Integrated understanding of acidic deep eutectic solvent choline chloride: Oxalic acid pretreatment to enhance the enzymatic hydrolysis of rape straw

Using choline chloride (ChCl) as hydrogen-bonded-acceptor (HBA) and oxalic acid (OA) as hydrogen-bonded-donor (HBD), deep eutectic solvents (DESs) with different HBD-to-HBA molar ratios were synthesized to pretreat rape straw (RS). The influence of DES treatment on the RS chemical composition, structure change, and enzymatic saccharification were explored. DESs with low molar ratio of HBD-to-HBA exhibited low viscosity, which enhanced the pretreatment efficiency. When HBA-to-HBD molar ratio was 1:3, the delignification and enzymatic digestion efficiency were only 60.6% and 41.8%. As the molar ratio of OA decreased, delignification and enzymatic digestion efficiency increased to 83.6% and 96% (ChCl:OA = 3:1, mol/mol). After pretreatment (severity factor LogR0 =2.66), the cellulose accessibility and crystallinity increased from 353.0 to 691.1 mg/g and 35.4–64.2%, respectively. The lignin surface area decreased from 596.3 to 201.9 m2/g. The significant increase in cellulose accessibility and the decrease in lignin surface area explained the efficient enzymatic hydrolysis efficiency.