Polycrystalline zeolite membranes

Polycrystalline zeolite membranes consist of inter-grown zeolite crystals with no apparent cracks or pinholes (Fig. lA). These films are composed of only zeolite (i.e., there are no non-zeolite components such as amorphous silica or polymer). They are normally supported on a substrate although free-standing films have also been synthesized. Membranes can be prepared on different substrates such as silicon wafer, quartz, porous alumina, carbon, glass, stainless steel (SS), gold, etc. Polycrystalline films are primarily prepared by hydrothermal synthesis methods including in situ crystallization, seeded growth,and vapor transport, " and have potential use in all of the applications discussed in this entry. 

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.

In the middle of the last century, the original form of zeolite membranes were synthesized by dispersing the zeolite crystals in polymer membrane matrixes, which were used for gas separation and pervaporative alcohol/water separations. In the last few decades, the researches of polycrystalline zeolite membranes that supported on ceramic, glass, or metal substrates have grown into an attractive and abundant field. Their applications for gas separation, pervaporation, membrane reactors, sensors, low-k films, corrosion protection coatings, zeolite modified electrodes, fuel cells, heat pumps et al. have been wildly explored. In the following text, the applications of supported polycrystalline zeolite membranes for energy and fuels will be presented. 

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. 

In this paper, we consider a zeolite membrane comprised of a single zeolite crystal. While real zeolite membranes are polycrystalline, this approximation is useful because many experimental studies aim to characterize transport that occurs purely through zeolite pores [6], The steady state flux, J, through this membrane is... 

Zeolite membranes are formed by a polycrystalline layer, which means that the permeation is not only through the ordered nanopores of the zeolite crystal, intracrystalline pathways, but also through the spaces between the crystals, intercrystaUine pathways (see Eigure 10.20). Nomura et al. [116] proposed a model taking into account this consideration. 

The third problem associated with the measurement of diffusivities in zeolite membranes is related to the fact that the membrane is a polycrystalline layer that possesses intercrystalline defects and diffusion also takes place through these random defects. 

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]. 

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. 

Jia and Noble and co-workers et al. reported a 10 pm thick silicalite membrane on a composite support of a-Al203 [27,77]. Finally, Xiang and Ma [76] partially filled the pores of a microporous a-alumina support with ZSM5 crystals. All the authors used an in situ hydrothermal crystallisation method to grow directly polycrystalline zeolite layers. 

Snyder MA, Vlachos DG, Nikolakis V. Quantitative analysis of membrane morphology, microstmcture, and polycrystallinity via laser scanning confocal microscopy Application to NaX zeolite membranes. J Membr Sci 2007 290(1-2) 1-18. 

Fluorescence confocal optical microscopy gives the possibility of observing the polycrystalline network of the zeolite membrane and so the internal defects not observable with SEM analysis ... 

These membranes are polycrystalline zeolite layers deposited on porons inorganic supports, and they offer several advantages over polymeric manbranes, which are as follows ... 

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