46874-AC9
Adsorption-Induced Changes in Zeolite Membrane Structures
Zeolite membranes are composed of thin layers of intergrown crystals on porous supports. The intercrystalline regions (defects) are pores or slits that can have diameters larger than the zeolite pores. Even a small number of defects can significantly decrease separation selectivity. Selectivities as high as 54,000 have been obtained for the removal of water from ethanol/water mixtures by pervaporation through hydrophilic A-type zeolite membranes. Because of their high selectivities, zeolite A membranes have been commercialized for dehydration of alcohols. In spite of their high pervaporation selectivities, zeolite A membranes are not effective for gas separations because they have significant flow through defects (intercrystalline regions) that are larger than the zeolite A pores. The objective of this study is to understand why the membranes are selective for the alcohol/water separations in spite of their high defect concentrations. A series of zeolite A membranes were prepared and permporosimetry measurements were carried out by measuring the flux of a weakly-adsorbing or non-adsorbing gas as a function of the activity of a molecule that adsorbs in the zeolite A pores. A molecule was also used that was too large to adsorb in the zeolite A pores, but it was able to adsorb in larger pores (defects). H helium, methane, isobutane, and sulfur hexafluoride were used as the permeating gases and water, methanol, ethanol, and isopropanol as adsorbing vapors. The flux versus activity measures indicate that water and methanol swell zeolite A crystals, and this causes the defects to shrink so that the flow through defects decreases. The flux of sulfur hexafluoride decreased at a lower activity than the flux of isobutane, which decreases at a lower activity than the flux of methane and helium. That is, the defect size decreased more as the amount adsorbed in the zeolite A structure increased. When water or methanol were adsorbed at higher activities, the helium flow through one membrane decreased to 0.1% of its original value. In contrast, the helium flux only decreased to 25% of its original value in the presence of ethanol, even for an activity near one. That is, the helium flux was 250 times higher in the presence of ethanol than in the presence of methanol. Isopropanol had almost no effect on the helium flux, even at activities near one. Similar behavior was observed for a second membrane, but the helium flux decreased four orders of magnitude for both methanol and water adsorption. These permporosimetry results can be explained by assuming that methanol and water expanded the size of zeolite A crystals, and this expansion decreased the size of the defects sufficiently to essentially block more than 99% of the flow through defects. The decreased flux through defects did not appear to be due to capillary condensation of water and methanol in the defects because both ethanol and isopropanol would be expected to undergo capillary condensation more readily than methanol, but they had a relatively small effect on helium flux. Also, the flux of SF6 decreased dramatically at a significantly lower water activity than the helium flux decreased. This behavior is expected if water caused the zeolite A crystals to expand and the defects to shrink. As water activity increased, the defects shrank so that they were too small to allow SF6 to permeate. At higher activities more water adsorbed, the defects shrank more, and they became too small to allow He and CH4 to permeate. If capillary condensation were responsible for the decrease in helium and SF6 fluxes, smaller pores would fill at lower activities and larger pores at the higher activities. In that case the helium flux would decrease some at lower activities, as some of the smaller pores were blocked, before the SF6 flux decreased, and at higher activities both He and SF6 fluxes would decrease, but this is not what was observed. Swelling of zeolite A crystals may be responsible, at least in part, for the high water/ethanol selectivities reported for separation of liquid mixtures. Since the crystals do not swell when exposed to various gas molecules, the membranes are not selective for gas separations because much of the gas flow is through defects.
