Cu^IIlocation and adsorbate interaction in Cu^II-exchangedsynthetic 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°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.

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