Types of Zeolite Membranes

Zeolites are crystalline microporous aluminosilicate solids with a regular three-dimensional pore structure that is stable at high temperatures. Their atomic structures are based on the three-dimensional frameworks of silica and alumina tetrahedral, where the silicon or aluminum ions are surrounded by four oxygen ions in a tetrahedral configuration. Clusters of tetrahedra form different box-like polyhedral units, further linked to build up various frameworks with different channel sizes. 

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. 

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Fig. 1 Schematic of the three types of zeolite membranes (A) a polycrystalline zeolite membrane (B) a zeolite matrix composite membrane and, (C) a zeolite crystal layer.

Type of Reaction Type of zeolite membrane Preferential permeating species References... 

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]. 

Kita et al. (2003) reported on a tubular-type PV and vapor permeation module with zeolite membranes for fuel EtOH production. They used two types of zeolite membranes (i) NaA-type zeolite membrane, which was grown on the surface of a porous cylindrical mullite support and (ii) T-type zeolite membrane, which was also grown hydrothermally on the mullite support. Both membranes were studied for the flux and the separation factor of PV and vapor permeation for water-alcohol mixtures at 50°C and 75°C. The membranes were selective for permeating water preferentially with the high permeation flux. The separation factor of the T-type zeolite membrane was slightly smaller than the NaA zeolite membrane. They also claimed that this can provide more energy-efficient concentration of the EtOH to fuel grade EtOH. 

Several types of zeolite membranes such as A-type, Y-type, silicalite, ZSM-5, etc. have been developed, and have been applied mainly to gas and pervapo-ration separations. Kumakiri et al. [43] prepared A-type zeolite membranes by hydrothermal synthesis with seed growth, and applied these to the reverse osmosis separation of water/ethanol mixtures. The zeolite A membrane showed a rejection of 40% and a permeate flux of 0.06 kg m h for 10 wt% ethanol at a pressure difference of 1.5 MPa, while a permeate flux of 0.8 kgm h and a separation factor of 80 were obtained in PV. 

Type of zeolite membrane Aperture-free diameter (nm) Separation... 

In this chapter, we Hmit ourselves to the topic of zeolite membranes in catalysis. Many types of membranes exist and each membrane has its specific field where it can be appHed best. Comparing polymeric and inorganic membranes reveals that for harsher conditions and high-temperature applications, inorganic membranes outperform polymeric membranes. In the field of heterogeneous catalYsis, elevated temperatures are quite common and therefore this is a field in which inorganic membranes could find excellent applications. 

Some types of zeolite-based catalytic membrane configurations are schematically depicted in Figure 30 ... 

The use of zeolitic membranes in separation or combined reaction and separation processes is very appealing. Advantages of using this type of membrane include not only their ability to discriminate between molecules based on molecular size but also their thermal stability. The large variety of zeolite types could provide a tailor-made separation medium for specific processes. Moreover, the properties of zeolites are often easily adjustable (ion exchange, Si/Al ratio, etc.). This makes zeolitic membranes also very promising for use as catalytic membranes. 

To assess about the quality and purity of the synthesized membranes, several experimental techniques and procedures are available many of those are commonly employed in catalyst characterization. Thus, XRD (x-ray diffraction) analysis of the supported samples is conventionally used to identify the type of zeolite, the proportion of amorphous material and impurities, and the preferential orientation of the crystals (XRD-pole figure). However, for the vast majority of the synthesis procedures described, the XRD spectra of the scrapped membrane or the resulting powder from the liquid phase is supplied to avoid the support contribution. 

Zeolite membranes are not the only kind of membranes that have been used in pervaporation, organic and other types of inorganic membranes, different from zeolites, have been employed. Polymeric membranes of PVA (polyvinyMcohol) have been widely employed for dehydratation and separation of organic mixmres however, their main limitations are related to their low thermal and chemical stability. When the water content in the feed mixmre is high, polymeric membranes suffer from swelling moreover, in the separation of organic mixtures they usually present a low selectivity. 

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]. 

Another form of zeolite membranes is a zeolite crystal layer that consists of isolated crystals deposited on a solid substrate (Fig. 1C). The substrate can be a variety of materials such as metal, ceramic, or silicon wafer. Crystal layers have to be supported. There has been exciting fundamental research carried out in this area, however, demonstrated applications have been limited to sensors. The organic linker approach appears very promising for the preparation of these types of membranes. ... 

A series of original synthesis strategies has been also reported recently such as flow-through reactors for the homogeneous synthesis of zeolite membranes [77], centrifugal force field [114] or electrophoresis [115] for the preparation of A-type membranes, and pulse laser deposition (PLD) for the secondary growth of oriented MCM-22 membranes [116]. 

Typical applications of zeolite membranes in reactors include i) conversion enhancement either by equilibrium displacement (product removal) or by removal of catalyst poisons/ inhibitors and ii) selectivity enhancement either by control of residence time or by control of reactant traffic. A large number of examples are reported and discussed in [49,50,52], Several of them are reported in fable 3. The use of a zeolite membrane as a distributor for a reactant has been attempted for the partial oxidation of alkanes such as propane to propene [137], or n-butane to maleic anhydride [138]. Limited performances were obtained because the back-diffusion of the alkane is hardly controllable with this type of microporous membrane [139]. 

Zeolite membranes and films have been employed to modify the surface of conventional chemical electrodes, or to conform different types of zeolite-based physical sensors [49]. In quartz crystal microbalances, zeolites are used to sense ethanol, NO, SO2 and water. Cantilever-based sensors can also be fabricated with zeolites as humidity sensors. The modification of the dielectric constant of zeolites by gas adsorption is also used in zeolite-coated interdigitaled capacitors for sensing n-butane, NH3, NO and CO. Finally, zeolite films can be used as barriers (for ethanol, alkanes,...) for increasing the selectivity of both semiconductor gas sensors (e.g. to CO, NO2, H2) and optical chemical sensors. 

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