(639e) Mechanistic Insights into Acid Catalysis: The Myths and Challenges of Small Voids

The sieving and confinement properties that confer upon zeotypes their remarkable catalytic diversity also interfere with direct inquiries into the mechanism of proton-catalyzed chemical transformations within their molecular-sized voids. The carbocation chemistry that mediates acid catalysis must often be inferred from data of blurred chemical origins, leading to imaginative concepts that have become part of the canon; some of these deserve reinterpretation or quantification, assisted by theory and experiments of evolving fidelity. This lecture covers some specific examples in acid catalysis, but the concepts are relevant in general to catalytic chemistries within confined environments.

Selectivity and reactivity differences among Al-based zeotypes are often attributed to their different acid strength. The acid strength of aluminosilicates is, in fact, essentially the same for all frameworks. Instead, these differences in their function as catalysts reflect confinement and sieving effects that favor specific transition states and/or the preferential diffusion of certain molecules that the observer then detects in the extracrystalline fluid phase. The effects of acid strength (a precise and useful term that remains frustratingly inaccessible to experimental assessments for solids) on reactivity merely reflect differences in charge among the relevant transition states, inextricably linked to the consequents of confinement brought forth by van der Waals contacts that reflect differences in size and shape among transition states. The preferential location of methyl branches near the end of hydrocarbon chains among alkane isomerization products formed on 1D zeolites has been quaintly ascribed to “pore mouth catalysis”, but merely reflects a preference for effusing them over other isomers. One last example addresses whether zeotypes can activate H₂; in fact, protons catalyze hydrogenation turnovers at rates predicted by those of the well-known reverse reaction (monomolecular dehydrogenation), and reaction-derived organic residues also catalyze hydrogenation-dehydrogenation events but with kinetic fingerprints that are clearly discernable from those of proton-catalyzed routes.