Metal-organic Framework - MOFs For Catalysis

MOFs For Catalysis

MOFs have large potential in numerous catalyst applications. Catalysts are used to manufacture the majority of the most used chemicals in the world. The high surface area, tunable porosity, diversity in metal and functional groups of MOFs makes them especially suited for use as catalysts. The study of MOFs for catalysts has only recently begun with the majority of the work achieved during the last few years. The set geometry of the MOFs internal framework allows for their use as size selective catalysts. Previous work in this area was achievable only using zeolites. But zeolites are limited by the fixed tetrahedral coordination of the Si/Al connecting points and the oxide linker and there are only less than 200 zeolites present indicating its limitation in structure tuning whereas MOFs use versatile coordination chemistry, polytopic linkers, and terminating ligands (F-, OH-, H₂O among others) which makes it possible to design an almost infinite variety of MOF structures. It is also difficult to obtain zeolites with pore sizes larger than 1 nm, which limits the catalytic applications of zeolites to relatively small organic molecules (typically no larger than xylenes). Furthermore, mild synthetic conditions typically employed for MOF synthesis allow direct incorporation of a variety of delicate functionalities into the framework structures. Such a process would not be possible with zeolites or other microporous crystalline oxide-based materials because of the harsh conditions typically used for their synthesis (e.g., calcination at high temperatures to remove organic templates). Again, zeolites still cannot be obtained in enantiopure form, which prevents applications of zeolites in catalytic asymmetric synthesis of value-added chiral molecules for the pharmaceutical, agrochemical, and fragrance industries. For example, either enantiopure chiral ligands or their metal complexes can be incorporated directly into the frameworks of MOFs to lead to efficient asymmetric catalysts. Even some MOF materials may bridge the gap between zeolites and enzymes when they combine isolated polynuclear sites, dynamic host-guest responses, and a hydrophobic cavity environment. MOFs might be useful for making semi-conductors. Theoretical calculations show that MOFs are semiconductors or insulators with band gaps between 1.0 and 5.5 eV which can be altered by changing the degree of conjugation in the ligands indicating its possibility for being photocatalysts.