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However, even though the membrane did not show an absolute separation selectivity for trans-2-butene over cis-2-butene, it still displayed a balanced performance between sufficient product removal and reactant loss since the preferentially adsorbing trans-2-butane was able to block the pores for the permeation of propene. In addition, the use of a supplementary reactor for equilibration of the feed mixture before feeding the zeolite membrane reactor reduced the reactant loss. Furthermore, the authors evaluated the perspectives for industrial applications [93]. It was stated for high permeating silicalite-l membranes this requirement was fulfilled. However, to the best of our knowledge no such industrial plant has ever been built up to now. 4.2. Product Removal: Enhanced Selectivity by Displacing the Chemical Equilibrium Xylene isomers with a typical composition 18% p-xylene, 40% m-xylene, 22% o-xylene, and 20% ethylbenzene are generally produced from petroleum reformate streams [218]. However, the further use of the obtained isomers requires their separation. Due to the close boiling points of p- and m-xylene the use of distillation for their separation is not effective so that industrially crystallization and adsorption techniques have been developed, e.g., by using ZSM-5 [219]. The o- and m-xylenes can be isomerized to obtain more p-xylene. As an energy-efficient alternative to the conventional techniques, membrane reactors coupling xylene isomerization and simultaneous selective recovery represent a research topic gaining in importance in the last years due to the increasing demand of xylenes estimated at approximately 22 Mtones in 2003 [220]. Applying this concept p-xylene, the raw material for production of polyester resins could be obtained as product at the permeate side. In a search of an appropriate membrane type, one should consider that p-xylene possesses a kinetic diameter of 5.8 Å which is significantly smaller than those of the m- and o- isomers being about 6.8 Å [221]. Due to the specific pore structure of the MFI-zeolite combining straight, circular pores (0.54 nm × 0.56 nm) and sinusoidal, elliptic pores (0.51 nm × 0.54 nm) [222] it became the membrane material of choice for xylene isomer separation as well as for the application in membrane reactors. Several groups have studied the separation of xylenes with MFI zeolite membranes demonstrating high permselectivity for p-xylene over the other isomers, attributed mainly to the preferential permeation of p-xylene, since the zeolite pores expose sterical hindrance for the permeation of the bulkier m- and o-xylene isomer molecules [104,221,223–225]. In an outstanding study, Lai et al. [226] reported on dramatic improvement of the p-/o-xylene separation by b-oriented silicalite-1 membranes. In Figure 7 a comparison of achievable permeances as well as separation factors (SP) as a function of the operating temperature using (a) c-oriented; (b) [h0h]-oriented; (c) a- and b-oriented; and (d) b-oriented MFI films is given. It is clearly evidenced that the oriented membrane possesses the best properties for 31PDF Image | Zeolite Catalysis
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