TRACING INDICATIONS OF ANOMALOUS DIFFUSION IN ADSORBENT ADSORBATE SYSTEMS BY PFG NMR-SPECTROSCOPY

PFG (pulsed field gradient) NMR spectroscopy is applied to measure the molecular mean square displacement in various adsorbent-adsorbate systems in dependence on the observation time. Irrespective of the fact that the pore size distribution yields fractal properties, for the investigated active carbons over the accessible space scale (greater-than-or-similar-to 1-mu-m), mass transfer of the adsorbed molecules is found to obey the laws of ordinary diffusion. When molecular mass transfer in beds of zeolite crystallites is investigated, deviations from ordinary diffusion are found. They are to be attributed, however, to the influence of the crystallite boundary and/or the intercrystalline space. It is most likely that the recently observed indication of anomalous diffusion in polycrystalline grains of zeolite ZSM-5 may be attributed to analogous microdynamic processes.