Copper(II) ionic species in CuII-exchanged K-offretite aluminosilicate and comparison with CuII-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 e-cage to form a distorted octahedral complex. Upon evacuation at increasing temperature, Cu II ions move from the main channel through the e-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 e-cage based on EPR parameters and ESEM data, which is the same for Cu II in CuK-offretite gallosilicate.