Zeolite-Templated Carbon Catalysts for Adsorption and Hydrolysis of Cellulose-Derived Long-Chain Glucans: Effect of Post-Synthetic Surface Functionalization

This manuscript quantitatively investigates the effect of weak acid site surface density on adsorption and catalytic hydrolysis of long-chain β-glucans, with post-synthetically modified zeolite-templated carbon (ZTC) catalysts. Our approach requires ZTC-surface modification and overcomes previous limitations of pore collapse in accomplishing this, which has previously necessitated electrochemical methods. We demonstrate that mild ZTC treatment in hydrogen peroxide preserves the 1.1 nm micropores of ZTC, which were previously shown to be ideal for β-glucan adsorption, while synthesizing surface-modified ZTC catalysts that hydrolyze adsorbed β-glucans to glucose in up to 87% yield. Our results demonstrate a direct increase in catalytic hydrolysis activity and glucose yield upon increasing acid site density via surface functionalization. Upon investigating the mechanism of catalytic hydrolysis under buffered conditions, we rule out the synthesis of acid sites with stronger acidity as a result of possible greater anion delocalization as well as the possibility of a cooperative acid-base bifunctional mechanism. Our data instead argue for the importance of a high density of surface carboxylic acid functionality as promoting the likelihood of pairing a surface acid site with a glycosidic oxygen of an adsorbed glucan on a length scale that is commensurate with that required for general acid catalysis. From this perspective, our ZTC catalysts function much like zeolites - wherein both achieve high rates with weak-acid sites by coupling adsorption of reactant into a confined domain containing the acid site, and general-acid catalyzed reaction.