(3ay) Understanding and Predicting Chemical Reactivity In Solid Acid Catalysis

Chemical reactions catalyzed by Brønsted acids involve cationic transition states and the separation of charge into ion-pairs, which become more stable as acid sites become stronger. Acid sites in aluminosilicate zeolites are weaker and less diverse in composition and strength than in mesoporous or liquid acids, but confinement effects can selectively stabilize transition states relative to the relevant reactants and lead to turnover rates that depend sensitively on the nanostructure of the confining voids. Efforts to examine the individual consequences of acid strength and confinement for catalysis have been limited, however, because these effects are inextricably linked in rate constants and because a microporous structure can contain a diverse range of local environments with significant consequences for turnover rates.

Here, I will discuss mechanistic studies of reactions of alkanes (cracking, dehydrogenation), alkenes (hydrogenation, alkylation, oligomerization), and alkanols (dehydration) on solid acids and protocols to characterize the number, strength and location of acid sites within microporous voids. Fundamental relations between the structures of reactants and catalysts and those of ion-pairs formed at transition states have been established using Born-Haber thermochemical cycles, Marcus theory for charge-transfer reactions and De Donder relations for non-equilibrium thermodynamics. These insights have enabled precise predictions of rate and equilibrium constants for elementary steps and, in turn, of overall catalytic rates and selectivities and of site requirements as reactants, catalysts and reaction paths change. These principles apply generally to acid catalysis and thus provide a methodology also useful in my future research plans to address issues relevant in the production of fuels and chemicals from renewable and fossil-based resources. This research program will integrate fundamental mechanistic studies, methods to characterize active site structure and techniques for the targeted synthesis of catalytic materials.