Fractionation of rice straw by a single-step solvothermal process: Effects of solvents, acid promoters, and microwave treatment

Abstract

Fractionation of lignocellulosic components is a primary step for maximizing value-added conversion of plant biomass to biofuels and chemicals. In this study, different organic solvents (methyl isobutyl keone, ethyl acetate, toluene and diethyl ether) in the ternary water/alcohol/organic solvent system and acid promoters (H2SO4, HCl, and H3PO4) were studied on a single-step solvothermal fractionation of rice straw. Ethyl acetate was found to be a superior composite solvent to the conventional solvent MIBK by providing higher glucan and lignin yields. Glucan and xylan could be further solubilized and lignin recovery increased at higher temperature and acid concentration. The optimal fractionation conditions were found to be water/ethanol/ethyl acetate (62.5%:25%:12.5%) mixture using 0.05 M H2SO4 at 160 °C for 1 h, which gave cellulose-enriched solid with 71.4 wt% glucan yield. Hemicellulose (53.2 wt%) was obtained as sugars, and dehydration products in the aqueous/alcohol phase were minimal. 63.1 wt% lignin was recovered in the organic solvent phase with no cross-contamination of sugars. Microwave-assisted heating at 300 W, corresponded to the temperature of 160 °C for 1 h resulted in comparable glucan and lignin yields in the respective phases to conventional heating with the advantages of increased pentose yield and lower furan formation in the aqueous phase.