Zeolite Membranes for Separation Processes: synthesis methods, applications and transport mechanisms

The zeolite membranes constitute a special class of inorganic membranes that have been developed very rapidly in the recent years. The zeolite membranes have distinctive advantages such as molecular sieving properties, uniform molecular-sized pores, high thermal stability, chemical inertness, and high mechanical strength. Several zeolite membrane synthesis procedures have been reported in literature including in situ hydrothermal synthesis method, secondary growth method, dry gel conversion method, microwave heating and the embodiment of zeolite crystals into a matrix. The pioneering works in literature on the synthesis of zeolites confirmed that the hydrothermal synthetic technique has become the basic route for zeolites synthesis. Hydrothermal synthesis usually refers to reactions occurring under conditions of high temperature–high pressure in aqueous solutions in a closed system. In general, zeolite membranes are prepared through hydrothermal synthesis method, which leads to the growth of a zeolite film over a porous support system. Coating the zeolite seeds on the support surface before hydrothermal synthesis, which is also named secondary growth method, is an effective approach to synthesize a highquality zeolite membrane. It is well known that the presence of seeds on support surface plays an important role in the membrane layer formation. Synthesis with seeds gives a better control of the membrane layer formation process by separating the crystal nucleation and growth with a shortened crystallization time. Different applications were presented for zeolite membranes during the last decades. In our research groups, MFI zeolite membranes were applied in the pervaporation processes for the removal of organic materials from water, in gas separation processes for the fractionation of hydrocarbons isomers, etc. Also, hydrophilic zeolite membranes (A-type, Sodalite) were used for dehydration of organic solvents via pervaporation process. In the recent years, DDR zeolite membranes synthesis and its application in the removal of natural gas impurities (N۲ and CO۲) was developed by NMRC as one of the research centers in all over the world. The MFI zeolite membranes were also used in isomerization membrane reactors, resulting in a high potential of this membranes to reach high octane number gasoline and P-Xylene with high purity which both of them are crucial for our country and the middle-east countries. The adsorption-diffusion mechanism is the dominant mechanism in the separation of different components by microporous zeolite membranes. However, more detailed studies on the separation mechanisms of the different zeolite membranes in the various separation processes are appreciated.