SELF-DIFFUSION OF PROPANE AND PROPYLENE IN 5A AND 13X ZEOLITE CRYSTALS STUDIED BY THE TRACER ZLC METHOD

A new variant of the zero length column method (tracer ZLC) has been d eveloped 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 tr acer diffusivities are consistent with the corrected diffusivities fro m gravimetric measurements with large zeolite crystals and with the li miting diffusivity at zero concentration derived from traditional ZLC measurements. The trend with loading is similar to that observed for t he p.f.g.n.m.r. self-diffusivities, although the actual values are som ewhat smaller. By contrast, in 13X the tracer diffusivities are much s maller than the n.m.r. values (for propane), and they show the opposit e 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 resist ance. Similarly, there can be no significant thermal effect in a trace r exchange experiment, so the discrepancy cannot be attributed to heat effects.