Bereket Tassew Bekele, Rajamani Gounder
The secondary environment surrounding Brønsted acid (H + ) sites in zeolites influences the stability of confined intermediates and transition states in acid catalysis. Extra-framework aluminum (Al ex ) moieties are commonly found within zeolitic voids, either formed intentionally via hydrothermal treatments that remove framework Al sites (Al f ) or adventitiously during synthesis and subsequent treatments. Al ex moieties have been documented to influence catalytic and adsorptive properties of zeolites via chemical interactions and mechanisms that remain debated. Although Al ex species have been proposed to increase H + site acid strength based on analogies to Lewis acid–base interactions in aqueous-phase superacid systems, we present experimental evidence demonstrating that the preeminent role of Al ex species is instead to decrease effective void spaces in zeolitic micropores, which strengthens dispersive stabilization of both adsorbed neutral intermediates and their cationic transition states alike. We combine site-specific spectroscopic, kinetic, and adsorption studies to quantify entropy-enthalpy trade-offs for adsorbed charge-neutral alkanes and their carbocationic transition states mediating protolytic cracking and dehydrogenation, with increasing Al ex content in model chabazite (CHA) zeolite materials containing isolated H + sites. Entropy-enthalpy trade-offs with increasing Al ex content are quantitatively identical to those describing changes in the size of confining micropore environments among zeolite topologies, a behavior characteristic of changes in the strength of dispersive forces. These findings enable catalyst design strategies that preferentially position extra-framework oxide moieties within confining voids containing H + sites to alter dispersive interactions and influence catalytic reactivity, complementing strategies based on varying framework topology or the location of active sites among distinct voids of a given topology.