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 5-30μm thick FAU films with Si/Al ∼1-1.5. The separation of saturated/unsaturated hydrocarbon mixtures is demonstrated over a range of temperatures (40-160°C). 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 (5kPa/5kPa benzene/cyclohexane) and in the temperature range 65-160°C, the membranes exhibit separation factor of 20-160 with the benzene flux in the range 10-4-10-3molm-2s-1. Decreasing the total feed partial pressure (0.31/0.31kPa benzene/cyclohexane) reduces both separation factor (12) and benzene flux. Similar trend is observed when the benzene/cyclohexane ratio in the feed mixture (0.5/9.5kPa 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.
Bibliographical noteFunding Information:
The authors would like to acknowledge the W.M. Keck Polymer Morphology Laboratory and the Department of Geology of University of Massachusetts for the use of their Electron Microscopy Facilities. Financial support was provided by NETI, NSF (CAREER, CTS-9624613 and CTS-9702615) and the David and Lucille-Packard Foundation through a fellowship in Science and Engineering to M.T.
- Zeolite membranes