Abstract:
Zeolites MgNaX and CaNaX with varying concentrations of NaL 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, t/r, 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 ft of ~ 40-50%. With a further rise in the concentration of Me2’ cations in the zeolite the ns and values for ethane and especially for ethylene become much higher and the values of qv are practically invariable over a certain interval of ns rising with ft. The amount of adsorbed ethylene molecules,Azi’, corresponding to the range of ns over which the values of qv are still high is approximately equal to the amount of Me ’ 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.
