Effect of constituents molar ratios of deep eutectic solvents on rice straw fractionation efficiency and the micro-mechanism investigation

Effective obtaining fermentable sugars and other platform chemicals from rice straw mediated by choline chloride- oxalic acid dihydrate (CO) and the corresponding micro-mechanism investigation were conducted in this work. Choosing a suitable constituents molar ratio of CO appeared to be a feasible method to achieve an appropriate pretreatment severity for a good biomass fractionation. Pretreatment by using CO with high oxalic acid dihydrate to choline chloride molar ratio could afford easily digestible cellulose-rich materials (CMRs, >80% of enzymatic digestion) and lignin-rich materials (LRMs) of high purity (>82%) due to extensive hemicellulose removal (>93%) and delignification (around 70%). Cell wall microstructure characterizations based on confocal laser scanning microscopy (CLSM) and transmission electron microscopy (TEM) verified the stronger xylan and lignin removal ability of CO with higher oxalic acid dihydrate content. Moreover, the considerable removal of xylan confirmed by CLSM was beneficial to delignification and the subsequent cellulose enzymatic hydrolysis. Furthermore, ultra-structural changes of lignin distribution in cell walls based on TEM indicated the occurrence of partial delignification, preferentially in cell corner (CC) and compound middle lumen (CML) rather than in the secondary cell walls. These findings provides a new insight into the detailed mechanism of acidic DESs mediated biomass deconstruction.