Electrical conductivity of beech sawdust using graphite catalytic coating: unlocking the microwave-assisted thermolysis efficiency of lignocellulosic biomass.

The coating of the beech sawdust using a catalytic amount of graphite (as low as 0.25 wt%) allowed a step improvement in the microwave-assisted thermolysis. Results demonstrated that the pyrolysis performance was linked to an electrical conductivity threshold of the coated samples rather than a gradual increase. With as low as 0.13 mS m-1 of electrical conductivity, the 0.75 wt% graphite coated sawdust (250-500 µm) was efficiently gasified with up to 43 wt% of gas (30 wt% of carbon monoxide, 25 vol% of hydrogen). Initial particle size impacted the thermolysis performance where optimal size (250-500 µm) provided high heat homogeneity due to efficient graphite coating and low temperature gradient between the outer and inner part of the sawdust. The small initial particle size (75-250 µm) was unsuitable for microwave pyrolysis, exhibiting a too large surface area for efficient coating with 0.75 wt% of graphite which was confirmed by the absence of electrical conductivity (<0.003 mS m-1). The electrical conductivity can be used as a marker to evaluate the suitability of the sample for microwave-assisted pyrolysis. Unlike simple graphite mixing, the mechanical coating allowed more than 20-fold decrease of susceptor quantity, providing more homogeneous samples with higher reproducibility.

Microwave-assisted catalytic depolymerization of lignin from birch sawdust to produce phenolic monomers utilizing a hydrogen-free strategy.

Catalytic hydrogenolysis of lignin to obtain value-added phenolic chemicals is a sustainable and cost-effective strategy for the efficient valorization of biomass derived wastes. Herein, an innovative approach by using a single-step microwave assisted depolymerization of lignin from birch sawdust without external hydrogen in the mixture of water-alcohol (methanol, ethanol, isopropanol) co-solvents over commercial catalysts (Pd/C, Pt/C, Ru/C) was investigated. A 65 wt% yield of phenolic monomers was obtained based on 43.8 wt% of delignification (190 °C, 3 h). The solid residues retained 92.0 wt% of cellulose and 57.3 wt% of hemicellulose, which could be further used for fermentation or in the pulp industry. Analysis of the lignin oil revealed that in-situ hydrogen generated from methanol decomposition promoted the hydrogenolysis of ßO4 ether linkage and selective hydrogenation of unsaturated side-chains of phenolic monomers. This work introduces new perspectives for the efficient and cost-effective production of value-added phenolic compounds from lignin in agro-industrial wastes without external hydrogen assisted by microwave heating.

Recent Advances in the Catalytic Depolymerization of Lignin towards Phenolic Chemicals: A Review.

The efficient valorization of lignin could dictate the success of the 2nd generation biorefinery. Lignin, accounting for on average a third of the lignocellulosic biomass, is the most promising candidate for sustainable production of value-added phenolics. However, the structural alteration induced during lignin isolation is often depleting its potential for value-added chemicals. Recently, catalytic reductive depolymerization of lignin has appeared to be a promising and effective method for its valorization to obtain phenolic monomers. The present study systematically summarizes the far-reaching and state-of-the-art lignin valorization strategies during different stages, including conventional catalytic depolymerization of technical lignin, emerging reductive catalytic fractionation of protolignin, stabilization strategies to inhibit the undesired condensation reactions, and further catalytic upgrading of lignin-derived monomers. Finally, the potential challenges for the future researches on the efficient valorization of lignin and possible solutions are proposed.

Isolation of high quality lignin as a by-product from ammonia percolation pretreatment of poplar wood.

A two-step process combining percolation-mode ammonia pretreatment of poplar sawdust with mild organosolv purification of the extracted lignin produced high quality, high purity lignin in up to 31% yield and 50% recovery. The uncondensed fraction of the isolated lignin was up to 34%, close to that the native lignin (40%). Less lignin was recovered after pretreatment in batch mode, apparently due to condensation during the longer residence time of the solubilised lignin at elevated temperature. The lignin recovery was directly correlated with its molecular weight and its nitrogen content. Low nitrogen incorporation, observed at high ammonia concentration, may be explained by limited homolytic cleavage of ß-O-4 bonds. Ammonia concentrations from 15% to 25% (w/w) gave similar results in terms of lignin structure, yield and recovery.

Organosolv pretreatment of Sitka spruce wood: conversion of hemicelluloses to ethyl glycosides.

A range of Organosolv pretreatments, using ethanol:water mixtures with dilute sulphuric acid, were applied to Sitka spruce sawdust with the aim of generating useful co-products as well as improving saccharification yield. The most efficient of the pretreatment conditions, resulting in subsequent saccharification yields of up to 86%, converted a large part of the hemicellulose sugars to their ethyl glycosides as identified by GC/MS. These conditions also reduced conversion of pentoses to furfural, the ethyl glycosides being more stable to dehydration than the parent pentoses. Through comparison with the behaviour of model compounds under the same reaction conditions it was shown that the anomeric composition of the products was consistent with a predominant transglycosylation reaction mechanism, rather than hydrolysis followed by glycosylation. The ethyl glycosides have potential as intermediates in the sustainable production of high-value chemicals.

Study of a specific lignin model: γ-oxidation and how it influences the hydrolysis efficiency of alcohol-aldehyde dehydrogenation copolymers.

Six coniferyl alcohol-coniferaldehyde dehydrogenation copolymers (DHcoPs) were synthesized in order to determine the influence of an increased number of aldehyde functions on hydrolysis. After heterogeneous hydrolysis using acidic Montmorillonite K10 clay, the DHcoPs were thioacidolyzed and analyzed by gel permeation chromatography (GPC). Comparison of the thioacidolyzed products, with and without the hydrolysis step, showed that there was a greater proportion of condensation reaction in the absence of aldehyde. When the coniferaldehyde content in the initial synthetic mixture was more than 30% (w/w), only a low fraction of condensed products was generated during the K10 clay hydrolysis step. This suggests that condensation pathways are mainly due to the alcohol present in the γ-position in the DHcoPs. Investigation of the reactivity and the potential condensation of aldehyde and alcohol monomers under hydrolysis conditions showed the important conversion of coniferyl alcohol and conversely the stability of coniferaldehyde.

Direct conversion of xylan into alkyl pentosides.

Xylan has been used as a raw material in the synthesis of butyl, octyl and decyl glycosides. Mixtures of D-xylose-, L-arabinose- and D-glucose-based surfactants were obtained under smooth conditions with high yields in a one-pot process. The surface activities of octyl and decyl glycosides thus obtained have been studied and compared with that of pure alkyl D-xylosides. The results have confirmed that the new synthetic approach described in this paper is a potentially economical and efficient method for the preparation of environmentally friendly surfactants.

Acidolysis of a lignin model: investigation of heterogeneous catalysis using Montmorillonite clay.

The use of heterogeneous conditions involving Montmorillonite K10 clay was investigated as a mild alternative to homogeneous acidolysis of a lignin model. Guaiacyl Dehydrogenation Oligomers (DHOs) synthesized by horseradish peroxidase were selected as starting material. Hydrolysis products were analyzed by gel permeation chromatography and by HPLC/mass spectrometry. Several experimental parameters were studied such as catalyst and substrate concentration, as well as reaction solvent composition in order to minimize high molar mass product formation generated by recombination mechanisms. In both catalytic modes, the best hydrolysis conditions were similar in terms of solvent composition with dioxane/water (90/10, v/v) and catalyst H(+) concentration of about 0.01 mol/L. Although the homogeneous catalysis generated only 28% of low molecular weight (LMW) products (monomers and dimers), clay catalysis generated 35%. In light of the qualitative analysis, both catalytic methods gave the same products, thus supporting similar hydrolysis mechanisms.