The rate and selectivity of reactions catalyzed by acids depend on the stability of ion-pairs at transition states that mediate kinetically-relevant steps. Rates and selectivities for alkanol dehydration and homologation, alkene and cycloalkene isomerization, and alkoxide scission and hydrogen transfer on polyoxometalate and zeolitic acids show that sensitivity to acid strength reflects differences in the amount and diffuseness of the charge in the relevant precursors and the transition states. The effects of solvation by confinement on rates and selectivities depend, in turn, on their respective differences in size.  The known structures of these acids allow rigorous comparisons between experiment and theory, which confirm the mechanistic interpretations of rate data and the relevance of deprotonation energies as theoretical proxies of acid strength.  These studies and insights suggest a rigorous reactivity-based ranking of acid strength that can be used to assess the strength of solid acids with uncertain or non-uniform structures.

Award Address  (Gabor A. Somorjai Award for Creative Research in Catalysis sponsored by the Gabor A. and Judith K. Somorjai Endowment Fund). Consequences of acid strength and confinement in catalytic reactions mediated by ion-pair transition states

243rd National Meeting (2012)

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