(51c) Synergistic Effect Between Defect Sites and Functional Groups in the Hydrolysis of Cellulose Using Activated Carbon

Carbon with Brønsted acid sites can function as a stable and effective solid acid catalyst for reactions in hot liquid water. It is able to hydrolyze cellulose, an abundant and renewable biomass, into glucose, which is a versatile platform molecule. However, there are limited reports on the effect of different chemical treatments and the nature of active sites. In this study, the effect of chemical oxidation by H₂O₂ and ­H₂SO₄ on activated carbon was investigated, and the materials were used in the hydrolysis of cellulose for comparison. The oxidation procedure was characterized by XRD, FTIR, ¹³C DP MAS NMR, Boehm titration, N₂ physisorption, CO₂ physisorption and Raman spectroscopy. Oxidation by H₂O₂ imparts acidic functional groups such as phenol, lactone and carboxylic acid, while treatment with H₂SO₄ imparts all of those oxygen functional groups along with sulfonic acid groups. Characterization reveals that H₂O₂ is capable of generating and enlarging defect sites, while H₂SO₄ creates more edges in the carbon material, and this effect is more pronounced with higher temperature.

Adsorption isotherms demonstrated that sulfonic groups enhance the adsorption of glucose monomers more than phenols, lactones and carboxylic acids by hydrogen bonding, and the influence of this enhanced-adsorption effect decreases as the length of glucan chain increases. This can be explained by the increasing influence of van der Waals interactions between the CH groups and the polyaromatic rings. All of the chemically treated materials exhibit catalytic activities in the hydrolysis of cellulose despite the presence of weakly acidic functional groups. Carbon treated by H₂SO₄ showed higher reactivity even though it has less functional groups and lower pore volume distribution. This can be attributed to the synergistic effect between the edge/defect sites and functional groups in the carbon material, where the glucan chains are immobilized and the exposed glycosidic bonds are forced to interact with the in-plane functional groups.