Cu-II location and adsorbate interaction in Cu-II-exchanged synthetic Na-omega gallosilicate - EPR and electron spin echo modulation studies

Abstract

The location of Cu-II and its interaction with adsorbates in Cu-II-exchanged synthetic Na-omega gallosilicate have been studied by EPR and electron spin echo modulation (ESEM) spectroscopies. These results are compared with those of Cu-II-exchanged Na-omega aluminosilicate and also those of L and offretite gallosilicates of similar channel type and size, and the differences are discussed. In general, similar results to those for CuNa-omega gallosilicate are obtained for CuNa-omega aluminosilicate. It is concluded that, in fresh hydrated omega material, Cu-II is in a main channel coordinating to three water molecules and to framework oxygens in the main channel wall. A minor Cu-II diaquo species not seen in the gallosilicate is observed in the aluminosilicate. Upon evacuation at increasing temperature, Cu-II moves from the main channel to a gmelinite cage. Dehydration at 410 degrees C produces one Cu-II species located in a six-ring window of a gmelinite cage, based on a lack of broadening of its EPR lines by oxygen. In L and offretite gallosilicates, there is evidence for back migration of Cu-II from a hexagonal prism into a main channel to coordinate with adsorbates. However, in omega the back migration from a gmelinite cage to a main channel seems to be blocked, as shown by very slow changes in the EPR spectra and differing coordination numbers for methanol, ethanol and propanol to Cu-II when alcohols are adsorbed. Cu-II does not form a complex with propanol or larger adsorbates in omega gallosilicate. It is suggested that, in omega, small adsorbates must diffuse into a gmelinite cage where Cu-II is located, to form Cu-II-adsorbate complexes. The slow changes in the EPR spectra correspond to the time for adsorbate diffusion into a gmelinite cage. Cu-II interacts with one molecule each of ethylene and acetonitrile, based on EPR and ESEM analyses. Cu-II forms a square-planar complex containing four molecules of ammonia, based on resolved nitrogen superhyperfine coupling.