Silicalite membrane

MFI zeolite membranes (silicalite-1, ZSM-5), on either flat or tubular porous supports, have been the most investigated for gas separation, catalytic reactors, and pervaporation applications. The structural porosity of MFI zeolite consists of channels of about 5.5 A, in diameter, the sihca-rich compositions induce... 

In general, when carrying out a new separation, the kinetic diameter and the heat of adsorption of the gases, which compose the mixture, are the main variables used to select the most adequate zeolite. MFI, FAU, LTA, SOD, ANA, DDR, MOR, BEA, CHA, FER, KFI are zeolite structures widely used as membranes for different separations. In gas separation, MFI zeolite membranes (silicalite-1, ZSM-5, and with Al, Fe, B, and Ge isomorphously substituted into their stmctures) are the most commonly used membranes because their pores (-0.55 nm diameter) are in the size range of many industrial mixtures furthermore, their synthetic chemistry is well established in the literature. 

As an example of the selective removal of products, Foley et al. [36] anticipated a selective formation of dimethylamine over a catalyst coated with a carbon molecular sieve layer. Nishiyama et al. [37] demonstrated the concept of the selective removal of products. A silica-alumina catalyst coated with a silicalite membrane was used for disproportionation and alkylation of toluene to produce p-xylene. The product fraction of p-xylene in xylene isomers (para-selectivity) for the silicalite-coated catalyst largely exceeded the equilibrium value of about 22%. 

As described in the previous section, the silica-alumina catalyst covered with the silicalite membrane showed exceUent p-xylene selectivity in disproportionation of toluene [37] at the expense of activity, because the thickness of the sihcahte-1 membrane was large (40 pm), limiting the diffusion of the products. In addition, the catalytic activity of silica-alumina was not so high. To solve these problems, Miyamoto et al. [41 -43] have developed a novel composite zeohte catalyst consisting of a zeolite crystal with an inactive thin layer. In Miyamoto s study [41], a sihcahte-1 layer was grown on proton-exchanged ZSM-5 crystals (silicalite/H-ZSM-5) [42]. The silicalite/H-ZSM-5 catalysts showed excellent para-selectivity of >99.9%, compared to the 63.1% for the uncoated sample, and independent of the toluene conversion. 

This paper describes the morphological and transport properties of a composite zeolite (silicalite) - alumina membrane. Some advantages obtained in combining the membrane with a conventional fixed-bed catalyst are also reported. 

The silicalite-alumina membrane was prepared after adding a solution containing the silicalite precursor (i e silica + template) to the above-mentioned porous tube (hereafter called support) and a specific hydrothermal treatment performed [8], under the chosen conditions no material is formed in the absence of the porous support. The tube is then calcined at 673 K for removing the template. 

Another challenging and industrially important separation that utihzes pervapo-ration through zeolite membranes is acid removal from H2O. In this case, the zeolite must have a high Si/Al ratio due to leaching of A1 by the acid. Both Ge-ZSM5 and silicalite have demonstrated significant stability and separation capability for the removal of acetic acid from H2O [35]. 

Liu et al. [42] reported permeation of mixture of hexanes and octanes through silicalite membranes. It was found that the permeances of the mixture could not be predicted by the single-component data. In the separation of alkane isomers, the permeance of 2,2-DMB is significantly reduced in the presence of n-hexane resulting in a permselectivity much higher than the ideal separation factor [7]. 

Arruebo, M., Coronas, J., Menendez, M., and Santamaria, J. (2001) Separation of hydrocarbons from natural gas using silicalite membranes. Sep. Purif. Tech., 25, 275-286. 

Lovallo, M.C. and Tsapatsis, M. (1996) Preferentially oriented submicron silicalite membranes. Separations, 42, 3020-3024. 

Jia and coworkers prepared thin-film composite zeolite-filled silicone rubber membranes by a dip-coating method [82]. The membranes have a thin silicalite-1/ silicone rubber mixed-matrix selective layer on top of a porous polyetherimide support. 

Chen, H Li, Y and Yang, W. (2007) Preparation of silicalite-1 membrane by solution-filling method and its alcohol extraction properties. J. Membr. Sci., 296, 122-130. 

Silica-alumina particles coated with a permselective silicalite membrane is almost completely selective in the formation of p-xylene in the disproportionation of toluene.402 Friedel-Crafts alkylations were performed in ionic liquids. The strong polarity and high electrostatic fields of these materials usually bring about enhanced activity.403 404 Easy recycling is an additional benefit. Good characteristics in the alkylation of benzene with dodecene were reported for catalysts immobilized on silica or MCM-41 405... 

Pervaporation with ceramic membranes is less well understood in terms of transport mechanisms. Consequently, modeling of ceramic pervaporation is still less mature, although the performance of the process was reported to be good [89]. Nomura et al. [90] studied the transport mechanism of ethanol/water through silicalite membranes in... 

Krishna and Paschek [91] employed the Maxwell-Stefan description for mass transport of alkanes through silicalite membranes, but did not consider more complex (e.g., unsaturated or branched) hydrocarbons. Kapteijn et al. [92] and Bakker et al. [93] applied the Maxwell-Stefan model for hydrocarbon permeation through silicalite membranes. Flanders et al. [94] studied separation of C6 isomers by pervaporation through ZSM-5 membranes and found that separation was due to shape selectivity. 

Improved selectivity in the liquid-phase oligomerization of i-butene by extraction of a primary product (i-octene C8) in a zeolite membrane reactor (acid resin catalyst bed located on the membrane tube side) with respect to a conventional fixed-bed reactor has been reported [35]. The MFI (silicalite) membrane selectively removes the C8 product from the reaction environment, thus reducing the formation of other unwanted byproducts. Another interesting example is the isobutane (iC4) dehydrogenation carried out in an extractor-type zeolite CMR (including a Pt-based fixed-bed catalyst) in which the removal of the hydrogen allows the equilibrium limitations to be overcome [36],... 

Figure 2. Zeolite silicalite membrane [34], Separation behavior of a n-hexane 2-2-dimethylbutane mixture (1 1) as a function of temperature.

Jia et al. [1993] have prepared thin, dense pure silicalite zeolite membranes on porous ceramic supports by an in-situ synthesis method. A sol consisting of silica, sodium hydroxide, tctrapropylammonium bromide and water is prepared with thorough mixing. A ceramic support is immersed in the sol which is then heated and maintained at 180X... 

Similarly, Sano et al. [1994] added colloidal silica to a stirred solution of tetrapropylammonium bromide and sodium hydroxide to synthesize a hydrogel on a stainless steel or alumina support with a mean pore diameter of 0.5 to 2 pm. The composite membrane is then dried and heat treated at 500 C for 20 hours to remove the organic amine occluded in the zeolite framework. The silicalite membranes thus obtained are claimed to be free of cracks and pores between grains, thus making the membranes suitable for more demanding applications such as separation of ethanol/water mixtures where the compound molecules are both small. The step of calcination is critical for synthesizing membranes with a high permselectivity. 

Effect of membrane treatment conditions on the permeate flux and separation factor of a 5% ethanol/water mbcture at 60 C by silicalite membranes... 

Ad Figure 2.6. A fimdamentally different system is to load a polymer film (e g. a siloxane) with zeolite crystals. Especially the Twente group studied such composite membranes. The zeolite crystals then just contribute to the permeation by acting as selective reservoirs of components. Hydrophobic (silicalite-1) as well as hydrophilic (zeolite A) zeolites have been studied in such a configuration. 

Zeolite single crystals may serve as zeolite membrane models. We mention the early elegant work of Hayhurst and Paravar [10] on an oriented large silicalite-1 crystal embedded in epoxy resin. 

In a more recent membrane model study [11] a large (twinned) silicalite-1 crystal (100 100 300 p.m) was embedded in an epoxy matrix, using an aluminium gasket as a support. Polishing improved the crystal surface exposure. Mcropore diffiisitivities were measured for benzene, toluene and the xylenes. 

Silicalite-1, are known to grow relatively easy on various surfaces, Moreover, some principles have been developed by Jansen et al. [8] how to govern the zeolite crystal orientation with respect to the support. Two articles with a reviewing character [7,44] as to MFI-based membranes are recommended to the reader. 

MFI-based membranes may be categorized as to Al-containing (ZSM-5) membrane layers and Al-free (Silicalite-1) layers, or as to the support applied (alumina, metal). The first division will be used in this chapter. 

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