Defective MFI zeolite membranes with high selectivities

Zeolite membranes prepared by different methods and in different labs have shown a range of separation behavior, with a membrane that is selective for one mixture not being selective for another. Characterization of these membranes have been difficult. One reason for the differences between membranes is that separations have been made over a range of concentrations. As concentration on the feed side increases, the flux through non-zeolitic pores is shown to increase dramatically. Another reason for the differences in behavior is because polycrystalline MFI zeolite membranes are flexible; the intercrystalline spaces between the MFI crystals shrink in size upon n-hexane adsorption in the MFI pores. This happens because n-hexane expands the MFI crystals. Pure component pervaporation and vapor permeation measurements through MFI membranes that have significant flow through non-zeolite pores demonstrate this. The pervaporation flux of n-hexane, which can adsorb in the MFI pores, was an order of magnitude smaller than the flux of molecules that are too large to adsorb in the MFI pores. The flux of isooctane or dimethylbutane as a function of partial pressure during vapor permeation, but up to high partial pressures, is shown to be a good characterization of non-zeolitic pores. The DMB and isooctane fluxed exhibit dramatic increases over narrow pressure ranges due to capillary condensation in nanometer-sized non-zeolite pores.