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

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%.