Mechanistic role of γ-valerolactone co-solvent to promote ethyl levulinate production from cellulose transformation in ethanol

Direct utilization of cellulosic material to produce specialty chemicals such as levulinate ester could greatly increase biorefinery profitability, but low-level efficiency due to its structural nature limits the strategy for industrial deployment. We report herein that adding γ-valerolactone (GVL) co-solvent during the acid-catalyzed conversion of cellulose in ethanol leads to a high reaction rate and improved catalytic activity towards ethyl levulinate (EL) production. With GVL as the co-solvent (the volume ratio of GVL to ethanol in 1:4), the yield of EL highly enhanced from 31% to 56% under mild operating conditions (170 °C for 2 h) over a low loading Al(OTf)3 of 0.005 mol/L. The mechanistic role of GVL co-solvent to accelerate EL formation from cellulose has been fully elucidated via the combination of experimental and theoretical approaches. It demonstrates that GVL intervention not only favored the dissolution and depolymerization of cellulose, but also enabled the formation of Al species with higher acidity and catalytic activity by modulating the solvation environment, both of which are in charge of promoting cellulose conversion to EL. The activation energy barriers for cellulose depolymerization and glucoside alcoholysis have decreased by 30.8 kJ/mol and 24.8 kJ/mol, respectively. The understanding of this solvation effect can be employed to optimize the rate and yield for production of the versatile bio-based ester compound straight from cellulose valorization.