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

Abstract

The interaction of Cu" with deuteriated adsorbates in Cu"-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"-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" is octahedrally coordinated to six water molecules. This species is located in the main channels and rotates rapidly at room temperature. A minor Cu" 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" 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" moves from the main channel towards recessed sites. Dehydration at 400 "C produces one Cu" 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" indicating migration into the main channels where adsorbate coordination can occur. Ethene also coordinates with Cu" but causes less migration. Cu" 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" in CuK-L aluminosilicate zeolite.

Adsorbate interactions of Cu(II) in Cu(II)-exchanged K-L gallosilicate studied by CW and Pulsed electron spin resonance

“…4c). Similar spectra were observed upon adsorption of methanol and ethanol in earlier work [34]. The ESR spectrum of dehydrated CuK-L gallosilicate measured after dimethyl sulfoxide adsorption is shown in Fig.…”

“…la). This was assigned to a recessed site, i.e., site A in the center of a hexagonal prism, in earlier work [34].…”

“…The synthesis of a gallium analog of LTL zeolite was described in previous work [34]. The chemical composition of the material determined by an inductively coupled plasma spectrometer in combination with ah atomic absorption spectrometer is K10.0Ga10.0Si26.0072.0.49.1H20 per unit cell in the fully hydrated state.…”

“…A value of g• = 2.07 was estimated for the perpendicular component of the g tensor. This species was assigned to Cu(II) in the center of a hexagonal prism in a 6-ring channel based on a lack of broadening of its ESR line by oxygen in earlier work [34]. Figs.…”

“…When that transition metal is paramagnetic as in the case of Cu(II), electron spin resonance (ESR) coupled with electron spin echo modulation (ESEM) spectroscopy have been proved very revealing, yielding coordination numbers and geometries [24][25][26][27][28][29][30][31][32][33]. In recent previous work [34], the location of Cu(II) in hydrated and dehydrated CuK-L gallosilicate and its coordination structure with water, alcohols and ethylene adsorbates were studied for the first time by ESR and ESEM spectroscopies. Zeolite Lis a channel type zeolite with a main channel diameter of about 0.75 nm [35][36][37].…”