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

The self-diffusion of gases within 5A- and 13X-type zeolites has been investigated using pulsed field gradient (PFG) NMR. Self-diffusion coefficients for pure carbon monoxide, methane, and nitrogen within commercially available zeolitic adsorbents have been measured as a function of pressure and temperature. In addition, we elucidate the effect of gas composition on transport rate by measuring diffusion of the corresponding binary and ternary gas mixtures within these zeolites. The diffusion process in both the pure and multicomponent systems is well-described by a long-range diffusion model (LRDM) that requires only adsorbent porosity and isotherm data. The general applicability of this model is critically evaluated by (1) analyzing pure gas self-diffusion within porous media as a function of pressure; (2) predicting self-diffusion coefficients while simultaneously varying pressure, temperature, and sorbate loading; and (3) extending these predictions to address diffusion in binary and ternary gas systems. The tortuosity factors derived from this study are reasonable, characteristic of the adsorbent, and gas-independent.