Figure 1 - SEM image of zeolite membrane
Advantages of zeolites:
These membranes have advantages over other types of membranes in that they are highly stable under thermal cycling, high temperatures, and harsh physical and chemical environments which other membranes cannot withstand. The chemistry of the zeolites can be modified to provide catalytic properties, to change them between hydrophobic and hydrophilic surfaces, to change the pore size and structure (creating different types of zeolites), etc., which make them useful for many different applications.
Diffusion through zeolite membranes:
Molecules diffuse through the pores via various diffusion mechanisms. Zeolites can be size and shape selective - more easily allowing straight-chain than branched hydrocarbons to pass through, for example. When used for catalytic reactions, they can be intermediate or product shape selective as well. However, separations using zeolite membranes are not always based simply on size or shape of the diffusing species. When interactions between the surface and the diffusing molecules are important, adsorption
or surface diffusion can dominate the transport. In these cases, separations where the larger molecules preferentially pass through the membrane can occur. Additionally, "non-zeolite" pores - pathways through the membrane such as those between crystals or any pathway other than the well-defined zeolite pores - can exist in the membrane. Transport of molecules through these non-zeolite pores can be in series with or in parallel to zeolite pore diffusion. Thus, various models are being developed to describe transport of gases and liquids through
zeolites.
The most commonly studied zeolite structure for membrane synthesis is MFI-type. (Figure 2 shows a diagram of this structure.) Because they are relatively inexpensive to synthesize in the laboratory, and because of their approximately 0.55 nm pore size, MFI-type zeolites have been studied extensively for use as separation devices. Silicalite-1 is an MFI-type structure whose framework consists of only silicon and oxygen atoms. ZSM-5 zeolites have exactly the same structure, but some of the silicon atoms are replaced by aluminum atoms. These aluminum atoms in the framework lead to a net negative charge, and therefore must have cations associated with them. Na-ZSM-5 (with sodium as the cation) and H-ZSM-5 (with hydrogen as the associated cation) are the most common forms of this structure.
In addition to ZSM-5 zeolites, we have prepared membranes that consist of layers of X, Y, Beta, SSZ, and SAPO structures. These have different pore sizes and adsorption/separation properties.
Figure 2 - Three dimensional structure of MFI-type zeolite
Source - Atlas of Zeolite
Structure Types