Mechanistic Insights into Acid Catalysis: The Myths and Challenges of Small Voids
The author acknowledges the technical contributions from Aditya Bhan, Robert Carr, Rajamani Gounder, Stanley Herrmann, Andrew Jones, Phillip Kester, William Knaeble, Josef Macht, Gina Noh, Michele Sarazen, and Stacey Zones and the financial support of BP, Chevron, and the U.S. Department of Energy.
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, assisted by theory and experimentation of evolving fidelity. This lecture covers some examples for 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. Instead, they reflect confinement and sieving effects that favor specific transition states and/or the diffusion of certain molecules that the observer then detects. The acid strength of aluminosilicates is, in fact, essentially the same for all frameworks; it can be varied, with strong consequences for catalysis, only by replacing Al with other trivalent heteroatoms. Certain zeolites form multibranched isomers or β-scission products in reactions of n-alkanes, preferences often taken as evidence of their different “acidity” (an imprecise and not useful term). Such trends reflect instead the selective retention of the more highly branched isomers that act as required precursors in β-scission. In fact, the relative formation rates of monomethyl and dimethyl isomers are not affected by acid strength, and β-scission is favored over isomerization on weaker acids. The effects of acid strength (a precise and useful term that is frustratingly inaccessible to experimental quantification for solid acids) on reactivity merely reflect differences in charge among the relevant transition states, inextricably linked, in the case of small voids, to confinement effects mediated 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 one-dimensional zeolitic acids has been quaintly ascribed to "pore mouth catalysis", a comforting heuristic visual of how the end of a reactant molecule selectively accesses intracrystalline protons. Such terminal methyls merely reflect a preference for effusing terminal isomers, brought to an extreme by the large intracrystalline diffusional barriers imposed by one-dimensional channels. The final example addresses whether zeotypes can activate H2, an observation often ascribed to adventitious metal impurities or to certain types of “coke”. In fact, protons catalyze hydrogenation turnovers; they do so at rates precisely predicted by those of their well-known reverse reactions (monomolecular dehydrogenation). Reaction-derived organic residues do catalyze hydrogenation-dehydrogenation events; they do so quite competently, but with kinetic fingerprints that are clearly discernable from those of proton-catalyzed routes.