Nuclear magnetic resonance measurement of mass transfer in molecular sieve crystallites

Abstract

The fundamentals of pulsed field gradient (PFG) NMR applied to studying molecular diffusion in microporous adsorbent crystallites are presented. Depending on the structural and microdynamic peculiarities of the adsorbate–adsorbent system intracrystalline molecular diffusion is found to follow at least five different patterns of concentration dependence. The order of magnitude of the intracrystalline diffusivities is in satisfactory agreement with the result of molecular dynamics (MD) calculations and quasielastic neutron scattering experiments, while some divergence with the results of sorption measurements is observed.

Combining the intracrystalline mean residence times of the adsorbed molecules with the intracrystalline diffusivities yields direct information about the existence of transport resistances at the outer surface of the crystallites. Such surface barriers depend on both the adsorbent and the diffusant. Studying their intensity may provide direct information about structural changes during adsorbent modification.

As it is sensitive to certain molecular species, PFG NMR permits selective diffusion measurements in multicomponent adsorbate–adsorbent systems. Examples of such measurements in the regimes of both intracrystalline and long-range diffusion are given. In non-cubic adsorbent crystallites, PFG NMR may be applied to study diffusion anisotropy. For methane in HZSM-5, the factor of anisotropy is found to be of the order of five.