Adsorption of ethane and ethylene by zeolites MgNaX and CaNaX with different degrees of ion exchange

Abstract

Zeolites MgNaX and CaNaX with varying concentrations of Na⁺, Mg²⁺ and Ca²⁺ were prepared from zeolite NaX. Adsorption isotherms of ethane and ethylene were measured on these zeolites at different temperatures, and the dependence of differential heats of adsorption, q_v, on the amount of adsorption, n^s, was determined. The adsorption properties of zeolites MgNaX and CaNaX are similar to those of zeolite NaX up to a degree of exchange β of ∼40–50%. With a further rise in the concentration of Me²⁺ cations in the zeolite the n^s and q_v values for ethane and especially for ethylene become much higher and the values of q_v are practically inveriable over a certain interval of n^s rising with β. The amount of adsorbed ethylene molecules, Δn^s, corresponding to the range of n^s over which the values of q_v are still high is approximately equal to the amount of Me²⁺ cations situated in the supercage on the sites II. These results are interpreted on the basis of the nature of the distribution of Me²⁺ cations in dehydrated zeolites assuming that these cations are situated preferentially on sites inside the sodalite units and hexagonal prisms, inaccessible to hydrocarbons. Only after occupying these sites did Me²⁺ cations appear in the supercages, giving rise to a sharp increase in the specific adsorption of ethylene.