Preparation of Zeolite Membranes

Zeolite membranes generally have a thickness of 2-50 pm, formed on porous materials such as AI2O3 and stainless steel tubes or disks owing to their greater structural stability and reduced mass-transfer resistance. The quality of zeolite membranes in terms of permeability and selectivity is determined by their intercrystalline porosity (defects), the crystal orientation relative to the membrane layer, the size of zeolite crystals, and the thickness and uniformity of the zeolite layer. 

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. 

Dead-end filtration causes excessive crystal accumulation whereas the cross-flow mode allows a more uniform and compact zeolite layer, because the suspension flows tangentially along the support surface. In cross-flow filtration separation systems, a high velocity is typically used for increasing the sweeping action through the membrane. This is not desired for seeding purposes which 

Coverage uniformity problems can happen when tubular supports are used horizontally and without any rotation. In this case, zeoUte seeds will be deposited preferentially in the bottom of the support owing to the effect of the gravitational force. To overcome this problem a new seeding procedure for tubular membranes has been designed combining for the first time  

Suited tilting allows air bubbles, eventually formed inside the support during filtration and responsible for local defects, to go up. The rotation is required to achieve the deposition over the whole membrane area. These three different procedure aspects guarantee a uniform and sufficient coverage of the support with zeolitic seeds. 

In the last 20 years, great progress on the zeolite membrane has been made, but only 20 structures are used for membrane preparation (Table 17.4) even if 170 zeolitic structures are indicated by the International Zeolite Association. 

The penetration of siliceous species in the porous support is investigated by means of energy dispersive X-ray. Phase identification and determination of any preferred orientation is performed by using X-ray diffraction. 

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In the preparation of zeolite membranes, in situ crystallization in an autoclave is used to deposit a zeolite layer on a macroporous metallic membrane substrate. For example, Geus [96] reported the preparation of ZSM-5 zeolite on a macroporous metal substrate, which can be used in separations at high temperatures. 

In spite of all these hurdles, there are already industrial-scale applications of zeolite membranes for solvent dehydration [106] by pervaporation plants using tubular zeolite A membranes with 0.0275 m of permeation area each (see Section 10.2.3). Li et al. [280] have prepared large area (0.0260 m ) ZSM-5 membranes on tubular a-alumina supports. This work is also interesting from the industrial point of view because the authors used inexpensive n-butylamine as template. Indeed, the cost required for industrial modules, on a general basis, is still far from clear. However, it must be noted that most of the costs can be ascribed to the module, and only 10%-20% to the membrane itself [3]. This underlines again the importance of preparation of zeolite membranes on cheaper, alternative supports that can also pack more area per unit volume. 

Nevertheless, the availability of procedures allows the preparation of zeolite membranes and layers with sufficient quality, reproducibility, and reliability only up to a few hundred square centimeters in surface, delaying the industrial implementation of zeolite membrane-based technology. To be realistic, the lack of module reliability under extreme temperature cycling or harsh environment and the necessary raw material cost reductions (supports and chemicals) are two of the main challenges toward which strong efforts must be targeted. 

Richter H, Voight I, Fischer P, and Puhtfiir BP. Preparation of zeolite membranes on the inner surface of ceramic tubes and capillaries. Sep PurifTechnol 2003 32 133-138. 

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]. 

More promising for reactive separations involving gas phase reactions appears to be the development and use in such applications of microporous zeolite and carbon molecular sieve (Itoh and Haraya [2.25] Strano and Foley [2.26]) membranes. Zeolites are crystalline microporous aluminosilicate materials, with a regular three-dimensional pore structure, which are relatively stable to high temperatures, and are currently used as catalysts or catalyst supports for a number of high temperature reactions. One of the earliest mentions of the preparation of zeolite membranes is by Mobil workers (Haag and Tsikoyiannis... 

Although the reproducibility in the preparation of zeolite membranes and layers is stiU a challenge, the methods developed for seeding and for synthesizing make that zeolite... 

Figure 8.2 Process flow diagram for the preparation of zeolite membrane-catalyst platelets.

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