Multi-level dissolution and hydrolysis of lignocellulosic waste with a semi-flow hydrothermal system

•Multi-level hydrothermal process was studied to optimize lignocellulosic conversion.•Semi-flow system offers separate and long residence times to undissolved materials.•Dissolution of amorphous compounds at 195–200 °C without affecting cellulose.•Cellulose dissolution ratios higher than 75% are achieved at 245–250 °C.•Soluble sugars can be further obtained after the hydrolysis of dissolved cellulose.

The hydrothermal process is efficient in lignocellulosic conversion and is beneficial to potential bioethanol production. In batch- and flow-type processes, concurrent dissolution and hydrolysis of lignocellulose result in product loss and inhibitory intermediates. Therefore, multi-level hydrothermal conversion of corn stalks was implemented with a semi-flow system to provide different residence times to undissolved compounds and facilitate dissolution or hydrolysis at respective optimal conditions. First-stage dissolution dissolved amorphous hemicellulose and lignin at 195–200 °C. Xylan, acid soluble lignin, and part of Klason lignin were dissolved without affecting glucan. In second-stage dissolution, the crystallinity of the undissolved materials suddenly decreased at 245–250 °C. The cellulose dissolution ratio was higher than 75%. Soluble sugars were obtained after the hydrolysis of dissolved cellulose at 280 °C. The results provide significant information on the multi-level hydrothermal process and its potential applications for recovering valuable chemicals from lignocellulosic waste.