Lignin Structure Research Articles

One-Pot Production of Cinnamonitriles from Lignin β-O-4 Segments Induced by Selective Oxidation of the γ-OH Group.

The construction of N-containing aromatic compounds from lignin is of great importance to expanding the boundary of the biorefinery and meeting the demand for value-added biorefinery. However, it remains a huge challenge due to the complex lignin structure and the incompatible catalysis for C-O/C-C bond cleavage and C-N formation. Herein, sustainable synthesis of cinnamonitrile derivatives from lignin β-O-4 model compounds in the presence of 2,2,6,6-tetramethylpiperidine oxide (TEMPO), (diacetoxyiodo)benzene (BAIB), and a strong base has been achieved in a one-pot, two-step fashion under transition-metal-free conditions. Mechanistic studies suggest that this transformation starts from selective oxidation of Cγ-OH of the β-O-4 model compound, followed by retro-aldol condensation, resulting in the cleavage of the Cα-Cβ bond to afford veratraldehyde. Whereafter, the aldol condensation reaction allows coupling of veratraldehyde with nitriles to provide cinnamonitriles. With this protocol, 3,4-dimethoxycinnamonitrile and 3,4-dimethoxyphenyl-2-phenylacrylonitrile were synthesized from lignin β-O-4 model compounds and showed good antibacterial or antifungal activity, showcasing the application potential of lignin in pharmaceutical synthesis.

A comprehensive review of the application of cold plasma technology in lignocellulosic biomass pretreatment

AbstractLignocellulosic biomass (LCB) is a promising feedstock for the sustainable production of biofuels and other biobased products, owing to its abundance, low cost, and minimal competition with food crops for resources. It is mainly composed of cellulose, hemicellulose, and lignin, forming a complex biomass matrix. Its utilization is hindered by its complex structure and pretreatment is required to break down the chemical resistance caused by the strong association between the cellulose, hemicellulose, and lignin structures. Conventional methods such as acid and alkali pretreatment are used frequently but they require the use of high temperatures, high pressure, and corrosive chemicals. This has encouraged research into environmentally friendly pretreatments to overcome these challenges. Recently, cold plasma (CP) technology has emerged as a promising alternative to the aggressive conventional methods for producing value‐added products, such as biofuels, and renewable chemicals from lignocellulosic biomass. Cold plasma technology uses electricity to generate a highly reactive, ionized gas in a variety of applications without producing any dangerous or polluting compounds. This review discusses the application of cold atmospheric pressure plasma in the biochemical conversion of biomass, with a focus on delignification, detoxification of biomass hydrolysate, surface modifications, and process intensification of cellulosic ethanol production.

Regulation and function of a polarly localized lignin barrier in the exodermis

AbstractMulticellular organisms control environmental interactions through specialized barriers in specific cell types. A conserved barrier in plant roots is the endodermal Casparian strip (CS), a ring-like structure made of polymerized lignin that seals the endodermal apoplastic space. Most angiosperms have another root cell type, the exodermis, that is reported to form a barrier. Our understanding of exodermal developmental and molecular regulation and function is limited as this cell type is absent from Arabidopsis thaliana. We demonstrate that in tomato (Solanum lycopersicum), the exodermis does not form a CS. Instead, it forms a polar lignin cap (PLC) with equivalent barrier function to the endodermal CS but distinct genetic control. Repression of the exodermal PLC in inner cortical layers is conferred by the SlSCZ and SlEXO1 transcription factors, and these two factors genetically interact to control its polar deposition. Several target genes that act downstream of SlSCZ and SlEXO1 in the exodermis are identified. Although the exodermis and endodermis produce barriers that restrict mineral ion uptake, the exodermal PLC is unable to fully compensate for the lack of a CS. The presence of distinct lignin structures acting as apoplastic barriers has exciting implications for a root’s response to abiotic and biotic stimuli.

Efficient fractionation of lignin from Camellia oleifera shell by acidic deep eutectic solvent under mild conditions

In this study, ethylene glycol-oxalic acid deep eutectic system (DES) was used for delignification of Camellia oleifera shell (COS). The effects of treatment temperature and time on lignin removal rate, lignin purity and yield were investigated. At the treatment temperature and treatment time of 90 °C and 6 h, the residue rate, lignin removal rate and yield values of 42.33 %, 67.79 % and 45.84 %, respectively. The structure of lignin was ascertained by performing Fourier transform infrared spectroscopy (FT-IR) and two-dimensional nuclear magnetic resonance spectroscopy (2D HSQC NMR), the spectra of which indicated that COS lignin mainly comprises guaiacyl (G) units, followed by syringyl (S) units. Thermal analysis of lignin was conducted based on thermogravimetric analysis (TG) and differential scanning calorimetry (DSC), and the glass transition temperature of lignin was determined to be between 90 and 130 °C. The results in this study indicated that ethylene glycol-oxalic DES system can be used to facilitate the efficient delignification of COS.

Fractionation of lignin from corncob with high−yield p − coumaric acid production in deep eutectic solvents followed by pyrolysis to produce monophenols

Two distinctive aromatic units, p − coumarate and ferulate, exist in corncob lignin, which have the potential to yield p − coumaric acid (pCA) and ferulic acid (FA). Although pCA and FA are primarily extracted from corncob lignin utilizing strong acids and bases, extremely acidic or alkaline conditions result in the disruption of the aromatic unit structure of the residual lignin. Herein, lactic acid coupled with choline chloride was utilized as acidic deep eutectic solvent (DES), while K2CO3 with glycerin was used as alkaline DES, thereby facilitating the extraction of pCA, FA and lignin from corncob in a mild environment. Furthermore, the differences in monophenol yields from pyrolysis of lignin were investigated. The alkaline DES exhibited a stronger extraction capacity for these acids. The yields of pCA and FA were 15.47 mg/g and 7.44 mg/g (based on the weight of corncob). Contrastively, the subsequent pyrolysis process yielded a higher amount of monophenol from the lignin extracted using acidic DES, with notably greater quantities of low methoxy phenolic monomers. This renders it a preferred option for subsequent processing into high−calorific biofuels. This work presents a straightforward and efficient strategy for the deconstruction of the lignin from corncob to enhance the utilization value of agricultural waste.

The Feasibility of Using Lignin as Modifier for Asphalt Cement

Modifying asphalt cement is an effective approach to enhance the performance of asphalt concrete. This study evaluates the physical properties of lignin-modified asphalt (LMA) at various lignin percentages. The initial step involved extracting lignin from the pinus wood sawdust using the soda process. Subsequently, the extracted Soda Lignin Powder (SLP) was characterized through Fourier transformation infrared spectroscopy (FTIR), and Scanning Electron Microscopy-Energy Dispersive X-ray Analysis (SEM/EDX) to examine the changes in the chemical structure of lignin after extraction by soda process. Following this, three asphalt blends containing varying amounts of SLP (2%, 4%, 6%) by weight of asphalt cement were produced to assess physical properties such as penetration, softening point, ductility, and rotational viscosity test. The findings indicate that incorporating SLP into asphalt cement increases stiffness and viscosity compared to virgin asphalt cement, with increasing SLP content. These results suggest that lignin presents significant enhancement to the performance of asphalt cement under high service temperature conditions.

Effects of Silicon Concentration and Synthesis Duration on Lignin Structure: A Spectroscopic and Microscopic Study.

Silicon (Si) is a highly abundant mineral in Earth's crust. It plays a vital role in plant growth, providing mechanical support, enhancing grain yield, facilitating mineral nutrition, and aiding stress response mechanisms. The intricate relationship between silicification and lignin chemistry significantly impacts cell wall structure. Yet, the precise influence of Si on lignin synthesis remains elusive. This study investigated the interaction between Si and lignin model compounds during invitro synthesis. Employing spectroscopic and microscopic analyses, we delineated how Si concentrations modulate lignin polymerization dynamics, particularly affecting molecular conformation and aggregation behavior over time. Fluctuations in the polymer structure are directly related to both the synthesis time and the concentration of silica. Our results demonstrate that lower Si concentrations promote the aggregation of lignin oligomers into larger particles, while higher concentrations increase the possibility of oligomer repulsion, thus preventing particle growth. These findings elucidate the intricate interplay between Si and lignin, which is crucial for understanding plant cell wall structure and stress resilience. Moreover, the results provide insights for developing lignin-silica materials with increasing applications in industry and medicine.

Controllable preparation of lignin-based photothermal composites based on fractionation treatment

Using natural renewable lignin to prepare new photothermal materials can improve the value of lignin, reduce costs, and promote sustainable development. However, the heterogeneity of lignin is an important factor limiting the stability and tunability of lignin-based photothermal materials. In this paper, lignin after fractionation treatment was blended with the Polybutylene succinate (PBS) to obtain an environmental-friendly, tunable and multifunctional photothermal material. The modulation and enhancement of the compatibility, mechanical, thermal, and photothermal properties of PBS/Lignin composites were achieved by adjusting the structure of lignin. The abundant conjugated structure and high near-infrared (NIR) light absorption capability endowed the high molecular-weight lignin with 71.34 % photothermal conversion efficiency, which led to the maximum surface temperature of the as-prepared composite reached 291 °C under NIR light of 0.9 W/cm2. Meanwhile, the composite exhibited favorable light-triggered shape memory behavior (98.2 % fixation rate and 92.6 % recovery rate), effective light-controlled self-healing capability (91.6 % self-repairing efficiency), and significant photothermal antimicrobial activity (90.6 % inhibition of Escherichia coli, 88.1 % inhibition of Staphylococcus aureus) under the stimulation of NIR light. This study serves as a reference for regulating and enhancing the photothermal conversion efficiency of lignin and developing environmental-friendly photothermal materials.

Effects of ultrasonic-alkali integrated extraction combined with Mannesi reaction on the antimicrobial properties of rice straw lignin and enhancement strategies

Lignin is one of the most abundant and underused biopolymers in nature with limited antimicrobial activities. Herein, this work aimed to enhance the antimicrobial activity of lignin extracted from waste rice straw by ultrasonic-alkali integrated extraction (USP-AT) and modify the alkali lignin through Mannich reaction to improve its antimicrobial properties. The effects of ultrasonic pretreatment (USP) time on the chemical structure, morphology, antioxidant, and antibacterial activities of lignin were studied. The results demonstrated that the total phenolic content of USP-AT lignin was higher than that of lignin extracted by alkali treatment. Moreover, the antioxidant activity of USP-AT lignin was increased by 49.69 %–69.42 %. The antibacterial activity of USP-AT lignin against Escherichia coli and Staphylococcus aureus increased by more than 40 %. However, the antibacterial capacity of USP-AT lignin is far from meeting the requirements of antibacterial food packaging. The alkali lignin was modified by Mannich reaction in order to improve its antimicrobial properties against common spoilage microorganisms or pathogenic bacteria in food and packaging. The modified USP-AT lignin exhibit remarkable antimicrobial capacities to representative bacteria, molds, and yeasts. The antifungal capacity of modified USP-AT lignin against A. niger and P. citrinum were improved by more than 40 %.

On the nature of the selectivity of oxygen delignification

Abstract This work has focused on oxygen’s role in the delignification process within the context of pulp production. We have investigated the role of oxygen in a complex set of chemical reactions taking place during this process, including both oxidative and non-oxidative reactions. This study explores the impact of pH changes during the oxygen delignification process and the characteristics of the resulting pulps. Additionally, this research examines the effect of oxygen, by comparing conventional oxygen delignification with trials using air and nitrogen. Industrial softwood kraft pulps with a kappa number of 35 were subjected to delignification for 20–120 min under alkaline conditions. The resulting pulps were assessed for kappa number, intrinsic viscosity, fiber charge, and ISO brightness. An important observation from this research is the reduction in lignin molecular weight upon exposure to oxygen and air, suggesting depolymerization reactions facilitated by oxygen species, whereas nitrogen exposure results in less pronounced changes. This finding underscores the impact of oxygen in altering lignin structure, thus informing the selectivity and effectiveness of the delignification process.

Microstructural optimization of lignin‐based carbon fiber: Effects of purification and high‐temperature heat treatment

AbstractLignin is a renewable biomass polymer with a carbon content of more than 60% and has been extensively studied for making lignin‐based carbon fibers (CFs). However, impurities and high carbohydrate content in lignin materials severely limit their molding in melt spinning, and pre‐oxidation and carbonization processes also affect the subsequent preparation of CFs. This paper presents a detailed study of lignin‐based carbon fiber preparation. Pretreatment increased lignin's decomposition temperature to 302.1°C and enabled melt‐spun fiber preparation. Then, pre‐oxidation and carbonization of lignin fibers were further discussed. The pre‐oxidized fibers prepared at 280°C and 0.4°C min−1 have high thermal stability and excellent carbonization performance. The lignin‐based carbon fibers (LCFs) prepared at 1100°C and 3°C min−1 have the highest degree of graphitization, and the surface is smooth without obvious holes. Under these conditions, LCFs have a graphitization degree of 1.50 and conductivity of 62.50 S cm−1, making them suitable for sensors and capacitors.Highlights Effects of pretreatment on lignin structure and properties have been studied. Effects of pre‐oxidation and carbonization on the lignin fibers were studied. Lignin‐based carbon fibers with high conductivity were obtained.

Investigating the depolymerization behavior of structurally modified kraft lignin in deep eutectic solvents

Deep eutectic-like solvents (DESs) are ideal green solvents for achieving depolymerization of kraft lignin under mild conditions. In this study, the impact of changes in chemical structures in hardwood kraft lignin (HKL), softwood kraft lignin (SKL), hydroxypropyl-modified hardwood kraft lignin (PHKL), and acetylated hardwood kraft lignin (AHKL) on depolymerization was investigated. The results indicated that structural modification can effectively adjust the depolymerization reaction of lignin. Under the depolymerization conditions (150 °C, 3 h, CHCl:p-TSOH), the depolymerization extent of HKL was 44.8%, which was 9.4% higher than that of SKL (35.4%). It was found that after hydroxypropyl modification of phenolic hydroxyl in HKL, the depolymerization extent of PHKL decreased to 29.5%, confirming that phenolic hydroxyl was the key functional group promoting lignin depolymerization. Acetylation modification of HKL can effectively increase the lignin depolymerization extent from 44.8% to 54.2%. The experimental results of lignin model compound addition confirmed that acetylation modification can effectively inhibit condensation reactions. The GPC analysis showed that the average molecular weight (Mw) of depolymerized oil was less than 800 g/mol and had a narrow polydispersity index (Mw/Mn < 1.2), making it an important precursor for high-value applications such as the preparation of aviation fuel.

Modulation of improved lignin structure for conversion of iron oxides on the metal artifact

This research reveals the structural analysis and antioxidant properties of unmodified ethanol organosolv lignin (AH EOL) and modified ethanol organosolv lignin (AHD EOL and AHPB EOL) upon incorporation with 1,5-dihydroxy naphthalene and p-benzoquinone, respectively. The extracted lignin derived from OPF was then evaluated with FTIR NMR, GPC, thermal analyses (TGA and DSC), solubility test, and antioxidant activity. Modified ethanol organosolv lignin incorporated with p-benzoquinone (AHPB EOL) exhibited the smallest lignin matrix, which leads to the highest water solubility and antioxidant activity compared to other ethanol organosolv lignin (EOL) samples (D% AHPB EOL: 24.25 ± 0.13 % > D% AHD EOL: 22.50 ± 0.16 % > D% AH EOL: 20.50 ± 0.22 %). The extracted EOL samples derived from OPF were then utilized in a preliminary rust conversion study of a metal artifact, which indicated that 7 wt% AHPB EOL possessed the highest RT% with 88.81 ± 0.14 %. The XRD and surface analysis of the treated rust also showed that it had transformed the archeological rust into an amorphous phase.

Exploring the Photocatalytic Cleavage Pathway of the β-5 Linkage Lignin Model Compound on Carbon Nitride.

As a globally abundant source of biomass, lignocellulosic biomass has been the centre of attention as a potential resource for green energy generation and value-added chemical production. A key component of lignocellulosic biomass, lignin, which is comprised of aromatic monomers, is a potential feedstock for value added chemical production. The cleavage processes of the linkages between monomers to obtain high value products, however, requires significant investigation as it is a complex, non-facile process. This study focuses on the photocatalytic valorization of a β-5 lignin model compound, a key linkage in the lignin structure. It was found that greater yields of aromatic products were obtained from the photocatalytic conversion of β-5 lignin model compound using carbon nitride (CN) when compared to Evonik P25 titanium dioxide (TiO2). Products of the β-5 model compound photocatalytic conversion were determined and C-C bond cleavage was observed. It was also determined that the solvent participated in the reactions with the introduction of a cyano group to one of the products. Radical quenching experiments revealed that superoxide radicals participated in the CN photocatalytic conversion. These results reveal for the first time the products and possible mechanism of the photocatalytic transformation of β-5 model compounds using CN photocatalysis.

Mild degradation of corncob extraction residue in a novel sodium metavanadate and aqueous hydrogen peroxide/acetic anhydride system for monophenolic compounds

A novel sodium metavanadate and aqueous hydrogen peroxide/acetic anhydride system, was used to convert corncob extraction residue (CCER) for producing monophenolic compounds (MPCs). The system demonstrates excellent performance for producing MPCs in a high yield from CCER and CCER conversion to soluble portion (SP) is ca. 97.7 wt%. According to multiple analyses the SP is rich in aromatics. The system is also effective for oxidatively degrading other lignin-rich raw materials to produce aromatics as the main products. In the initial step, H2O2 breaks a >C-O- bond in a peroxy chain of lignin structure to produce phenylpropanoid, followed by H2O2 attacking Cβ and Cα to produce H, G, and S-type aromatics.

A new strategy to prepare bio-based polylactic acid modifier for multi-functionality: Dopamine-grafted modified lignin

Currently, to address the performance deficiencies associated with high brittleness, inadequate thermal stability, and susceptibility to hydrolysis in polylactic acid (PLA), it is common practice to incorporate multiple or high proportions of additives. However, this approach raises concerns regarding interface compatibility, energy consumption, and environmental impact. In this context, we introduce an environmentally friendly, bio-based PLA modifier that is facile to prepare and facilitates multi-functional applications at a minimal dosage. The presence of a benzene ring structure in lignin endows the material with ultraviolet (UV) resistance, thermal stability, and hydrophobic characteristics, while the exceptional adhesion properties of polydopamine (PDA) enhance interfacial compatibility. A novel biomass polylactic acid modifier, designated as OL@PDA, has been synthesized by modifying low-molecular-weight lignin (OL) with PDA. Comparative analysis of OL@PDA/PLA composite materials reveals a 14.08 % increase in tensile strength and a 42.00 % enhancement in elongation at break relative to pure PLA. The incorporation of the lignin-derived benzene ring structure effectively transforms PLA from a hydrophilic to a hydrophobic material. Furthermore, the UV transmittance at 400 nm (T400 nm%) is recorded at 65.8 % for pure PLA, which significantly decreases to below 5 % for OL@PDA/PLA composites. The integration of OL@PDA as a modifier in PLA materials yields comprehensively performance enhancements while maintaining a low additive concentration.

Assessing the potential for upscaling the continuous production of lignin oils through reductive catalytic depolymerization

Lignin represents up to one third of lignocellulosic biomass and is a sustainable carbon source available at a sufficient scale to replace an impactful amount of fossil resources when converted into materials and chemicals. The molecular structure of lignin renders it ideal to obtain renewable aromatic building blocks for the polymer industry. Nevertheless, in polymeric form, lignin poses serious challenges for material applications and chemical conversion, namely poor solubility, and low reactivity. This problem can be circumvented by depolymerization of isolated lignins. To meet the demand for renewable aromatic building blocks, it is crucial to utilize technical lignin streams generated on large scale. This will eventually allow the development of continuous, high-throughput processes which are required for viable industrial-scale operations. Our work demonstrates for the first time that “technical grade” hydrolysis lignin from an operating biorefinery can be successfully subjected to continuous reductive catalytic depolymerization (RCD) in a packed bed reactor using a commercially available Pd/Al2O3 catalyst. The impact of catalyst amount, feed concentration (in methanol), and temperature with regards to process stability and product quality was investigated. The product was characterized by NMR, GPC, and GC-FID. Feed concentrations up to 7 wt% could be continuously processed for 77 h at 200 °C whereas at 225 °C, the operation time was restricted to 21 h. In all cases, a stable output was obtained over time in terms of molar mass, OH-group distributions, and monomer content. At 200 °C, carbon yields above 90 % of the soluble lignin fraction were feasible.

Exploiting Lignin Structure and Reactivity to Design Vitrimers with Controlled Ratio of Dynamic to Non-Dynamic Bonds.

Lignin is an abundant biobased feedstock, representing the first source of renewable aromatic structures. Thanks to its high functionality in aliphatic hydroxyls (Al-OH), phenolic hydroxyls (Ph-OH) and carboxylic acids (COOH), lignin is an attractive precursor to crosslinked polymer materials. Different biobased macromolecular architectures can be designed from lignins, whose end-of-life should also be considered in the context of a circular bioeconomy. To enhance the recyclability of crosslinked polymer networks, the introduction of dynamic linkages to design vitrimers is a promising strategy. In this study, Kraft lignin was chemically modified with succinic anhydride, to prepare a series of modified lignins with a controlled COOH/Ph-OH ratio, exploiting the difference in reactivity between Al-OH and Ph-OH groups. Upon crosslinking with a diepoxy, mixed vitrimer networks with variable ratios between dynamic ester bonds and non-dynamic ether bonds were synthesized. The analysis of their properties evidenced the impact of the non-dynamic linkages on the materials behaviors, including their dynamicity and reprocessing ability. Although the activation energy for bond exchange is increased, non-dynamic linkages do not hinder the reprocessability of these adaptable materials, and provide them high creep resistance. The controlled introduction of non-dynamic linkages appears as a promising strategy to enhance the properties of lignin-based vitrimers.

Unlocking full potential of bamboo waster: Efficient co-production of xylooligosaccharides, lignin, and glucose through low-dosage mandelic acid hydrolysis with alkaline processing

Mandelic acid (MA), a natural and environmentally friendly organic acid, demonstrates high selectivity and efficiency in hydrolyzing hemicellulose, making it an excellent candidate for xylooligosaccharides (XOS) production at low acid dosages. Despite its potential, the application of MA for XOS production has not been evaluated. The study first investigated the effectiveness of MA in hydrolyzing hemicellulose in bamboo into XOS. Under optimized conditions (50 mM MA, 180 °C, 45 min), a high XOS yield of 65.9 % was achieved, with a total xylobiose and xylotriose yield of 43.5 %. Subsequent alkaline pretreatment enabled 92.1 % lignin removal from MA-pretreated bamboo. The recovered lignin exhibited a high purity of 95.2 % and retained fundamental structure and functional groups of native lignin. The resulting residue displayed enhanced crystallinity and accessibility, with reduced hydrophobicity and surface area lignin compared to untreated bamboo. At high substrate concentration of 20 %, cellulase hydrolysis resulted in a glucose conversion efficiency of 83.9 %. Overall, this integrated strategy offered an efficient approach for the co-production of valuable XOS, lignin, and glucose from bamboo. The efficient energy utilization and economic viability further highlighted the potential of this method for large-scale industrial applications, making it an attractive option for biomass valorization.

Enhancement of hydrogen production in dark fermentation of corn stover hydrolysates through composite electric field pretreatment: Improving enzymatic efficiency and regulating microbial metabolic balance

This study investigates the effects of composite electric field pretreatment (CEP) on hydrogen production from corn stover enzymatic hydrolysates in dark fermentation. The findings reveal that under optimal conditions, the CEP group achieved a cumulative hydrogen yield of 77.3 ± 2.6 mL/g TS, marking a 55.3 % increase compared to the control group without the electric field. CEP significantly enhances the fermentation capacity of enzymatic hydrolysates during the acid-stage of dark fermentation by disrupting the lignin structure and optimizing cellulase hydrolysis efficiency. Additionally, pH self-regulation is facilitated through the interaction between volatile fatty acids (VFAs) and ammonia nitrogen. Microbial community analysis revealed that CEP shifts the metabolic balance between Clostridium_sensu_stricto_1 and Lactococcus, leading to increased hydrogen yield and concentration during acid fermentation. Notably, Terrisporobacter exhibited a superior acid-producing capability compared to Bacteroides during the dark fermentation of enzymatic hydrolysates. This study provides a new perspective for the practical application of corn stover.