Direct observation of N-2 self-diffusion in zeolitic adsorbents using N-15 PFG NMR

Standard methodologies used for studying hydrocarbon diffusion (using H-1 PFG NMR) were applied for the first time to the diffusion of N-15(2) in beds of finely powdered commercial zeolites 5A and 13X. Unlike H-1 PFG NMR on hydrocarbon adsorbents, crystallite size, gradient strength, observation time, and gyromagnetic ratio are more critical in the measurement of N-2 self-diffusion in commercial-grade crystals. In fact, with the available hardware configuration and crystallite diameter constraints, we were restricted to the measurement of long-range diffusivities. These diffusivities were found to be similar for both adsorbents, varying from 3.2 x 10(-6) to 1.3 x 10(-8) m(2)/s over the temperature range studied (289-167 K). We also found the intercrystalline diffusion (D-inter) from the measured long-range diffusivities (D-long-range) and adsorption isotherms. We found them to be similar to bulk diffusion of N-2 at the conditions studied. Finally, the experimentally determined D-long-range results were shown to compare favorably to those calculated from simple gas kinetic theory.