Jeffrey Raymond Hufton

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.

Adsorption and diffusion of hydrocarbons in silicalite at very low concentration: effect of defect sites

Abstract Adsorption and diffusion of benzene, p-xylene, ethylbenzene, toluene, isobutene, cis-but-2-ene, trans-but-2-ene, isobutane and n-butane has been studied in large silicalite crystals, at very low sorbate concentration, by non-isobaric concentration pulse chromatography. At these low concentrations the heats of adsorption and Henry constants for the aromatic and olefinic hydrocarbons are substantially greater than previously reported values, which were derived from measurements at higher loading levels, while the intracrystalline diffusivities of these species are substantially smaller with higher activation energies. In contrast, the results for saturated hydrocarbons are similar to the values obtained at higher loadings. This behaviour parallels the previously reported results of detailed calorimetric measurements which, for olefins and aromatics, show a rapid increase in heat of adsorption at very low loadings, suggesting the presence of a small fraction of highly favourable adsorption sites. Such sites are probably associated with defects in the crystal structure and, although the precise nature of these sites is not established, it seems plausible to suggest that they may be associated with the presence of silanol groups.

Measurement of Intracrystalline Diffusion by Zero Length Column Tracer Exchange

The zero length column (ZLC) technique has been adapted to the measurement of tracer self-diffusivities in zeolite crystals. In this new variant the method is applicable over the entire range of sorbate loadings and the resulting diffusivity values should be directly comparable with PFG NMR self-diffusivities. The application of the technique is illustrated by a detailed study of the C 3 H 8 -5A system.