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

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 CO₂ (i.e., scCO₂) 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 °C and 100–300 bar-CO₂, the hydrolysis kinetics remarkably increased by 3× for conversion and 5× for glucose, resulting in ∼90% conversion and ∼85% total sugar selectivity at 20 min. The hydrolysis rate obtained with scCO₂ here was higher than conventional ones with toxic and unrecyclable homogeneous catalysts (e.g., HCl) under harsh reaction conditions (i.e., 180–220 °C and pH of 1–2). A comprehensive reaction engineering study (e.g., temperature, CO₂ pressure, stirring speed, catalyst acid properties) combined with the estimation of the solution pH by the CO₂ phase equilibrium model and the in-situ and ex-situ monitoring of the phase behavior of the H₂O/scCO₂ solution were conducted to quantify the activity promotion by scCO₂ and understand the acid-solvent roles of scCO₂ toward the enhanced hydrolysis of cellulose. Specifically, the formation of the Pickering emulsions at the interface between scCO₂ and water and their impact on the enhancement of the cellulose-carbon contact were proposed and verified in detail.