Interactions of chemical species with acid sites in zeolites

A detailed understanding of reaction pathways in zeolite solid acids requires knowledge about the structure and thermochemistry of all chemical species which could potentially exist along the reaction coordinate. Because some high-silica zeolites, such as HZSM-5, consist of a collection of nearly identical, Bronsted-acid sites, equal in concentration to the Al content, the 1 : 1 stoichiometric adsorption complexes formed by various molecules when they interact with the acid sites are well defined and relatively easy to characterize using temperature-programmed desorption, microcalorimetry, and NMR. This paper briefly reviews what is known about the reactivity, thermochemistry, and structure of complexes formed by amines, pyridines, imines, alcohols, thiols, olefins, aldehydes, ketones, and nitriles. It is shown that the thermochemistry and structure of the complexes are affected by both local effects (proton transfer or hydrogen-bonding interactions) and nonlocal effects (van der Waals interactions with the zeolite cavity). However, solvation effects in zeolites are very different from that found in acidic solutions so that the use of gas-phase reference conditions provides a much better starting point for understanding adsorption complexes, and therefore acid-catalyzed reactions, in the zeolite.