Understanding the role of defect sites in glucan hydrolysis on surfaces

Though unfunctionalized mesoporous carbon consisting of weakly Brønsted acidic OH-defect sites depolymerizes cellulose under mild conditions, the nature of the active site and how this affects hydrolysis kinetics--the rate-limiting step of this process--has remained a puzzle. Here, in this manuscript, we quantify the effect of surface OH-defect site density during hydrolysis catalysis on the rate of reaction. Our comparative approach relies on synthesis and characterization of grafted poly(1→4-β-glucan) (β-glu) strands on alumina. Grafted β-glu strands on alumina have a 9-fold higher hydrolysis rate per glucan relative to the highest rate measured for β-glu strands on silica. This amounts to a hydrolysis rate per grafted center on alumina that is 2.7-fold more active than on silica. These data are supported by the lower measured activation energy for hydrolysis of grafted β-glu strands on alumina being 70 kJ/mol relative to 87 kJ/mol on silica. The observed linear increase of hydrolysis rate with increasing OH-defect site density during catalysis suggests that the formation of hydrogen bonds between weakly Brønsted acidic OH-defect sites and constrained glycosidic oxygens (i.e., those juxtaposed adjacent to the surface) activates the latter for hydrolysis catalysis. Altogether, these data elucidate crucial structural requirements for glucan hydrolysis on surfaces and, when coupled with our recent demonstration of long-chain glucan binding to mesoporous carbon, present a unified picture, for the first time, of adsorbed glucan hydrolysis on OH-defect site-containing surfaces, such as unfunctionalized mesoporous carbon.