MEASUREMENT OF INTRACRYSTALLINE DIFFUSION BY THE FREQUENCY-RESPONSE METHOD - ANALYSIS AND INTERPRETATION OF BIMODAL RESPONSE CURVES

In this paper we present a detailed investigation of alternative possible explanations for the bimodal behavior of frequency response curves observed experimentally by Shen and Rees for diffusion of p-xylene in silicalite-1. It is shown that a nonisothermal single-diffusion model can well represent the experimental data using consistent equilibrium data, suggesting that the beat effect provides a possible explanation for the bimodal behavior. On the other hand, it is shown that such a behavior could also be described by a diffusion-rearrangement model which considers the straight and sinusoidal channels of silicalite-1 as the transport and storage channels. However, with this model, good agreement with the experimental data could be obtained only when the crystals are assumed to be spherical, which is not consistent with the actual geometry.