FAU zeolite membranes

Different techniques can be used in order to evaluate the morphological quality of the zeolite membranes. Scanning electron microscopy is used to study the surface morphology and the cross section. The top view shows the shape and size of the crystals and also the eventual presence of large defects and a cross-sectional view shows the thickness of the zeolite layer. Figure 17.4 shows cross section and top-view of a supported FAU zeolite membrane. 

Several studies concerning the separation of propylene from propane base on faujasite (FAU zeolite) membranes or titanosilicate ETS have been published recently. The olefin/paraffin selectivity of all those materials was comparable to glassy polymers and always below 10. 

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. 

Jeong BH, Sotowa KI, and Kusakabe K. Catalytic dehydrogenation of cyclohexane in an FAU-type zeolite membrane reactor. J Membr 5d2003 224 151-158. 

The multistep synthesis is classically used to repair defects or to increase the zeolite membrane thickness. This technique also allows to superimpose layers with different zeolite structures. We have also to notice that for certain zeolite structures such as FAU or LTA, when the synthesis duration is too long, other phases can appear by transformation of the initial zeolite structure. 

Microwave heating has become one of the most successful approaches in the synthesis of zeolite membranes in terms of energy efficiency by reducing the synthesis times. LTA, MFI, AFl, FAU, SOD, and ETS-4 types of zeolite membranes have been successfully synthesized by microwave heating [5,20-24]. 

Figure 11.19b. The most studied structure is the MFI, followed by LTA, and a similar number of publications could be found about FAU and MOR. The section others corresponds to FER, BETA, MEL, ZSM-11, and related materials like ETS-10 or ETS-4. The distribution of the zeotypes studied in the period 2(X)5-2011 does not change that much, although the proportion of mixed matrix membranes or composites decreases to approximately 10%. The number of publications referred to MFI, FAU, LTA, or MOR is multiplied by three compared to the period 1995-2(X)5 and CHA structure has also been introduced. After the excellent review published by Falconer and Noble in 2004 [158], the work on pervaporation and zeolites has been reviewed in specific or general reviews of zeolite membranes and pervaporation [1,3,159].

Zeolite membranes are amenable by surface modification with a variety of chemical functional groups using simple silane chemistry that may provide alternative surface chemistry pathways for enzyme immobilization. In this context, Shukla et al. [338] have recently used a chemically modified zeolite-clay composite membrane for the immobilization of porcine lipase using glutaraldehyde to provide a chemical linkage between the enzyme and the membrane. The effects of pH, temperature, and solvent on the performance of such biphasic zeolite-membrane reactors have been evaluated in the hydrolysis of olive oil to fatty acids. Similarly, Algieri et al. [339] have immobilized tyrosinase on FAU membranes for the enzymatic conversion of the 1-tyrosine to 1-DOPA as an effective drug for Parkinson s disease treatment. This approach combines the active role of zeolite membrane as enzyme support and inhibitor suppressor. Moreover,... 

Sato K, Sugimoto K, Sekine Y, Takada M, Matsukata M, Nakane T. Application of FAU-type zeolite membranes to vapor/gas separation under high pressure and high temperature up to 5 MPa and 180°C. Micropor Mesopor Mater 2007 101 312-318. 

Zhu G, Li Y, Chen H, Liu J, Yang W. An In situ approach to synthesize pure phase FAU-type zeolite membranes Effect of ageing and formation mechanism. J Mater Sci 2008 43 3729-3288. 

Figure 17.4 SEM micrographs of (a) cross section and (b) top view of a FAU supported zeolite membrane.

Figure 17.6 Temperature effect on the selectivity for FAU membrane A (A A) and FAU membrane B ( ). Reprinted from K. Kusakabe, T. Kuroda, A. Murata and S. Morooka, Formation of a Y-type zeolite membrane on a porous a-alumina tube for gas separation, Industrial and Engineering Chemistry Research, 36, 649-655, 1997, with permission from ACS.

Figure 17.7 Gas permeance and CO2/N2 separation factor as a function of the temperature for equimolar CO2-N2 mixture in dry and moist conditions. Reprinted from X. Gu, J. Dong and T. M. Nenoff, Synthesis of defect free FAU-type zeolite membranes and separation for dry and moist CO2/ N2 mixtures, Industrial and Engineering Chemistry Research, 44, 937-944, 2005, with permission from ACS.

Zeolite membranes show high thermal stability and chemical resistance compared with those of polymeric membranes. They are able to separate mixtures continuously on the basis of differences in the molecular size and shape [18], and/or on the basis of different adsorption properties [19], since their separation ability depends on the interplay of the mixture adsorption equilibrium and the mixture. Different types of zeolites have been studied (e.g. MFI, LTA, MOR, FAU) for the membrane separation. They are used still at laboratory level, also as catalytic membranes in membrane reactors (e.g. CO clean-up, water gas shift, methane reforming, etc.) [20,21]. The first commercial application is that of LTA zeolite membranes for solvent dehydration by pervaporation [22], Some other pervaporation plants have been installed since 2001, but no industrial applications use zeolite membranes in the GS field [23]. The reason for this limited application in industry might be due to economical feasibility (development of higher flux membranes should reduce both costs of membranes and modules) and poor reproducibility. 

Jeong, Sotowa, and Kusakabe (2004) simulated the catalytic dehydrogenation of cyclohexane in an FAU-type zeolite membrane reactor. The cyclohexane conversion enhanced in the zeohte membrane reactor, which was more dependent on the permeance than the separation factor. Table 21.3 presents a summary of some of the membrane reactors used for cyclohexane dehydrogenation. 

FAU-type zeolite membrane on a porous a-ALOs support tube Tubular reactor 39.1% increase in cyclohexane conversion (the conversion in the membrane reactor approaches 72.1% whereas it is 32.2% in the conventional one) Jeong, Sotowa, and Kusakabe (2003)... 

Jeong, B.-H., Sotowa, K.-L, Kusakabe, K. (2004). Modeling of an FAU-type zeolite membrane reactor for the catalytic dehydrogenation of cyclohexane. Chemical Engineering Journal, 103, 69—75. 

When zeolites are grown as films, zeolite membranes are formed. Efforts to prepare polycrystalline zeolite membranes started in the late 1980s, but not until the early 1990s were MFI-type zeolite membranes (ZSM-5 and silicalite-1) successfully prepared with very good permeation and separation properties [3]. Since then, zeolite membranes have constantly attracted considerable attention because of their unique properties in terms of size uniformity, shape selective separation behavior, and good thermal/chemical stabilities. So far, more than 20 different types of zeolite membranes have been prepared - such as LTA, FAU, MOR, FER, MEL, CHA, DDR, and AFI - with significant separation interest [4, 5]. Table 3.1 lists a few typical zeolite membranes and their potential applications for separation of fluid mixtures. 

The majority of zeolite MR applications reported in the literature to date fall into the category of PBMRs. The reactor consists of a zeolite membrane with a conventional catalyst present in the form of a packed bed of pellets. The reaction takes place in the catalyst bed while the zeolite membrane serves mainly as a product separator (for H2 or H2O separation) [27] or a reactant distributor (for O2 distribution) [28]. Figure 3.5 illustrates a FAU-type zeolite PBMR combined with a packed bed reactor for dehydrogenation of cyclohexane [29]. Half of the catalyst is packed in the area upstream of the permeation portion to enhance the conversion, otherwise cyclohexane will preferentially permeate at the front end of the zeolite membrane, resulting in a decrease in conversion. 

Treatment of the a-alumina support by APTES as shown on the right, switches the zeta potential of the support from negative to neutral/slightly positive at the pH of zeolite FAU membrane formation. As a result, the negatively charged precursor macro molecules from the FAU zeolite synthesis solution can better adsorb and crystallize to a dense membrane layer [33]. 

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