Deep eutectic solvents for pretreatment of lignocellulosic biomass: Towards circular bioeconomy

This review critically evaluates the potential of lignocellulosic biomass (LCB) for sustainable biofuels and biochemicals production, with a focus on pretreatment of LCB using deep eutectic solvents (DESs). DESs emerged as a green pretreatment for LCB's complex structure since they exhibit high selectivity in lignin removal while, preserving cellulose and hemicellulose. This is achieved by breaking ether bonds within lignin-carbohydrate complex and ether/ester bonds between hemicellulose and lignin. From existing studies, choline chloride-lactic acid was the most effective DESs for biomass pretreatment able to achieve lignin content as low as 18.8% as compared to untreated biomass of 31.6%. However, limited research focuses on comprehensive evaluation to understand the impacts of DESs and LCB structure under varying conditions such as temperature, pressure, pH, solvent concentration, reaction time, mixing and agitation, substrate compatibility and environmental factors which are crucial for scaling up and downstream application. Further research is needed to optimize and assess performance of DESs under diverse conditions with a focus on economic and scalability by evaluating non-degradative DESs recycling methods, varying water content in DESs reactions, enhancing reusability through encapsulation of DESs and synthesizing low-cost DESs from naturally available components. By addressing these challenges, use of DESs can shift towards sustainable biofuel production through efficient biomass delignification and conversion, advancing circular bioeconomy goals. This review discusses pretreatments of LCB emphasizing on DESs for green conversion of LCB into biofuels and biochemicals and is particularly unique in its integrated discussion towards circular bioeconomy concepts, employing green chemistry principles and techno-economic assessment.