Oxidative Carboxylation of Lignin: Exploring Reactivity of Different Lignin Types.

The increased interest in the utilization of lignin in biobased applications is evident from the rise in lignin valorization studies. The present study explores the responsiveness of lignin toward oxidative valorization using acetic acid and hydrogen peroxide. The pristine lignins and their oxidized equivalents were analyzed comprehensively using NMR and SEC. The study revealed ring opening of phenolic rings yielding muconic acid- and ester-end groups and side-chain oxidations of the benzylic hydroxyls. Syringyl units were more responsive to these reactions than guaiacyl units. The high selectivity of the reaction yielded oligomeric oxidation products with a narrower dispersity than pristine lignins. Mild alkaline hydrolysis of methyl esters enhanced the carboxylic acid content of oxidized lignin, presenting the potential to adjust the carboxylic acid content of lignin. While oxidation reactions in lignin valorization are well documented, this study showed the feasibility of employing optimized oxidation conditions to engineer tailored lignin-based material precursors.

Industrial Kraft Lignin Based Binary Cathode Interface Layer Enables Enhanced Stability in High Efficiency Organic Solar Cells.

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.

Impact of Extraction Method on the Structure of Lignin from Ball-Milled Hardwood.

Understanding the structure of hardwoods can permit better valorization of lignin by enabling the optimization of green, high-yield extraction protocols that preserve the structure of wood biopolymers. To that end, a mild protocol was applied for the extraction of lignin from ball-milled birch. This made it possible to understand the differences in the extractability of lignin in each extraction step. The fractions were extensively characterized using 1D and 2D nuclear magnetic resonance spectroscopy, size exclusion chromatography, and pyrolysis-gas chromatography-mass spectrometry. This comprehensive characterization highlighted that lignin populations extracted by warm water, alkali, and ionic liquid/ethanol diverged in structural features including subunit composition, interunit linkage content, and the abundance of oxidized moieties. Moreover, ether- and ester-type lignin-carbohydrate complexes were identified in the different extracts. Irrespective of whether natively present in the wood or artificially formed during extraction, these complexes play an important role in the extractability of lignin from ball-milled hardwood. Our results contribute to the further improvement of lignin extraction strategies, for both understanding lignin as present in the lignocellulosic matrix and for dedicated lignin valorization efforts.

A flexible physical protection process for lignin extraction.

Research on lignin valorization has gained ground, driven by its potential to replace fossil-based phenolics in bio-based applications. Technical lignins are structurally complex and still poorly characterized, prompting the need for novel extraction processes for lignin of high analytical quality. In this context, a two-step cyclic extraction process for lignin was contrasted with a one-step cyclic extraction. The latter was shown to preserve the native structure of the spruce lignin product better and improved the yields of both the extracted lignin and residual fiber fraction. The application of the one-step cyclic extraction process to birchwood resulted in a similar protection of the lignin structure. Overall, a flexible physical protection (FPP) process for extraction of lignin with an abundance of native bonds is presented. The lignin product has a high abundance of ether bonds and hydroxyl functionalities, which are of interest in biochemical, polymer, and material applications.

Effect of Molecular Organization on the Properties of Fractionated Lignin-Based Thiol-Ene Thermoset Materials.

In this study, the combination of sequential solvent fractionation of technical Kraft lignin was followed by allylation of most OH functionalities to give highly functional thermoset resins. All lignin fractions were highly functionalized on the phenolic (≥95%) and carboxylic acid OH (≥85%) and to a significant extent on the aliphatic OH moieties (between 43 and 75%). The resins were subsequently cross-linked using thiol-ene chemistry. The high amount of allyl functionalities resulted in a high cross-link density. Dynamic mechanical analysis measurements showed that the thioether content, directly related to the allyl content, strongly affects the performance of these thermosets with a glass transition temperature (Tg) between 81 and 95 °C and with a storage modulus between 1.9 and 3.8 GPa for all thermosets. The lignin fractions and lignin-based thermosets' morphology, at the nanoscale, was studied by wide-angle X-ray scattering measurements. Two π-π stacking interactions were observed: sandwich (≈4.1-4.7 Å) and T-shaped (≈5.5-7.2 Å). The introduction of allyl functionalities weakens the T-shaped π-π stacking interactions. A new signal corresponding to a distance of ≈3.5 Å was observed in lignin-based thermosets, which was attributed to a thioether organized structure. At the same time, a lignin superstructure was observed with a distance/size corresponding to 7.9-17.5 Å in all samples.

Kraft Lignin: A Valuable, Sustainable Resource, Opportunities and Challenges.

Kraft lignin, a by-product from the production of pulp, is currently incinerated in the recovery boiler during the chemical recovery cycle, generating valuable bioenergy and recycling inorganic chemicals to the pulping process operation. Removing lignin from the black liquor or its gasification lowers the recovery boiler load enabling increased pulp production. During the past ten years, lignin separation technologies have emerged and the interest of the research community to valorize this underutilized resource has been invigorated. The aim of this Review is to give (1) a dedicated overview of the kraft process with a focus on the lignin, (2) an overview of applications that are being developed, and (3) a techno-economic and life cycle asseeements of value chains from black liquor to different products. Overall, it is anticipated that this effort will inspire further work for developing and using kraft lignin as a commodity raw material for new applications undeniably promoting pivotal global sustainability concerns.

Molecular understanding of the morphology and properties of lignin nanoparticles: unravelling the potential for tailored applications.

Studies have shown that the size of LNP depends on the molecular weight (Mw) of lignin. There is however need for deeper understanding on the role of molecular structure on LNP formation and its properties, in order to build a solid foundation on structure-property relationships. In this study, we show, for similar Mw lignins, that the size and morphology of LNPs depends on the molecular structure of the lignin macromolecule. More specifically, the molecular structure determined the molecular conformations, which in turn affects the inter-molecular assembly to yield size- and morphological-differences between LNPs. This was supported by density functional theory (DFT) modelling of representative structural motifs of three lignins sourced from Kraft and Organosolv processes. The obtained conformational differences are clearly explained by intra-molecular sandwich and/or T-shaped π-π stacking, the stacking type determined by the precise lignin structure. Moreover, the experimentally identified structures were detected in the superficial layer of LNPs in aqueous solution, confirming the theoretically predicted self-assembly patterns. The present work demonstrates that LNP properties can be molecularly tailored, consequently creating an avenue for tailored applications.

Lignin Structure and Reactivity in the Organosolv Process Studied by NMR Spectroscopy, Mass Spectrometry, and Density Functional Theory.

There is need for well-defined lignin macromolecules for research related to their use in biomaterial and biochemical applications. Lignin biorefining efforts are therefore under investigation to meet these needs. The detailed knowledge of the molecular structure of the native lignin and of the biorefinery lignins is essential for understanding the extraction mechanisms as well as chemical properties of the molecules. The objective of this work was to study the reactivity of lignin during a cyclic organosolv extraction process adopting physical protection strategies. As references, synthetic lignins obtained by mimicking the chemistry of lignin polymerization were used. State-of-the-art nuclear magnetic resonance (NMR) analysis, a powerful tool for the elucidation of lignin inter-unit linkages and functionalities, is complemented with matrix-assisted laser desorption/ionization-time-of-flight-mass spectrometry (MALDI-TOF MS), to gain insights into linkage sequences and structural populations. The study unraveled interesting fundamental aspects on lignin polymerization processes, such as identifications of molecular populations with high degrees of structural homogeneity and the emergence of branching points in lignin structure. Furthermore, a previously proposed intramolecular condensation reaction is substantiated and new insights into the selectivity of this reaction are introduced and supported by density functional theory (DFT) calculations, where the important role of intramolecular π-π stacking is emphasized. The combined NMR and MALDI-TOF MS analytical approach, together with computational modeling, is important for deeper fundamental lignin studies and will be further exploited.

Fundamental Insights on the Physical and Chemical Properties of Organosolv Lignin from Norway Spruce Bark.

The interest in the bark and the attempt to add value to its utilization have increased over the last decade. By applying an integrated bark biorefinery approach, it is possible to investigate the recovery of compounds that can be used to develop green and sustainable alternatives to fossil-based materials. In this work, the focus is on extracting Norway spruce (Picea abies) bark lignin via organosolv extraction. Following the removal of the extractives and the subcritical water extraction to remove the polysaccharides, a novel cyclic organosolv extraction procedure was applied, which enabled the recovery of lignin with high quality and preserved structure. Main indicators for low degradation and preservation of the lignin structure were a high ß-O-4' content and low amounts of condensed structures. Furthermore, high purity and low polydispersity of the lignin were observed. Thus, the obtained lignin exhibits high potential for use in the direct development of polymer precursors and other bio-based materials. During the extraction sequence, around 70% of the bark was extracted. Besides the lignin, the extractives as well as pectic polysaccharides and hemicelluloses were recovered with only minor degradation, which could potentially be used for the production of biofuel or other high-value products such as emulsifiers or adhesives.

Toward a Consolidated Lignin Biorefinery: Preserving the Lignin Structure through Additive-Free Protection Strategies.

As part of the continuing efforts in lignin-first biorefinery concepts, this study concerns a consolidated green processing approach to obtain high yields of hemicelluloses and lignin with a close to native molecular structure, leaving a fiber fraction enriched in crystalline cellulose. This is done by subcritical water extraction of hemicelluloses followed by organosolv lignin extraction. This initial report focuses on a detailed characterization of the lignin component, with the aim of unravelling processing strategies for the preservation of the native linkages while still obtaining good yields and high purity. To this effect, a static cycle process is developed as a physical protection strategy for lignin, and advanced NMR analysis is applied to study structural changes in lignin. Chemical protection mechanisms in the cyclic method are also reported and contrasted with the mechanisms in a reference batch extraction process where the role of homolytic cleavage in subsequent repolymerization reactions is elucidated.

Lignin-Based Epoxy Resins: Unravelling the Relationship between Structure and Material Properties.

Here we investigate the relationship between thermomechanical properties and chemical structure of well-characterized lignin-based epoxy resins. For this purpose, technical lignins from eucalyptus and spruce, obtained from the Kraft process, were used. The choice of lignins was based on the expected differences in molecular structure. The lignins were then refined by solvent fractionation, and three fractions with comparable molecular weights were selected to reduce effects of molar mass on the properties of the final thermoset resins. Consequently, any differences in thermomechanical properties are expected to correlate with molecular structure differences between the lignins. Oxirane moieties were selectively introduced to the refined fractions, and the resulting lignin epoxides were subsequently cross-linked with two commercially available polyether diamines (Mn = 2000 and 400) to obtain lignin-based epoxy resins. Molecular-scale characterization of the refined lignins and their derivatives were performed by 31P NMR, 2D-NMR, and DSC methods to obtain the detailed chemical structure of original and derivatized lignins. The thermosets were studied by DSC, DMA, and tensile tests and demonstrated diverse thermomechanical properties attributed to structural components in lignin and selected amine cross-linker. An epoxy resin with a lignin content of 66% showed a Tg of 79 °C from DMA, Young's modulus of 1.7 GPa, tensile strength of 66 MPa, and strain to failure of 8%. The effect of molecular lignin structure on thermomechanical properties was analyzed, finding significant differences between the rigid guaiacyl units in spruce lignin compared with sinapyl units in eucalyptus lignin. The methodology points toward rational design of molecularly tailored lignin-based thermosets.

Nativity of lignin carbohydrate bonds substantiated by biomimetic synthesis.

The question of whether lignin is covalently linked to carbohydrates in native wood, forming what is referred to as lignin-carbohydrate complexes (LCCs), still lacks unequivocal proof. This is mainly due to the need to isolate lignin from woody materials prior to analysis, under conditions leading to partial chemical modification of the native wood polymers. Thus, the correlation between the structure of the isolated LCCs and LCCs in situ remains open. As a way to circumvent the problematic isolation, biomimicking lignin polymerization in vivo and in vitro is an interesting option. Herein, we report the detection of lignin-carbohydrate bonds in the extracellular lignin formed by tissue-cultured Norway spruce cells, and in modified biomimetic lignin synthesis (dehydrogenation polymers). Semi-quantitative 2D heteronuclear singular quantum coherence (HSQC)-, 31P -, and 13C-NMR spectroscopy were applied as analytical tools. Combining results from these systems, four types of lignin-carbohydrate bonds were detected; benzyl ether, benzyl ester, γ-ester, and phenyl glycoside linkages, providing direct evidence of lignin-carbohydrate bond formation in biomimicked lignin polymerization. Based on our findings, we propose a sequence for lignin-carbohydrate bond formation in plant cell walls.

Tunable Thermosetting Epoxies Based on Fractionated and Well-Characterized Lignins.

Here we report the synthesis of thermosetting resins from low molar mass Kraft lignin fractions of high functionality, refined by solvent extraction. Such fractions were fully characterized by 31P NMR, 2D-HSQC NMR, SEC, and DSC in order to obtain a detailed description of the structures. Reactive oxirane moieties were introduced on the lignin backbone under mild reaction conditions and quantified by simple 1H NMR analysis. The modified fractions were chemically cross-linked with a flexible polyether diamine ( Mn ≈ 2000), in order to obtain epoxy thermosets. Epoxies from different lignin fractions, studied by DSC, DMA, tensile tests, and SEM, demonstrated substantial differences in terms of thermo-mechanical properties. For the first time, strong relationships between lignin structures and epoxy properties could be demonstrated. The suggested approach provides unprecedented possibilities to tune network structure and properties of thermosets based on real lignin fractions, rather than model compounds.

Lignin-Retaining Transparent Wood.

Optically transparent wood, combining optical and mechanical performance, is an emerging new material for light-transmitting structures in buildings with the aim of reducing energy consumption. One of the main obstacles for transparent wood fabrication is delignification, where around 30 wt % of wood tissue is removed to reduce light absorption and refractive index mismatch. This step is time consuming and not environmentally benign. Moreover, lignin removal weakens the wood structure, limiting the fabrication of large structures. A green and industrially feasible method has now been developed to prepare transparent wood. Up to 80 wt % of lignin is preserved, leading to a stronger wood template compared to the delignified alternative. After polymer infiltration, a high-lignin-content transparent wood with transmittance of 83 %, haze of 75 %, thermal conductivity of 0.23 W mK-1 , and work-tofracture of 1.2 MJ m-3 (a magnitude higher than glass) was obtained. This transparent wood preparation method is efficient and applicable to various wood species. The transparent wood obtained shows potential for application in energy-saving buildings.

In muro deacetylation of xylan affects lignin properties and improves saccharification of aspen wood.

BACKGROUND: Lignocellulose from fast growing hardwood species is a preferred source of polysaccharides for advanced biofuels and "green" chemicals. However, the extensive acetylation of hardwood xylan hinders lignocellulose saccharification by obstructing enzymatic xylan hydrolysis and causing inhibitory acetic acid concentrations during microbial sugar fermentation. To optimize lignocellulose for cost-effective saccharification and biofuel production, an acetyl xylan esterase AnAXE1 from Aspergillus niger was introduced into aspen and targeted to cell walls. RESULTS: AnAXE1-expressing plants exhibited reduced xylan acetylation and grew normally. Without pretreatment, their lignocellulose yielded over 25% more glucose per unit mass of wood (dry weight) than wild-type plants. Glucose yields were less improved (+7%) after acid pretreatment, which hydrolyses xylan. The results indicate that AnAXE1 expression also reduced the molecular weight of xylan, and xylan-lignin complexes and/or lignin co-extracted with xylan, increased cellulose crystallinity, altered the lignin composition, reducing its syringyl to guaiacyl ratio, and increased lignin solubility in dioxane and hot water. Lignin-associated carbohydrates became enriched in xylose residues, indicating a higher content of xylo-oligosaccharides. CONCLUSIONS: This work revealed several changes in plant cell walls caused by deacetylation of xylan. We propose that deacetylated xylan is partially hydrolyzed in the cell walls, liberating xylo-oligosaccharides and their associated lignin oligomers from the cell wall network. Deacetylating xylan thus not only increases its susceptibility to hydrolytic enzymes during saccharification but also changes the cell wall architecture, increasing the extractability of lignin and xylan and facilitating saccharification.

A glucuronoyl esterase from Acremonium alcalophilum cleaves native lignin-carbohydrate ester bonds.

The Glucuronoyl esterases (GE) have been proposed to target lignin-carbohydrate (LC) ester bonds between lignin moieties and glucuronic acid side groups of xylan, but to date, no direct observations of enzymatic cleavage on native LC ester bonds have been demonstrated. In the present investigation, LCC fractions from spruce and birch were treated with a recombinantly produced GE originating from Acremonium alcalophilum (AaGE1). A combination of size exclusion chromatography and (31) P NMR analyses of phosphitylated LCC samples, before and after AaGE1 treatment provided the first evidence for cleavage of the LC ester linkages existing in wood.

A molecular dynamics study of the effect of glycosidic linkage type in the hemicellulose backbone on the molecular chain flexibility.

The macromolecular conformation of the constituent polysaccharides in lignocellulosic biomass influences their supramolecular interactions, and therefore their function in plants and their performance in technical products. The flexibility of glycosidic linkages from the backbone of hemicelluloses was studied by evaluating the conformational freedom of the φ and ψ dihedral angles using molecular dynamic simulations, additionally selected molecules were correlated with experimental data by nuclear magnetic resonance spectroscopy. Three types of ß-(1→4) glycosidic linkages involving the monosaccharides (Glcp, Xylp and Manp) present in the backbone of hemicelluloses were defined. Different di- and tetrasaccharides with combinations of such sugar monomers from hemicelluloses were simulated, and free energy maps of the φ - ψ space and hydrogen-bonding patterns were obtained. The glycosidic linkage between Glc-Glc or Glc-Man (C-type) was the stiffest with mainly one probable conformation; the linkage from Man-Man or Man-Glc (M-type) was similar but with an increased probability for an alternative conformation making it more flexible, and the linkage between two Xyl-units (X-type) was the most flexible with two almost equally populated conformations. Glycosidic linkages of the same type showed essentially the same conformational space in both disaccharides and in the central region of tetrasaccharides. Different probabilities of glycosidic linkage conformations in the backbone of hemicelluloses can be directly estimated from the free energy maps, which to a large degree affect the overall macromolecular conformations of these polymers. The information gained contributes to an increased understanding of the function of hemicelluloses both in the cell wall and in technical products.

Phoma herbarum, a soil fungus able to grow on natural lignin and synthetic lignin (DHP) as sole carbon source and cause lignin degradation.

The fungus Phoma herbarum isolated from soil showed growth on highly pure lignin extracted from spruce wood and on synthetic lignin (DHP). The lignin remaining after cultivation was shown to have a lower molecular weight. The reduction in the numbers of ether linkages of the extracted lignins was also observed by derivatization followed by reductive cleavage (DFRC) in combination with (31)P NMR studies. The fungal strain showed an ability to degrade synthetic lignin by extracellular catalysts. GC-MS was applied to study the evolution of low molar mass adducts, e.g., monolignols and it was shown that a reduced coniferyl alcohol product was produced from DHP in a cell-free environment. The work has demonstrated the ability of soil microbes to grow on lignin as sole carbon source. The potential impact is in the production of low molar mass renewable phenols for material application.

Modification of Kraft Lignin to Expose Diazobenzene Groups: Toward pH- and Light-Responsive Biobased Polymers.

A pH- and light-responsive polymer has been synthesized from softwood kraft lignin by a two-step strategy that aimed to incorporate diazobenzene groups. Initially, styrene oxide was reacted with the phenolic hydroxyl groups in lignin, to offer the attachment of benzene rings, thus creating unhindered reactive sites for further modifications. The use of advanced spectroscopic techniques ((1)H and (31)P NMR, UV and FTIR) demonstrated that the reaction was quantitative and selective toward the phenolic hydroxyl groups. In a second step, the newly incorporated benzene rings were reacted with a diazonium cation to form the target diazobenzene motif, whose formation was again thoroughly verified. As anticipated, the diazobenzene-containing kraft lignin derivatives showed a pH-dependent color change in solution and light-responsive properties resulting from the cis-trans photoisomerization of the diazobenzene group.

On the formation of lignin polysaccharide networks in Norway spruce.

In this study we were mirroring suggested in vivo phenomena of lignin-hemicellulose complex formation in vitro, by cross-linking Norway spruce (Picea abies) galactoglucomannans, xylans and lignin moieties to high molecular weight complexes by laccase treatment. We were able to observe the oxidation and cross-linking of non-condensed guaiacyl-type phenolic moieties attached to both of the hemicelluloses by (31)P NMR and size-exclusion chromatography. We suggest that hemicelluloses-lignin complexes form covalently linked structural units during the early stages of lignification via radical enzymatic cross-linking catalyzed by laccase. This work shows that the hemicellulose molecules in wood are covalently linked to two or more lignin units thereby making them suited for forming network structures.