Adsorption of ethane and ethylene on X-zeolites containing Li+, Na+, K+, Rb+ and Cs+ cations

Abstract

The adsorption isotherms of ethane and ethylene on LiNaX, NaX, KNaX, RbNaX and CsNaX zeolites have been measured at different temperatures and the isosteric heats of adsorption have been determined for various coverages. The amount a of adsorption and the heats Q of adsorption of ethylene are greater than those of ethane due to the contribution of the energy of specific interaction of the π-bond of ethylene with the zeolite cations to the total adsorption energy. The amount of this contribution decreases with increasing cation radius. The virial equation of the adsorption isotherm was applied to the experimental data. The first coefficient C₁ and the second coefficient C₂ of the virial equation may be considered as physicochemical constants in so far as these coefficients are independent of the number of terms in the series and of the length of the a interval. From the temperature dependence of C₁(Henry's constant) the heat of adsorption, Q₁, at a= 0, which characterizes adsorbate-adsorbent interactions, was determined for ethane and ethylene. From the temperature dependence of C₂ the constant Q₂, which characterizes adsorbate-adsorbate interactions, was determined. The determination of four coefficients C_i of the virial equation and their dependence on temperature allows us to calculate the values of a for different pressures and temperatures as well as the values of Q for various coverages. These calculated values coincide with the experimental values over a wide range of temperature and coverage (up to 70 %). The dependence of C₁, Q₁ and Q₂ on the radius of the exchange cation of the zeolite is discussed.