Copper(II) ionic species in Cu^II-exchanged K-offretite aluminosilicate and comparison with Cu^II-exchanged K-offretite gallosilicate determined by electron paramagnetic resonance and electron spin echo modulation spectroscopies

Abstract

The location of Cu^II and its interaction with deuteriated adsorbates in Cu^II-exchanged K-offretite aluminosilicate zeolite have been investigated by electron paramagnetic resonance (EPR) and electron spin echo modulation (ESEM) spectroscopies and compared with those in Cu^II-exchanged K-offretite gallosilicate. Basically similar Cu^II locations to those in CuK-offretite gallosilicate are observed in CuK-offretite aluminosilicate, but there are some interesting differences. It is found that in the fresh hydrated sample, Cu^II is, in the main channel, coordinated to three water molecules and three framework oxygens in a six-ring window of an ε-cage to form a distorted octahedral complex. Upon evacuation at increasing temperature, Cu^II ions move from the main channel through the ε-cages to hexagonal prism sites. However, the water coordinated to Cu^II is more tightly bound in the aluminosilicate than in the gallosilicate. Dehydration produces two different Cu^II species in the aluminosilicate, both believed to be located in recessed sites owing to the lack of broadening of its EPR lines by oxygen, while only one Cu^II species is located in a recessed site in the gallosilicate. Adsorption of polar molecules such as water, alcohols, dimethyl sulfoxide, acetonitrile and ammonia cause changes in the EPR spectrum of the Cu^II indicating migration into cation positions in the main channels where adsorbate coordination can occur. However, non-polar ethene does not cause migration of Cu^II. Cu^II forms complexes with two molecules of methanol, ethanol and propanol, and one molecule of dimethyl sulfoxide based on ESEM data. Cu^II forms a trigonal bipyramidal complex with two ammonias in axial positions and three framework oxygens in a six-ring window of an ε-cage based on EPR parameters and ESEM data, which is the same for Cu^II in CuK-offretite gallosilicate.