Evaluation of relative catalytic activities of Cu(II)-exchanged L, offretite and omega gallosilicates based on Cu(II) location and adsorbate interactions determined by CW and pulsed electron spin resonance

Abstract

Three structurally related gallosilicates with L, offretite and omega zeolite structures were synthesized and cupric ion was ion-exchanged into each of these gallosilicates. The location of Cu(II) and its interaction with adsorbates in these Cu(II)-exchanged gallosilicates have been studied by electron spin resonance (ESR) and electron spin echo modulation (ESEM) spectroscopies. In fresh hydrated state, Cu(II) is located in a main channel for these materials but with different coordination numbers to water. Cu(II) is octahedrally coordinated to six water molecules in L, while Cu(II) is coordinated to three water molecules and to framework oxygens in the main channel in offretite and omega. During evacuation at increasing temperature, Cu(II) moves from the main channel towards recessed sites. Dehydration produces predominantly one Cu(II) species located in recessed sites such as hexagonal prism for L and offertite and six-ring window of a gmelinite cage for omega based on a lack of broadening of its ESR lines by added oxygen. In L and offretite gallosilicates, there is evidence for back migration of Cu(II) from a hexagonal prism into a main channel to coordinate with adsorbates. Cu(II) in L gallosilicate forms complexes with bigger molecules such as benzene, pyridine and aniline. However, no complex formation between Cu(II) and each of these bulky molecules was observed due to a distorted 12-ring entrance in offretite which makes diffusion of these bulky molecules into the main channel difficult. In omega, the back migration of Cu(II) from a gmelinite cage to a main channel was blocked and thus coordination was possible only with small molecules which can diffuse into the gmelinite cage. The L structure is likely to be the best and comparable to ZSM-5 zeolite, offretite the next and omega the least good for catalytic efficiency for exchanged transition metal ions.