PhotochemistrY of Organic Molecules Adsorbed on Faujasite Zeolites: Steric Effects on Product Distributions

Summary

Zeolites are robust, crystalline, porous aluminosilicates possessing an enormous internal surface area that is capable of adsorbing large quantities of guest molecules, the size and shape of whose structures allow them to pass from the external to the internal zeolitic surface and to diffuse onto the internal surface. The framework composition, the presence of cations associated with the framework, and the topology of the void space internal to the zeolite all contrive to imbue these materials with special properties that contribute to their widespread use as catalysts, ion exchange materials and molecular sieves. Photochemical probes have been developed to explore the structure of zeolites near the site of adsorption and to examine the dynamics of reactions of molecules adsorbed on the internal zeolitic surface. The basic composition of the internal framework of alkali ion exchanged zeolites is extremely chemically inert. As a result, very reactive photochemically generated intermediates such as radicals do not react with the framework, but react with each other via radical-radical reactions. However, in contrast to the results of radical-radical reactions in solution, the products of radical-radical reactions in zeolites may be controlled by the geometry and occurrence of the zeolite surface. In this report, we review the structure of zeolites in general and then survey the structure of an important class of zeolites, the faujasites. We then show how a photochemical probe, the photochemistry of dibenzyl ketone, can yield information on how intracrystalline dynamics can be influenced by cation type, cation number density and coadsorbed guests and, in turn, how intracrystalline dynamics can determine the products of photoreactions.