Evaluation of effective routes and sizes for permeation through polycrystalline MFI-type zeolite membranes prepared by secondary growth

This study proposes a method for evaluating permeation routes and sizes of zeolite membranes. Effective pore shapes and sizes for permeation through polycrystalline zeolite membranes were determined by a perm porometer in conjunction with the Horvath–Kawazoe (HK) model. Slit-shaped, cylindrical, and spherical micropores have different average potential energies, even if the effective pore sizes are the same. The effective routes and sizes for permeation can be decided using these differences in the average potential energies. By evaluating the effective permeation routes and sizes using the permeation properties for binary mixtures of helium and propane, it was understood that spherical spaces having diameter of ca. 1 nm were important for permeation through the polycrystalline MFI-type zeolite membranes than cylindrical zeolitic channels having diameter of 0.5–0.6 nm. The permeation behavior for butane isomers also supported the evaluation results. Furthermore, to estimate the pore sizes more accurately, we discussed the influences of coexisting components on propane adsorption and the possibility of extending the HK model to the adsorption of propane at 350 K.