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Technical lignins are complex mixtures of polymers with molecules of different molar masses, number of functional groups and carbohydrate residues. This heterogeneity induces variance in lignin properties and thus limits lignin's applicability in products where constant quality is required. Lignin fractionation into more homogeneous fractions is an attractive approach to valorize lignin feedstock. In this paper, solvent fractionation of lignin was applied to produce lignin fractions with improved purity and homogeneity. The main lignin fraction consisted 86% of the original lignin feedstock and this fraction was practically free from carbohydrates. In addition, a lignin fraction with very low molar mass, high solubility, high number of reactive free phenolic hydroxyl groups and a low glass transition temperature, was produced. The water solubility of lignin fractions depended not only on the molar mass and functional groups, but also on carbohydrates bound to lignin. The molecules with the highest solubility were most likely covalently linked to arabinogalactan. The applied aqueous acetone fractionation method with solvent evaporation required significantly less solvents than precipitation fractionation processes with nonsolvent addition and therefore is considered technically, economically and environmentally more sound process than alternative methods.

The demands of energy sustainability drive efforts to bio-chemical conversion of biomass into biofuels through pretreatment, enzymatic hydrolysis, and microbial fermentation. Pretreatment leads to significant structural changes of the complex lignin polymer that affect yield and productivity of the enzymatic conversion of lignocellulosic biomass. Structural changes of lignin after pretreatment include functional groups, inter unit linkages and compositions. These changes influence non-productive adsorption of enzyme on lignin through hydrophobic interaction and electrostatic interaction as well as hydrogen bonding. This paper reviews the relationships between structural changes of lignin and enzymatic hydrolysis of pretreated lignocellulosic biomass. The formation of pseudo-lignin during dilute acid pretreatment is revealed, and their negative effect on enzymatic hydrolysis is discussed.

As the attraction of creating biofuels and bio-based chemicals from lignocellulosic biomass has increased, researchers have been challenged with developing a better understanding of lignin structure, quantity and potential uses. Lignin has frequently been considered a waste-product from the deconstruction of plant cell walls, in attempts to isolate polysaccharides that can be hydrolyzed and fermented into fuel or other valuable commodities. In order to develop useful applications for lignin, accurate analytical instrumentation and methodologies are required to qualitatively and quantitatively assess, for example, what the structure of lignin looks like or how much lignin comprises a specific feedstock׳s cellular composition. During the past decade, various diverse strategies have been employed to elucidate the structure and composition of lignin. These techniques include using two-dimensional nuclear magnetic resonance to resolve overlapping spectral data, measuring biomass with vibrational spectroscopy to enable modeling of lignin content or monomeric ratios, methods to probe and quantify the linkages between lignin and polysaccharides, or refinements of established methods to provide higher throughput analyses, less use of consumables, etc. This review seeks to provide a comprehensive overview of many of the advancements achieved in evaluating key lignin attributes. Emphasis is placed on research endeavored in the last decade.

An advanced strategy for the improvement of conservation techniques applied to archaeological wood requires a deep insight in its structure. The authors report here a new approach to the structural characterisation of ancient wood. A sample of an ancient Egyptian artefact was studied by means of scanning electron microscopy, quantitation of the residual amount of lignin and carbohydrates. Its residual lignin was isolated and characterised by 31 P-, 1 H-NMR and 2D-Homonuclear NMR HOHAHA techniques. These analyses allowed the description of structural details of the ancient wood.

12 páginas.-- 6 figuras.-- 5 tablas.-- 27 referencias, Xylan–lignin (XL), glucomannan–lignin (GML) and glucan–lignin (GL) complexes were isolated from spruce wood, hydrolyzed with xylanase or endoglucanase/β-glucosidase, and analyzed by analytical pyrolysis and 2D-NMR. The enzymatic hydrolysis removed most of the polysaccharide moieties in the complexes, and the lignin content and relative abundance of lignin–carbohydrate linkages increased. Analytical pyrolysis confirmed the action of the enzymatic hydrolysis, with strong decreases of levoglucosane and other carbohydrate-derived products. Unexpectedly it also revealed that the hydrolase treatment alters the pattern of lignin breakdown products, resulting in higher amounts of coniferyl alcohol. From the anomeric carbohydrate signals in the 2D-NMR spectra, phenyl glycoside linkages (undetectable in the original complexes) could be identified in the hydrolyzed GML complex. Lower amounts of glucuronosyl and benzyl ether linkages were also observed after the hydrolysis. From the 2D-NMR spectra of the hydrolyzed complexes, it was concluded that the lignin in GML is less condensed than in XL due to its higher content in β-O-4′ ether substructures (62 % of side chains in GML vs 53 % in XL) accompanied by more coniferyl alcohol end units (16 vs 13 %). In contrast, the XL lignin has more pinoresinols (11 vs 6 %) and dibenzodioxocins (9 vs 2 %) than the GML (and both have ~13 % phenylcoumarans and 1 % spirodienones). Direct 2D-NMR analysis of the hydrolyzed GL complex was not possible due to its low solubility. However, after sample acetylation, an even less condensed lignin than in the GML complex was found (with up to 72 % β-O-4′ substructures and only 1 % pinoresinols). The study provides evidence for the existence of structurally different lignins associated to hemicelluloses (xylan and glucomannan) and cellulose in spruce wood and, at the same time, offers information on some of the chemical linkages between the above polymers., This study was supported by the EU projects LIGNODECO (KBBE-2009-3-244362), the ProLignin project of the WoodWisdom-Bioenergy ERA-Net, and the Spanish project AGL 2011-25379. J.R. thanks the CSIC for a JAE-DOC contract, cofinanced by Fondo Social Europeo.

Three non-wood lignins differing in their molecular structure were isolated and characterized by 2D-NMR. The Musa textilis milled-wood lignin (MWL), with a syringyl-to-guaiacyl (S/G) ratio of 9, is strongly acylated (near 85% of side-chains) at the γ-carbon by both acetates and p-coumarates, as estimated from 13C-1H correlations in Cγ-esterified and Cγ-OH units. The p-coumarate C3,5-H3,5 signal was completely displaced by acetylation, and disappeared after alkali treatment, indicating that p-coumaric acid was esterified maintaining its free phenolic group. By contrast, the Cannabis sativa MWL (S/G ~0.8) was free of acylating groups, and the Agave sisalana MWL (S/G ~4) showed high acylation degree (near 80%) but exclusively with acetates. Extensive Cγ-acylation results in the absence (in M. textilis lignin) or low abundance (4% in A. sisalana lignin) of β-β' resinol linkages, which require free Cγ-OH to form the double tetrahydrofuran ring. However, minor signals revealed unusual acylated β-β' structures confirming that acylation is produced at the monolignol level, in agreement with chromatographic identification of γ-acetylated sinapyl alcohol among the plant extractives. In contrast, resinol substructures involved 22% side-chains in the C. sativa lignin. The ratio between β-β' and β-O-4' side-chains varied from 0.32 in the latter lignin to 0.02-0.07 in the two other MWL, and was inversely correlated with the degree of acylation. The opposite was observed for the S/G ratio that was directly correlated with the acylation degree. Monolignol acylation in these and other angiosperms is discussed as a mechanism contributing to regulate the structure of lignin., The study was funded by the BIORENEW EU-project (NMP2-CT-2006-26456), and the Spanish projects BIO2005-3569, BIO2007-28719-E, BIO2007-28720-E, BIO2008-01533, AGL2005-01748 and AGL2008-00709/FOR. CELESA (Tortosa, Spain) is acknowledged for providing the plant materials analyzed. J.I. Santos (CIB, CSIC, Madrid) is acknowledged for help in the NMR studies. D.I. and J.R. thank I3P Fellowships of the Spanish CSIC, and G.M. thanks the Spanish MEC for an FPI Fellowship

This paper presents the results about the lignin from Pints pinaster, the only softwood used in Portuguese pulp and paper industries. Six samples of lignin were collected and studied. The first three samples were precipitated from kraft black liquors at different H-factors. using conventional batch pulping. The other three corresponded to the lignin obtained from black liquors of flow-through reactor at the same H-factors. The residual sugars from the lignin obtained were hydrolysed according to the trifluoroacetic acid method and quantified by HPLC. The main residual sugars on the lignin samples from both reactors were found to be xylose, galactose and arabinose, which indicates that the lignin-carbohydrate linkages are similar to other common softwoods. The elemental analyses of these samples showed that, in the initial stage of delignification, the HS' anions are strongly involved in the delignification mechanism. It was also shown that the C/H ratio was relatively constant, which indicated the weak participation of condensation reactions. The infrared spectra of these samples evidenced the presence of the main peaks corresponding to different functional groups of softwood lignin structure. They also showed that the content of conjugated carbonyl groups decreased with the reaction time. Finally, the permanganate potassium oxidation of the lignin samples showed the presence of the principal fragments present in softwood lignin. It was also shown that. when the time of delignification increased, the amount of non-condensed units decreased, whereas that corresponding to condensed ones increased.

A facile approach to a corncob biorefinery, focusing on both carbohydrate valorization and lignin stabilization, was proposed to coproduce platform chemicals (glucose, xylose, arabinose, and furfural) and lignin. Different metal chloride prehydrolyses of corncob in the biphasic system (2-methyltetrahydrofuran/H2O) were first carried out, followed by enzymatic hydrolysis of treated corncob. It was found that the dissolution and recovery of carbohydrate and lignin were dependent on the prehydrolysis conditions (metal chloride concentration, temperature, and time); 82.9% xylose with 56.2% arabinose was produced at 140 degrees C for 20 min using 25 mM FeCl3, and subsequently, furfural was generated in a yield of 60.0% from this hydrolysate-containing biphasic system by increasing the temperature to 180 degrees C for 120 min. The FeCl3 prehydrolysis of corncob released 99% xylan, retained 91% cellulose, and showed a significant enhancement in the cellulose enzymatic hydrolysis rate of 4.9-fold as compared to that for raw corncob. The chemical structure of the leftover lignin-linked tricin was similar to that of native lignin according to gas permeation chromatography and two-dimensional C-13-H-1 correlation heteronuclear single-quantum coherence nuclear magnetic resonance characterization, which provided a useful substrate for the production of fine and bulk chemicals.

The first aim of this study was to investigate the delignification kinetics of alkaline cooking for autohydrolyzed birch wood (Betula pendula). A series of cooking trials, at temperatures ranging between 130°C and 170°C were performed on untreated (P-factor 0) and autohydrolyzed (P-factor 200) wood. The results of carbohydrate and lignin analyses were fitted to a kinetic model and it was observed that autohydrolysis doubles the delignification rate, during bulk delignification. However, a slight decrease was noticed in the delignification rate during residual pulping. Thus, the second aim of this study was to elucidate the reasons behind accelerated delignification. For this purpose two different autohydrolysis treatments (P-factors 200 and 750) were applied to birch wood. Lignin was isolated from both the native and autohydrolyzed wood and subjected to elemental analysis, methoxyl group determination, carbohydrate analysis and quantitative 1H, 13C and 31P nuclear magnetic resonance (NMR) spectroscopy. The results indicate that, during autohydrolysis, P-O-4 linkages were cleaved, the methoxyl group content was reduced and the syringyl/guaiacyl (S/G) ratio decreased. In addition, a decrease in primary and secondary aliphatic hydroxyl groups and an increase in phenolic hydroxyl groups were detected. It could be concluded that condensation reactions were also occurring during autohydrolysis. Although lignin reactions such as the cleavage of (i-O-4-cleavages and the increase in phenolic hydroxyl groups partly account for the pulping behaviour the reason behind the significantly accelerated bulk delignification is not entirely explained through this study. More research concerning reactions during autohydrolysis is needed.

Eucalyptus wood is among the most important biomass resource in the world. Wood mechanical defibration and fibrillation are energy-intensive processes utilized not only to produce pulp for papermaking, but also to produce reinforcement fibers for biocomposites, nanocellulose, or pretreat lignocellulosic material for biofuels production. The structural features of different Eucalyptus hybrids affecting the refining energy consumption and produced fiber furnish properties were evaluated. The defibration and fiber development were performed using an alkaline peroxide mechanical pulping (APMP) process, which included chelation followed by an alkaline peroxide treatment prior to wood chip defibration. Despite the similar wood densities and chemical compositions of different Eucalyptus hybrids, there was a clear difference in the extent of defibration and fibrillation among the hybrids. The high energy consumption was related to a high amount of guaiacyl lignin. This observation is of major importance when considering the optimal wood hybrids for mechanical wood defibration and for understanding the fundamental phenomena taking place in chemi-mechanical defibration of wood.