High-yield selective conversion of pretreated milled softwood mixture to glucose by oxalic acid impregnation and microwave-assisted hydrolysis – Application to bio-ethanol production

In this study, a deep eutectic solvent pretreated milled softwood mixture was impregnated with oxalic acid before performing a water hydrolysis to obtain glucose using a microwave. Analysis of variance of a central composite design (CCD) showed that both temperature and time were significant parameters to consider when studying the cellulose conversion, glucose yield and glucose selectivity. The optimal conditions were 174 °C and 2 h, which resulted in 27.7% cellulose conversion, 22.4% glucose yield and 81.1% glucose selectivity. Oxalic acid affected the amorphous domains of cellulose at low hydrolysis temperature, while both the amorphous and the crystalline regions were affected at high temperature. Successive treatments of the solid hydrolysis residues resulted in 98.4% cellulose conversion, 79.0% glucose yield and 80.3% selectivity. Bioethanol was produced from the recovered glucose within 24 h with a fermentation efficiency of 42.9%.