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Zeolitic imidazolate framework

Zeolitic imidazolate frameworks (ZIFs) are a class of metal-organic frameworks that are topologically isomorphic with zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions (e.g. Fe, Co, Cu, Zn) connected by imidazolate linkers. Since the metal-imidazole-metal angle is similar to the 145° Si-O-Si angle in zeolites, ZIFs have zeolite-like topologies. As of 2010, 105 ZIF topologies have been reported in the literature. Due to their robust porosity, resistance to thermal changes, and chemical stability, ZIF’s are being investigated for applications such as carbon capture.Oxidation of aldehyde groupsHydrogenation of n-hexene Zeolitic imidazolate frameworks (ZIFs) are a class of metal-organic frameworks that are topologically isomorphic with zeolites. ZIFs are composed of tetrahedrally-coordinated transition metal ions (e.g. Fe, Co, Cu, Zn) connected by imidazolate linkers. Since the metal-imidazole-metal angle is similar to the 145° Si-O-Si angle in zeolites, ZIFs have zeolite-like topologies. As of 2010, 105 ZIF topologies have been reported in the literature. Due to their robust porosity, resistance to thermal changes, and chemical stability, ZIF’s are being investigated for applications such as carbon capture. ZIFs are prepared by solvothermal or hydrothermal techniques. Crystals slowly grow from a heated solution of a hydrated metal salt, an ImH (imidazole with acidic proton), a solvent, and base. Functionalized ImH linkers allow for control of ZIF structure. This process is ideal for generating monocrystalline materials for single-crystal X-ray diffraction. A wide range of solvents, bases, and conditions have been explored, with an eye towards improving crystal functionality, morphology, and dispersity. Prototypically, an amide solvent such as N,N-dimethylformamide (DMF) is used. The heat applied decomposes the amide solvent to generate amines, which in turn generate the imidazolate from the imidazole species. Methanol, ethanol, isopropanol, and water have also been explored as alternative solvents for ZIF formation but require bases such as pyridine, TEA, sodium formate, and NaOH. Polymers such as poly(ethylene oxide)–poly(propylene oxide)–poly(ethylene oxide), polyvinylpyrrolidone, and poly-(diallyldimethylammonium chloride) have been found to act as crystal dispersants, imparting particle-size and morphology control. Due to their promising material properties, significant interest lies in economical large-scale production methods. Sonochemical synthesis, which allows nucleation reactions to proceed rapidly through acoustic generation of localized heat and pressure, has been explored as a way to shorten synthesis times. As with the case of zeolites, microwave-assisted synthesis has also been of interest for the rapid synthesis of ZIFs. Both methods have been shown to reduce reaction times from days to hours, or from hours to minutes. Solvent-free methods, such as ball-milling or chemical vapor deposition, have also been described to produce high-quality ZIF-8. Chemical vapor deposition is of particular promise due to the high degree of uniformity and aspect ratio control it can offer, and its ability to be integrated into traditional lithographic workflows for functional thin films (e.g. microelectronics). Environmentally-friendly synthesis based on supercritical carbon dioxide (scCO2) have been also reported as a feasible procedure for the preparation of ZIF-8 at an industrial scale. Working under stoichiometric conditions, ZIF-8 could be obtained in 10 hours and does not require the use of ligand excess, additives, organic solvents or cleaning steps. ZIF’s exhibit some properties relevant to carbon capture, but commercial technology is based on amine solvents.

[ "Metal-organic framework" ]
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