Is diffusion controlled by crystal morphology? Inclusion of morphology to modelling the n-hexane diffusion in MFI-type zeolites

Diffusion of n-hexane in large crystals of the ZSM-5 and KVS-5 (V-Sil-1) zeolites (the MFI structure type) is discussed. The theoretical description takes into account morphology of the microporous crystals. Two diffusion models are used to compare theoretical uptake curves with the experimental data and to determine the diffusion coefficient values. The results are related to the classic diffusion model assuming the spherical crystal shape. The model comprising the actual crystal morphology is more realistic and reflects the course of uptake curves clearly better than the classic spherical model does. It yields diffusion coefficient values of ca. 4 × 10−11 m2 s−1, which are more than twice higher than the classic ones and, at the same time, somewhat lower than those from the measurements based on the equilibrium-state methods, including the crystal morphology that can bring closer together the results of non-equilibrium and equilibrium methods. This approach may have a more common applicability for the description of diffusion in crystalline microporous solids.