PFG NMR-STUDY OF THE INFLUENCE OF THE EXCHANGEABLE CATIONS OF THE SELF-DIFFUSION OF HYDROCARBONS IN ZEOLITES

The molecular mobility of methane and n-hexane adsorbed in X- and A-type zeolites with different calcium contents is studied by pulsed-field gradient (p.f.g.) n.m.r. In contrast to sodium, the calcium ions are found to be of substantial influence on the intracrystalline diffusivities. As a consequence of this interaction, the diffusivities of methane in NaCaA and NaCaX are found to be quite close to each other in both the absolute values and the dependence on temperature and concentration. For n-hexane, the diffusivities in NaCaA are significantly below the diffusivities in NaCaX. This can be attributed to the larger critical diameter of the n-hexane molecules, which is of the order of the diameters of the windows between adjacent supercages in zeolite A. The proposed model of molecular interaction is confirmed by comparison with the results of MD simulations and by n.m.r. tracer desorption measurements with zeolite alpha.