Deciphering ‘water-soluble lignocellulose’ obtained by mechanocatalysis: new insights into the chemical processes leading to deep depolymerization

Recently, the mechanocatalytic depolymerization of lignocelluloses yielding ‘water-soluble lignocelluloses’ was demonstrated. Water-soluble C5 & C6 sugars and sulfur-free lignins are formed through the saccharification of the oligosaccharides, allowing for the fractionation of biomass by simple filtration. Herein, we present an in-depth analysis of water-soluble products obtained from beechwood. The complex nature of the ‘water-soluble beechwood’ is investigated by 2D HSQC NMR, HPLC and gel filtration chromatography. The detailed analysis of the ‘water-soluble beechwood’ lends significant insights into the chemical nature of the lignocellulose depolymerization driven by the mechanical forces

Mechanocatalytic depolymerization of cellulose and raw biomass and downstream processing of the products

The utilization of lignocelluloses (e.g. wood, grass, crops residues and several others) shows great potential as part of the solution for decreasing the dependence of modern societies on fossil resources. In spite of this, the catalytic conversion of these renewable carbon resources via chemical and biotechnological processes is hindered by their complex polymeric nature. For this reason, chemical or enzymatic processes for hydrolysis of cellulose suffer from low efficacy due to harsh reaction conditions and high byproduct formation in case of the chemical methods, or high costs and long reaction times for the enzymatic methods. There is thus an urgent need for processes able to convert the whole plant biomass, which allow the formation of fermentable sugars and technical sulfur-free lignins. Recently, we demonstrated the combination of acid-catalysis with mechanical forces to be an efficient approach to fully overcome the recalcitrance of lignocellulose. As a result, the solvent-free depolymerization of lignocellulose (in solid-state) forms ‘water-soluble lignocellulose’ in quantitative yield. In this article, we present an overview of the mechanocatalytic depolymerization of lignocellulose and downstream processing of the ‘water-soluble’ lignocellulose’ to sugar alcohols and furfurals. The water-soluble products appear to be the ideal platform at the beginning of advanced value chains of biorefining, starting with ‘real’ lignocellulosic substrates

Fractionation of ‘water-soluble lignocellulose’ into C₅/C₆ sugars and sulfur-free lignins

Recently, we demonstrated the mechanocatalytic depolymerization of lignocellulosic substrates as a powerful methodology that fully converts lignocellulosic substrates into ‘water-soluble lignocellulose’. We now show that the saccharification of the aqueous solution of depolymerized beechwood, pinewood and sugarcane bagasse (at 140 °C for 1 h) produces a high yield of sugars (e.g. 88–92% glucose, 3.5–8% glucose dimers and 93–98% xylose relative to the glucan and xylan fractions, respectively) and leads to precipitation of sulfur-free lignins. Noteworthy, the formation of furfurals is suppressed because the ‘water-soluble lignocelluloses’ undergo hydrolysis at relatively low temperatures. At 140 °C, 5-hydroxymethylfurfural and furfural are formed in yields not exceeding 1.4 and 5.7%, respectively. The separation of the carbohydrate fraction (as C5 and C6 sugars) from the lignin fraction is thus feasible by simple filtration

Utilization of mechanocatalytic oligosaccharides by ethanologenic Escherichia coli as a model microbial cell factory

Mechanocatalysis is a promising method for depolymerization of lignocellulosic biomass. Microbial utilization of the resulting oligosaccharides is one potential route of adding value to the depolymerized biomass. However, it is unclear how readily these oligosaccharides are utilized by standard cell factories. Here, we investigate utilization of cellulose subjected to mechanocatalytic depolymerization, using ethanologenic Escherichia coli as a model fermentation organism. The mechanocatalytic oligosaccharides supported ethanol titers similar to those observed when glucose was provided at comparable concentrations. Tracking of the various oligomers, using maltose (alpha-1,4) and cellobiose (beta-1,4) oligomers as representative standards of the orientation, but not linkage, of the glycosidic bond, suggests that the malto-like-oligomers are more readily utilized than cello-like-oligomers, consistent with poor growth with cellotetraose or cellopentaose as sole carbon source. Thus, mechanocatalytic oligosaccharides are a promising substrate for cell factories, and microbial utilization of these sugars could possibly be improved by addressing utilization of cello-like oligomers

Correction: Deciphering ‘water-soluble lignocellulose’ obtained by mechanocatalysis: new insights into the chemical processes leading to deep depolymerization

Correction for ‘Deciphering ‘water-soluble lignocellulose’ obtained by mechanocatalysis: new insights into the chemical processes leading to deep depolymerization’ by Mats Käldström et al., Green Chem., 2014, 16, 3528–3538. On page 3532, second column, the second paragraph should read: “The last important information that can be extracted from the HSQC spectra is the relative composition of lignin in terms of coumaryl (H), coniferyl or guaiacyl (G) and sinapyl (S) units. To obtain the composition of H, G and S units, the aromatic region of the HSQC spectra (Fig. 3b and 3d) was analyzed. The 1H–13C pairs considered in this estimation are H2/6 (δC/δH, 128.0/7.2 ppm, not detected in the beechwood samples studied here), G2 and S2/6 (the contribution of S ′2,6 included) because of their similar chemical environment.22 Hence, by using the half-value of the volume integral of the correlation signals S2/6 (note that they correspond to two 1H–13C pairs), in addition to the entire integral value for G2, the lignin composition can be estimated in terms of S and G units as given by eqn (2) and (3), respectively. The values for S- and G-contents (Table 2) are correct. The discussion and conclusions remain unchanged. The Royal Society of Chemistry apologises for these errors and any consequent inconvenience to authors and readers

Mechanocatalytic Depolymerization of Dry (Ligno)cellulose As an Entry Process for High-Yield Production of Furfurals

Driven by mechanical forces, the acid-catalyzed depolymerization of solid biomass completely overcomes the problems posed by the recalcitrance of lignocellulose. The solid-state reaction leads to water-soluble oligosaccharides, which display higher reactivity than cellulose and hemicellulose. Here, we show that water-soluble oligosaccharides are useful feedstock for the high-yield production of 5-hydroxymethylfurfural (HMF) and furfural in biphasic reactors. This is because they readily undergo hydrolysis upon microwave heating, selectively forming monosaccharides as intermediates in the aqueous phase. Short reaction times are possible with the use of microwave heating and limit the extent of degradation reactions. This work provides an ionic-liquid-free approach to process lignocellulosic substrates into HMF and furfural with high yields. In fact, starting this novel approach with alpha-cellulose, yields of HMF of 79% and furfural of 80% at 443 K for 9 min were obtained. The processing of real lignocellulose (e.g., beechwood and sugar cane bagasse) also achieved high yields of HMF and furfural. Thereby, the current results indicate that the process limitation lies no longer in the recalcitrance of lignocellulose, but in the extraction of highly reactive HMF and furfural from the aqueous phase in the biphasic reactor