303734 New Composite Membranes Based On Zeolitic Imidazolate Framework ZIF-7 for Gas Separation
Creation of new inorganic nanoporous membrane materials is one of the most perspective direction for gas separation processes. As a new family of nanoporous materials, metal–organic frameworks (MOFs) are considered versatile materials for widespread applications. The incorporation of MOFs into a polymeric membrane matrix is attractive due to the MOF's high surface areas, adjustable pore dimensions and tunable surface functionality. Although incorporation of MOFs in organic polymer might enable the fabrication of membranes exhibiting properties that are optimized for a specific separation, it is still early stage of the investigation. It is because of membrane defects and related processing issues, use of MOFs with low selectivity, unfavorable orientation of crystals in the membrane, and defective morphologies characterized by void spaces between the MOFs and the polymeric matrix. All of previous causes leads to poor gas-separation performance since the gas molecules bypass the MOFs nor selective free volumes of the polymer matrix. However, using MOFs as filler, controlling the interface morphology between filler and polymer matrix is easier due to the presence of organic linkers into the MOF structure. These have better affinity and compatibility with polymer chains, and their surface can be easily functionalized by choosing the functional linkers. Among numerous MOFs, zeolitic imidazole frameworks (ZIFs) draw lots of attention due to their superior chemical and thermal stability . They are very promising materials for several important applications, such as gas storage, chemical sensor, and separation process. Furthermore, several ZIFs can be synthesized easily and fast at low cost, such as ZIF-8 and ZIF-7 . Therefore, several studies has investigated ZIFs, such as ZIF-8 , ZIF-90  and ZIF-7 , as porous fillers with various polymers, including polysulfone, Matrimid, PBI and polyimide, for MMMs for gas separation and pervaporation. Almost every report showed improved permeability, sometimes coupled with enhanced selectivity for gas separation.
ZIF-7 possesses an open-framework structure with sodalite topology in hexagonal symmetry, formed by bridging benzimidazolate (bim) anions and zinc cations . The pore size of ZIF-7 is about 0.3 nm, which is just in between the size of H2 (0.29 nm) and CO2 (0.33 nm). Therefore, ZIF-7 membranes are expected to achieve a high selectivity of H2 over CO2 through molecular sieving effect, which has been successfully proved by a recent study . A compact ZIF-7 thin layer was synthesized onto a porous alumina support using microwave-assisted secondary growth. In gas permeation tests, the ZIF-7 membrane showed a clear cut-off between H2 and CO2, with a H2/CO2 separation factor of 6.5, showing that a ZIF-7 membrane is very promising for hydrogen separation.
The 6FDA–ODA (4,4-(hexafluoroisopropylidene)diphthalic anhydride-4,4-oxydianiline) and 6FDA–(ODA:DABA) polyimide was chosen in this work because numerous studies had shown that fluorinated polyimides containing 6FDA exhibit good combination of gas separation factors and permeability coefficients for CO2 separation application. In the present work, aiming at the potential applications in hydrogen purification and recycling and integrating ZIF-7 membranes into the polyimide, a detailed investigation on single gas permeations through the ZIF-7 membrane was performed, including H2, CO2, CH4. Nano-sized ZIF-7 has been successfully synthesized with well-defined size distribution, and directly mixed with polyimide solution at different ZIF/polymer ratios.
Our results clearly demonstrate that ZIF-7 membranes have an intrinsic high H2/CO2 selectivity and a promising application in hydrogen separation, which is based on its very narrow and well-defined crystal pore structure. It was shown that obtained new composite membranes shown high mechanical and thermal stability. Therefore ZIF-7 can be considered as effective material for production of gas separation membranes with stable gas separation and mechanical properties.
1. Park K. S., Ni, Z., Cote A. P., Choi J. Y., Huang R. D., Uribe-Romo F. J., Chae H. K., O'Keeffe M.; Yaghi O. M. Exceptional Chemical and Thermal Stability of Zeolitic Imidazolate Frameworks, Proc. Natl. Acad. Sci. U. S. A., 103, 10186-10191, (2006).
2. Goh P.S., Ismail A.F., Sanip S.M., Ng B.C., Aziz M., Recent advances of inorganic fillers in mixed matrix membrane for gas separation, Separation and Purification Technology, 81, 243–64, (2011).
3. Song, Q.; Nataraj, S. K.; Roussenova, M. V.; Tan, J. C.; Hughes, D. J.; Li, W.; Bourgoin, P.; Alam, M. A.; Cheetham, A. K.; Al-Muhtaseb, S. A.; Sivaniah, E. Zeolitic Imidazolate Framework (ZIF-8) Based Polymer Nanocomposite Membranes for Gas Separation. Energy Environ. Sci., 5, 8359-8369, (2012).
4. Bae T.-H., Lee J.S., Qiu W., . Koros W.J, Jones C.W., Nair S., Angew. Chem. Int. Ed., 49, 9863–9866, (2010).
5. Li Y., F. Liang, H. Bux, W. Yang, J. Caro, Zeolitic imidazolate framework ZIF-7 based molecular sieve membrane for hydrogen separationJ. Membr. Sci., 354, 48–54, (2010).
6. Li Y.-S., Liang F.-Y., Bux H., Feldhoff A., Yang W.-S., Caro J., Molecular sievemembranes: supported metal-organic framework with high hydrogen selectivity, Angew. Chem. Int. Ed., 49, 548–551, (2010).