Growth of a faujasite-type zeolite membrane and its application in the separation of saturated/unsaturated hydrocarbon mixtures

Faujasite type zeolite membranes were synthesized on porous ceramic alumina supports by using direct (in situ) and secondary (seeded) growth methods. In the secondary growth method a seed layer of ZSM-2 nanocrystals (prepared according to a report by Schoeman et al. J. Colloid Interface Sci. 1995, 170, 449-456) was deposited on the surface of the support before the hydrothermal growth. For both in situ and secondary growth, the mixture composition was 4.17 Na2O:1.0 Al2O3:10 TEA (triethanol ammonium):1.87 SiO2:460 H2O. X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron microprobe analysis (EPMA), indicate well intergrown 5-30 mum thick FAU films with Si/Al similar to1-1.5. The separation of saturated/unsaturated hydrocarbon mixtures is demonstrated over a range of temperatures (40-160 degreesC). The mixtures examined (and the corresponding equimolar mixture separation factors) are benzene/cyclohexane (160), benzene/n-hexane (144), toluene/n-heptane (45), propylene/propane (6.2), and ethylene/methane (8.4). In all cases, the membranes are unsaturated hydrocarbon permselective. With equimolar feed mixtures (5 kPa/5 kPa benzene/cyclohexane) and in the temperature range 65-160 degreesC, the membranes exhibit separation factor of 20-160 with the benzene flux in the range 10(-4)-10(-3) mol m(-2) s(-1). Decreasing the total feed partial pressure (0.31/0.31 kPa benzene/cyclohexane) reduces both separation factor (12) and benzene Bur. Similar trend is observed when the benzene/cyclohexane ratio in the feed mixture (0.5/9.5 kPa benzene/cyclohexane) is reduced. A sorption diffusion model based on the Stefan-Maxwell formulation has also been employed to show that the benzene/cyclohexane separation can mainly be attributed to differences of their adsorption properties. (C) 2001 Elsevier Science B.V. All rights reserved.