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The conventional fractionation of lignocellulose into cellulose, hemicellulose, and lignin was generally carried out at high temperatures and under highly acidic conditions owing to the rigid matrix of lignocellulose and crystallinity of cellulose, causing the degradation of carbohydrates and condensation of lignin, thus hindering their value-added utilization. In this study, a stepwise fractionation method was proposed in which microwave-assisted fractionation of hemicellulose under mild conditions contributed to 81.9% of xylose yield; then, the cellulose part solid was rapidly dissolved in LiBr molten salt hydrate (LiBr-MSH) solution, followed by a filtration process to isolate lignin solid. It was found that the removal of hemicellulose provided more pores for the next dissolution of cellulose, and LiBr-MSH caused the cleavage of the hydrogen bonds of cellulose. Consequently, the cellulose part solid was rapidly dissolved in LiBr-MSH. The results showed that under the optimal fractionation conditions, 78.0% yield of glucan oligomers and 89.7% purity of noncondensed lignin were obtained from cellulose fractionation and filtration, respectively. A high β-O-4 bond content close to 87.3% of the theoretical maximum yield was detected in the obtained lignin, indicating that most β-O-4 bond structure of lignin was well preserved. Additionally, the isolated lignin can be selectively depolymerized into aromatic monomers with a high yield of 20.4%, which is 74.2% of the theoretical maximum yield. [ABSTRACT FROM AUTHOR]

Selective production of glucose from cellulose in a molten salt hydrate (MSH) shows high efficiency but the separation of glucose is still challenging. To address this issue, we proposed a stepwise method, where cellulose was first hydrolyzed into glucan oligomers and separated via anti-solvent precipitation, followed by being hydrolyzed into glucose under mild conditions. For potential industrial applications, using raw biomass as a substrate is more attractive due to its commercial advantages. However, directly hydrolyzing raw corn stover in an MSH resulted in a low glucan oligomer yield and sugar concentration, together with insufficient separation efficiency. This work demonstrated that the generation of acetic acid during hemicellulose hydrolysis is the critical factor affecting glucan oligomer selectivity. To address this issue, a pre-extraction method using auto-hydrolysis was proposed to remove hemicellulose before MSH hydrolysis. After pre-extraction, the obtained glucan oligomer yield could be enhanced from 59.9% to 79.5%, with the separation efficiency increased from 53.7% to 80.6% by methanol precipitation. Besides, increasing the raw corn stover concentration led to an increase in acetic acid and resulted in glucan oligomer degradation. However, after removing hemicellulose, the concentration of glucan oligomers could be significantly enhanced from less than 72.1 to 121.5 mg mL−1. Thus, pre-extraction of hemicellulose followed by MSH hydrolysis provides a potential application for directly producing glucan oligomers from raw biomass with improved yield and separation efficiency. [ABSTRACT FROM AUTHOR]