Effect of microwave-assisted deep eutectic solvent pretreatment on lignocellulosic structure and bioconversion of wheat straw

Abstract

In this study, it was aimed to investigate the efficacy of microwave assisted deep eutectic solvent (MW-DES) pretreatment on the structure of wheat straw as well as the sugar and ethanol yields. The samples were treated with choline chloride: formic acid at different mole ratios (1:2, 1:3, and 1:4), microwave power (270, 360, and 450 W) and pretreatment time (2, 5, and 8 min). MW-DES pretreatment has a significant impact on lignocellulosic structure, which was also verified by XRD and SEM analysis. It was found that xylan is the most affected constituent of biomass during MW-DES pretreatment. Severe conditions can solubilize up to 90% of the xylan into liquid phase. The removal of xylan and lignin during MW-DES pretreatment led to highly digestible and fermentable wheat straw fibers. Enzymatic hydrolysis of pretreated wheat straw fibers revealed that MW-DES pretreatment was vastly effective at achieving very high sugar yields (99% glucose, 85% xylose yield). A linear correlation between xylan removal and glucose release during enzymatic hydrolysis was observed. The maximum total sugar release (619 mg/g pretreated wheat straw fibers) being twice as much of conventional deep eutectic solvent (DES) pretreatment, was observed at 1:3 mol ratio, 360 W microwave power and 8 min retention time. The theoretical ethanol yield using Escherichia coli KO11 was 81.5%.