Catalytic Hydrolysis of Cellulose to Glucose Using Weak-Acid Surface Sites on Postsynthetically Modified Carbon

Abstract

We demonstrate depolymerization of adsorbed (1 → 4)- β-d-glucans (β-glu) derived from crystalline cellulose (Avicel), using weak-acid sites of postsynthetically surface-functionalized mesoporous carbon nanoparticle (MCN) catalysts HT5-HSO3-MCN and COOH-MCN and investigate the role of acid-site density and β-glu molecular weight on this depolymerization. Both HT5-HSO3-MCN and COOH-MCN hydrolyze adsorbed β-glu strands and afford glucose yields of 73% and 90%, respectively, at a buffered pH of 2.0 after 3 h treatment at 180 °C. These yields are significantly higher than the 16% yield of an unfunctionalized MCN-control catalyst under otherwise identical conditions, demonstrating the importance of postsynthetic surface functionalization for achieving weak-acid catalytic hydrolysis. Highlighting the important role of confinement in this catalysis, all yields are generally depressed when using a lower rather than higher molecular weight of adsorbed β-glu strands on the same catalyst. The catalytic hydrolysis rate also generally increases upon decreasing buffer pH—particularly so for the more acidic carboxylic acid-functionalized catalyst COOH-MCN. This is interpreted on the basis of a higher local density of surface weak-acid sites upon protonation of surface conjugate-base functionality, as demonstrated by a comparison of zeta potential measurements of catalysts COOH-MCN and MCN.