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Summary: 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.

Lignin is a valuable bio-resource in the manufacturing of carbon-based functional materials, because of its large carbon content (~60%), various phenolic structural units, abundancy and sustainability. Here, we explored its use in photocatalytic and self-propelling applications. First, hydroxyl-abundant lignin-based carbon precursor particles, HCLSs, were produced by hydrothermal carbonization of lignin-based microcapsules (LCs). Then, by heating urea coated HCLSs, carbon spheres with a layer of graphitic carbon nitride (g-C3N4) were produced. The presence of surface available -OH groups on the HCLSs, were critical in the formation mechanism. Under visible-light irradiation, the photocatalytic spheres exhibited enhanced activity (49% of the model pollutant remained after 60 min, at 100 mW cm−2) and possessed a three times higher average removal rate constant compared to that of g-C3N4 powder. The g-C3N4 powder was obtained when heating urea only. Additionally, by introducing a Pt/Pd coating on only one side of the composite spheres, the spheres were made self-propelling in the presence of a fuel (H2O2). This work provides new insights into the preparation principles of lignin-based photocatalytic spheres for effective solar photocatalysis applications.

Abstract 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 simple method to simultaneously recover polymeric carbohydrates, mainly galactoglucomannans (GGM), lignin, and lignin-carbohydrate complex (LCC) from hot-water-extracted Norway spruce wood is presented. The isolation method consists of cross-flow filtration, where high and low molecular mass species are removed, followed by fixed-bed adsorption on a hydrophobic polymeric resin (XAD-16) to remove lignins and lignans. In the second step of fixed-bed adsorption, a phenylic reversed-phase analytical chromatography column, where mass transport resistance is minimized and a very high selectivity towards aromatic compounds have been observed, was used to separate LCC from GGM. The isolated LCC fraction contained about 10% aromatics, whereas the upgraded GGM fraction contained about 1.5% aromatics and the lignin fraction contained about 56% aromatics. Polymeric xylan was accumulated in the GGM fraction, while mannose was the dominant sugar found in the LCC fraction. As products, approximately 7% was recovered in the lignin fraction in the first adsorptive step, 5% was recovered as LCC, and 88% as upgraded hemicelluloses.

This paper describes a method for the removal of acid-soluble lignin from acid hydrolyzed hemicelluloses extracted from a mixture of northern hardwood chips, by using Amberlite XAD-4 resin, which was shown to remove 100% of furan derivatives and 90% of acid-soluble lignin. Subsequent fermentation of the resin treated hydrolyzates gave ethanol yields as high as 97% of theoretical and showed a marked increase in fermentation rate. Regeneration of resin performed with 75% acetone was 85% efficient with respect to acid soluble lignin.

Extraction of hemicelluloses from wood prior to pulping is being pursued to generate more value-added products, while still producing high quality pulp from the pre-extracted wood. For a better understanding of the factors limiting selective extraction, enzymatic hydrolysis in combination with size exclusion chromatography (SEC) was applied to milled wood and hydrothermally treated wood. Complete dissolution was achieved in a lithium chloride/dimethylacetamide solvent system after mild ball-milling of a Southern Hardwood Mixture (SHM), of SHM extracted using auto-hydrolysis, and of enzyme-treated SHM. SEC tests showed that severe degradation of wood polymers occurred after a milling time of 3 hours. The SEC data also confirmed the presence of lignin-carbohydrate complexes. Based on the results, it is suggested that linkages between lignin and polysaccharides may play an important role in limiting extraction of hemicelluloses.

AbstractPretreatment of wood or kraft pulp with endoglucanase followed by swelling in urea leaves a non-crystalline residue that can be dissolved in strong aqueous sodium hydroxide-sodium borate solution. A stepwise precipitation process employing acid and barium ions can separate lignin-carbohydrate complexes enriched in individual polysaccharides. This procedure has been applied to eucalypt and birch wood and to the corresponding kraft pulps. Thioacidolysis of the various lignin-carbohydrate complexes was used as the major analytical technique to obtain information about the structure and structural changes in lignin. A combination of thioacidolysis and size exclusion chromatography was used to obtain knowledge on the degree of depolymerisation and repolymerisation of lignin when going from wood to chemical pulp. In contrast to spruce wood and kraft pulp, complete recovery of the lignin-carbohydrate complexes could not be obtained from hardwood samples. [ABSTRACT FROM AUTHOR]

Lignin-carbohydrate complexes (LCCs) were isolated from unbleached acid sulfite pulp by selective enzymatic hydrolysis followed by fiber swelling and extraction. Approximately 80% of the lignin in the pulp was found to be chemically linked to carbohydrates in three major fractions, viz. as one glucan-lignin complex (with 8% lignin) and two hemicellulose-lignin complexes (with 72% lignin). The latter two were further separated into one glucomannan-lignin complex (with ∼25% lignin) and one xylan-lignin-glucomannan complex (with 45–50% lignin). Based on model experiments, it can be concluded that the lignin and carbohydrate moieties are probably linked together through benzyl ether linkages. Xylan is more stable towards acidic hydrolysis than galactoglucomannan, and this finding may explain the greater amount of xylan-rich LCC in the pulp. [ABSTRACT FROM AUTHOR]

A method for the quantitative isolation of lignin-carbohydrate complexes (LCCs) in a softwood is presented. The isolation steps involve partial enzymatic hydrolysis of cellulose, subsequent swelling in urea, and quantitative dissolution into four major fractions: (1) a galactoglucomannan LCC containing ∼8% of the wood lignin; (2) a glucane LCC containing ∼4% of the wood lignin; (3) a xylan-lignin-glucomannan network LCC (xylan>glucomannan) containing ∼40% of the wood lignin; and (4) a glucomannan-lignin-xylan network LCC (glucoman-nan>xylan) containing ∼48% of the wood lignin. Endoglucanase Novozyme 476, with only cellulase activity, and Ecopulp XM, with only xylanase and mannanase activities, were used as an enzymatic tool. From mildly ball-milled wood, all the lignin was isolated as LCCs. As a control, LCC was prepared from partially chlorite-delignified wood meal without ball milling, also in a mild procedure. The results were very similar to those obtained after ball milling. Thus, it can be safely concluded that the formation of new chemical linkages between lignin and carbohydrates during ball milling is improbable. Studies on isolated milled wood lignin (MWL) supported this conclusion and clearly showed that covalent linkages between lignin and carbohydrates are present. The study provide conclusive evidence of covalent linkages between lignin and carbohydrates in the native lignin in wood. It is concluded that carbohydrate-free lignin, i.e., lignin without covalent bonds to carbohydrates, probably cannot be present in spruce wood. [ABSTRACT FROM AUTHOR]