Photochemical and magnetic resonance investigations of the supramolecular structure and dynamics of molecules and reactive radicals on the external and internal surface of MFI zeolites

Photochemical, magnetic resonance, adsorption isotherms, and surface area m easurements have been integrated to investigate the surface coverage depend ence of the supramolecular structure and dynamics of two isomeric ketones, oMeDBK (molecular cross section similar to that of o-xylene) and pMeDBK (mo lecular cross section similar to that of p-xylene),adsorbed an three forms of zeolites with the MFI structure:silicalite, ZSM-5, and; LZ-105. For each zeolite, the two isomeric ketones display striking qualitative differences in the experimental responses as a function of surface coverage. These dif ferences are assigned to coverage-dependent changes in the supramolecular s tructural, dynamic, and binding characteristics of the adsorbed ketones and of the adsorbed reactive intermediates produced by photolysis of the keton es. In the case of pMeDBK, this ketone is adsorbed into the cages and chann els of the internal surface. The external surface-consists of two,sites for the binding of oMeDBK: pores or "holes", and framework surface between the holes. The coverage dependence of the measured parameters for oMeDBK is co nsistent with a two-site model for adsorption on the external surface. Furt hermore, a Langmuir expression for the adsorption isotherm of oMeDBK fits t he experimental data for two binding sites. The entire array of data for oM eDBK is consistent with this model when the data are normalized for the dif ferent surface areas of the zeolites. Persistent radicals produced by the p hotolysis of oMeDBK and pMeDBK are observed directly by EPR analysis.