A comparative study of effects of microwave-assisted deep eutectic solvents pretreatment of rice straw on cellulose enrichment and enzymatic digestibility

Lignocellulosic biomass (LCB), such as agricultural residue rich in cellulose and hemicellulose, can serve as a feedstock for bioethanol production due to its environmental benefits. Deep eutectic solvents (DES) are low-cost and greener solvents for LCB pretreatment. The present study investigated the efficiency of two DES types for rice straw pretreatment under microwave conditions: choline chloride:glycerol (ChCl:Gly) and choline chloride:formic acid (ChCl:FA). DES pretreatment was performed under microwave conditions (100-140 °C and 5-15 min) at 10% solids loading and constant power 200W. After pretreatment, the samples were characterized, and enzymatic digestibility was investigated at 50 °C for 72 h using enzyme loading of 6 filter paper unit g^-1 cellulose and 10% solids loading. Untreated rice straw consists of cellulose (41.8%), hemicellulose (24.9%), lignin (17.0%) and ash (15.0%). The cellulose content increased to 59.8% and 59.2% after ChCl:Gly and ChCl:FA pretreatment, respectively, while hemicellulose decreased to 15.6% and 10.1% after ChCl:Gly and ChCl:FA pretreatment, respectively. Lignin content decreased to 8.2% after ChCl:Gly pretreatment compared to 9.6% after ChCl:FA pretreatment. Ash content (20.4%) obtained after ChCl:FA pretreatment was higher than 14.0% obtained after ChCl:Gly pretreatment. The cellulosic and hemicellulose fractions from ChCl:Gly pretreatment were effectively hydrolyzed with glucose and xylose yield of 86.0% and 68.4%, respectively, compared to glucose and xylose yield of 58.2% and 62.8%, after ChCl:FA pretreatment. ChCl:Gly pretreatment enriched cellulosic content and preserved hemicellulose fraction, achieving a higher yield of fermentable sugars than ChCl:FA pretreatment; hence, it can potentially support a biorefinery.