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

Abstract

The interaction of CuII with deuteriated adsorbates in CuII-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 CuII-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, CuII is octahedrally coordinated to six water molecules. This species is located in the main channels and rotates rapidly at room temperature. A minor CuII diaquo species seen in the aluminosilicate is not observed in the gallosilicate. Evacuation at room temperature removes three of these water molecules, leaving the CuII 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, CuII moves from the main channel towards recessed sites. Dehydration at 400 °C produces one CuII 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 CuII indicating migration into the main channels where adsorbate coordination can occur. Ethene also coordinates with CuII but causes less migration. CuII 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 CuII in CuK-L aluminosilicate zeolite.