Hydrolysis of cellulose by carbon catalysts with weak acid sites

Abstract

The conversion of cellulose to glucose is a major challenge in biorefinery. This paper summarizes our work on the hydrolysis of cellulose by carbon catalysts with weak acid sites. Among solid catalysts carbon materials show a unique good catalytic activity for this reaction. The ball-milling pretreatment of a mixture of cellulose and a carbon material produces tight solid-solid contact, leading to a remarkable improvement in the hydrolysis rate of solid cellulose. As a result, the reaction achieves an up-to 70% yield of cello-oligosaccharides in water and an 88% yield of glucose in a 0.012% HCl aqueous solution. We have also succeeded in the hydrolysis of a real woody biomass containing lignin. A carbon catalyst is prepared from the biomass itself and the spent catalyst can be regenerated by an air oxidation process with the unconverted lignin from the hydrolysis reaction, making a self-contained system. A mechanistic study shows that the carbon adsorbs cellulose by CH-π dispersive forces and hydrophobic interactions. Afterwards, surface weak acid sites hydrolyze the adsorbed cellulose, where neighboring oxygenated groups increase the frequency factor by making strong hydrogen bonds. Accordingly, the structural characteristics of carbon, a combination of polycyclic aromatics and multiple oxygenated groups, is remarkably suited for the hydrolysis of cellulose.