Myths and Challenges in Acid Catalysis within Small Voids

The sieving and confinement phenomena that confer remarkable functional diversity on solids with voids of molecular size also act to blur the chemical origins of the molecular transformations that they catalyze. The mechanistic conjectures imposed by such barriers to direct observation deserve reconsideration through theory and experiments of increasing precision and fidelity. This lecture focuses on acid catalysis, because it is ubiquitous in practice, but the myths and the challenges are evident for many reactions that occur within small voids. Selectivity and reactivity differences among heterosilicates that differ in void topology are frequently ascribed to their diverse acid strength, but such properties are, in fact, similar for these framework structures. Their different catalytic properties reflect instead confinement and sieving effects that favor specific transition states, and, in some cases, the preferential diffusion of certain molecules thought small apertures based on their size. Acid strength, a precise chemical property, can be estimated from theory for solids with well-defined structure, but cannot be measured. It influences turnover rates when the amount (and distribution) of charge differs between cationic transition states and their relevant precursors. In small voids, the consequences of acid strength are inextricably linked to those brought forth by host-guest interactions, through non-covalent van der Waals contacts that depend on their respective size and shape. The strong preference for terminal methyls in skeletal alkane isomerization within small voids, quaintly ascribed to “pore mouth catalysis”, merely reflects the preferential sieving of such isomers by apertures of molecular dimensions. Despite the prevailing paradigms, the selectivity towards more demanding reactions is not inherently preferred on stronger acids and very strong acids are not essential to activate H2 and CH4 reactants in alkene hydrogenation and alkane alkylation, respectively. The clarity that emerges from dissecting chemistry from transport (and binding from solvation) is enabled by an increasingly seamless theory-experiment nexus; it provides essential guidance in designing solids that exploit synergies between binding sites and their outer sphere environments.

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Enrique Iglesia is a Distinguished Professor and the Vermeulen Chair (Emeritus) at the University of California at Berkeley. He holds degrees from Princeton and Stanford and doctor honoris causa from the Universidad Politecnica-Valencia and the Technical University-Munich. His research addresses the synthesis and structural/functional assessment of solids as catalysts for the production and use of energy carriers and chemicals with minimal environmental footprints. He is a member of the National Academy of Engineering, the American Academy of Arts and Sciences, and the National Academy of Inventors. He has been recognized by ACS (Olah, Somorjai, Murphree awards), AIChE (Wilhelm, Alpha Chi Sigma, Walker awards), and chemical and catalysis societies worldwide (Emmett, Burwell, Boudart, Distinguished Service awards; Gault and Cross Canada Lectureships). He received the ENI Energy Prize, the Kozo Tanabe Prize, and the International Natural Gas Conversion Award. He served as Editor-in-Chief of Journal of Catalysis and President of the North American Catalysis Society. His teaching has been recognized by several awards, notably the Noyce Prize, the highest teaching award at Berkeley.