Understanding the effect of zeolite crystal expansion/contraction on separation performance of NaA zeolite membrane: A combined experimental and molecular simulation study

Experimental measurements and molecular simulations were used to investigate the effect of adsorption-induced changes in zeolite crystal size on the separation performance of NaA zeolite membrane for dehydration of alcohols. The vapor permeation (VP) separations of water/IPA showed a dramatic decrease in selectivity due to increase of IPA flux as the feed water concentration decreased. However, in the case of water/methanol and water/ethanol mixtures, the alcohol fluxes were almost independent of the feed water concentration. Permporosimetry measurements as well as molecular simulations show that at low loading of water, NaA crystals contract slightly, while they expand at higher loadings, which significantly affects the intercrystalline defects. In addition, at low water loadings intercrystalline defects are not blocked by water adsorbed at the defect sites. Both methanol and ethanol can enter the zeolite to reduce the crystal contraction. However, isopropanol cannot enter the NaA crystal and is thus unable to mitigate the effects of low water loadings. Based on this knowledge, the presence of methanol or ethanol in the water/isopropanol mixtures with low water content was expected to improve the dehydration performance of NaA zeolite membrane. This result was also observed for the dehydration of water/other large molecular mixture. Our studies here provide an improved understanding of the permeation and separations for NaA zeolite membrane.