Diffusion in zeolites. Comparison of sorption and nuclear magnetic resonance diffusivities

Abstract

Corrected sorption diffusivities, calculated from uptake rate experiments, are compared with self diffusivities measured by the n.m.r. pulsed-field-gradient method. For butane and propane in 5A zeolite and for triethylamine in 13X zeolite there is good agreement, both in the magnitude of the diffusivities and the trends with concentration and temperature. A re-appraisal of some of the earlier sorption data suggests that the dramatic differences previously noted between n.m.r. and sorption data are due primarily to two causes: (i) failure to take proper account of the effects of external heat- and mass-transfer resistance in the analysis of uptake curves; (ii) the use of different zeolite crystals, subjected in some cases to quite different dehydration procedures, for the sorption and n.m.r. experiments.

However, for benzene in 13X there is a large difference between n.m.r. and sorption diffusivities even though the measurements were performed under apparently similar conditions on samples from the same original batch of zeolite. Uptake rates measured with different sizes of crystal are consistent with the simple diffusion model and on this basis the intrusion of external heat- or mass-transfer resistance seems improbable. The reason for the discrepancy for this system remains uncertain.