Transition-state Scaling Relations in Zeolite Catalysis: Influence of Framework Topology and Acid-site Reactivity

Microporous acid catalysts are extensively used in the chemical industry because of their high activity and unique shape-selectivity induced by frameworks with molecular-sized voids. However, it remains a challenge to tailor catalytic activity by choice of pore structure and acid strength. In this theoretical work we aim at extracting trends in how zeotype-catalyzed reactions are influenced by reactivity of Brønsted acid sites and framework topology. Using olefin-methanol reactions as an example, we consider a number of elementary steps across 21 zeotype materials of three different topologies. Density Functional theory was employed to calculate the enthalpies of transition states (TSs) and establish scaling relations across catalyst acid strength and framework topology, using the ammonia heat of adsorption as a descriptor of acid-site reactivity. The results indicate that when cation-like TSs are increasingly stabilized by dispersion interactions with the framework, they become less sensitive to changes in the reactivity of acid sites. It is crucial to further investigate this finding to ultimately be able to tailor the activity of zeotype acid catalysts, appreciating the influence of both framework topology and acid strength.