Direct Observation of N2Self-Diffusion in Zeolitic Adsorbents Using15N PFG NMR

Standard methodologies used for studying hydrocarbon diffusion (using 1 H PFG NMR) were applied for the first time to the diffusion of 15 N 2 in beds of finely powdered commercial zeolites 5A and 13X. Unlike 1 H 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 × 10 -6 to 1.3 × 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.