Ion-exchange equilibria of synthetic 4A zeolite with Ni2+, Co2+, Cd2+ and Zn2+ ions

Linde 4A zeolite was converted to Ni, Co, Zn and Cd forms by replacing sodium ions with divalent cations. The replacement was from 82%(Ni) up to 100%(Cd). Chemical, T.G.A., D.T.A. and X-ray analysis of the divalent metal zeolites have been made and data on the composition of the unit cells and the thermal stability of the lattices were obtained. Ni-zeolite is unstable, its crystal lattice being completely destroyed above 70°C, whereas Co-, Zn- and Cd-zeolites are stable up to 750–850°C. The adsorption of water on the latter zeolites is reversible. Ion-exchange isotherms for Co2+, Zn2+ and Cd2+ at three temperatures were obtained and the thermodynamic functions, ΔG°, ΔH° and ΔS°, of the ion-exchange equilibria calculated. The thermodynamic data are discussed with respect to hydration and size of the cations. A possible explanation of the instability of Ni-zeolite is suggested.