Exploring Alkyl-O-Alkyl Ether Structures in Softwood Milled Wood Lignins

Recent studies have suggested that there are significant amounts of various alkyl ether (Alk-O-Alk; Alk = alkyl) moieties in a spruce native lignin preparation, milled wood lignin (SMWL). However, the comprehensive NMR assignment to these moieties has not been addressed yet. This study focused on investigating different types of Alk-O-Alk structures at the alpha-and gamma-positions of the lignin side chain in an heteronuclear single-quantum coherence (HSQC) spectrum of SMWL using experimental NMR data of lignin and synthesized model compounds. Ambiguous structural features were predicted by computer simulation of 1H and 13C NMR spectra to complement the experimental NMR data. As a result, specific regions in the HSQC spectrum were attributed to different Alk-O-Alk moieties of Alk-O-Alk/beta-O-4 and Alk-O-Alk/beta-beta ' structures. However, the differences between the specific regions were rather subtle; they were not well separated from each other and some major lignin moieties. Furthermore, SMWL contained a large variety of Alk-O-Alk moieties but in minute individual amounts, resulting in rather broad, superimposing resonances. Thus, evaluation did not allow assigning individual types of Alk-O-Alk moieties from the HSQC spectra; instead, they were quantified as total (alpha-and gamma-linked) Alk-O-Alk based on the balance of structural units in the 13C NMR spectra. At last, potential formation mechanisms of various Alk-O-Alk ether structures in lignin biosynthesis, lignin aging, and during ball milling of wood were and discussed.Peer reviewe

Understanding sulfonated kraft lignin re-polymerization by ultrafast reactions in supercritical water

Producción CientíficaRe-polymerization reactions are a commonly reported issue on the way to the higher recovery of monomers from lignin. The reactivity of monomers obtained from lignin depolymerization and their contribution to the re-polymerization in supercritical water (SCW) was investigated. Sulfonated Kraft lignin (SKL) was used along with four monomers: vanillin, vanillic acid, vanillyl alcohol and acetovanillone. Indulin Kraft lignin was also employed as the reference to understand the re-polymerization of SKL in SCW. The formation of diarylmethane structures was detected in HSQC spectra of solid residue after the SCW process. Lignin released fragments with free phenolic β-O-4 structures, as well as the monomeric product vanillyl alcohol are involved with the formation of o-o´ and o-p´ diarylmethane structures. Chemical structure of Kraft lignin and its polymeric product after the SCW process was remarkably similar, as shown by HSQC, indicating that re-polymerization reactions occur through cross-linking polymerization, mainly in their fractions of low molecular weight products.Ministerio de Ciencia, Innovación y Universidades - Fondo Europeo de Desarrollo Regional (projects CTQ2016-79777-R and PID2019-105975GB-I00)Junta de Castilla y León (project CLU-2019-04 T

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)-, P-31 -, and C-13-NMR spectroscopy were applied as analytical tools. Combining results from these systems, four types of lignin-carbohydrate bonds were detected; benzyl ether, benzyl ester, gamma-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.Peer reviewe

A feasibility study on green biorefinery of high lignin content agro-food industry waste through supercritical water treatment

Producción CientíficaThis work discusses hydrolysis of defatted grape in supercritical water (SCW) at 380 °C and 260 bar from 0.18 s to 1 s focusing attention to sugars recovery in the liquid phase of the product and detailed characterization of remaining solid phase enriched in polyaromatics (e.g. lignin, flavonoids, etc.). After the longest reaction time of 1 s, 56% of carbohydrates could be recovered in the liquid phase, as a result of carbohydrate hydrolysis. The high content of insoluble lignin in biomass (36%), acts as a mass transfer limitation and presents an important feature in the hydrolysis process, slowing down the conversion of carbohydrate fraction, as after the maximum time of 1s, 10% of carbohydrates still remained in the solid phase. Milled wood lignin, extracted from biomass and dioxane extract from the solid phase were characterized in order to understand the main structural changes during the SCW hydrolysis process. Dioxane (80%) extraction of solids produces a very complex mixture of lipophilic extractives, flavonoids and lignin with a certain amount of chemically linked carbohydrates. 2D NMR analysis of dioxane extract shows remarkably subtle changes in the amounts of main lignin moieties (β-O-4′, β-β’ (resinol) and β-5 (phenylcoumaran)). This subtle change of the main lignin structures is an important feature in the further valorisation of this sulfur-free lignin residue.Ministerio de Ciencia, Innovación y Universidades - Fondo Europeo de Desarrollo Regional (projects CTQ2016-79777-R and PID2019-105975 GB-I00)Junta de Castilla y León - Fondo Europeo de Desarrollo Regional (project VA277P18

Machine Learning Optimization of Lignin Properties in Green Biorefineries

Novel biorefineries could transform lignin, an abundant biopolymer, from side-stream waste to high-value-added byproducts at their site of production and with minimal experiments. Here, we report the optimization of the AquaSolv omni biorefinery for lignin using Bayesian optimization, a machine learning framework for sample-efficient and guided data collection. This tool allows us to relate the biorefinery conditions like hydrothermal pretreatment reaction severity and temperature with multiple experimental outputs, such as lignin structural features characterized using 2D nuclear magnetic resonance spectroscopy. By applying a Pareto front analysis to our models, we can find the processing conditions that simultaneously optimize the lignin yield and the amount of beta-O-4 linkages for the depolymerization of lignin into platform chemicals. Our study demonstrates the potential of machine learning to accelerate the development of sustainable chemical processing techniques for targeted applications and products

New opportunities in the valorization of technical lignins

Commercialization of lignins for high-value products should dramatically improve the biorefinery economy and help the growing industry to overcome current economical challenges. However, this requires lignin engineering to obtain products with optimized characteristics for specific applications. This paper reviews important issues of lignin engineering, such as developments in lignin structural analysis, application of small-scale high throughput methods to evaluate lignin performance as well as new achievements in valorization of biorefinery lignins (lignin-cellulose synergism, green and cost efficient methods to upgrade crude biorefinery lignins). These recent developments allow a reconsideration of biorefinery lignins as highvalue products for different applications.</p

Application of Omno polymers in PF wood adhesives

The Plantrose® technology is a promising biorefinery method which enables the production of C5 and C6 sugars from different lignocellulosics using sub- and supercritical water in a two-step process. The lignin rich solids after carbohydrate hydrolysis containing various amounts of residual cellulose are trademarked as OmnoTM polymers. The reactivity and bonding performance of different Omno polymers in direct partial substitution of phenol-formaldehyde adhesive resins (PF) for the manufacture of oriented strand board (OSB) and softwood plywood were evaluated by a fast bench screening test using the Automatic Bond Evaluation System (ABES) and by pilot trials on the production and testing of wood panels. The results showed that about 1/3 of commercial glues could be successfully substituted by Omno polymers without any significant drop in the adhesive reactivity and properties of the resulting wood panels. Selected Omno polymers had superior performance as compared to high-purity pulping lignins (Kraft, soda and organosolv) due to a positive effect of the residual cellulose in the Omnopolymers on the adhesive performance. Hardwood lignins had no disadvantages as compared to various softwood lignins, in strict contrast to the current dogma.</p