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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, text-decoration:overlineqv, on the amount of adsorption, ns, 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 ns and text-decoration:overlineqv values for ethane and especially for ethylene become much higher and the values of text-decoration:overlineqv are practically inveriable over a certain interval of ns rising with β. The amount of adsorbed ethylene molecules, Δns, corresponding to the range of ns over which the values of text-decoration:overlineqv 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. |
