Lignin Source and Structural Characterization

The structure and properties of lignin isolated from various lignocellulosic biomass by different treatment processes

The structure and properties of lignin can vary depending on the type of lignocellulosic biomass it comes from and the separation techniques used, and also affects its suitability for different applications. In this work, the structure and properties of lignin isolated from moso bamboo, wheat straw, and poplar wood by different treatment processes were compared. Results show that deep eutectic solvent (DES) extracted lignin exhibits well-preserved structures (including β-O-4, β-β, and β-5 linkages), a low molecular weight (M_n = 2300-3200 g/mol), and relatively homogeneous lignin fragments (1.93 < PDI < 2.33) compared to dealkaline lignin (DL) and milled wood lignin (MWL). Besides, lignin samples extracted by DES have a regular nanostructure, higher carbon residue content (>40 %), and excellent antioxidant properties (the free radical scavenging index >20). Among the three types of biomass, the structural destruction of lignin in straw is the most obvious, which is due to the degradation of β-O-4 and β-β linkages during DES treatment. These findings can contribute to a better understanding of the structural changes that occur in various treatment processes from different lignocellulosic biomass, and help maximize the targeted development of their applications based on the characteristics of lignin.

Unveiling lignin structures and lignin-carbohydrate complex (LCC) linkages of bamboo (Phyllostachys pubescens) fibers and parenchyma cells

Bamboo (Phyllostachys pubescens) is an attractive biomass block to develop biorefining industry, however, less emphasis has been placed on elucidating the chemical linkage variations of lignin and LCC between different bamboo cell walls. Here, purified milled wood lignin (MWLp) and lignin-carbohydrate complex (LCC) were isolated from bamboo (Phyllostachys pubescens) fibers (BF) and parenchyma cells (PC), respectively. The variations of structure features and chemical linkages of lignin and LCC were investigated via FT-IR, 2D HSQC NMR, and ³¹P NMR techniques. 2D HSQC NMR revealed that β-O-4 alkyl-aryl ether linkages and resinol (β-β) substructure were the main substructures in BF-MWLp and PC-MWLp. β-1 linkages existed in the PC-MWLp (3.18/100 Ar), but not in BF-MWLp. Moreover, tricin, as a flavonoid compound, was only detected in the BF-MWLp. The amount of the syringyl (S) units of PC-MWLp was higher than BF-MWLp. The results indicated that phenyl glycoside (PhGlc) bonds (mainly lignin and xylan) were the predominant chemical linkage type of LCC bonds in BF-LCC and PC-LCC, and the high contents of PhGlc bonds (45.53/100 Ar) were presented in PC. Our finding can provide a reference for the structural variations of lignin and LCC between the different bamboo cell walls.

Functional surfaces, films, and coatings with lignin - a critical review

Lignin is the most abundant polyaromatic biopolymer. Due to its rich and versatile chemistry, many applications have been proposed, which include the formulation of functional coatings and films. In addition to replacing fossil-based polymers, the lignin biopolymer can be part of new material solutions. Functionalities may be added, such as UV-blocking, oxygen scavenging, antimicrobial, and barrier properties, which draw on lignin's intrinsic and unique features. As a result, various applications have been proposed, including polymer coatings, adsorbents, paper-sizing additives, wood veneers, food packaging, biomaterials, fertilizers, corrosion inhibitors, and antifouling membranes. Today, technical lignin is produced in large volumes in the pulp and paper industry, whereas even more diverse products are prospected to be available from future biorefineries. Developing new applications for lignin is hence paramount - both from a technological and economic point of view. This review article is therefore summarizing and discussing the current research-state of functional surfaces, films, and coatings with lignin, where emphasis is put on the formulation and application of such solutions.

Lignosulfonate-Based Ionic Liquids as Asphaltene Dispersants

Asphaltenes are recognized as being troublesome from upstream to downstream in the oil industry due to their tendency to precipitate and self-associate. Their extraction from asphaltenic crude oil for a cost-effective refining process is a crucial and critical challenge in the oil and gas sector. Lignosulfonate (LS), as a by-product of the wood pulping process in the papermaking industry, is a highly available and underutilized feedstock. This study aimed to synthesize novel LS-based ionic liquids (ILs) by reacting lignosulfonate acid sodium salt [Na]₂[LS] with different alkyl chains of piperidinium chloride for asphaltene dispersion. The synthesized ILs, 1-hexyl-1-methyl-piperidinium lignosulfonate [C₆C₁Pip]₂[LS], 1-octyl-1-methyl-piperidinium lignosulfonate [C₈C₁Pip]₂[LS], 1-dodecyl-1-methyl-piperidinium lignosulfonate [C₁₂C₁Pip]₂[LS] and 1-hexadecyl-1-methyl-piperidinium lignosulfonate [C₁₆C₁Pip]₂[LS] were characterized using FTIR-ATR and ¹H NMR for functional groups and structural confirmation. The ILs depicted high thermal stability because of the presence of a long side alkyl chain and piperidinium cation following thermogravimetric analysis (TGA). Asphaltene dispersion indices (%) of ILs were tested by varying contact time, temperature and ILs concentration. The obtained indices were high for all ILs, with a dispersion index of more than 91.2% [C₁₆C₁Pip]₂[LS], representing the highest dispersion at 50,000 ppm. It was able to lower asphaltene particle size diameter from 51 nm to 11 nm. The kinetic data of [C₁₆C₁Pip]₂[LS] were consistent with the pseudo-second-order kinetic model. The dispersion index (%), asphaltene particle growth and the kinetic model agreed with the molecular modeling studies of the HOMO-LUMO energy of IL holds.

Designing Lignin-Based Biomaterials as Carriers of Bioactive Molecules

There is a need to develop circular and sustainable economies by utilizing sustainable, green, and renewable resources in high-tech industrial fields especially in the pharmaceutical industry. In the last decade, many derivatives of food and agricultural waste have gained considerable attention due to their abundance, renewability, biocompatibility, environmental amiability, and remarkable biological features. Particularly, lignin, which has been used as a low-grade burning fuel in the past, recently attracted a lot of attention for biomedical applications because of its antioxidant, anti-UV, and antimicrobial properties. Moreover, lignin has abundant phenolic, aliphatic hydroxyl groups, and other chemically reactive sites, making it a desirable biomaterial for drug delivery applications. In this review, we provide an overview of designing different forms of lignin-based biomaterials, including hydrogels, cryogels, electrospun scaffolds, and three-dimensional (3D) printed structures and how they have been used for bioactive compound delivery. We highlight various design criteria and parameters that influence the properties of each type of lignin-based biomaterial and corelate them to various drug delivery applications. In addition, we provide a critical analysis, including the advantages and challenges encountered by each biomaterial fabrication strategy. Finally, we highlight the prospects and future directions associated with the application of lignin-based biomaterials in the pharmaceutical field. We expect that this review will cover the most recent and important developments in this field and serve as a steppingstone for the next generation of pharmaceutical research.

Horse manure as resource for biogas and nanolignocellulosic fibres

Nanofibrillated cellulose (NFC) has key applications in composites, water filters and as emulsifiers. The affinity of NFC to water is a challenge, as it negatively influences its integrity. Lignin, a major component of plant biomass, is a natural hydrophobiser. Anaerobic digestion (AD) of biomass to produce biomethane allows to up-concentrate lignin in the fermentation residue containing lignocellulosic fibres. Horse manure was used as substrate for biogas production from which nanolignocellulose fibres (LCNF) were extracted. A biogas yield of 207 L_N kg_VS^-1 with a methane concentration of 65 % was achieved. From the fermentation residue LCNFs, in yields of up to 41 %, with lignin contents between 23 and 29 wt% depending on fermentation time were obtained. Nanopapers produced from LCNFs possessed tensile strengths and moduli of 45 to 91 MPa and 7 to 8 GPa, respectively. The increased lignin content was responsible for decreased water absorption capacity of nanopapers.

Journey of lignin from a roadblock to bridge for lignocellulose biorefineries: A comprehensive review

The grave concerns arisen as a result of environmental pollution and diminishing fossil fuel reserves in the 21st century have shifted the focus on the use of sustainable and environment friendly alternative resources. Lignocellulosic biomass constituted by cellulose, hemicellulose and lignin is an abundantly available natural bioresource. Lignin, a natural biopolymer has over the years gained much importance as a high value material with commercial importance. The present review provides an in-depth knowledge on the journey of lignin from being considered a roadblock to a bridge connecting diverse industries with widescale applications. The successful valorization of lignin for the production of bio-based platform chemicals and fuels has been the subject of intensive investigation. A deeper understanding of lignin characteristics and factors governing the biomass conversion into valuable products can support improved biomass consumption. The components of lignocellulosic biomass might be totally transformed into a variety of value-added products with the improvements in bioprocess techniques that valorize lignin. In this review, the recent advances in the lignin extraction and depolymerization methods that may help in achieving the cost-economics of the bioprocess are summarized and compared. The industrial potential of lignin-derived products such as aromatics, biopolymers, biofuels and agrochemicals are also outlined. Additionally, assessment of the recent research trends in lignin valorization into value-added chemicals has been done and present scenario of technological-industrial applications of lignin with economic perspectives is highlighted.

Nanogrinding/ethanol activation facilitating lignin fractionation for preparation of monodispersed lignin nanoparticles

Lignin nanoparticles (LNPs), as one of green and sustainable biological macromolecules, have attracted great attention owing to their promising potentials in many valorized fields. However, the lignin heterogeneity seriously restricts the controllable preparation of LNPs. Herein, a facile nanogrinding activation combining anhydrous ethanol dissolution process was developed to efficiently homogenize lignin prior to gradient ethanol fractionation. Two lignin fractions were obtained from nanogrinding activation/ethanol dissolution followed by gradient ethanol fractionation: L-fractions and S-fractions. Therefore, monodispersed LNPs with unique concave hollow nanostructure and large particle size, and monodispersed LNPs with solid core nanostructure and small particle size were successfully prepared from L-fractions and S-fractions, respectively, via a GVL/water anti-solvent method. The proposed LNPs formation mechanisms facilitated by nanogrinding activation/ethanol dissolution treatment were demonstrated. This study put forwards a facile and green integrated approach for monodispersed LNPs preparation with controllable morphology and particle size.

Enzyme-Catalyzed Polymerization of Kraft Lignin from Eucalyptus globulus: Comparison of Bacterial and Fungal Laccases Efficacy

Kraft lignin, a side-stream from the pulp and paper industry, can be modified by laccases for the synthesis of high added-value products. This work aims to study different laccase sources, including a bacterial laccase from Streptomyces ipomoeae (SiLA) and a fungal laccase from Myceliophthora thermophila (MtL), for kraft lignin polymerization. To study the influence of some variables in these processes, a central composite design (CCD) with two continuous variables (enzyme concentration and reaction time) and three levels for each variable was used. The prediction of the behavior of the output variables (phenolic content and molecular weight of lignins) were modelled by means of response surface methodology (RSM). Moreover, characterization of lignins was performed by Fourier-transform infrared (FTIR) spectroscopy and different nuclear magnetic resonance (NMR) spectroscopy techniques. In addition, antioxidant activity was also analyzed. Results showed that lignin polymerization (referring to polymerization as lower phenolic content and higher molecular weight) occurred by the action of both laccases. The enzyme concentration was the most influential variable in the lignin polymerization reaction within the range studied for SiLA laccase, while the most influential variable for MtL laccase was the reaction time. FTIR and NMR characterization analysis corroborated lignin polymerization results obtained from the RSM.

Major Phytochemicals: Recent Advances in Health Benefits and Extraction Method

Recent scientific studies have established a relationship between the consumption of phytochemicals such as carotenoids, polyphenols, isoprenoids, phytosterols, saponins, dietary fibers, polysaccharides, etc., with health benefits such as prevention of diabetes, obesity, cancer, cardiovascular diseases, etc. This has led to the popularization of phytochemicals. Nowadays, foods containing phytochemicals as a constituent (functional foods) and the concentrated form of phytochemicals (nutraceuticals) are used as a preventive measure or cure for many diseases. The health benefits of these phytochemicals depend on their purity and structural stability. The yield, purity, and structural stability of extracted phytochemicals depend on the matrix in which the phytochemical is present, the method of extraction, the solvent used, the temperature, and the time of extraction.

Emerging Modification Technologies of Lignin-based Activated Carbon toward Advanced Applications

Lignin-based activated carbon (LAC) is a promising high-quality functional material due to high surface area, abundant porous structure, and various functional groups. Modification is the most important step to functionalize LAC by altering its porous and chemical properties. This Review summarizes the state-of-the-art modification technologies of LAC toward advanced applications. Promising modification approaches are reviewed to display their effects on the preparation of LAC. The multiscale changes in the porosity and the surface chemistry of LAC are fully discussed. Advanced applications are then introduced to show the potential of LAC for supercapacitor electrode, catalyst support, hydrogen storage, and carbon dioxide capture. Finally, the mechanistic structure-function relationships of LAC are elaborated. These results highlight that modification technologies play a special role in altering the properties and defining the functionalities of LAC, which could be a promising porous carbon material toward industrial applications.

Structural materials with afterglow room temperature phosphorescence activated by lignin oxidation

Sustainable afterglow room temperature phosphorescence (RTP) materials, especially afterglow RTP structural materials, are crucial but remain difficult to achieve. Here, an oxidation strategy is developed to convert lignin to afterglow materials with a lifetime of ~ 408 ms. Specifically, lignin is oxidized to give aromatic chromophores and fatty acids using H₂O₂. The aromatic chromophores are locked by a fatty acid-based matrix by hydrogen bonds, triggering enhanced spin orbit coupling and long afterglow emission. More interestingly, motivated by this discovery, an auto fabrication line is built to convert wood (natural structural materials) to wood with afterglow RTP emission (RTP wood) via in situ oxidation of naturally-occurring lignin located in the wood cell walls to oxidized lignin (OL). The as-prepared RTP wood exhibits great potential for the construction of sustainable afterglow furniture. With this research we provide a new strategy to promote the sustainability of afterglow RTP materials and structural materials.

Sustainable afterglow room temperature phosphorescence (RTP) Structural materials are difficult to achieve. Here, the authors demonstrate a wood based RTP material by oxidation of lignin to realize an afterglow RTP material with a lifetime of ~ 408 ms.

Recent Studies on the Preparation and Application of Ionic Amphiphilic Lignin: A Comprehensive Review

As the second most abundant natural polymer after cellulose, lignin has received considerable attention recently due to its reproducibility, safety, and biodegradability. Studies are now focusing on the development of new lignin applications to replace petroleum-based chemicals. Unfortunately, lignin has several inherent problems, such as poor water solubility and a tendency to agglomerate. However, after chemical modification, lignin can gain new functions through the introduction of new functional groups. For example, amphiphilic lignin is a polymer that is soluble in both water and organic solvents. Amphiphilic lignin polymers can be divided into anionic, cationic, and anionic-cationic amphoteric lignin-based polymers, according to the ions contained in their molecular structure. Amphiphilic lignin polymers also have a wide range of applications in various industrial fields and can be used as wetting agents, detergents, controlled release fertilizers, adsorbents, and emulsifiers. Thus, this article reviews research progress on the synthesis and applications of amphiphilic lignin-derived polymers over the past 10 years, providing a theoretical reference for the utilization of high-added-value and high-performance lignin.

Sustainable lignin and lignin-derived compounds as potential therapeutic agents for degenerative orthopaedic diseases: A systemic review

Lignin, the most abundant natural and sustainable phenolic compound in biomass, has exhibited medicinal values due to its biological activities decided by physicochemical properties. Recently, the lignin and its derivatives (such as lignosulfonates and lignosulfonate) have been proven efficient in regulating cellular process and the extracellular microenvironment, which has been regarded as the key factor in disease progression. In orthopaedic diseases, especially the degenerative diseases represented by osteoarthritis and osteoporosis, excessive activated inflammation has been proven as a key stage in the pathological process. Due to the excellent biocompatibility, antibacterial and antioxidative activities of lignin and its derivatives, they have been applied to stimulate cells and restore the uncoupling bone remodeling in the degenerative orthopaedic diseases. However, there is a lack of a systemic review to state the current research actuality of lignin and lignin-derived compounds in treating degenerative orthopaedic diseases. Herein, we summarized the current application of lignin and lignin-derived compounds in orthopaedic diseases and proposed their possible therapeutic mechanism in treating degenerative orthopaedic diseases. It is hoped this work could guide the future preparation of lignin/lignin-derived drugs and implants as available therapeutic strategies for clinically degenerative orthopaedic diseases.

A Value-Added Utilization Method of Sugar Production By-Products from Rice Straw: Extraction of Lignin and Evaluation of Its Antioxidant Activity

To value-added utilization of the rice straw, two types of lignin were extracted from the by-products of sugar production. The ether-extracted lignin with a purity of 98.7% was extracted from the pretreatment filtrate with two times the concentrated filtrate volume of ether, where the lignin yield was 6.62 mg/g of the rice straw. The ball-milled lignin with a purity of 99.6% was extracted from the milled enzymatic hydrolysis residue with a 1,4-dioxane solution, where the revolution speed and grinding time were 300 rpm and 12 h, respectively. The yield of ball-milled lignin was 34.52 mg/g of the rice straw, which was 421.5% higher than that extracted from extract-free rice straw. In the process of rice straw pretreatment and lignin extraction, 76.43% by mass of phosphotungstic acid catalyst and approximately 98% by volume of 1,4-dioxane solution could be recycled and reused. Compared with the soda lignin extracted from papermaking black liquor, the scavenging rates of DPPH radical and ABTS+ radical of ether-extracted lignin increased by 36.26% and 41.18%, respectively, while the above scavenging rates of ball-milled lignin increased by 30.22% and 37.75%, respectively. Moreover, the reducing power of the two extracted lignins was also stronger than that of soda lignin. The ether-extracted lignin and ball-milled lignin have the potential to be developed as natural macromolecular antioxidants.

A New Structural Model of Enzymatic Lignin with Multiring Aromatic Clusters

Lignin is a natural aromatic compound in plants. Several lignin structural models have been proposed in the past years, but all the models cannot be converted to benzene carboxylic acids (BCAs) for all aromatic rings connected to oxygen. This inspired us to explore the structures of lignin. Based on the yields of BCAs, the results of ¹³C NMR and ethanolysis residues, and gas chromatography-mass spectrometry and electrospray ionization mass spectrometry of ethanolysis of lignin, we have constructed a structural model of lignin with a formula C₆₄₀₇H₆₇₃₆O₂₅₉₀N₁₄₇S₃. The model not only satisfies the results of analyses, but also explains the generation of BCAs from lignin oxidation and the ethanolysis products. Importantly, double-ring and triple-ring aromatic clusters are found in lignin, and some of them are connected by alkyl bridges, which results in conventional low conversions of lignin. Our findings in the structures of lignin may significantly influence the structures and applications of lignin.

Ultrastructural elucidation of lignin macromolecule from different growth stages of Chinese pine

Understanding of the morphological changes at different growth stages and lignin accumulation pattern for pine biomass plays the key role in facilitating the further development of value-added utilization and downstream conversion processes. This work systematically revealed the morphological change and lignin accumulation pattern in Chinese pine branches cell walls via confocal Raman microscopy (CRM) technology. Meanwhile, the structural characteristics of isolated lignin samples from different growth stages were synthetically characterized by nuclear magnetic resonance (NMR) and gel permeation chromatography (GPC) techniques. The results indicated that the content of pith in adult pine new branch was bigger than juvenile trees. With the increase of physiological age, the branches in adult pine could accumulate more lignin both in overall content and the concentration of cell corner middle layer. Moreover, the significantly increases of molecular weights and the β-O-4, β-β linkages content revealed that the lignin macromolecule of pine would polymerize faster in the adult stage (14, 35 years). The panorama generated from the structural and chemical features of pine native lignin not only benefited to understand the biosynthetic pathways and lignin macromolecules structural variation in plant cell walls from different growth stages but also contributed to the valorization and deconstruction of biomass.

Optimized preparation of spruce kraft lignin/ZnO composites and their performance analysis in polyurethane films

Spruce kraft lignin (SKL) has received considerable attention in recent years for its application in the field of polymer materials. However, its structural complexity and polydispersity pose significant challenges for commercial applications. In this study, molecular structure models of acetone soluble kraft lignin (ASKL) and acetone insoluble kraft lignin (AIKL) were proposed based on quantitative calculations of the connection mode of functional groups and structural units. The lignin was modified by quaternization and synthesized in situ as lignin/ZnO composites. Based on the detailed characterization of the micromorphology and structure of the composites, waterborne polyurethane (WPU) films were optimally prepared using 0.6 wt% of the composites. The results showed that the composite films prepared from ASKL have the best UV-blocking performance and mechanical properties. The highest UVA and UVB blocking rates were 98% and 100%, respectively, and the highest tensile strength and elongation at break were 31.2 MPa and 732%, respectively. The differences in the structure and functional groups of the different types of SKL were accurately identified. ASKL is advantageous for the development of UV-blocking films because of its low molecular weight and the presence of abundant phenolic hydroxyl groups, which facilitate the formation of hydrogen bonds, improve the compatibility, and ensure uniform dispersibility. The results of this study are of practical importance in the field of nano-functional materials for the high-value application of industrial lignin.

Lignin-first biorefinery of corn stalk via zirconium(IV) chloride/sodium hydroxide-catalyzed aerobic oxidation to produce phenolic carbonyls

Lignin-first biorefinery of corn stalk via ZrCl₄/NaOH-catalyzed aerobic oxidation for phenolic carbonyls production was reported. Under the co-catalysis of ZrCl₄ and NaOH, lignin in corn stalk was oxidized into phenolic aldehydes (p-hydroxybenzaldehyde, vanillin, and syringaldehyde), ketones (p-hydroxyacetophenone, acetovanillone, and acetosyringone), acids (p-hydroxybenzoic acid and vanillic acid), and other derivatives. Reaction conditions, including time, temperature, ZrCl₄ dosage, NaOH dosage, MeCN/H₂O ratio, and initial O₂ pressure were comprehensively screened, and the optimal lignin-derived monomer yields of 13.2 wt% was obtained. Among these aromatic compounds, phenolic aldehydes were the main products, and the overall selectivity of phenolic carbonyls was as high as 93%. Cellulose-rich residues after lignin-first oxidation were further characterized by thermogravimetry and analytical pyrolysis with corn stalk as the control, proving the good fragmentation and dissolution of lignin streams. In general, ZrCl₄/NaOH-catalyzed lignin-first oxidation provided a novel approach for lignin valorization, and achieved the highest reported phenolic carbonyls selectivity.

Efficient fractionation of bamboo residue by autohydrolysis and deep eutectic solvents pretreatment

Bamboo processing residue, which is rich in parenchyma cells, was treated as huge waste in bamboo processing industry, such as reassemble bamboo and bamboo flooring. Herein, autohydrolysis and rapid different deep eutectic solvents (DES) delignification strategy were consecutively performed to remove hemicelluloses and lignin from bamboo processing residue. The xylooligosaccharides (XOS) with high yield (34.35%) was achieved in the autohydrolysis process. Results showed that alkaline DES pretreatment resulted in the highest glucose yield (88.22%) and relatively high delignification rate (83.75%) as well as well-preserved lignin structures. However, the lignin fractions obtained under acidic DES conditions were tending to assemble into lignin nanoparticles (LNPs) and having excellent antioxidant activity as compared to those obtained from alkaline DES system. In brief, the combination of autohydrolysis and rapid DES delignification can achieve orientated fractionation of the components from the industrialized bamboo.

Preparation of amine-functionalized lignins for the selective adsorption of Methylene blue and Congo red

Research on low-cost bio-adsorbents for the removal of harmful substances from effluents has recently attracted significant attention. In this study, three types of amino-silane-modified lignins (ASLs) with primary, secondary, and tertiary amine groups were prepared, and their adsorption behavior toward cationic and anionic dyes was investigated. Chemical structural analyses indicated that the three amino-silane reagents resulted in different molecular self-assembly structures on the lignin surface. The ASLs exhibited enhanced thermal stabilities and increased surface areas with different surface charges in different pH ranges. Owing to the high density of primary, secondary, and tertiary amine groups, the ASLs exhibited excellent adsorption capacities for cationic and anionic dyes. Additionally, they selectively adsorb anionic and cationic dyes according to the pH conditions. The ASL with primary amine had the highest adsorption capacity for Methylene blue and Congo red, reaching 187.27 and 293.26 mg·g^-1, respectively, followed by ASLs with the secondary amine and tertiary amine. All adsorption processes followed the Langmuir and Temkin isotherms and had pseudo-second-order kinetics. The hypothesized adsorption mechanism mainly involves electrostatic interaction, NH-π interaction, hydrogen bonding interaction and π-π interaction.

Synergistic effect of lignin and ethylene glycol crosslinked epoxy resin on enhancing thermal, mechanical and shape memory performance

Lignosulfonate (LS) was successfully introduced into the epoxy resin matrix with the aid of ethylene glycol (EG) dissolution. Both the rigid LS and soft EG segments were linked into the cross-linked network structure of epoxy resin via esterification of hydroxyl groups in LS and EG molecules with anhydride. The ultimate properties of cured samples were adjusted effectively by changing the proportion of LS and EG components. Curing reaction and kinetics were analyzed, by which the optimal curing process parameters were determined. Although thermal stability of LS itself was relatively lower than that of neat epoxy, the thermal performance was significantly enhanced for the modified sample of epoxy/LS0.5-EG0.5. At the same time, the flexural strength, flexural modulus and impact strength were found to be increased by 23.1, 35.7 and 15.1% respectively compared with the neat epoxy. In addition, the excellent shape memory behavior and improved mechanical stability with LS addition were exhibited by the cured LS-EG modified specimens. This work reveals that lignin can be used as an efficient functional additive to regulate thermal, mechanical and shape memory properties of epoxy resin.

Preparation and properties of novel bio-based epoxy resin thermosets from lignin oligomers and cardanol

A series of lignin-based epoxy resins (LEPs) were prepared by the reaction of epichlorohydrin with lignin oligomers derived from partial reductive depolymerization of lignin. To overcome the high viscosity and brittleness defects in practical applications, the LEPs were blended with renewable epoxied cardanol glycidyl ether (ECGE) and then cured with methyltetrahydrophthalic anhydride (MeTHPA) to form high-performance epoxy thermosets. The effects of degree of lignin depolymerization, chemical composition of lignin oligomers and dosage of ECGE on thermal and mechanical properties of the cured products were investigated. The LEP/MeTHPA thermosets exhibited good thermal and mechanical properties. Especially, by separating monomer-rich fractions from lignin oligomers, the thermal and mechanical properties of the cured product were improved obviously. Notably, the incorporation of ECGE also possessed a positive effect on reinforcing and toughening the cured products. With 20 wt% ECGE loadings, the tensile, flexural and impact strength of the cured product reached the maximum value of 77 MPa, 115 MPa and 14 kJ/m², respectively, which were equivalent to the commercial bisphenol A epoxy resins thermosets. These findings indicated that the novel bio-based epoxy resins from lignin oligomers and cardanol could be utilized as renewable alternatives for BPA epoxy resins.

Structural characterization of poplar lignin based on the microwave-assisted hydrothermal pretreatment

Increasing application for lignin due to its unique aromatic structures has encouraged the development of pretreatment techniques for lignin separation and recovery. In this work, the integration of microwave-assisted hydrothermal pretreatment (MW-HTP) and alkaline post-treatment was proposed for separating lignin from poplar wood and revealing its structural characteristics. Results showed that the yields of the lignins fractionated via the integrated treatment based on MW-HTP were increased up to 52.6%, and their associated sugars contents were clearly decreased to 0.19-0.09%, as compared to the fractionated lignins without the microwave pretreatment (29.8% and 0.29%). Interestingly, the integrated treatment based on MW-HTP promoted the cracking of β-O-4 ethers in the lignin macromolecules of poplar wood, resulting in the raise of their phenol OH groups up to 2.36 mmol/g. Overall, the fulfillment of this work will be conducive to improve the fractionation and efficient utilization of lignin in biorefinery industry.

Polycyclic Aromatics Observed in Enzymatic Lignin by Spectral Characterization and Ruthenium Ion-Catalyzed Oxidation

It is generally considered that lignin is a three-dimensional amorphous polymer consisting of methoxylated phenylpropane structures. However, high yields of monomer structural units of lignin cannot be obtained through various ways, which inspired us to gain insights into the structures of lignin. Herein, enzymatic lignin (EL) was directly characterized by a solid-state ¹³C nuclear magnetic resonance spectrometer and Fourier transform infrared spectrometer and then subjected to ruthenium ion-catalyzed oxidation. According to the spectral characterization, it can be inferred that multi-ring aromatic clusters exist in EL because of the aromatic bridgehead carbon ratio of 0.136. Based on the results of ruthenium ion-catalyzed oxidation of the EL, it can be deduced that (1) double- and triple-aromatic ring clusters exist in the EL besides the traditional phenylpropane single-aromatic ring clusters, and (2) some aromatic rings with long-alkyl chain substituents exist in the EL, which is quite different from the traditional cognition of lignin. This investigation provides a new insight into the structure of EL.

Dopant-assisted atmospheric pressure photoionization Orbitrap mass spectrometry - An approach to molecular characterization of lignin oligomers

Lignin is the second most abundant biopolymer in nature and is considered an important renewable source of aromatic compounds. One of the most promising analytical methods for molecular characterization of lignin is Orbitrap high-resolution mass spectrometry with atmospheric pressure photoionization (APPI), proved itself in the study of lignins of various origins and their depolymerization products. In this work, the photoionization of lignin using acetone, 1,4-dioxane, and THF as solvents for the biopolymer and APPI dopants providing the generation of protonated and deprotonated molecules of lignin oligomers has been studied. The ionization conditions were optimized on the basis of the dependences of the total ion current on temperature and the flow rate of the solution into the ion source. Lignin degradation processes under APPI conditions occur mainly with the cleavage of ether β-O-4 bonds between phenylpropane structural units, demethylation (negative ion mode), as well as the loss of water and formaldehyde (positive ion mode). Negative ion mode APPI provides a higher ionization efficiency in the region of high molecular weights, however, it is characterized by an increased fragmentation of β-O-4 ether bonds compared to APPI(+) leading to a partial depolymerization of lignin in the ion source. The combination of APPI with Orbitrap high-resolution mass spectrometry allows obtaining mass spectra of coniferous and deciduous wood lignins with resolved fine structure and containing signals of up to 3000 oligomers in the mass range of 300-1800 Da. This can be used for comprehensive characterization of lignins at molecular level and tracking changes in biopolymer chemical composition in various processes.

Sustainable production of benzene from lignin

Benzene is a widely used commodity chemical, which is currently produced from fossil resources. Lignin, a waste from lignocellulosic biomass industry, is the most abundant renewable source of benzene ring in nature. Efficient production of benzene from lignin, which requires total transformation of C_sp2-C_sp3/C_sp2-O into C-H bonds without side hydrogenation, is of great importance, but has not been realized. Here, we report that high-silica HY zeolite supported RuW alloy catalyst enables in situ refining of lignin, exclusively to benzene via coupling Bronsted acid catalyzed transformation of the C_sp2-C_sp3 bonds on the local structure of lignin molecule and RuW catalyzed hydrogenolysis of the C_sp2-O bonds using the locally abstracted hydrogen from lignin molecule, affording a benzene yield of 18.8% on lignin weight basis in water system. The reaction mechanism is elucidated in detail by combination of control experiments and density functional theory calculations. The high-performance protocol can be readily scaled up to produce 8.5 g of benzene product from 50.0 g lignin without any saturation byproducts. This work opens the way to produce benzene using lignin as the feedstock efficiently.

Efficient production of benzene from lignin is attractive and of great importance, but has not been realized. Here, the authors develop a strategy to transform lignin into benzene over a RuW/zeolite catalyst in water, and the yield of benzene can be as high as 18.8% on lignin weight basis.

A Review on the Utilization of Lignin as a Fermentation Substrate to Produce Lignin-Modifying Enzymes and Other Value-Added Products

The lignocellulosic biomass is comprised of three major components: cellulose, hemicellulose, and lignin. Among these three, cellulose and hemicellulose were already used for the generation of simple sugars and subsequent value-added products. However, lignin is the least applied material in this regard because of its complex and highly variable nature. Regardless, lignin is the most abundant material, and it can be used to produce value-added products such as lignin-modifying enzymes (LMEs), polyhydroxyalkanoates (PHAs), microbial lipids, vanillin, muconic acid, and many others. This review explores the potential of lignin as the microbial substrate to produce such products. A special focus was given to the different types of lignin and how each one can be used in different microbial and biochemical pathways to produce intermediate products, which can then be used as the value-added products or base to make other products. This review paper will summarize the effectiveness of lignin as a microbial substrate to produce value-added products through microbial fermentations. First, basic structures of lignin along with its types and chemistry are discussed. The subsequent sections highlight LMEs and how such enzymes can enhance the value of lignin by microbial degradation. A major focus was also given to the value-added products that can be produced from lignin.

Exploiting Nature's Most Abundant Polymers: Developing New Pathways for the Conversion of Cellulose, Hemicellulose, Lignin and Chitin into Platform Molecules (and Beyond)

The four most prominent forms of biomass are cellulose, hemicellulose, lignin and chitin. In efforts to develop sustainable sources of platform molecules there has been an increasing focus on examining how these biopolymers could be exploited as feedstocks that support the chemical supply chain, including in the production of fine chemicals. Many different approaches are possible and some of the ones being developed in the authors' laboratories are emphasised.