ELECTRON-SPIN ECHO ENVELOPE MODULATION STUDIES OF CU2+ INTERACTIONS WITH FRAMEWORK AL IN DEHYDRATED X-ZEOLITES, Y-ZEOLITES, AND A-ZEOLITES

The interactions of Cu2+ cations with framework Al in zeolites NaX, KX, NaY, and NaA in various stages of dehydration and after methanol adsorption were investigated by electron spin echo envelope modulation (ESEEM) spectroscopy. The FT-ESEEM spectra of all species studies showed contributions from two types of Al-27 nuclei. The first type, referred to as first shell, consists of Al-27 nuclei bonded to the oxygens to which the Cu2+ is coordinated. They exhibit relatively large isotropic hyperfine constants which manifest the firm binding of the Cu2+ to the framework. The second type consists of Al nuclei which are coupled to the Cu2+ by weak dipolar interactions and are termed distant Al. Orientation selective ESEEM experiments were carried out as well to obtain additional structural information. The modulation amplitude corresponding to distant Al did not show any dependence on the irradiation position within the EPR powder pattern as the arrangement of those nuclei around the cation is approximately isotropic. The modulation amplitudes of the first shell Al in some cases showed strong orientation dependence which was interpreted in terms of the site geometry. The amount of adsorbed methanol in Cu-NaX has a considerable effect on the relative intensities of the peaks of the first shell Al and distant Al. This dependence is explained in terms of the changes in the Al-27 quadrupole coupling constants of the two types of Al.