High selectivities in defective MFI membranes

Two polycrystalline MFI membranes with significant flow through defects (non-zeolitic pores that are gaps between the crystals) are shown to have high ideal and mixture selectivities. The membranes were characterized at room temperature by permporosimetry, pervaporation, separations, and single-gas permeation. These measurements indicate that one membrane (B-ZSM-5) had a relatively large number of smaller defects, whereas the second membrane (silicalite-1) had a smaller number of somewhat larger defects. The relative contributions of these defects to the overall flux changed dramatically in the presence of n-alkanes and SF₆. These molecules caused adsorption-induced expansion of the crystals, and this expansion shrank the defect sizes and thus changed the membrane permeation characteristics. The B-ZSM-5 membrane had 90% of its helium flux through defects at room temperature, but it had a H₂/SF₆ideal selectivity as high as 260 because of SF₆-induced swelling that stopped 99% of the flux through the defects. In contrast, the silicalite-1 membrane had only 9% of its helium flux through defects, but the defects were large enough that crystal swelling only decreased the flux through them by 30%. Thus its selectivities were lower. These studies show that n-hexane, n-pentane, n-butane, n-propane, and SF₆ swell MFI crystals when they adsorb, but benzene and CO₂ do not. The changes in membrane microstructure due to crystal expansion not only significantly affect membrane separation ability, but also have implications on how to select appropriate characterization techniques for evaluating MFI membrane quality.