Zeolite membranes types

Model separation values by molecular dynamics calculations for pure and mixed light gases interacting with differing zeolite type membranes (ie., comparing ZSM-5 to ZSM-22). 

With appropriate membrane pore size and a narrow distribution, membrane selectivity for smaller gas molecules can be high but the overall permeability is generally low due to a high flow resistance in fine pores. Several studies are being conducted to develop molecular sieve-type membranes using different inorganic materials, for example, those based on carbon (Liu, 2007), silica (Pex and van Delft, 2005), and zeolites (Lin, 2007). 

Morigami, Y., Kondo, M., Abe, J., Kita, H., and Okamoto, K. (2001) The first large-scale pervaporation plant using tubular-type module with zeolite NaA membrane separ. Purif. Tech., 25, 251-260. 

There are many different zeolite structures but only a few have been studied extensively for membrane applications. Table 10.1 lists some of these structures and their basic properties. One of the most critical selection criterion when choosing a zeolite for a particular application is the pore size exhibited by the material. Figure 10.1 compares the effective pore size of the different zeolitic materials with various molecule kinetic diameters. Because the pores of zeolites are not perfectly circular each zeolite type is represented by a shaded area that indicates the range of molecules that may stiU enter the pore network, even if they diffuse with difficulty. By far the most common membrane material studied is MFI-type zeolite (ZSM-5, Al-free siUcahte-l) due to ease of preparation, control of microstructure and versatility of applications [7]. 

M. Kondo, M. Komori, H. Kita and K. Okamoto, Tubular-type Pervaporation Module with Zeolite NaA Membrane, J. Membr. Sci. 133, 133 (1997). 

A-type zeolite pervaporation membranes have been developed by Mitsui Engineering Shipbuilding Co Ltd, which have been implemented in an industrial... 

Focus in the field of medium pore (ten-membered oxygen ring) zeolite-based membranes has been almost entirely on MFI-type zeolites. This zeolite and its all-silica form. 

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. 

Even if the problems of poor crystal intergrowth due to local exhaustion of reactants in the autoclave and synthesis of zeolite material in the bulk of the solution were solved, an important problem remains, related to the fact that several batch synthesis cycles (with their associated heating and cooling processes) are often required to achieve a zeolite membrane of good quality. Thus, a synthesis procedure in which reactants are continuously supplied to the synthesis vessel while this is maintained at a constant temperature would clearly be desirable not only for performance but also for the feasibility of the scale-up. This type of approaches has already been tested for inner MFI and NaA zeolite membranes [33-35], and the results obtained indicate that the formation of concomitant phases and the amount of crystals forming in the liquid phase are greatly reduced. Similarly, the continuous seeding of tubular supports by cross-flow filtration of aqueous suspensions [36-37] has been carried out for zeolite NaA membrane preparation. 

Kondo M, Komori M, Kita H, and Okamoto K. Tubular-type pervaporation module with zeolite NaA membrane. J Membr Sci 1997 133 133-141. 

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]. 

Supported zeolite Y, silicalite-1, A and a composite of silicalite-1 on A can be synthesised in a membrane configuration in a reproducible manner. Synthesis techniques using seeds were applied in the membrane preparation. The double layer type membrane has great potential in the bifunctional operation in one integrated unit. 

Zeolite NaA and FAU crystals were grown under the same hydrothermal condition using the same composition of clear solution. The only difference was the type of seed crystal used. This result suggested that the crystal growth did not occur by the attachment of nano-crystals. Zeolite films/membranes were prepared by the growth of seeded crystals. Based on the SEM and TEM observations and single gas permeation measurements, densification model of film/membrane is presented. 

FIGURE 11.19 Distribution of publications about zeolite membranes in pervaporation over the last decade (a) per year (b) per zeolite-type material. 

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