Intracrystalline self-diffusion of hydrocarbons in zeolite NaX in equilibrium with excess liquid phase

Abstract

The n.m.r.-pulsed field gradient technique is applied to study the molecular self-diffusion of the n-alkane homologues from n-hexane up to n-hexadecane in zeolite NaX saturated with the adsorbate and in contact with excess liquid phase. Measuring the temperature dependence over a temperature interval from 20° to 200°C gives apparent activation energies that significantly exceed the activation energies previously determined for gas phase adsorption. This difference may be attributed to the combined influence of the temperature dependence of the saturation capacity and the concentration dependence of the intracrystalline mobility. For the X-type zeolite crystallites under study, the mean molecular exchange times, as calculated on the basis of the measured intracrystalline self-diffusion coefficients, range from 0.5 s (for n-hexane) up to 60 s (for n-hexadecane). In the case of n-hexane, these data are shown to be in complete agreement with the results of n.m.r. tracer desorption studies.