ESR Analysis of Cupric Ion Species Exchanged into NaH-ZSM-5 Gallosilicate

Abstract

ZSM-5 gallosilicate molecular sieves was synthesized and cupric ion was ion-exchanged into the gallosilicate. The locations of Cu(II) species in the framework and their interactions with various adsorbates were characterized by combined electron spin resonance(ESR) and electron spin echo modulation(ESEM) methods. It was found that in a fresh hydrated material, Cu(II) is octahedrally coordinated to six water molecules. This species is located in the channel intersections of two sinusoidal channels and rotates rapidly at room temperature. Evacuation removes some of these water molecules, leaving the Cu(II) coordinated to less water molecules and anchored to oxygens in the channel wall. Dehydration produces two Cu(II) species, both of which are located in sites inaccessible to oxygen as evidenced by non-broadening of its ESR lines by oxygen. Adsorption of adsorbate molecules such as water, alcohols, ammonia, acetonitrile and ethylene on dehydrated CuNaH-ZSM-5 gallosilicate materials causes changes in the ESR spectrum of Cu(II), indicating the migration of Cu(II) into main channels to form complexes with these adsorbates there. Cu(II) forms a complex with two molecules of methanol, ethanol and propanol, respectively as evidenced by ESR parameters and ESEM data. Cu(II) also forms a square planar complex with four molecules of ammonia, based on the resolved nitrogen superhyperfine interactions and their ESEM parameters. Cu(II) forms a complex with two molecules of acetonitrile based on the ESR parameters and ESEM data. Interestingly, however, only part of Cu(II) interacts indirectly with one molecule of nonpolar ethylene based on ESR and ESEM analyses.