(520f) Alcohol and Water Adsorption and Capillary Condensation in MFI Zeolite Membranes
A membrane with the majority of its helium flux through defects at room temperature was used to study how MFI crystal expansion affects alcohol separations. Although this membrane is not the best membrane for alcohol/water separations, it allows the changes in flux through defects to be readily detected, and similar behavior is expected for membranes with less flow through defects. This study shows that alcohol adsorption expands MFI crystals and this shrinks the size of the membrane defects. This shrinkage, in addition to preferential adsorption, increases selectivity for removal of alcohols from water with MFI membranes.
We recently reported that n-alkanes (C3-C8) and SF6 swell MFI crystals and shrink membrane defects [1-4]. Single-component pervaporation fluxes of smaller molecules (n-hexane: 0.43 nm; acetone: 0.47 nm) were significantly lower than the fluxes of larger molecules (2,2 dimethylbutane (DMB), 1,3,5 trimethylbenzene (TMB), i-octane, benzene) in some MFI membranes, even though DMB (0.63 nm), TMB (0.73 nm), and i-octane (0.7 nm) are too large to adsorb at a significant rate in MFI pores. The objective of the current study was to determine if alcohols had similar effects on defects.
Silicalite membranes selectively separate alcohols from water, and separation factors as high as 106 have been reported for ethanol/water separations [5,6]. Studies found that increasing the alcohol percentage in the feed decreased the water flux, and separation was attributed to preferential adsorption of alcohols. Sano et al. observed that the separation factor decreased significantly as the ethanol feed concentration increased for a silicalite membrane . Since ethanol (0.43 nm) and water (0.265 nm) are both smaller than MFI pore (~0.6 nm), preferential adsorption was considered responsible for the selectivity.
A membrane containing isomorphously-substituted boron in the framework (B-ZSM-5) was synthesized by in situ crystallization onto the inside of a tubular α-alumina support (0.2-µm pores, Pall Corp.) . Permporosimetry was used to estimate the percentage of flow through defects . For most measurements the feed pressure was maintained at 185 kPa, but permporosimetry was also carried out for lower feed pressures. Pervaporation was done at room temperature with pure component and alcohol/water mixtures. The flux of i-octane vapor, which can only diffuse through defects that are larger than the MFI pores, was measured as a function of the activity of 2-propanol to determine how the defect sizes changed with adsorption.
Permporosimetry measurements showed that some alcohols dramatically decreased the helium flux through membrane defects. The flux decreased because defects shrank due to adsorption, not because of capillary condensation of the alcohols. Indeed, crystal swelling decreased the helium flux at much lower activities than where capillary condensation was observed. Moreover, capillary condensation strongly depended on the pressure drop across the membrane, and was only observed at low activities (indicating small defects) when the trans-membrane pressure drop was low. Vapor permeation of i-octane also showed that alcohols shrank the size of defects. X-ray diffraction measurements also showed that alcohols increased the MFI crystal size. Pervaporation measurements were consistent with the permporosimetry measurements: the separation selectivity for 2-propanol/water was 5 orders of magnitude higher than the ideal selectivity for certain concentration ranges because of crystal swelling due to alcohol adsorption. Thus, both crystal swelling and preferential adsorption contribute to the separation of alcohol/water mixtures by pervaporation.
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