Flexible nanostructure of MFI zeolite membranes

Pervaporation and vapor permeation of hydrocarbons were used to characterize three MFI zeolite membranes that had significant fluxes through non-zeolitic pores. The single-component pervaporation fluxes of n-hexane were only 5-55% of the fluxes of larger molecules (2,2-dimethylbutane (DMB), isooctane, and 1,3,5-trimethylbenzene (TMB)) at ∼300 K, all of which did not adsorb in the MFI pores under the conditions used. In spite of this behavior, these membranes were selective for C₆ isomer separation, and the n-hexane/DMB vapor-phase separation factor was 3200 for one membrane at low hydrocarbon partial pressures. The pervaporation behavior was consistent with the explanation that MFI membrane structures are flexible; MFI crystals increase in size when n-hexane adsorbs, and this expansion dramatically decreases the non-zeolitic pore sizes. Thus, less than 0.8 mol% n-hexane reduced DMB and TMB pervaporation fluxes to ∼5% of their single-component fluxes for one membrane. At high vapor concentrations, molecules condensed in the non-zeolitic pores, which had almost no selectivity, and this capillary condensation decreased the n-hexane/DMB separation factor. Non-zeolitic pores were shown to increase in size after a membrane was calcined at 1000 K.