Diffusion NMR characterization of catalytic silica supports: a tortuous path

Mesoporous silicas have found widespread application within the field of heterogeneous catalysis. Acid functionalization of such materials, through one-pot or postsynthetic grafting of sulfonic acid groups, imparts activity for fatty acid esterification, with the studious choice of pore geometry facilitating significant rate enhancements. Diffusion NMR has been utilized for the first time to characterize the structure of mesoporous silicas through the transport behavior of systematically related carboxylic acids confined within their mesopore networks. A reduced diffusion coefficient is obtained for species constrained within the 3-dimensional interconnected pores of KIT-6 relative to the 2-dimensional noninterconnected pore channels of SBA-15. The effective tortuosity of both porous silicas increases with the acid chain length, with the diffusion behavior of long-chain acids dominated by the alkyl chain and silica architecture. Carboxylic acid diffusion within these two pore networks is unlikely to be rate-limiting in catalytic esterification over sulfonic acid silica analogues. Physicochemical insights from diffusion NMR will aid the future design of optimal silica architectures for catalytic applications.