Hydrolysis of Crystalline Cellulose by Nano-Sized Carbon-Based Catalyst with Weak Acid Sites

Cellulose is the most abundant form of biomass and is expected to be a renewable resource for the production of a variety of chemicals. Natural cellulose is a recalcitrant material because it is insoluble in common solvents and exists in a rigid crystalline form. Accordingly, the direct hydrolysis of crystalline cellulose to its monomer, namely, glucose, is a grand challenge in biorefining. In this work, we propose an idea of using nano-sized catalysts to overcome this issue. Specifically, a small and highly mobile carbon-based catalyst can attach to rigid crystalline cellulose so that it can hydrolyze the attached cellulose. The catalyst was synthesized simply by oxidizing cellulose with air and nitric acid. The nanostructure of the catalyst was revealed by several physicochemical techniques. The nano-sized catalyst hydrolyzed crystalline cellulose to glucose in up to 58% yield by repeating the reaction. Moreover, the catalyst works even at a high cellulose concentration of 16 wt % and at a high substrate/catalyst ratio (32 w/w), giving a 6 wt % glucose solution in a one-pass reaction. The catalytic performance is far higher than that of conventional micron-sized catalysts. Adsorption of the catalyst on crystalline cellulose was confirmed by adsorption experiments and quantum calculations. These results demonstrate that designing nano-sized catalysts is a hopeful strategy to overcome this essential issue in biorefining.