Seminar Abstract:
Zeolites are crystalline inorganic oxides that contain microporous channels, cages and pockets that are typically sub-nanometer in dimension. They are indispensable catalysts in the petrochemical industry because their microporous voids can select molecules and reactions using size exclusion criteria. In many cases, the choice of zeolites to meet specific catalytic targets has relied on phenomenological concepts of shape selectivity. This reflects, in part, our incomplete understanding of confinement effects in zeolite catalysis, which has been limited by the combined effects of active site and surrounding environment properties to reaction rates and by the presence of different void structures within a given microporous solid. A more fundamental understanding of acid strength and confinement effects in zeolite catalysis is required to improve existing processes for petroleum-based fuel and chemical production. More importantly, this knowledge is also relevant to the development of new catalytic technologies for the selective conversion of renewable carbon sources (lignocellulosic biomass) that differ markedly from petroleum in terms of thermal stability, chemical functionality and oxygen content.
In this talk, I will discuss how kinetic and mechanistic studies of Brønsted acid-catalyzed reactions of hydrocarbons (alkanes, alkenes) and oxygenates (alkanols, ethers) can be used to independently assess acid strength and solvation effects in catalysis. These studies show how turnover rates and selectivities depend on specific catalyst and reactant properties, such as acid site deprotonation energies and reactant gas-phase proton affinities. They show that zeolites can behave as catalytically diverse materials, in some cases with enzyme-like reaction specificity, for reasons beyond simple considerations of shape selectivity. This diversity arises from the stabilization of reactive intermediates and transition states by dispersion forces upon confinement and prevails even for Brønsted acid-mediated reactions, which is remarkable in light of the similar acid strengths among aluminosilicate zeolites. These insights have clarified enduring controversies regarding the effects of thermal, chemical and cation-exchange treatments on the catalytic reactivity of faujasite zeolites used in fluid catalytic cracking. These concepts and findings give predictive guidance about which void environments provide the right fit for certain reactions and represent progress toward the design of catalytic materials that convert alternative feedstocks to fuels and chemicals derived historically from petroleum.