Structural Features Of Lignin Research Articles

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 depolymerization, 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 depolymerization, 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.

Efficient fractionation of corn stover using deep eutectic solvent from lignin-derived 3-phenylpropionic acid: Towards a closed-loop biorefinery

The search for renewable deep eutectic solvents (DESs) for efficient biomass pretreatment based on a closed-loop biorefinery is of great interest and importance. Herein, we reported three novel lignin-based DESs prepared by choline chloride (ChCl) and 3-(4-hydroxyphenyl)propionic acid (HPA), 3-(4-hydroxy-3-methoxyphenyl)propanoic acid (HMPA) and their combination for corn stover pretreatment. The effects of three lignin-based DESs pretreatments on the chemical compositions of raw material, structural features of lignin, and enzymatic digestibility of cellulose were comprehensively analyzed, among which ChCl/HPA DES exhibited superior pretreatment efficiency, affording 58% of delignification, 62% of hemicellulose removal, and 85% of glucose yield. A high abundance of p-coumaric acid in the regenerated and remaining lignin samples makes them a sustainable feedstock to produce hydrogen bond donors of ChCl/HPA DES, as demonstrated by the results from 2D HSQC NMR analysis and catalytic depolymerization of lignins. These results could be conducive to creating an integrated biorefinery strategy for biomass valorization. Mass balance analysis indicated 81% of cellulose, 90% of hemicellulose, and 91% of lignin components in corn stover could be collected after DES pretreatment. This contribution offers a novel and efficient pretreatment method for corn stover fractionation based on a closed-loop biorefinery.

Unraveling lignin's structural features for catalytic oxidative depolymerization and enhanced renewable resource utilization

The effective utilization of lignin, a complex and abundant biopolymer in biomass, holds great potential for sustainable biomass conversion and the development of renewable resources. This study focuses on the catalytic oxidative transformation of lignin, aiming to unravel its structural features and their impact on product diversity. The significance of the content of β-O-4 linkages in lignin becomes evident as a crucial factor influencing the yield of phenolic monomer products, emphasizing its utmost importance. Additionally, the presence of S-type structural units in lignin positively influences its catalytic conversion efficiency, as evidenced by higher S/G ratios and increased degradation yields. However, the complex nature of lignin, influenced by growth environment and pretreatment methods, presents challenges for its efficient depolymerization and utilization. Therefore, a comprehensive understanding of lignin's structure is crucial for advancing its effective utilization and the development of renewable resources. This research elucidates the catalytic conversion mechanisms of lignin, providing practical implications for promoting the utilization of renewable resources in relevant fields.

Effect of torrefaction pretreatment on the selectivity improvement of lignin pyrolysis products

Pre-regulating the molecular composition and structural characteristics of lignin before pyrolysis is of great significance for the lignin valorization. In this work, torrefaction pretreatment of softwood kraft lignin (SKL) is carried out at different temperatures, and the characterization results show that torrefaction pretreatment is beneficial for removing oxygen element from SKL, which increases the high heating value of torrefied lignins (TLs). In addition, torrefaction pretreatment has a destructive effect on Cβ-O-C4 linkages, and reduces the molecular weight of TLs. Torrefaction pretreatment also removes the thermally unstable components from SKL and increases the release rate of pyrolysis volatiles. Furthermore, torrefaction pretreatment degrades the methoxy group (-OCH3) in SKL into phenolic hydroxyl group, which reduces the yield of guaiacyl type phenols in TLs pyrolysis products, and significantly increases the yield of catechol type phenols from 23.59% in SKL to 47.50% in TL300.

Evaluating performance of CrCl3-catalyzed ethanol pretreatment of poplar on cellulose conversion

As green and effective depolymerization processes, CrCl3 and CrCl3-catalyzed ethanol pretreatment were comprehensively investigated to improve cellulose conversion from pretreated residues in poplar in the present study. The influence of pretreatment temperature on delignification, structural characteristics of lignin, and the yield of glucose and ethanol was comparatively analyzed to emphasize the synergy effect of CrCl3 and ethanol solvent. Particularly, CrCl3-catalyzed ethanol pretreatment at 200 °C resulted in the pretreated substrates with >94% removal of hemicellulose and lignin, consequently reaching 88.88% and 56.11% yields of glucose and ethanol, respectively. Lignin's structural change was observed by Gel permeation chromatography and 2D nuclear magnetic resonance spectroscopy and its relationship with the efficiency of enzymatic hydrolysis and fermentation was determined. Results indicated that the increased pretreatment temperature enhanced the lignin depolymerization with smaller molecular weight, fewer β-O-4 linkages, and higher S/G ratio, especially in CrCl3-catalyzed ethanol pretreatment, which was positively associated with poplar cellulose conversion. In addition to offering a comprehensive illustration of the sugar/ethanol generation during CrCl3-catalyzed organosolv pretreatment, this work also provided theoretical support for biofuel and biochemical production from poplar.

Beyond Yield Optimization: The Impact of Organosolv Process Parameters on Lignin Structure

AbstractWhen optimizing the process parameters of the acidic ethanolic organosolv process, the aim is usually to maximize the delignification and/or lignin purity. However, process parameters such as temperature, time, ethanol and catalyst concentration, respectively, can also be used to vary the structural properties of the obtained organosolv lignin, including the molecular weight and the ratio of aliphatic versus phenolic hydroxyl groups, among others. This review particularly focuses on these influencing factors and establishes a trend analysis between the variation of the process parameters and the effect on lignin structure. Especially when larger data sets are available, as for process temperature and time, correlations between the distribution of depolymerization and condensation reactions are found, which allow direct conclusions on the proportion of lignin's structural features, independent of the diversity of the biomass used. The newfound insights gained from this review can be used to tailor organosolv lignins isolated for a specific application.

Exploring the Potential of Vine Shoots as a Source of Valuable Extracts and Stable Lignin Nanoparticles for Multiple Applications.

Large amounts of vine shoots are generated every year during vine pruning. This residue still presents many of the compounds found in the original plant, including low molecular weight phenolic compounds and structural compounds such as cellulose, hemicellulose, and lignin. For wine-producing regions, the challenge is to develop alternatives that will increase the value of this residue. This work proposes the full valorization of vine shoots, focusing on the extraction of lignin by mild acidolysis for the preparation of nanoparticles. The effect of the pretreatment solvents (ethanol/toluene, E/T, and water/ethanol, W/E), on the chemical and structural features of lignin, was evaluated. The chemical analysis suggests similar composition and structure regardless of the pretreatment solvent, although lignin isolated after pretreatment of biomass with E/T showed a higher content of proanthocyanidins (11%) compared with W/E (5%). Lignin nanoparticles (LNPs) presented an average size ranging from 130-200 nm and showed good stability for 30 days. Lignin and LNPs showed excellent antioxidant properties (half maximal inhibitory concentration, IC50 0.016-0.031 mg/mL) when compared to commercial antioxidants. In addition, extracts resulting from biomass pretreatment showed antioxidant activity, with W/E presenting a lower IC50 (0.170 mg/mL) than E/T (0.270 mg/mL), correlated with the higher polyphenol content of W/E, with (+)-catechin and (-)-epicatechin being the main compounds detected. Overall, this work shows that the pre-treatment of vine shoots with green solvents can yield (i) the production of high-purity lignin samples with antioxidant properties and (ii) phenolic-rich extracts, promoting the integral reuse of this byproduct and contributing to sustainability.

Revealing adsorption of mixed enzymes onto lignin resulted from integration of hydrothermal and chemi-mechanical pretreatment

Lignin is the main factor inhibiting enzymatic hydrolysis of lignocellulosic biomass. Different pretreatments can alter chemical and structural characteristics of lignin, thus affecting its adsorption capacity of enzymes. Herein, enzymatic mild acid lignin (EMAL) was separated from poplar branch wood chips as substrates resulted from different combined pretreatments, and then characterized and adsorbed by enzymes, so as to study the effects of different pretreatments on enzyme adsorption capacity. The results showed that the phenolic hydroxyl content and CC bond number of lignin increased after hydrothermal pretreatment, which increased the unproductive adsorption of mixed enzymes through hydrophobic interaction and hydrogen bonds. Chemical pretreatment after hydrothermal pretreatment can further reduce the unproductive adsorption of lignin on mixed enzymes, improving the enzymatic hydrolysis of substrates. This study would help to unmask the adsorption of mixed enzymes on lignin resulted from combined pretreatments, and facilitate the rapid development of efficient pretreatment for lignocellulosic biomass.

Fractionation of lignin from rice straw using an acidified biphasic solvent system

To obtain lignin from lignocellulosic biomass, phenoxyethanol (EPH) was employed to construct a biphasic solvent system. The concentration of EPH in this biphasic solvent system was first studied to determine a pretreatment condition for fractionation of lignin. Then, the fractionation of lignin from rice straw was performed under the conditions of temperature 130 °C, cooking time 60 min and sulfuric acid concentration 0.1 M, in 70 % aqueous EPH solvent system. The results showed that 50.97 %, 49.52 % or 82.02 % of the removed lignin with the purity of 89.04 %, 91.30 % or 84.76 % was regenerated from EPH liquor using dimethyl carbonate (DMC), dimethoxymethane (DMM) or diethyl ether (DE) as precipitant, respectively. Additionally, the weight-average molecular weight (Mw) and dispersity index (Đ) of the regenerated lignin decreased to 4247–4809 g/mol and 1.26–1.60 compared with that of the original lignin (5654 g/mol and 4.78). Finally, the compositional and structural characteristics of lignin, e.g., molecular weight and molecular structure, were also investigated by DSC, HSQC and elemental analysis.

2-Naphthol modification alleviated the inhibition of ethanol organosolv lignin on enzymatic hydrolysis

It has been reported that the lignins deposited on surfaces of pretreated biomass directly affected the enzyme adsorption, thus impeding enzymatic hydrolysis of lignocellulose. Alleviating the surface lignin inhibition is urgently needed for establishing an efficient lignocellulose biorefinery process. In this study, ethanol organosolv lignins (EOLs) from masson pine and poplar were employed as representatives of surface lignins. Aiming to mitigate the EOLs inhibition on enzymatic hydrolysis, 2-naphthol as a carbocation scavenger was added in the organosolv pretreatment. The function of 2-naphthol for suppressing lignin condensation was verified by investigating the lignin structural changes with NMR analysis. Moreover, it was verified that 2-naphthol modification remarkably eliminated the negative effects of EOLs from both two wood species, leading to a 6.8–34.4% increase in hydrolysis yields. The effects of 2-napthol modified lignins were even reversed to slightly promote the enzymatic hydrolysis of pure cellulose. The mechanism of 2-naphthol modification for relieving lignin inhibition was revealed by evaluating the relationship between lignin structural features and hydrolysis performance with Pearson's correlation heatmap. Results enrich our understanding of surface lignin inhibition, thereby providing ideas for promoting enzymatic hydrolysis by alleviating the surface lignin inhibition.

New ternary deep eutectic solvents with cycle performance for efficient pretreated radiata pine forming to lignin containing cellulose nanofibrils

Ternary deep eutectic solvents (TDESs) as a new class of solvents play a key role in fabrication of lignin containing cellulose nanofibrils (LCNF), however, current work requires a considerable time and thus limits industrial application of TDES. Herein, a novel strategy was proposed to produce lignin containing cellulose nanofibrils (LCNF) by TDESs based on benzyltrimethylammonium chloride (BTMAC), formic acid (FA), and maleic acid (MA). The TDESs could remove 87.80% of lignin and 89.15% of hemicellulose from Radiata Pine while most of the cellulose reserved under optimum pretreatment conditions (TDES = 100 wt%, T = 130 °C, t = 1.5 h). Benefiting from the increase in cellulase accessibility, desired sugar yield of 98.9% was obtained. Moreover, the proposed TDESs could be regarded as a green medium for the production of containing cellulose nanofibrils (LCNF) in which lignin has been esterified. The LCNF displayed a well-individualized structure with a wide size distribution (32.3–146.9 nm), which was closely associated with lignin content. 13C−1H HSQC 2D NMR test was performed to illustrate the molecular − structural features of lignin and polysaccharide in LCNF. In addition, the self-standing LCNF films exhibited excellent UV-blocking, higher thermal stability, and mechanical strength. Most importantly, LCNF films showed strong barrier property to oil and air owing to lower surface free energy and smaller pore structure. Briefly, this work highlights the pretreatment performance of TDESs for cascade utilization of lignocellulosic biomass to high-value products.

Machine Learning Optimization of Lignin Properties in Green Biorefineries

Novel biorefineries could transform lignin, an abundant biopolymer, from side-stream waste to high-value-added byproducts at their site of production and with minimal experiments. Here, we report the optimization of the AquaSolv omni biorefinery for lignin using Bayesian optimization, a machine learning framework for sample-efficient and guided data collection. This tool allows us to relate the biorefinery conditions like hydrothermal pretreatment reaction severity and temperature with multiple experimental outputs, such as lignin structural features characterized using 2D nuclear magnetic resonance spectroscopy. By applying a Pareto front analysis to our models, we can find the processing conditions that simultaneously optimize the lignin yield and the amount of β-O-4 linkages for the depolymerization of lignin into platform chemicals. Our study demonstrates the potential of machine learning to accelerate the development of sustainable chemical processing techniques for targeted applications and products.

Lignin fractionation to realize the comprehensive elucidation of structure-inhibition relationship of lignins in enzymatic hydrolysis

A better understanding of the relationship between lignin structures and their inhibitory effects in enzymatic saccharification would facilitate the development of lignocellulose biorefinery process. However, the heterogeneity of lignins challenges the elucidation of lignin structure-inhibition correlation. In this study, two types of lignin fractions including ethanol soluble lignins and ethanol insoluble lignins were respectively isolated from the poplars pretreated with various severities. The impacts of pretreatment severities on the structural changes of lignin fractions were studied from the perspective of inter-units linkages, condensed aromatic substructure, and hydroxyl groups. Furthermore, it was observed that lignin addition strongly inhibited the enzymatic saccharification of pure cellulose by 13.3 ∼ 56.3%. Lignin inhibition extents were increased with the elevated pretreatment severity. The relationships between the lignin structural features and lignin inhibition were analyzed, which revealed that the contents of condensed aromatic units and phenolic hydroxyl were crucial factors determining the lignin inhibition.

Thermoplasticity reinforcement of ethanol organosolv lignin to improve compatibility in PLA-based ligno-bioplastics: Focusing on the structural characteristics of lignin

Commonly, lignin macromolecules have limitations in application to the thermoplastics industries due to poor dispersibility and interfacial compatibility within ligno-bioplastics. In this study, the dispersibility and interfacial compatibility of ethanol organosolv lignin (EOL) in PLA-based ligno-bioplastic were improved by enhancing the thermoplasticity via oxypropylation. Further, three types of EOLs extracted from different severity conditions were applied to investigate the effect of the structural characteristics of EOLs on the changes in the thermal properties. The thermal properties of oxypropylated EOL were dependent on the structural characteristics of the initial EOL as well as the degree of polymerization of propylene oxide. The thermoplasticity of EOLs extracted under mild condition was effectively increased as a new Tg and melting were observed. Based on increased thermoplasticity, the dispersibility and interfacial compatibility of EOL within PLA-based ligno-bioplastic were successfully improved, which compensates for the deterioration in mechanical strength of ligno-bioplastic due to the addition of unmodified EOL. Therefore, oxypropylation of EOL with suitable structural characteristics promises improved availability as a thermoplastic material.

Biobased rigid polyurethane foam using gradient acid precipitated lignin from the black liquor: Revealing the relationship between lignin structural features and polyurethane performances

Rigid polyurethane foams (RPFs) are a type of versatile polymers with a variety of applications, which can be produced by condensation polymerization of isocyanates with renewable polyols that are derived from biomass materials. This study investigates the characteristics of bio-sourced synthesized RPF using lignin fractionated from the alkaline black liquor of corn stover. GPC, FTIR, TGA, 2D-HSQC, and 31P NMR were used to characterize the isolated lignin specimens. The fractionated lignin specimens were then pre-polymerized followed by using them as the substitutes of RPFs. The characterization and properties of the RPFs substituted with the isolated lignins were analyzed. Compared with other lignin specimens, the specimen isolated in pH range of 5.5–2.0 afforded to higher compressive strength (0.39 ± 0.01 MPa), lower apparent density (41.35 ± 0.21 kg m-3), lower thermal conductivity (0.043 ± 0.0012 W m-1 K-1), and better thermal stability of the RPF product. In addition, the relationships between structural features of lignin specimens and the properties of the RPFs were investigated. Results show the content of β-O-4 bond inner lignin structure was the main factor that affected on the thermal conductivity of RPF, while a higher hydroxyl groups content in lignin would largely improve the compressive strength and leading to the reduction of the apparent density. This research helps to comprehensively understand the features of lignin on RPF’s performance, providing a new method for its functionalization which is significant in their commercialization.

Insights into alkaline choline chloride-based deep eutectic solvents pretreatment for Populus deltoides: Lignin structural features and modification mechanism

Deep eutectic solvent (DES) is a kind of green solvent for biorefinery, which favors the progress of being more environmentally friendly and effective. A better understanding of structural changes of lignin is necessary to optimize pretreatment conditions and efficient utilization of the resultant lignin. The current study reported the structural features of lignin recovered from alkaline ChCl/imidazole and ChCl/urea DES pretreatment, and the mechanism of lignin modification was revealed. The profiling demonstrated that lignin samples possessed a high purity (>94.4%), low molecular weight ranging from 1544 to 2562 g/mol and an excellent uniformity (PDI < 1.6). Noteworthy, the content of β-O-4′ linkages in lignin was over 75% (i.e. 72.2%–77.4% retention); S/G ratio was increased whereas the content of -OCH3 groups were decreased. It was revealed that slight cleavage of β-O-4′ linkages, preferential breakdown of G units, and demethylation reaction were occurred during alkaline ChCl-based DES pretreatment. Specifically, cleavage of ester linkages between PB and lignin macromolecule was taking place during ChCl/imidazole pretreatment at a high temperature; whereas oxidation only appeared in ChCl/urea system. Despite the modification, well β-O-4′ preserved and less condensed lignin samples were recovered after low-temperature pretreatment. Consequently, high contents of phenol derivatives (26.3–30.6%) were achieved in lignin oil. The present study provides critical information on alkaline ChCl-based DES pretreatment, which will contribute to the valorization of lignin by-products and will be beneficial to the development of biorefineries.

Characterization of lignin streams during ionic liquid/hydrochloric acid/formaldehyde pretreatment of corn stalk

This work investigated the role of formaldehyde (FA) in lignin anti-condensation during corn stalk pretreatment based on 1-butyl-3-methylimidazolium chloride ([C4C1im]Cl)/hydrochloric acid (HCl). As a result of the aldolization reactions between FA and lignin, the condensation of lignin fragments was inhibited, and lignin remained in soluble fragmental molecules. Characterizations on the compositional and structural changes of lignin and its degraded products during pretreatment (80 °C–100 °C, 2–5 h) with FA addition in comparison with those in DO/HCl/FA or [C4C1im]Cl/HCl were conducted. Results revealed that the structural features of lignin were affected by FA addition and solvent type. In the [C4C1im]Cl/HCl/FA system, FA stabilization was unfavorable for the cleavage of β-O-4′ bonds and lignin with low S/G ratio (3.4) and high molecular weight (Mw = 9920 g·mol−1) was extracted. The compositions of degraded products were considerably affected by FA addition.

Unlocking Structure-Reactivity Relationships for Catalytic Hydrogenolysis of Lignin into Phenolic Monomers.

Lignin depolymerization into aromatic monomers with high yields and selectivity is essential for the economic feasibility of biorefinery. However, the relationship between lignin structure and its reactivity for upgradeability is still poorly understood, in large part owing to the difficulty in quantitative characterization of lignin structural properties. To overcome these shortcomings, advanced NMR technologies [2D HSQC (heteronuclear single quantum coherence) and 31 P] were used to accurately quantify lignin functionalities. Diverse lignin samples prepared from Eucalyptus grandis with varying β-O-4 linkages were subjected to Pd/C-catalyzed hydrogenolysis for efficient C-O bond cleavage to achieve theoretical monomer yields. Strong correlations were observed between the yield of monomeric aromatic compounds and the structural features of lignin, including the contents of β-O-4 linkages and phenolic hydroxyl groups. Notably, a combined yield of up to 44.1 wt % was obtained from β-aryl ether rich in native lignin, whereas much lower yields were obtained from technical lignins low in β-aryl ether content. This work quantitatively demonstrates that the lignin reactivity for acquiring aromatic monomer yields varies depending on the lignin fractionation processes.

A simple fractionation method and GPC analysis of organosolv extracts obtained from lignocellulosic materials

The elucidation of lignin structural features is necessary for the efficient application of this polymer. Fractionation is an effective technique to achieve homogeneous lignin fractions, helping to better understand its composition and structure, and improving its use as source of phenolic compounds. This work develops a new and simple fractionation method of organosolv extracts obtained from lignocellulosic materials that enables the selective separation of lignin (from complex macromolecules to low molecular weight aromatic compounds originated from their degradation) from saccharides derived from hemicelluloses and cellulose. For this purpose, the non-catalytic organosolv extraction of lignin from almond shells was performed with dioxane/water mixtures and then the successful fractionation of the organosolv extracts with anhydrous tetrahydrofuran was achieved, resulting in the selective separation of lignin (lignin fraction) from saccharides (saccharide fraction). Gel permeation chromatography (GPC), thioacidolysis, and GC-MS were used as analytical methods to qualitatively determine the purity of both fractions. In addition, GPC was also used to quantify the content of lignin monomers and furfural in the organosolv extracts, thus allowing valuable information on the state of degradation of the lignin and hemicelluloses extracted to be obtained, respectively. The application of the GPC method also enabled the determination of the molecular weight distribution of the extracted lignin in the same analysis.