The calculation of the thermodynamic parameters for the adsorption of methane by zeolite NaX was carried out using the atom-atom approximation, in which the potential energy of the intermolecular interaction of a molecule with the zeolite lattice can be represented as the sum of the interaction energies of the constituent atoms of the molecule with all the oxygen ions and cations of the lattice. The configuration integrals were calculated numerically, different positions and orientations of the molecule in the zeolite cavity being taken into account. To simplify the calculations the large cavity of the zeolite was divided into small cells. The energies of interaction, at the centre of each cell, of C and H atoms with the lattice were calculated by computer. At any point inside a given cell the corresponding contribution to the energy of interaction was assumed to be equal to that for the centre of the cell. For very small (zero) filling of the zeolite cavity the thermodynamic adsorption characteristics (Henry's constants, changes in differential internal energy, isosteric heats of adsorption and heat capacities of the adsorbed substance) were calculated. By making a reasonable choice of the molecular parameters the atom-atom approximation gives thermodynamic quantities which are in satisfactory agreement with corresponding experimental values.
Zeolites MgNaX and CaNaX with varying concentrations of Na+, Mg2+ and Cat+ 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, ijv, 011 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 /3 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 / 3. The amount of adsorbed ethylene molecules, Ans, corresponding to the range of ns over which the values of ijV are still high is approximately equal to the amount of Me2+ cations situated in the supercage on the sites 11. 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.
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