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

Abstract

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.

Pure and Multicomponent Gas Diffusion within Zeolitic Adsorbents: Pulsed Field Gradient NMR Analysis and Model Development

“…The maximum gradient strength was127 G/cm. 8 In the case of a random walk, the measured spin attenuation, Ψ, due to the field gradient applied after the first and third π/2 rf pulses, can be described by the application of a Gaussian propagator, namely, 2 where 2D∆ represents the mean square displacement of the diffusing molecules, D is the experimental self-diffusion coefficient, and ∆ is the diffusion time (∆ ) t′ + τ). The parameter q represents the product δγg, where δ is the duration of the gradient pulse, g is the strength of the gradient pulses, and γ is the gyromagnetic ratio of the nucleus of interest (e.g., 1 H, 13 C, or 15 N).…”

Pulsed field gradient nuclear magnetic resonance study of long–range diffusion in beds of NaX zeolite: Evidence for different apparent tortuosity factors in the Knudsen and bulk regimes

“…However, in our opinion, the assumption that dϷ2.5 m leading to K Ϸ b can be definitely ruled out in view ͑i͒ of the relatively low fraction of the volume of the bed occupied by zeolite crystals (V inter /V intra Ϸ1.6) and ͑ii͒ of the mean crystal size of 25 m. The temperature dependencies of the long-range diffusivities in the Knudsen and the transition ͑between the Knudsen and the bulk͒ regimes were measured earlier by PFG NMR for n-heptane in beds of NaX 14 and for N 2 in beds of 5A zeolite. 15 In both cases the diffusivities were calculated using the same simple gas kinetic theory as in the present work by assuming that there is no difference between the tortousity factors in the bulk and the Knudsen regimes. It turned out that these theoretical estimates either underestimated the measured values at high temperatures and/or significantly overestimated them at low temperatures.…”

Predicting packed‐bed breakthrough behavior from PFG NMR diffusion data

“…The detailed sample preparation procedure has previously been described, and, therefore, shall be only summarized here (McDaniel, 1996; Rittig, 2002b). Pelletized zeolites, obtained from different manufactures, were activated (to remove adsorbed water) under dynamic vacuum (<10 −4 torr) with a 1 K/min thermal ramp.…”