Mechanisms of sequential dissolution and hydrolysis for lignocellulosic waste using a multilevel hydrothermal process

Hydrothermal conversion normally results in fermentable sugar decomposition and inhibitor accumulation through dissolving and hydrolyzing lignocellulose synchronously. A multilevel hydrothermal process was applied to sequentially dissolve and hydrolyze lignocellulosic waste, and the dissolution mechanisms were investigated. The crystallinity of treated corn stalks increased from 43.1% to 72.5% with temperatures increasing from 190 to 220 °C and reaction times from 5 to 40 min. FT-IR analysis under these conditions showed gradual hemicellulose disappearance and cellulose retention. For cellulose dissolution, temperatures above 240 °C presented significant reductions in crystallinity and degrees of polymerization. Intermolecular and intramolecular H-bonds were broken with the increase in temperature, and glycosidic bonds remained until 260 °C. Thus, the amorphous structures of lignin and hemicellulose, as well as crystalline structure of cellulose, could be sequentially dissolved around 190 °C and 240 °C, respectively. Kinetic analysis revealed that the lignocellulosic dissolution reaction above 190 °C did not agree with the surface reaction rate equation because of the high dissolution rate and interaction in the lignocellulosic structure. By contrast, the reaction kinetics of cellulose dissolution did not exhibit a sudden change in the test hydrothermal conditions, and the activation energy was calculated as 189.8 ± 8.6 kJ mol−1 according to the obtained reaction rate constants. These findings significantly enhance the understanding of the mechanism, and provide evidence for the sequential dissolution and hydrolysis of lignocellulosic waste using a multilevel hydrothermal process.