EPR and electron spin echo modulation spectroscopy of CuII ion species in CuII-exchanged K-L gallosilicate

Abstract

The interaction of Cu^II with deuteriated adsorbates in Cu^II-exchanged gallosilicate with the zeolite L channel-type structure has been investigated by EPR and electron spin echo modulation (ESEM) spectroscopies, and compared with that in Cu^II-exchanged K-L aluminosilicate zeolite. Similar results to those for CuK-L aluminosilicate were observed in CuK-L gallosilicate. It was found that, in the fresh hydrated material, Cu^II is octahedrally coordinated to six water molecules. This species is located in the main channels and rotates rapidly at room temperature. A minor Cu^II diaquo species seen in the aluminosilicate is not observed in the gallosilicate. Evacuation at room temperature removes three of these water molecules, leaving the Cu^II coordinated to three water molecules and anchored to the zeolite lattice by coordination to zeolitic oxygens in an eight-ring; the coordinated water is removed more easily in the gallosilicate than in the aluminosilicate. Upon further evacuation at increasing temperature, Cu^II moves from the main channel towards recessed sites. Dehydration at 400 °C produces one Cu^II species located in recessed sites, as evidenced by a lack of broadening of its EPR lines by oxygen. Adsorption of polar molecules such as water and alcohols causes changes in the EPR spectrum of the Cu^II indicating migration into the main channels where adsorbate coordination can occur. Ethene also coordinates with Cu^II but causes less migration. Cu^II forms complexes with two molecules of methanol and ethanol, and with one molecule of ethene, as evidenced by ESEM data, which is the same as for Cu^II in CuK-L aluminosilicate zeolite.