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-ty pe 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 diffusivi ties. As a consequence of this interaction, the diffusivities of metha ne in NaCaA and NaCaX are found to be quite close to each other in bot h the absolute values and the dependence on temperature and concentrat ion. For n-hexane, the diffusivities in NaCaA are significantly below the diffusivities in NaCaX. This can be attributed to the larger criti cal diameter of the n-hexane molecules, which is of the order of the d iameters of the windows between adjacent supercages in zeolite A. The proposed model of molecular interaction is confirmed by comparison wit h the results of MD simulations and by n.m.r. tracer desorption measur ements with zeolite alpha.