Highly efficient hydrolysis of cellulose to sugars using supercritical CO2 as a green acid catalyst and solvent

Abstract

Rapid depolymerization of cellulose into processable monomers (e.g., sugars) using solid acid catalysts is an important step for cost-effective biofuel and biochemical production, but has not yet been achieved due to the limited contact between solid cellulose and solid catalysts. Herein, the unique roles of supercritical CO 2 (i.e., scCO 2 ) as an in-situ acid catalyst and reaction solvent in achieving the ultra -fast full solid catalytic hydrolysis of cellulose are disclosed for the first time. When the ball-milling pretreated cellulose was hydrolyzed using oxidized carbon catalysts at 150 degrees C and 100 -300 bar-CO 2 , the hydrolysis kinetics remarkably increased by 3x for conversion and 5x for glucose, resulting in -90% conversion and -85% total sugar selectivity at 20 min. The hydrolysis rate obtained with scCO 2 here was higher than conventional ones with toxic and unrecyclable homogeneous catalysts (e.g., HCl) under harsh reaction conditions (i.e., 180 -220 degrees C and pH of 1 -2). A comprehensive reaction engineering study (e.g., temperature, CO 2 pressure, stirring speed, catalyst acid properties) combined with the estimation of the solution pH by the CO 2 phase equilibrium model and the in-situ and ex-situ monitoring of the phase behavior of the H 2 O/scCO 2 solution were conducted to quantify the activity promotion by scCO 2 and understand the acid-solvent roles of scCO 2 toward the enhanced hydrolysis of cellulose. Specifically, the formation of the Pickering emulsions at the interface between scCO 2 and water and their impact on the enhancement of the cellulose-carbon contact were proposed and verified in detail.