Performance of green solvents in microwave-assisted pretreatment of lignocellulose

To reveal the action mechanisms of microwave intensification and solvent properties in biorefinery, five typical green solvents (including glycerol (GO), ethyl lactate (EL), γ-valerolactone (GVL), deep eutectic solvents (DES) and water as a control) were selected to conduct biomass pretreatment under microwave irradiation. The pretreatment experiments of different solvents showed that microwave-assisted DES pretreatment had a superior degree of separation (DOS) of 8.64 with higher cellulose retention (88.64%) and lignin removal efficiencies (89.75%). By comprehensively considering the separation efficiencies of three major components in biomass, K-T parameters, and dielectric loss of different solvents, the synergism between microwave and solvents was a key factor in biomass pretreatment, which was further verified by comparative analysis with biomass pretreatment by conventional heating (CH). DES with high dielectric loss could quickly convert microwave energy into thermal energy, causing the effective deconstruction of lignocellulose (observed by SEM) and promoting the separation of three major components. This kind of intensification effect of microwaves contributed to the mild pretreatment process with a lower severity factor (R0) and led to a decrease of energy consumption by 31.56% relative to conventional heating. Benefitting from the excellent separation effect by DES pretreatment, enzymatic saccharification of 89.13% and lignin recovery rates of 67.76% were achieved with a total potential utilization efficiency of 67.98%, but the lignin condensation needed to be concerned.