Self-diffusion of propane and propylene in 5A and 13X zeolite crystals studied by the tracer ZLC method

Abstract

A new variant of the zero length column method (tracer ZLC) has been developed and used to study the self-diffusion of propane and propylene in 5A and 13X zeolite crystals at 358 K. Diffusivities of propane and propylene in both zeolites are similar; the diffusivities in 13X are about an order of magnitude greater than for 5A. For propane-5A the tracer diffusivities are consistent with the corrected diffusivities from gravimetric measurements with large zeolite crystals and with the limiting diffusivity at zero concentration derived from traditional ZLC measurements. The trend with loading is similar to that observed for the p.f.g. n.m.r. self-diffusivities, although the actual values are somewhat smaller. By contrast, in 13X the tracer diffusivities are much smaller than the n.m.r. values (for propane), and they show the opposite trend with concentration. Partial loading experiments show that the desorption process is controlled by intracrystalline diffusion so that the discrepancy cannot be ascribed to the intrusion of surface resistance. Similarly, there can be no significant thermal effect in a tracer exchange experiment, so the discrepancy cannot be attributed to heat effects.