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NMR Database of Lignin and Cell Wall Model Compounds
This database was designed to provide a coherent, single source of NMR data of lignin and other plant cell wall model compounds. The database exists as an Adobe pdf cross-platform file for viewing and printing. This is the latest public version of the Database, version 2024/08 updated from the 2009 version.
The effect of lignin degradation products on the generation of pseudo-lignin during dilute acid pretreatment
Pseudo-lignin is an insoluble material that tends to deposit on acidic pretreated fiber surface and negatively impacts the biological conversion of biomass to valued added products. Employing a laboratory generated acidic hydrolysate solution from dilute acid pretreatment of bamboo residues, we sought to seek an improved understanding of pseudo-lignin formation from fragmented carbohydrates. In addition, we also performed experiments to investigate how lignin model compounds interact with carbohydrates en route to pseudo-lignin’s formation. The yields of pseudo-lignin generated from xylose (CXPL) were higher than those generated from glucose (CGPL). Gas chromatography-Mass spectrometry and Foloin-Ciocalteau analysis suggested that there were more aromatic compounds in hydrolyzate during CGPL’s formation. The presence of lignin phenolic model compounds had both a positive and negative effect upon quantitative yields of pseudo-lignin. Gel permeation chromatography and NMR analysis ( 31 P and 2D-HSQC NMR) of the pseudo-lignin samples generated in the presence and absence of 4-hydroxybenzoic acid (4-HA), employed as a lignin model compound, revealed drastic differences in molecular weight, changes in hydroxyl group content, and hydrocarbon bonds, suggesting significant differences in the pseudo-lignin reaction pathways when certain lignin model compounds are present. Finally, our findings demonstrate the effects of both hemicellulose carbohydrate makeup and lignin model compound impact on the formation of pseudo-lignin and its chemical properties, suggesting a potential way to minimize pseudo-lignin formation.
Lignocellulosic Processing Stream Analytical: Cooperative Research and Development (Final Report)
The project will determine the detailed composition of process lignocellulosic biomass slurries and solids using NREL’s unique capabilities in the area.
Data-Centric Development of Lignin Structure–Solubility Relationships in Deep Eutectic Solvents Using Molecular Simulations
Lignin is a natural source of aromatic chemicals with significant potential as an abundant, renewable feedstock for value-added products. Deep eutectic solvents (DES)–solvents composed of a hydrogen bond donor (HBD) and acceptor (HBA) in varying ratios–have emerged as a highly tunable class of solvents for lignin solubilization. However, the variety of possible DES compositions and limited molecular-scale understanding of lignin solubility makes solvent selection a challenge without laborious trial-and-error experimentation. To address these challenges, we use classical molecular dynamics (MD) simulations to study the interactions of lignin model compounds with various DES–water systems. Quantitative parameters (descriptors) were calculated by postprocessing the MD results and used to train a regression model that predicts experimentally determined solubilities of lignin model compounds. This approach revealed that the most important descriptors of solubility are the system temperature, solute hydrophilicity, and metrics quantifying hydrogen bonding. Maximizing the interactions between solute–HBD (hydrophobic group), water–HBD (hydrophilic group), and water–HBA molecules led to the highest model compound solubility. Our results support a hydrotropic mechanism in which extensive DES–water hydrogen bonding and favorable HBD interactions with the solute promote high solubility. We applied the regression model derived using model compounds to predict the solubility of representative lignin oligomers. The model predicted lignin oligomers’ solubilities in good agreement with experiments, indicating that the simulations of model compounds can be extended to predict the solubility of larger lignin compounds across a range of solvent compositions and temperatures. Furthermore, these findings provide new molecular-scale insight into lignin solubilization mechanisms and a new method for computationally screening potential solvent systems for lignin valorization.
Mild, Electroreductive Lignin Cleavage: Optimizing the Depolymerization of Authentic Lignins
Among candidate strategies to depolymerize lignin, electroreduction has promise as a mild, environmentally friendly, low-cost method to cleave lignin’s most common linkage, the β-O-4 bond. Much of the prior work on lignin electrolysis has focused on anodic (oxidative) approaches, while most available electroreduction studies have studied lignin model compounds rather than authentic lignin. Here, after optimizing with a model system, we investigated the room-temperature electroreductive cleavage of four common lignin samples from different pretreatment approaches, including Kraft, Soda, GVL, and Cu-AHP lignin. This mild electrolysis process resulted in the solubilization of 45% of the GVL lignin and 60% of the Cu-AHP lignin, affording approximately 17.6 and 19.4 wt % of monomeric materials, respectively. Furthermore, the present work is an encouraging first step toward an electroreductive strategy for lignin depolymerization and upgrading.
Enzymatic cleavage of model lignin dimers depends on pH, enzyme, and bond type
Lignin is composed of phenylpropanoid monomers linked by ether and carbon-carbon bonds to form a complex heterogeneous structure. Bond-specific studies of lignin-modifying enzymes (LMEs; e.g., laccases and peroxidases) are limited by the polymerization of model lignin substrates and repolymerization of cleavage products. Here we present a high throughput platform to screen LME activities on four tagged model lignin compounds that represent the β-O-4’, β-β’, 5–5’, and 4-O-5’ linkages in lignin. We utilized nanostructure-initiator mass spectrometry (NIMS) and model lignin compounds with tags containing perfluorinated and cationic moieties, which effectively limit polymerization and condensation of the substrates and their degrading products. Sub-microliter sample droplets were printed on the NIMS chip with a novel robotics method. This rapid platform enabled characterization of LMEs across a range of pH 3–10 and relative quantification of modified (typically oxidized), cleaved, and polymerized products. All tested enzymes oxidized the four substrates and cleaved the β-O-4’ and β-β’ substrates to monomeric products. We discovered that the active pH range depended on both the substrate and the enzyme type. This has important applications for biomass conversion to biofuels and bioproducts, where the relative percentages of different bond types in lignin varies depending on feedstock and chemical pretreatment methods.
Mechanocatalytic Hydrogenolysis of the Lignin Model Dimer Benzyl Phenyl Ether over Supported Palladium Catalysts
This work demonstrates the mechanocatalytic hydrogenolysis of the ether bond in the lignin model compound benzyl phenyl ether (BPE) and hardwood lignin isolated by hydrolysis with supercritical water. Pd catalysts with 4 wt % loading on Al 2 O 3 and SiO 2 supports achieve 100% conversion of BPE with a toluene production rate of (2.6–2.9) × 10 –5 mol·min –1 . The formation of palladium hydrides under H 2 gas flow contributes to an increase in the turnover frequency by a factor of up to 300 compared to Ni on silica–alumina. While a near-quantitative toluene yield is obtained, some of the phenolic products remain adsorbed on the catalyst.
Solvation effect on binding modes of model lignin dimer compounds on MWW 2D-zeolite
Lignin as a potential renewable source of biofuels, chemicals, and other value-added products has gained much attention. However, the complexity of lignin structure poses a significant challenge for developing efficient valorization techniques. As most processes involve solvothermal conditions to minimize energy cost, lignin depolymerization is governed by reaction conditions (temperature and pressure), and solvents. In this work, binding of β -O-4 linkage consisting lignin dimers on MWW 2-dimensional (2D) zeolite is investigated using periodic density functional theory (DFT). Furthermore, the effect of different terminated surfaces (H:OH % = 100:0; 50:50; 0:100 %), different temperatures (323, 353, 373 K), and different solvents (water and methanol) on the binding modes is quantified. Here, our work shows that in the gas phase the binding strength increases 10 to 15 kcal/mol upon increasing the number of hydroxyl groups on the surface. Also, the phenolic dimer binds more strongly than the non-phenolic dimer, and the binding strength of model compounds increases in the presence of solvent. Analysis of structural changes in the presence of the solvent reveals that aromatic rings are parallel to the zeolite surface and primary interaction with zeolite is through the hydroxyl groups near the β -O-4 linkage. Furthermore, while the solvation energy decreases with increasing temperature the opposite trend is observed for the binding energy with the surface.
Efficient Synthesis of Pinoresinol, an Important Lignin Dimeric Model Compound
Pinoresinol is a high-value monolignol-derived lignan used in plant defense and with human health-supporting effects. The synthetic yield and isolation efficiency of racemic pinoresinol from coniferyl alcohol by conventional radical coupling methods is sub-optimal. In this work, a facile and efficient synthetic approach was developed to synthesize pinoresinol with much higher yield. By using 5-bromoconiferyl alcohol, which was synthesized in high yield from 5-bromovanillin, to make 5,5′-bromopinoresinol via a peroxidase-mediated radical coupling reaction takes advantage of the smaller variety of radical coupling products from the 5-substituted monolignol, producing simpler product mixtures from which 5,5′-bromopinoresinol may be readily crystalized with good yield (total yield of 44.1% by NMR; isolated crystalline yield of 24.6%). Hydro-debromination of the crystalline 5,5′-bromopinoresinol to pinoresinol was essentially quantitative. Gram quantities of pinoresinol were conveniently synthesized by using this approach. This simple alternative pathway to make pinoresinol will impact pinoresinol-related research including structural characterization and modification of lignins, as well as clinical applications of pinoresinol and its derivatives.
Advances in versatile nanoscale catalyst for the reductive catalytic fractionation of lignin
In the past five years, biomass derived-biofuel and biochemicals were widely studied both in academia and industry as a promising alternative to petroleum. In this review article, the latest progress of the synthesis and fabrication of porous nano-catalysts that are used in catalytic transformations involving hydrogenolysis of lignin is reviewed in terms of their textural property, catalytic activity, and stability. A particular emphasis is dedicated to the catalyst design for the hydrogenolysis of lignin and/or lignin model compounds. Furthermore, the effects of different supports on the lignin hydrogenolysis/hydrogenation are discussed in detail. Finally, the challenges and future opportunities of lignin hydrogenolysis over nanomaterials supported catalysts are also presented.
Effect of Solvents on Lignin–Surface Interactions via Molecular Dynamics Simulations
Lignin, an essential building block of lignocellulosic biomass, is a potential abundant source of aromatic monomers for the polymer and chemical industry. Reductive catalytic fractionation (RCF) is one promising process that can produce high yields of phenolic monomers and oligomers from lignin under different catalytic conditions. An important choice in optimizing RCF is the selection of solvent; however, detailed insights into solvent effects on lignin behaviors and interactions remain limited. Here, in this work, we perform all-atom molecular dynamics simulations to study the solvation of lignin, solvent-mediated conformational changes, and the interaction of solvated lignin oligomers with model surfaces. We focus on the behavior of an oligomeric lignin model compound in methanol, ethanol, a binary mixture of ethanol and water, and water at both the RCF reaction temperature (473 K) and room temperature. Analysis of structural features of lignin suggests that these three organic solvent systems favorably solvate lignin, resulting in a more extended conformation suitable for catalytic conversion to valuable chemicals. We further introduce model palladium (Pd) and carbon (C) surfaces to understand how solvent choice impacts adsorption onto a representative catalytic surface and support, and to quantify the competition among the reactant and solvent molecules for the surface. Unbiased simulations suggest strong adsorption of lignin on both Pd and C surfaces at 473 K, with notable solvent-mediated differences in adsorption energies. Additionally, our findings indicate that lignin adsorption is promoted by the entropy change resulting from the displacement of solvent molecules from the surface. This study provides a molecular perspective of adsorption of lignin onto varying surfaces, which is a step towards understanding and optimizing the catalytic conversion of lignin into valuable chemicals.
Electrochemical Aminoxyl-Mediated Oxidation of Primary Alcohols in Lignin to Carboxylic Acids: Polymer Modification and Depolymerization
An electrochemical process has been developed for chemoselective oxidation of primary alcohols in lignin to the corresponding carboxylic acids. The electrochemical oxidation reactions proceed under mildly basic conditions and employ 2,2,6,6-tetramethyl-1-piperidine Noxyl (TEMPO) and 4-acetamido-TEMPO (ACT) as catalytic mediators. Lignin model compounds and related alcohols are used to conduct structure-reactivity studies that provide insights into the origin of the reaction selectivity. Here, the method is applied to the oxidation of lignin extracted from poplar wood chips via a mild acidolysis method, and the reaction affords a novel polyelectrolyte material. Gel permeation chromatography data for the oxidized lignin shows that this material has a molecular weight and molecular weight distribution very similar to that of the extracted lignin, but notable differences are also evident. Base titration reveals a significant increase in the acid content, and the oxidized lignin has much higher water solubility relative to the extracted lignin. Lastly, treatment of the oxidized lignin under acidic conditions results in depolymerization of the material into characterized aromatic monomers in nearly 30 wt% yield.
Effect of Solvent Quality on Structure and Dynamics of Lignin in Solution
The conversion of lignin into useful chemicals and monomers requires that different linkages connecting monomers are exposed to the catalytic sites. As most conversion processes are expected to occur in the liquid phase, it is important to understand the structure and dynamics of lignin in solution. Here, we have examined the structure and dynamics of hardwood- and softwood-derived lignin model compounds with 61 monomers in methanol/water solution.
Role Evaluation of Active Groups in Lignin on UV-Shielding Performance
The contribution of the active groups in lignin to ultraviolet (UV)-shielding performance still remains unclear. In this study, we employed 16 lignin model compounds and theoretical calculations to ascertain the contribution of different functional groups to UV-shielding performance. These results showed that the order of UV-shielding performance on functional groups was -C=C–CHO > -C=C–COOH > -C=C–CH 2 OH > --C=C > quinone > -COOH > -OCH 3 ≈ -OH. Moreover, the synergism between the para-substituted electron-donating group -OH and the electron-withdrawing group allyl on the aromatic ring played a major role in UV-shielding performance of lignin models, whereas the additive effect of meta/ortho-substituted -OCH 3 played a secondary role. Furthermore, we found that the hydrophile–lipophile balance (HLB) value of lignin models also greatly affected the sunscreen protection factor (SPF) of lignin model-based sunscreens. A closer HLB value of lignin to 8.89 corresponded with a higher SPF of the lignin model-based sunscreen. The commercial photoprotective ingredient ethylhexyl methoxycinnamate (EHMC) and lignin models had a synergistic effect on UV-shielding performance caused by the π–π stacking interaction between the parallel-displaced stacked aromatic rings of lignin models and EHMC. Additionally, the UV irradiation test showed that these sunscreens prepared using lignin models and EHMC exhibited good photostability.
Mechanistic Insights into the Demethylation of Lignin‐Derived Structures Using Protic Ionic Liquids: A Density Functional Theory Study
Lignin valorization is restricted by the stability of its methoxy groups, creating a critical need for efficient demethylation strategies. Here, in this study, density functional theory (DFT) is employed to dissect the mechanistic pathways of demethylation in lignin model compounds, guaiacol and syringol, using protic ionic liquids (PILs) that act as both solvent and catalyst. Conductor‐like Screening Model for Real Solvents (COSMO‐RS) analysis identifies monoethanolammonium acetate ([MEOA][Ace]) as the most promising medium, attributed to its strong hydrogen bonding network and solvation ability. By integrating implicit and explicit solvation models, it is revealed that an acid‐catalyzed hydrolytic mechanism governs demethylation, with PILs stabilizing crucial transition states and intermediates. Complementary electronic structure evaluations, including highest occupied molecular orbital–lowest unoccupied molecular orbital (HOMO‐LUMO) gap analysis, charge distribution, and electrostatic potential mapping, demonstrate how PILs lower energetic barriers and enhance reactivity. To simulate realistic environments, this study is extended to lignin dimer complexes with varying water content, uncovering how water‐bridged solvation reverses demethylation preference from guaiacyl to syringyl units. This mechanistic shift aligns with experimental observations showing faster S‐unit reactivity in hydrated systems. Together, these findings provide atomic‐level insight into lignin demethylation dynamics and highlight how tuning acid concentration in PILs can accelerate kinetics, enabling a rational pathway toward next‐generation biomass conversion technologies.
A unique Co@CoO catalyst for hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran
The development of precious-metal-free catalysts to promote the sustainable production of fuels and chemicals from biomass remains an important and challenging target. Here, we report the efficient hydrogenolysis of biomass-derived 5-hydroxymethylfurfural to 2,5-dimethylfuran over a unique core-shell structured catalyst, Co@CoO that affords the highest productivity among all catalysts, including noble-metal-based catalysts, reported to date. Surprisingly, we find that the catalytically active sites reside on the shell of CoO with oxygen vacancies rather than the metallic Co. The combination of various spectroscopic experiments and computational modelling reveals that the CoO shell incorporating oxygen vacancies not only drives the heterolytic cleavage, but also the homolytic cleavage of H 2 to yield more active H δ- species, resulting in the exceptional catalytic activity. Co@CoO also exhibits excellent activity toward the direct hydrodeoxygenation of lignin model compounds. This study unlocks, for the first time, the potential of simple metal-oxide-based catalysts for the hydrodeoxygenation of renewable biomass to chemical feedstocks.