Lignin Depolymerization Research Articles

Quantification of Native Lignin Structural Features with Gel‐Phase 2D‐HSQC0 Reveals Lignin Structural Changes During Extraction

AbstractOur ability to study and valorize the lignin fraction of biomass is hampered by the fundamental and still unmet challenge of precisely quantifying native lignin's structural features. Here, we developed a rapid elevated‐temperature 1H−13C Heteronuclear Single‐Quantum Coherence Zero (HSQC0) NMR method that enables this precise quantification of native lignin structural characteristics even with whole plant cell wall (WPCW) NMR spectroscopy, overcoming fast spin relaxation in the gel phase. We also formulated a Gaussian fitting algorithm to perform automatic and reliable spectral integration. By combining HSQC0 measurements with yield measurements following depolymerisation, we can confirm the combinatorial nature of radical coupling reactions during biosynthesis leading to a random sequential organization of linkages within a largely linear lignin chain. Such analyses illustrate how this analytical method can greatly facilitate the study of native lignin structure, which can then be used for fundamental studies or to understand lignin depolymerization methods like reductive catalytic fractionation or aldehyde‐assisted fractionation.

Quantification of Native Lignin Structural Features with Gel-Phase 2D-HSQC0 Reveals Lignin Structural Changes During Extraction.

Our ability to study and valorize the lignin fraction of biomass is hampered by the fundamental and still unmet challenge of precisely quantifying native lignin's structural features. Here, we developed a rapid elevated-temperature 1H-13C Heteronuclear Single-Quantum Coherence Zero (HSQC0) NMR method that enables this precise quantification of native lignin structural characteristics even with whole plant cell wall (WPCW) NMR spectroscopy, overcoming fast spin relaxation in the gel phase. We also formulated a Gaussian fitting algorithm to perform automatic and reliable spectral integration. By combining HSQC0 measurements with yield measurements following depolymerisation, we can confirm the combinatorial nature of radical coupling reactions during biosynthesis leading to a random sequential organization of linkages within a largely linear lignin chain. Such analyses illustrate how this analytical method can greatly facilitate the study of native lignin structure, which can then be used for fundamental studies or to understand lignin depolymerization methods like reductive catalytic fractionation or aldehyde-assisted fractionation.

Transformation of organosolv lignin into sustainable aromatics: Catalytic hydrodeoxygenation using carbon-supported bimetallic MoRu catalyst

While lignin holds potential as a sustainable source for fuels and chemicals, it faces hurdles due to its large molecular size, low heating values, and high oxygen content. In this study, the reductive depolymerization of organosolv lignin was investigated using mono-/bimetallic carbon-supported catalysts. At 340 °C, the MoRu/AC catalyst significantly reduced organosolv lignin feed's molecular weight from 2600 to 460 g/mol, yielding ∼70 % depolymerized organosolv lignin with a higher heating value of 32.92 MJ/kg, mainly composed of substituted phenolic compounds/aromatic hydrocarbons. Conversely, the MoRu/ACP catalyst underperformed, possibly due to residual acidity triggering undesirable side reactions like condensation. Thorough investigations revealed that introducing Mo into Ru catalysts significantly influences the hydrodeoxygenation process, resulting in H/C and O/C ratios of 1.29 and 0.20, contrasting with the initial feed's 1.05 and 0.35 ratios. Furthermore, density functional theory calculations showed lower energy barriers for breaking Caryl−OH/Caryl−O* bonds in guaiacol, a model compound, due to MoRu interactions, implying a promising deoxygenation potential for organosolv lignin transformation. The successful production of high-value bio-oil from technical lignin using the MoRu catalyst showcases its potential to pave the way toward carbon neutrality by offering eco-friendly alternatives to traditional fossil fuels and chemicals.

Sustainable depolymerization of lignin into aromatic compounds using amphiphilic Anderson-type polyoxometalate catalysts

Lignin serves as a primary abundant source of renewable aromatic compounds. Achieving efficient breakdown of lignin and retaining its aromatic properties is highly desirable but remains a challenging task. To address this challenge, we synthesized Anderson-type polyoxometalate (POM) catalysts, particularly [CTAC]2[CoMo6]. We then investigated the effectiveness of the POM catalysts in the oxidative depolymerization of larch lignin. Under conditions of 160 °C, 1.0 MPa oxygen atmosphere, and a catalyst-to-substrate ratio of 1:5, we achieved a monomer yield of phenolic compounds at 12.43 wt%. The unsaturated coordination sites of Mo5+ within the catalysts were identified as active sites, facilitating enhanced O2 adsorption and activation. The enhanced O2 adsorption significantly influenced the production of aromatic monomers from lignin. We observed that the catalysts effectively cleaved CC bonds in β-O-4 dimer compounds using lignin dimer model compounds. Notably, the [CTAC]2[CoMo6] catalyst exhibited excellent stability across five cycles, maintaining its high efficiency in lignin depolymerization. This indicates that Anderson-type POM-based catalysts exhibit potential for sustainable conversion of biomass into valuable compounds and for enhancing lignin valorization processes.

Depolymerization of Native Lignin over Thiol Capped Ultrathin ZnIn2S4 Microbelts Mediated by Photogenerated Thiyl Radical.

The valorization of native lignin to functionalized aromatic compounds under visible light is appealing yet challenging. In this communication, colloidal mercaptoalkanoic acid capped ultrathin ZnIn2S4 (ZIS) microbelts was successfully fabricated, which was used as a superior catalyst for depolymerization of native lignin in birch woodmeal under visible light, with an optimum yield of 28.8 wt % to functionalized aromatic monomers achieved in 8 h. The capped mercaptoalkanoic acid not only enables a solvent modulated reversible interchange of ZIS between the colloidal state for efficient reaction and the aggregated state for facile separation, but also serves as a precursor for light initiated generation of reactive thiyl radical for highly selective cleavage of β-O-4 bond in native lignin. This work provides a green and efficient strategy for the depolymerization of native lignin to functionalized aromatic monomers under mild conditions, which involves a new mechanism for the cleavage of β-O-4 bonds in native lignin. The capability of cleavage of β-O-4 bonds in native lignin by photogenerated thiyl radicals also demonstrates the great potential of using photogenerated thiyl radicals in organics transformations.

N-doped MOF-lignin hybrid catalyst for highly selective hydrodeoxygenation of lignin-derived phenols

Upgrading of lignin-derived bio-oil to value-added biofuel was of great significance for efficient utilization of renewable resources and reducing the use of fossil fuels. Hydrodeoxygenation (HDO) was considered to be a promising technology for the conversion of lignin-derived bio-oil. Herein, we reported a novel method for the preparation of lignin-MOF derived N-doped catalysts, which was then successfully introduced into the upgrading of lignin-derived phenols and lignin depolymerization. All catalysts were well characterized and Co/CN-0.5–750 exhibited excellent performance during the catalytic HDO process. The optimized catalyst demonstrated highly efficient active hydrogen generation from isopropanol. Moreover, it could afford the best vanillin (VAN) conversion (100 %) and highest 2-Methoxy-4-methylphenol (MMP) yield (92 %) under the mild reaction conditions (180 °C, 0.5 MPa nitrogen, 2 h). It was confirmed that the excellent performance was related to nitrogen doping. This study could provide some useful insights for the hydrodeoxygenation of lignin and its derivative.

Direct conversion of lignin to liquid hydrocarbons for sustainable biomass valorization

Lignin, a principal biomass component, is a promising resource for manufacturing value-added products, tackling environmental concerns, and fostering sustainable energy alternatives. However, its intricate structure and recondensation reactions pose substantial conversion challenges. This study introduces an innovative one-pot catalytic technique for lignin’s direct conversion into liquid hydrocarbons over a Ru/C catalyst. The method integrates lignin depolymerization and the hydrogenolysis of resultant phenolics, achieving an 84.3 % carbon yield mainly constituting benzene, toluene, xylene, and cycloalkanes. The solvent-free concept in this system is proposed and tested, it streamlines downstream separation and boosts energy efficiency. These findings highlight the viability of repurposing waste lignin into liquid hydrocarbons and high-value chemicals, thereby supporting sustainable biomass utilization advancements.

Eugenol Hydrodeoxygenation Over Mixed Mo-W Carbides.

The modification of molybdenum carbide catalysts by another transition metal has raised an increasing research interest due to the significant improvement of catalyst activity in hydrodeoxygenation of lignin derivatives. At par with the commonly used Co and Ni that add a strong hydrogenation functionality, it was found that the addition of the more oxophilic W restricts ring hydrogenation while allowing the deoxygenation of oxygenated compounds and thus yielding higher selectivity toward the formation of non-oxygenated aromatic compounds. The coexistence of Mo2C with W2C along with metallic W altered the electronic properties of Mo2C which resulted in an increase of catalyst active site density and facilitated further total eugenol deoxygenation. Propyl-benzene selectivity of up to 83 % was reached at close to 100 % eugenol conversion. These findings will allow a better overview of the effect of different metal phases of mixed carbides on the catalyst performance and raise the prospect of optimizing catalyst design for a hydrodeoxygenation processing of lignin depolymerization products.

High-selectivity demethoxylation of lignin alkylmethoxyphenols enhanced by hydrogen spillover on Ag/Fe-CoOx nanocatalyst

This investigation focuses on the catalytic demethoxylation of alkylmethoxyphenols derived from lignin depolymerization, aiming to convert them with high selectivity into essential chemical intermediates, namely methoxy-free alkyl phenols. A silver-loaded iron-cobalt oxide (Ag/Fe-CoOx) nanocomposite catalytic system was developed, featuring a dual-active-site catalytic mechanism driven by a hydrogen-spillover effect that involves the activation of hydrogen at the metallic active site, while the support's secondary site serves as the reaction locus for the substrates. It was revealed that by integrating iron into cobalt oxide nanosheet supports, catalyst activity and selectivity were finely tuned, particularly to enhance the production of demethoxylated phenols. With the optimized Ag/Fe-CoOx catalyst, a 99.7 % conversion of 4-propylguaiacol and a 94.1 % selectivity for 4-propylphenol were achieved, indicating effective reductive demethoxylation at 280°C and 0.4 MPa H2. The fine structure of the support significantly affects demethoxylation selectivity, guiding the development of selective catalyst design in biomass conversion.

Catalytic Depolymerization of Lignin into Aromatic Monomers over Ru-Ni/TiO2 Catalysts

Catalytic Depolymerization of Lignin into Aromatic Monomers over Ru-Ni/TiO2 Catalysts

Exploring factors affecting the dissociation energies of C–O and C–C bonds in lignin oligomers

The bond dissociation energy (BDE) of lignin is a key factor for lignin depolymerization, assisting in revealing the depolymerization mechanism. However, the impact of substituent as well as solvent, which usually stay together, on BDE of lignin remains unclear. Here, density functional theory (DFT) calculation is employed to compute the BDE of C–O and C–C bonds in lignin oligomers with six substituents in four solvents. The relationship models between multiple factors and BDE are established using random forest algorithm, achieving a predictive accuracy of 0.97. The results show that oxidating hydroxyl to ketone benefits the cleavage of β-O-4 and β-1 bonds. Hydroxyl and methoxy effectively contribute to breaking the Cβ-1 and Cα–Cβ bonds in the β-1 dimer. Polar solvents combined with hydroxyl promote C–O bond dissociation, while alkyl-containing solvents favor C–C bond dissociation. These findings provide valuable insights into lignin depolymerization, pre-depolymerization structural modifications, and the prediction of depolymerization pathways.

Evaluation of OxiOrganosolv pretreated hardwood and softwood lignocelluloses as substrates for the chemoenzymatic production of 5-hydroxymethylfurfural (HMF)

Furans, such as 5-hydroxymethylfurfural (HMF), are compounds of great importance that can serve as starting materials for the synthesis of polymers. Their production from lignocellulose-derived sugar streams offers a promising alternative to fossil fuels, while enabling biomass transformation to chemicals with higher value. In the present work, the production of HMF from OxiOrganosolv pretreated beechwood and pine was assessed by integrating a three-step process of enzymatic saccharification and isomerization followed by catalytic dehydration. The use of isobutanol in the pretreatment solvent and the addition of polyoxometallates (POMs) as oxidative catalysts were evaluated. The results showed that isobutanol leads to high delignification rates for both beechwood and pine, yielding cellulose-rich pulps with high susceptibility to enzymatic hydrolysis and isomerization. A fructose production up to 51.2 and 53.4 g/g of pretreated material was achieved for beechwood and pine, respectively, corresponding to 14 and 11.3 g of HMF/g of pretreated material. Regarding the use of POMs, the commercially available phosphomolybdic acid (HPMO) and POMs modified with oxidation metals (Fe-PMO, Cu-PMO) were tested, verifying their beneficial effect to lignin depolymerization and the composition of the final pulp. Hydrolysates produced from HPMo and Cu-PMo-assisted OxiOrganosolv pretreatment were efficiently used for the production of HMF, while severe inhibition of the dehydration reaction was observed with the hydrolysates from Fe-PMo pretreated biomass due to the presence of residual metals. This is the first systematic report comparing two lignocellulosic materials subjected to different pretreatment conditions for their potential to produce fructose and, subsequently, HMF.

Depolymerization of lignin over heterogeneous Co-NC catalyst

Green and atom-economic depolymerization of lignin remains a great challenge due to its complex non-repetitive structure and the inert property for chemo-digestion. A redox-neutral lignin depolymerization system without the use of extra oxidant and/or reductant over a Co-NC catalyst has been developed in this work, providing the first non-noble metal heterogeneous catalytic system for redox-neutral valorization of lignin. Mechanistic studies based on control reactions and deuterium labeling experiments suggest that the reaction proceeds via a metal-catalyzed dehydrogenation of Cα-OH to afford a carbonyl intermediate, followed by Cβ-O bond cleavage (via hydrogenolysis) to afford monophenols and aromatic ketone products. The hydrogen used for the cleavage of the Cβ-O bond originates from the alcohol moiety in the substrate, and the cascade dehydrogenation and hydrogenolysis steps are highly coupled, rendering it an efficient and atom-economic process.

Promotion of lignin depolymerization and cellulose utilization via the coupling effects of DDQ pre-oxidation and HCOOH extraction

The full utilization of biomass plays an important role in the environment and energy. In this study, the coupling pretreatment of DDQ pre-oxidation and HCOOH extraction was employed to realize the separation and modification of three components of sugarcane bagasse for full utilization. Especially, the modified lignin was depolymerized to obtain aromatic monomers, and cellulose-rich residue was used to prepare lignocellulose straws. Furthermore, the coupling effects between DDQ pre-oxidation and HCOOH extraction on chemical and physical properties of modified lignin and lignocellulose straws were studied. In addition, the mechanism of lignin depolymerization and lignocellulose straw synthesis were explored as well as the mass balance of the biorefinery process. The results showed that the total yield of aromatic monomers increased by nearly 2 times and the conversion of lignin was 95.1 % after DDQ pre-oxidation. The mechanism showed that DDQ selectively oxidized Cα-OH bond of lignin to Cα = O bond, which not only avoided the formation of carbocation, but also reduced the bond dissociation energy of β-O-4 bond. Moreover, the tensile strength of hydrophobic lignocellulose straw increased by nearly 2 times and its flexural strength was 10.36 MPa. Modified lignin and modified cellulose were connected via hydrogen bonds, which enhanced the intermolecular crosslinking of molecules, thus improving the mechanical properties of lignocellulose straws. Besides, the maximum xylose relative content and concentration of xylose were 61.16 % and 17.57 g/L, respectively. This study provides a method for high-value and green utilization of sugarcane bagasse.

Unlocking the potential of phenolated kraft lignin as a versatile feed additive

Lignin, a renewable natural antioxidant and bacteriostat, holds promise as a versatile, cost-effective feed additive. However, traditional industrial lignin faces limitations, including low reactivity, poor uniformity, and unstable properties, necessitating chemical modification. Complex modification methods pose economic and toxicity challenges, so this study adopted a relatively simple alkali-catalyzed phenolization approach, using phenol, catechol, and pyrogallol to modify kraft lignin, and characterized the resulting products using various techniques. Subsequently, their antioxidant, antibacterial, adsorption properties for heavy metal ions and mycotoxins, growth-promoting properties, and antiviral abilities were assessed. The phenolation process led to lignin depolymerization and a notable increase in phenolic hydroxyl content, particularly in pyrogallol-phenolated lignin (Py-L), rising from 3.08 to 4.68 mmol/g. These modified lignins exhibited enhanced antioxidant activity, with over 99 % inhibition against E. coli and S. aureus, and remarkable adsorption capacities for heavy metal ions and mycotoxins. Importantly, Py-L improved the growth performance of mice and reduced influenza mortality. Furthermore, density functional theory calculations elucidated the mechanism behind the enhanced antioxidant properties. This study presents a promising avenue for developing versatile feed additives to address challenges related to animal feed antioxidant supplementation, bacterial control, and growth promotion.

Microwave Depolymerization of Lignin via Dynamic Vapor Flow Reaction System: HCOOH as Pretreatment Solvent or Reforming Solvent Vapor.

Different forms of HCOOH in the depolymerization system play an important role in governing the monomeric products from lignin. We reported two strategies for the introduction of HCOOH to enrich the monophenols from kraft lignin by microwave-assisted depolymerization. The reaction of lignin models showed that HCOOH was in favor of the cleavage of C-O bonds (β-O-4 typically) and partial C-C bonds (Cα-Cβ). Subsequently, Microwave-assisted depolymerization of lignin with two strategies was conducted via a designed dynamic vapor flow reaction system. Strategy A with HCOOH as pretreatment solvent showed excellent monophenols enrichment with total mass yields of 193.71 mg/g (lignin basis). Strategy B using HCOOH as reforming solvent vapor significantly increased the monophenols selectivity. It presented unique reforming and upgrading performance by generating catechol (42.59 mg/g, lignin basis) and homovanillic acid (17.58 mg/g, lignin basis). This study provided potential strategies for the efficient conversion of kraft lignin into high-value platform chemicals.

Mesoporous SBA‐15 Supported CdS through Organosilane Ligand for Photo Driven Cleavage of Aryl Ether in Lignin and its Model Compound

AbstractMesoporous silica (SBA‐15) was synthesized hydrothermally followed by calcination and then functionalization with 3‐(triethoxysilyl)‐1‐propanethiol through surface grafting. The thiol moiety of the grafted ligand was utilized for the uniform anchoring of cadmium into the channels of the material. The synthesised material was subjected for its photo catalytic applications after characterization through standard characterization protocol. The synthesised mesoporous silica (i.e. SAB‐15), ligand modified material (i.e.SBA‐15‐S and Cd loaded ( i.e. SAB‐15‐CdS) materials were characterized through standard characterization protocol like powder XRD, FT‐IR, UV‐DRS, MAS NMR, N2 sorption, FE‐SEM, HR‐TEM and TG‐DTA studies. After successful characterization, the screening of the catalyst was carried out on a model reaction involving photo catalytic cleavage of α‐O‐4 aryl ether linkage present in benzyl phenyl ether, which is an important lignin model compound. The catalyst exhibited good efficiency along with the selectivity for desired monoaromatic platform chemicals. The synthesized catalyst was also carried out for the photo catalytic depolymerization of alkali lignin at the optimized reaction conditions which further showed the promising results for getting value added chemicals from lignin. The reaction products were characterized by using Gas chromatography‐mass spectrometry (GC‐MS) techniques.

Effect of alkaline deep eutectic solvents pretreatment on CH4 yield from anaerobic digestion of corn stover

Deep eutectic solvents (DES), comprising hydrogen bond donors (HBD) and acceptors (HBA), are gaining prominence in biomass pretreatment due to their excellent lignin depolymerization capabilities. This study explored the impact of alkaline DES pretreatment, specifically focusing on CH4 yield from corn stover anaerobic digestion. Batch experiments were conducted to assess the influence of ethylene glycol (EG) concentration, pretreatment time, and organic loading rate (OLR) on CH4 yield. Results indicated that these factors markedly affected the cumulative CH4 yield. Subsequent optimization using Box-Behnken design (BBD) significantly enhanced the CH4 yield by 1.16 times, achieving the optimal conditions at 0.4 v/v EG concentration, 33.8 min of pretreatment, and 22.2 w/v OLR. Notably, DMER64 and Methanobacterium emerged as the dominant bacterial and archaeal communities, constituting 40.2 % and 71.9 % respectively. Spearman correlation analysis underscored that enhanced substrate accessibility fostered a hydrogen-based, CH4-producing synergistic metabolic pathway. Alkaline DES pretreatment notably resulted in 20.5 % of the carbon flow being directed to CH4, demonstrating its efficacy in converting corn stover into a viable clean energy source and mitigating the environmental impact of agricultural waste.

A review on chemical mechanisms of kraft pulping

Abstract Kraft pulping of wood is based on efficient depolymerization and solubilization of lignin, while cellulose is relatively undamaged. Non-cellulose cell wall polysaccharides are however in some cases heavily degraded, especially pectin and to a lesser degree also glucomannan while, xylan is relatively stable. In this mini-review, the most important reactions in lignin and polysaccharide degradation in kraft pulping are described, both the technically favorable and the problematic reactions, and the chemical background to discuss the advantages and drawbacks of the process. An attempt to put the different reactions in the perspective of the goals of the pulping process is made and a special focus is on the development of color in the pulp fiber during the kraft pulping.

Catalytic hydrogenolysis of organosolv lignin: Cleaving C–O bonds over CuMgAlOx-layered porous metal oxide catalysts for oriented monophenols production

To understand the catalytic conversion of lignin into high-value products, lignin depolymerization was performed using a layered polymetallic oxide (CuMgAlOx) catalyst. The effects of the conversion temperature, hydrogen pressure, and reaction time were studied, and the ability of CuMgAlOx to break the C–O bond was evaluated. The CuMgAlOx (Mg/Al = 3:1) catalyst contained acidic sites and had a relatively homogeneous elemental distribution with a high pore size and specific surface area. The β-O-4 was almost completely converted by disassociating the C–O bond, resulting in yields of 14.74% ethylbenzene, 47.58% α-methylphenyl ethanol, and 36.43% phenol. The highest yield of lignin-derived monophenols was 85.16% under reaction conditions of 280 °C and 3 Mpa for 4 h. As the reaction progressed, depolymerization and condensation reactions occurred simultaneously. Higher temperatures (> 280 ℃) and pressures (> 3 Mpa) tended to produce solid char. This study establishes guidelines for the high-value application of industrial lignin in the catalytic conversion of polymetallic oxides.