optimize its productivity and profitability. Recently, formaldehyde stabi...

Effect of HCOOK/Ethanol on Fe/HUSY, Ni/HUSY, and Ni-Fe/HUSY Catalysts on Lignin Depolymerization to Benzyl Alcohols and Bioaromatics.

We have investigated the production of benzyl alcohols and bioaromatics via the reductive lignin depolymerization process over Fe/H-style ultrastable Y (HUSY), Ni/HUSY, and Ni–Fe/HUSY catalysts using HCOOK/ETOH in air. Synergy effect between HCOOK and the catalysts improved the depolymerization process, resulting in a higher bio-oil recovery. HCOOK does not act solely as an in situ hydrogen source; it also interacts with lignin to enable its initial depolymerization via a base-catalyzed mechanism to low-molecular-weight fragments, and in tandem with the catalyst, the hydrogenolysis rate of the depolymerized lignin monomers was enhanced. Fe/HUSY displayed an excellent activity for the catalytic reductive step in contrast to Ni/HUSY and Ni–Fe/HUSY by facilitating methoxy group removal via hydrogenolysis, thereby contributing to the yield and stabilization of the low-molecular-weight aromatics [diethyl ether (DEE)-soluble products]. Fe/HUSY gave the highest DEE product yield of >99 wt % and a total benzyl alcohol yield of 16 wt % with a total selectivity of 47 wt % (60 wt % for aromatic alcohols). Fe/HUSY was reused for the lignin depolymerization reaction without much loss of its initial activity, giving 13 wt % yield of benzyl alcohols with a selectivity of 58 wt % (77 wt % for aromatic alcohols).

Direct lignin depolymerization process from sulfur-free black liquors

Agricultural residues (olive tree pruning and almond shell) were subjected to different delignification treatments (organosolv and soda) and the obtained liquors were treated in a high-pressure reactor at 300 °C for 80 min to depolymerize the dissolved lignin. In this way, the step of precipitating lignin from the liquor was avoided. The phenolic oil obtained after liquors treatment was around 20% of the organic matter contained in previous liquors in all cases. However, phenolic monomeric compounds varied in function of the liquor source. Soda black liquors produced higher quantity of catechols, phenol and cresols whereas using organosolv black liquors, more guaiacol and syringol were obtained, highlighting the higher potential enabled by base catalyst for demethoxylation, demethylation and dealkylation reactions. Furthermore, the NaOH present in soda black liquors prevented undesirable repolymerization reactions by inhibiting the char formation and noticeably dropping the molecular weight of residual lignin. However, organosolv liquors presented a significant higher yield of phenolic monomers, about three times higher than the one obtained in the soda process. Residual lignin, which was not only unconverted lignin, was proved to be different from the initial lignin, pointing out the totally conversion of the initial lignin samples.