Porous support, zeolite membranes

There are basically two approaches to synthesize supported zeolite membranes liquid-phase synthesis and vapor-phase transport (or dry-gel conversion) methods [2,12], The liquid-phase-synthesis approach is to bring the surface of a porous support in contact with a zeolite synthesis solution (sol or gel) and keep the system under controlled conditions so that the zeolite can nucleate and grow to a continuous film on the support surface. 

Kusakabe K, Kuroda T and Morooka S (1998), Separation of carbon dioxide from nitrogen using ion-exchanged faujasite-type zeolite membranes formed on porous support tubes , Membrane Sci, 148,13-23. 

Zeolite membranes are generally synthesized as a thin, continuous film about 2-20 xm thick on either metallic or ceramic porous supports (e.g., alumina, zirco-nia, quartz, siHcon, stainless steel) to enhance their mechanical strength. Typical supported membrane synthesis follows one of two common growth methods (i) in situ crystallization or (ii) secondary growth. Figure 10.2 shows the general experimental procedure for both approaches. 

Zeolite/polymer mixed-matrix membranes can be fabricated into dense film, asymmetric flat sheet, or asymmetric hollow fiber. Similar to commercial polymer membranes, mixed-matrix membranes need to have an asymmetric membrane geometry with a thin selective skin layer on a porous support layer to be commercially viable. The skin layer should be made from a zeohte/polymer mixed-matrix material to provide the membrane high selectivity, but the non-selective porous support layer can be made from the zeohte/polymer mixed-matrix material, a pure polymer membrane material, or an inorganic membrane material. 

The esterification of acetic acid with ethanol has been investigated using zeolite membranes grown hydrothermally on the surface of a porous cylindrical alumina support (the catalyst used was a cation exchange resin) [37]. The conversion exceeded the equilibrium limit, by the selective removal through the membrane of water and reached to almost 100% within 8h [37]. 

Membranes are classified as organic or inorganic, taking into account the material used for their syntheses porous or dense, based on the porosity of the material applied and symmetric and asymmetric for a membrane made of a single porous or dense material or for a membrane made of a porous support and a dense end, respectively [16,64], We are fundamentally interested here in asymmetric inorganic membranes made of a porous end to bring mechanical stability to the membrane and made of alumina, silica, carbon, zeolites, and other materials, and a dense end to give selectivity to the membrane (see Chapter 10). However, we also analyze the performance of porous polymers. 

It is evident that the ceramic membrane, which is represented in the XRD pattern (see Figure 10.6) by the amorphous component of the XRD profile, was covered by the AlP04-5 molecular sieve, since the crystalline component of the obtained XRD pattern fairly well coincides with the standards reported in the literature [107]. Consequently, the porous support was successfully coated with a zeolite layer, which was shaped by the hydrothermal process as previously described. Thus, a composite membrane, that is, an AlP04-5 molecular sieve thin film zeolite-based ceramic, was produced. 

Different ways have been proposed to prepare zeolite membranes. A layer of a zeolite structure can be synthesized on a porous alumina or Vycor glass support [27, 28]. Another way is to allow zeolite crystals to grow on a support and then to plug the intercrystalline pores with a dense matrix [29], However, these two ways often lead to defects which strongly decrease the performance of the resulting membrane. A different approach consists in the direct synthesis of a thin (but fragile) unsupported monolithic zeolite membrane [30]. Recent papers have reported on the preparation of zeolite composite membranes by hydrothermal synthesis of a zeolite structure in (or on) a porous substrate [31-34]. These membranes can act as molecular sieve separators (Fig. 2), suggesting that dcfcct-frcc materials can be prepared in this way. The control of the thickness of the separative layer seems to be the key for the future of zeolite membranes. 

Zeolite membranes have also been made atop of porous supports. To make a zeolite membrane on a porous support, a sol or gel layer of metal oxides (such as alumina or... 

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... 

Hardly any research has been performed on ferrierite in zeolite membrane configurations. Matsukada et al. [50,51] prepared a ferrierite-based membrane by the frequently used Vapour-phase Transport Method. By using ethylenediamine, triethylamine and steam (under hydrothermal conditions), a porous alumina support, covered with the proper aluminosilicate gel, was transformed into a alumina supported (30 pm thick) ferrierite layer. No permeation with 1,3,5-triisopropylbenzene coirld be observed, proving the layer to be defect-free. Fluxes of small gases were found in the order of 10" -10 mol.m. s. Pa and decreased in the order H2>He>CH4>N2>02>C02... 

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... 

Since then, layers of grown-together zeolite crystals have been prepared on porous supports of stainless steel [93] or of porous alumina [69,72,94], showing very promising results (see Table 2). However, major steps still have to be taken in order to render these highly selective porous membranes reliable and cheap enough to be produced at an industrial scale. If these problems are solved, the porous IMR technology will probably make its way toward practical success. 

Different supports are used, (see Section 10.6.4) with different geometry (discs or tubes), thickness, porosity, tortuosity, composition (alumina, stainless steel, silicon carbide, mullite, zirconia, titania, etc.), and symmetry or asymmetry in its stmcture. Tubular supports are preferable compared to flat supports because they are easier to scale-up (implemented as multichannel modules). However, in laboratory-scale synthesis, it is usually found that making good quality zeolite membranes on a tubular support is more difficult than on a porous plate. One obvious reason is the fact that the area is usually smaller in flat supports, which decreases the likelihood of defects. In Figure 10.1, two commercial tubular supports, one made of a-alumina (left side) and the other of stainless steel (right side) used in zeolite membrane synthesis, are shown. Both ends of the a-alumina support are glazed and both ends of the stainless steel support are welded with nonporous stainless steel to assure a correct sealing in the membrane module and prevent gas bypass. 

In addition to porous ceramic and stainless steel plates and tubes commonly employed as supports of zeolite membranes and films, a wide variety of alternative supports have been used. Among these are steel [250], ceramic [251,252] monoliths... 

Dong JH, Lin YS, Hu MZC, Peascoe RA, and Payzant EA. Template-removal-associated microstmctural development of porous-ceramic-supported MFI zeolite membranes. Micropor Mesopor Mater 2(K)0 34(3) 241-253. 

Li Y, Wang J, Shi J, Zhang X, Lu J, Bao Z, and Yan D. Synthesis of ZSM-5 zeolite membranes with large area on porous, tubular alpha-A1203 supports. Sep PurifTechnol 2003 32(l-3) 397-401. 

So far, essentially three different approaches have been reported for the preparation of zeolitic membranes [119]. Tsikoyiannis and Haag [120] reported the coating of a Teflon slab during a "regular" synthesis of ZSM-5 by a continuous uniform zeolite film. Permeability tests and catals ic experiments were carried out with such membranes after the mechanical separation of the coating from the Teflon surface [121]. Geus et al. [122] used porous, sintered stainless steel discs covered with a thin top layer of metal wool to crystallize continuous polycrystalline layers of ZSM-5. Macroporous ceramic clay-type supports were also applied [123]. 

Silicalite-1 membranes, supported on porous alumina ceramic discs, have been prepared by two different routes. In the first the zeolite membrane has been formed by in situ hydrothermal synthesis. Secondly a layer has been formed by controlled filtration of zeolite colloids. To optimise membrane stability, conditions have been established in which penetration of zeolite into the support sublayer occurs. The pore structure of these membranes has been characterised by a combination of SEM and Hg-porosimetry. The permeabilities of several gases have been measured together with gas mbeture separation behaviour. 

A polycrystalline Y-type zeolite membrane was formed by hydrothermal synthesis on the outer surface of a porous a-alumina support tube, which was polished with a finely powdered X-type zeolite for use as seeds. When an equimolar mixture of CO2 and N2 was fed into the feed side, the CO2 permeance was nearly equal to that for the singlecomponent system, and the N2 permeance for the mixture was greatly decreased, especially at lower permeation temperatures. At 30"C, the permeance of CO2 was higher than 10- mol m-2 s- Pa-, and the permselectivity of CO2 to N2 was 20-100. 

---