THE INFLUENCE OF ACID SITES ON INTRACRYSTALLINE DIFFUSIVITIES WITHIN MFI-TYPE ZEOLITES

The intracrystalline diffusivities of benzene, toluene and para-xylene in MFI-type zeolites with different amounts of acid sites were measur ed using the constant volume method at temperatures between 423 and 72 3 K and a pressure below 0.70 kPa. The intracrystalline diffusivities for the MFI-type zeolites with acid sites showed a different temperatu re dependence in high- and low-temperature regions. Ir: the high-tempe rature 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-tempera ture 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 equ al to the adsorption enthalpy of the diffusion molecules, which was ca lculated from the adsorption isotherm. The critical temperature T-C be tween the high-temperature region (where the effect of acid sites on t he diffusivity was negligibly small) and the low-temperature region (w here the effect of acid sites on the diffusivity was significant) incr eased as the acid amount of the zeolites increased, and T-C also incre ased in the order benzene <toluene <para-xylene, especially for zeolit es with a large number of acid sites. Semi-empirical equations were de veloped to predict the diffusivities within MFI-type zeolites with aci d sites. These equations include the diffusivity within silicalite, th e adsorption enthalpy of the diffusion molecules and the acid amount a s parameters, The predicted values of the diffusivities were in fairly good agreement with the experimental data. (C) 1998 Elsevier Science B.V. All rights reserved.