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 measurements have been integrated to investigate the surface coverage dependence of the supramolecular structure and dynamics of two isomeric ketones, oMeDBK (molecular cross section similar to that of o-xylene) and pMeDBK (molecular 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 differences are assigned to coverage-dependent changes in the supramolecular structural, dynamic, and binding characteristics of the adsorbed ketones and of the adsorbed reactive intermediates produced by photolysis of the ketones. In the case of pMeDBK, this ketone is adsorbed into the cages and channels 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 consistent with a two-site model for adsorption on the external surface. Furthermore, a Langmuir expression for the adsorption isotherm of oMeDBK fits the experimental data for two binding sites. The entire array of data for oMeDBK is consistent with this model when the data are normalized for the different surface areas of the zeolites. Persistent radicals produced by the photolysis of oMeDBK and pMeDBK are observed directly by EPR analysis.