Lignin is a highly abundant, renewable biopolymer with the potential to replace fossil-based aromatic building blocks in polymeric materials. However, the structural complexity of lignin, arising from its heterogeneous interunit linkages and irregular architecture, complicates the assessment of structure–property relationships. In this work, a lignin with a high content of the β-O-4′ interunit linkage, prevalent in native lignin, is utilized to reveal how the molecular structure governs the mechanical performance of lignin-based thermosets. The lignin was modified with allyl functionalities and thermally cross-linked using thiol–ene chemistry. A comprehensive structural analysis and characterization show that maintaining a high β-O-4′ content results in networks that exhibit both high modulus and ductile behavior (E: 1–3 GPa, and ε_b > 10%). This effectively addresses the trade-off between stiffness and ductility often encountered in technical lignin-based materials, having strain-at-break values below 4%. These findings highlight molecular structure as a crucial parameter for optimizing mechanical performance and tailoring lignin-based materials for high-performance applications, such as organic coatings or adhesives.

During kraft pulping of wood, a considerable part of biomass is solubilized, forming a black liquor from which material can be taken out as by-products. Of these, extractive-derived fractions such as tall oil and raw turpentine has long seen technical utilization, and presently, lignin degradation products have garnered a large interest. The carbohydrate degradation products, however, have seen considerably less focus. In this work, we have investigated the structure of a high molecular-weight fraction of the carbohydrate degradation products using nuclear magnetic resonance spectroscopy, finding it to be a conjugated aromatic structure rich in methyl, methylidine, alcohol and carboxylic acid groups. Based on this information, we suggest a structure based on hydroxymethylfurfural as the repeating unit, with sugar acid substituents providing additional functionality. Additionally, UV-vis data of the polymer is compared with data from the kraft cooking of cotton linters and other model systems to corroborate the hypothesis that this polymer is indeed present in black liquor and potentially responsible for some of its characteristic colour. It also reacts in the kappa number analysis, exhibiting 40 % of the permanganate consumption predicted for pure lignin. Finally, the technical significance of these carbohydrate degradation products is discussed based on the structural findings.

The elucidation of the structures of individual lignin molecules in heterogeneous lignin isolates poses a challenge, underscoring the need for the development of robust analytical methods. Herein, we report the combined use of NMR and MALDI-TOF MSn as a facile approach for distinguishing and determining the detailed structures of individual oligomeric molecules in heterogeneous lignin mixtures. Supported by NMR, MALDI-TOF analysis of acetylated lignins provides precision by enabling the facile discernment of inter-unit linkages in lignin molecules. Furthermore, the progression of lignin linkages could be tracked through population studies, yielding a structural progression map that elucidates the chemical features of individual oligomers in milled wood lignins and synthetic lignins. By unmasking lignin’s molecular heterogeneity, this study marks an essential milestone in lignin analytics with possibility to advance the frontiers of molecular-level research in both fundamental and applied lignin studies.

Lignin-carbohydrate complexes, in which lignin and polysaccharides are directly connected, have been identified and extensively analyzed. To date, however, the origin of these structures has not been unequivocally established. That notwithstanding, it has been found that delignification, whether by conventional pulping and bleaching processes or in the biorefinery context, is effected by the presence of lignin-carbohydrate complexes. Using density functional theory calculations, the current work has evaluated the thermodynamics of bond dissociation as a function of structure and chemical composition. Among the lignin-carbohydrate complexes that have been identified, the homolytic bond dissociation energy is highest for the α-benzyl ethers and γ-ester, with phenyl glycosides being markedly less endothermic. This is consistent with observations on the recalcitrance of these compounds. Heterolytic cleavage reactions of the α-benzyl ethers are less endothermic, due to water solvation of the ions. The latter observation may provide support for the proposed homolytic cleavage reaction, since if heterolysis were operative, the α-benzyl ethers would not exhibit the level of recalcitrance that is observed experimentally.

Chemical modification of lignin is widely used to improve its reactivity and solubility in bio-based thermosetting systems. However, the relationship between apparent solubility and true network compatibility remains poorly understood. Here, a hierarchical, multi-scale approach was used to study how lignin structure evolves from molecular modification to nanoscale organization and, ultimately, thermoset performance in fully bio-based epoxy networks derived from epoxidized linseed oil (ELO). Carboxymethylated and oxidative carboxylated lignins were examined as case studies to elucidate how different modification pathways link molecular structure, nanoscale organization, and thermoset performance. Both modification routes increased carboxylic acid content and enabled ester bond formation with ELO, as confirmed by FTIR. Despite these similarities, wide-angle X-ray scattering (WAXS) revealed distinct differences in nanoscale organization. Carboxymethylation caused a moderate change in T-shaped π–π stacking interactions, while preserving a substantial fraction of sandwich π–π stacking, resulting in partial solubility. In contrast, oxidative carboxylation produced a pronounced expansion of T-shaped distances, reflecting a more open and weakly interconnected aromatic network, leading to complete solubility but weaker network cohesion after curing. DSC and FTIR confirmed curing across all systems, although variations in thermal transitions reflected differences in resin-lignin interactions. The resulting materials formed brittle thermosets with heterogeneous network structures. NMR analysis further indicated the occurrence of side reactions, including epoxide ring opening by lignin hydroxyl groups. Leaching tests further indicated that increased carboxylic acid content enhanced solvent accessibility, particularly in oxidized lignin systems. Incorporation of triacetin as a bio-based plasticizer yielded free-standing thermosets suitable for thermomechanical characterization, confirming network formation and structure–property relationships consistent with the multi-scale structural analysis. Overall, this work demonstrates that increasing lignin functionality and solubility alone is insufficient to achieve high-performance bio-based epoxy thermosets, highlighting the critical role of nanoscale packing and short-range order in determining macroscopic properties.

Molecular heterogeneity in lignin remains poorly understood due to the lack of analytics robust enough to determine the precise structure of individual molecules in isolated lignin samples. Recently, we showed that MALDI-TOF MS analysis, combined with a range of NMR techniques, facilitated comprehensive structural studies of oligomer populations present in milled wood lignin from spruce, without prior fractionation. Here, the developed methodology is applied to study populations in milled wood lignin from birch. We report a dominance of linear aryl ether homo-oligomers subdivided into three categories: oligomers exclusively of sinapyl units, oligomers exclusively of guaiacyl units, and oligomers with both guaiacyl and sinapyl units. Linkage progressions in oligomeric structures with the other common interunits are also elucidated. Unlike previous reports, the study enabled the determination of molecule-specific syringyl/guaiacyl ratios. All of the elucidated oligomers contain enone or enal groups at the aliphatic end and phenolic hydroxyls on the other, highlighting homolytic cleavage reactions that occur during lignin procurement. Overall, the study provides a comprehensive framework for an atomistic understanding, offering significant potential for both fundamental and applied research.

This study explores paired electrolysis, leveraging the oxygen reduction reaction (ORR) and industry-relevant lignosulfonate oxidation to enhance sustainable electrochemical processes. The anode reaction is driven by the direct oxidation of lignosulfonate, an abundant biopolymer derived from sulfite pulping, on bare graphite electrodes, eliminating the need for costly catalysts. This process occurs in a membrane electrolyzer, where the cathode catalyst dictates ORR selectivity: a carbon paper cathode modified by the conducting polymer poly(3,4-ethylenedioxythiophene) (PEDOT) favors hydrogen peroxide formation via a 2-electron pathway, while a platinum-modified carbon paper cathode facilitates full oxygen reduction to water via a 4-electron pathway. When applying a cell voltage of 0.7 V (a geometrical current density of 0.04 mA cm^–2), the air-saturated catholyte had an 8-fold decrease in dissolved oxygen, which corresponded to 68% faradaic efficiency and an electrical energy consumption of 0.0233 W hour l^–1. Removing the low molecular weight lignosulfonate (<3.5 kDa) via dialysis minimizes membrane crossover but also reduces oxygen consumption rates. The oxidation process preserves the lignosulfonate backbone while enriching its quinone content, offering a novel, energy-efficient approach to biomass valorization. By integrating lignosulfonate oxidation with ORR, this work presents a cost-effective and sustainable alternative to conventional anodic processes, with potential applications in green hydrogen peroxide production and biobased electrochemical systems.

Lignin, as a renewable source of aromatics has garnered interest for several applications. In this chapter, recent progress on the application of lignin as a component in thermosets and thermoplastics is summarized and discussed. In the named applications, two routes for lignin valorization are typically explored, namely: a) The depolymerization of lignin to monomeric precursors. b) The direct use of pristine lignin macromolecule, or through subsequent chemical modification, as precursors. This report will focus on the latter of the two routes. The study is further refined to address lignin as a structural component and its influence on the mechanical properties of derived thermosets and thermoplastics. The challenges with using available lignin in these applications are discussed. Finally, a future outlook on the use of lignin as a component for the construction of polymeric materials is provided.

Herein, a binary cathode interface layer (CIL) strategy based on the industrial solvent fractionated LignoBoost kraft lignin (KL) is adopted for fabrication of organic solar cells (OSCs). The uniformly distributed phenol moieties in KL enable it to easily form hydrogen bonds with commonly used CIL materials, i.e., bathocuproine (BCP) and PFN-Br, resulting in binary CILs with tunable work function (WF). This work shows that the binary CILs work well in OSCs with large KL ratio compatibility, exhibiting equivalent or even higher efficiency to the traditional CILs in state of art OSCs. In addition, the combination of KL and BCP significantly enhanced OSC stability, owing to KL blocking the reaction between BCP and nonfullerene acceptors (NFAs). This work provides a simple and effective way to achieve high-efficient OSCs with better stability and sustainability by using wood-based materials.

Amidst declining fossil-based resources and environmental challenges, the focus on biobased materials has intensified. Carboxymethylation is one way to introduce reactive functionality to enhance the reactivity of lignin for a specified application. This research investigates the carboxymethylation of four lignin sources: eucalyptus kraft lignin, spruce kraft lignin, birch cyclic extracted organosolv lignin, and spruce cyclic extracted organosolv lignin. Our aim is to elucidate the role of the lignin structure in its reactivity. Using the advanced analytical techniques NMR spectroscopy, Fourier transform infrared spectroscopy, density functional theory, and size-exclusion chromatography, we provide a comprehensive characterization of the modified lignin. The findings offer valuable insights into how the chemical and physical properties of molecular lignin affect the selectivity and efficiency of the carboxymethylation reaction. These fundamental findings hold great potential for guiding considerations on the selection of lignin sources for specific applications based on their molecular properties.