Towards a multi-scale understanding of dilute hydrochloric acid and mild 1-ethyl-3-methylimidazolium acetate pretreatment for improving enzymatic hydrolysis of poplar wood

Sole pretreatment methods always have limitations in improving enzymatic hydrolysis of wood due to the multi scale biomass recalcitrance. Herein, dilute acid pretreatment (DAP) with 2% hydrochloric acid at 160 degrees C and mild ionic liquid pretreatment (MILP) with 1-ethyl-3-methylimidazolium acetate at 90 degrees C were combined to increase cellulose accessibility in poplar wood. Additionally, the compositional, structural, and topochemical changes of wood during pretreatment and their effect on cellulose digestibility were comprehensively investigated and well understood. The sequential DAP and then MILP led to the highest yield of fermentable sugar (87.15%) after 72 h enzymatic hydrolysis when compared with the sole pretreatments and the sequential MILP and then DAP (21.31-73.64%). We suggested that the significant hemicellulose removal and cell wall deconstruction during first-step DAP exposed more cellulose fibrils to ionic liquid, thereby improving the efficacy of subsequent MILP to delignify wood and change cellulose crystal structure. The considerable removal of both hemicellulose and lignin, the damage of wood structure, and the crystal transformation from cellulose I to cellulose II dramatically improved the enzymatic hydrolysis of the DA + MIL sample. This study provides a better understanding of decomposition mechanism of cell walls during DAP and MILP, which can offer valuable insights into the development of promising pretreatment strategies.