The influence of acid sites on intracrystalline diffusivities within MFI-type zeolites

Abstract

The intracrystalline diffusivities of benzene, toluene and para-xylene in MFI-type zeolites with different amounts of acid sites were measured using the constant volume method at temperatures between 423 and 723 K and a pressure below 0.70 kPa. The intracrystalline diffusivities for the MFI-type zeolites with acid sites showed a different temperature dependence in high- and low-temperature regions. In the high-temperature region, the magnitude of the diffusivity agreed well with that of an MFI-type zeolite without any acid sites (silicalite), regardless of the existence of acid sites. On the other hand, in the low-temperature region, the magnitude of the diffusivity decreased significantly with increasing amounts of acid sites in the zeolites. The activation energy of the diffusivity in the low-temperature region was almost equal to the adsorption enthalpy of the diffusion molecules, which was calculated from the adsorption isotherm. The critical temperature TC between the high-temperature region (where the effect of acid sites on the diffusivity was negligibly small) and the low-temperature region (where the effect of acid sites on the diffusivity was significant) increased as the acid amount of the zeolites increased, and TC also increased in the order benzene<toluene<para-xylene, especially for zeolites with a large number of acid sites. Semi-empirical equations were developed to predict the diffusivities within MFI-type zeolites with acid sites. These equations include the diffusivity within silicalite, the adsorption enthalpy of the diffusion molecules and the acid amount as parameters. The predicted values of the diffusivities were in fairly good agreement with the experimental data.