Cupric ion species in Cu(II)-exchanged K-offretite gallosilicate determined by electron spin resonance and electron spin echo modulation spectroscopies

Abstract

The location of Cu(II) and its interaction with deuterated adsorbates in Cu(II)-exchanged gallosilicate with the offretite channel-type structure were investigated by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. It is suggested that in fresh, hydrated offretite gallosilicate Cu(II) is in the main channel coordinated to three water molecules and three framework oxygens in a six-ring window of an epsilon-cage to form distorted octahedral coordination. Upon evacuation at increasing temperature, Cu(II) moves from the main channel through an epsilon-cage to a hexagonal prism, Dehydration at 400 degrees C produces one Cu(II) species located in a recessed site in a hexagonal prism based on a lack of broadening of its ESR lines by oxygen. Adsorption of polar molecules such as water, alcohols, dimethyl sulfoxide, and ammonia causes changes in the ESR spectrum of the Cu(II), indicating migration into cation positions in the main channels where adsorbate coordination can occur. However, nonpolar ethylene does nor 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) is suggested to form a trigonal-bipyramidal complex with two ammonias in axial positions and three framework oxygens in a six-ring window of an epsilon-cage based on its ESR parameters and ESEM data.