Electron Spin Resonance and Electron Spin Echo Modulation Spectroscopic Studies of Cupric Ion−Adsorbate Interactions in Synthetic Clinoptilolite

Abstract

The site locations and adsorbate interactions of Cu2+ in Cu2+-exchanged synthetic clinoptilolite have been determined by electron spin resonance and electron spin echo modulation spectroscopies. Strong effects of the H+, Li+, Na+, and K+ cocations have been found on the coordination number and on the location of the cupric ion. Cu2+ coordinates three molecules of water in hydrated CuH−clinoptilolite (CuH−Clino) and in CuLi−Clino, but only two molecules of water in hydrated CuNa−Clino and CuK−Clino. Two cupric ion sites are observed in many cases and are attributed to sites in ten-ring and eight-ring main channels. Activation to 400 °C is sufficient to remove these water molecules and cause migration of Cu2+ to an eight-ring intersecting channel. Adsorption of polar molecules such as water, ammonia, alcohols, and acetonitrile causes the migration of Cu2+ into the main channels to coordinate with the adsorbates. Cu2+ forms complexes with three molecules of ethanol and four of ammonia in CuH−Clino, but only coordinates to two molecules of ethanol or methanol and three of ammonia in CuNa−Clino, and to one molecule of ethanol or methanol and three of ammonia in CuK−Clino. Cu2+ coordinates four molecules of acetonitrile at the center of a main channel in clinoptilolite, and this coordination number is unaffected by the alkali metal cocations.