Interpretation and correlation of zeolitic diffusivities obtained from nuclear magnetic resonance and sorption experiments

Abstract

The application of the n.m.r. pulsed field gradient technique to molecules adsorbed in zeolites, provides information on molecular diffusion both in the interior of the crystallites and in the inter-crystalline space. Comparison of the intra-crystalline self-diffusion coefficients obtained by this method with sorption data, reveals striking discrepancies. While the validity of the pulsed field gradient technique results can be checked in a number of experiments, the diffusion coefficients obtained from uptake measurements on zeolite crystallites of different radii exhibit an unreasonable radius dependence. It must therefore be concluded that sorption is limited by elementary processes different from intracrystalline diffusion and that, as a consequence, the diffusion coefficients obtained by sorption experiments are best regarded as apparent coefficients.Comparing, however, molecular transport parameters deduced by sorption and n.m.r. experiments more qualitatively, remarkable correlations are obtained.Since with increasing crystallite size the influence of surface effects on sorption kinetics decreases, uptake experiments with sufficiently large (i.e., natural) zeolite crystals provide diffusion coefficients comparable with those obtained by the n.m.r. pulsed field gradient technique.