Analysis of bimodal frequency-response behaviour of p-xylene diffusion in silicalite-1

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Abstract
The bimodal frequency-response spectra of p-xylene diffusion in silicalite-1 at 373 K have been analysed using the two independent diffusion processes, diffusion–rearrangement and non-isothermal diffusion models. The FR parameters and equilibrium constants derived from these three models have been compared. It was found that the two independent diffusion and diffusion–rearrangement models can well represent the experimental data and give similar equilibrium constants and diffusion time constants. It may be due to the similar principles used in these two models that the molecules in the straight channels are mobile, while those absorbed in the sinusoidal channels are ‘immobile’. On the other hand, the non-isothermal diffusion model gives a poor fit of the experimental data. The non-isothermality parameter γ obtained from data fitting is not consistent with the value calculated from the K constant obtained from the same fit. The frequency-response spectra obtained within the hysteresis loop region of the p-xylene isotherm at 348 K are also discussed.

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