by What are Metal Organic Frameworks?
Metal organic frameworks (MOFs) are a relatively new class of porous crystalline materials composed of metal ions or clusters coordinated to organic ligands. The organic ligands act as linkers between the metal constituents, forming multidimensional frameworks with adjustable and highly tunable structures at the molecular level.
Chemical Structure and Properties
MOFs are constructed from metal centers/clusters that act as secondary building units connected to organic struts. This results in frameworks with regular arrays of nanometer-sized cavities and channels. The metal-oxygen/nitrogen/carbon clusters act as nodes and the organic ligands function as struts to link these metal clusters. This allows for the formation of crystalline porous solids with very high surface areas. Some key properties Metal Organic Framework include their:
– Nanoporous crystalline structures with pores ranging from the microporous to the mesoporous region, allowing size- and shape-selective sorption of guest molecules.
– Extremely high surface areas often exceeding 5000 m2/g which provides abundant accessible surface area for gas/vapor adsorption or heterogeneous catalysis.
– Adjustable pore sizes and functionalities by choosing appropriate metal-organic building blocks and linkers during synthesis, allowing for tailored properties.
– Lightweight nature and biodegradability of some MOFs.
Gas Storage Applications
Given their extraordinarily high surface areas and tunable pore sizes/structures, MOFs have attracted significant attention for gas storage applications such as hydrogen storage and capture of greenhouse gases like carbon dioxide. Some MOF materials have been shown to adsorb unprecedented amounts of hydrogen or carbon dioxide gases at room temperature, significantly exceeding current DOE targets for onboard vehicular storage. Key advantages of MOFs for gas storage include their crystalline permanent porosity, ability to tailor pore sizes on the angstrom scale for optimal gas-framework interactions, and potential for low pressure/room temperature adsorption. Ongoing research is exploring new MOF frameworks and activation/functionalization methods to further enhance gas storage capacities.
Separations Using MOFs
Another major application area of MOFs is in gas separations and molecular sieving. Their nanoscale uniform pores can be selectively adjusted to sieving gases/vapors based on molecular interactions and kinetic diameters. MOFs with appropriately sized channels have been used for separation of industrial gas mixtures like separation of carbon dioxide from natural gas or flue gases, propylene/propane separations and oxygen/nitrogen separations from air. Some MOFs can selectively adsorb specific hydrocarbons from complex mixtures, showing promise for separations in petroleum refining as well. The tunability of MOF pores and functionalities allows tailoring frameworks for optimal separations of specific gas pairs of interest.
Catalysis with MOFs
Beyond gas storage and separations, MOFs also show much promise as heterogeneous catalysts due to their exceptionally high surface areas, modular design, and potential for facile inclusion of catalytically active metal sites and organic functional groups within their porous frameworks. Well-defined and reticular MOF structures allow for highly dispersed and accessible metal active sites, while their engineered pores can provide confinement effects to reactants and selectivity in reactions. MOF-based catalysts have been developed for a variety of important organic reactions and transformations like oxidations, reductions, C-C coupling reactions and more. Post-synthetic modification expands their catalytic repertoire. Ongoing efforts aim to better understand structure-activity relationships for rational design of new solid catalysts based on MOF platforms.
Biomedical Applications of MOFs
MOFs are also being evaluated for biomedical uses including drug delivery, imaging, and separation of biomolecules. Their customizable pore sizes and functionalities allow encapsulation and controlled release of therapeutic agents. Some MOFs are biodegradable, slowly releasing drugs inside the body. They show potential as MRI contrast agents due to the presence of paramagnetic metal ions in the frameworks. Selective adsorption of biomolecules by MOFs could enable diagnostic/ analytical separations. Toxicity evaluation is however needed before in vivo biomedical uses. Overall, MOF versatility enables potential applications from targeted drug/gene delivery to bioimaging and analytical biotechnology.
In metal-organic frameworks represent an exciting new class of highly tunable porous materials with diverse applications in gas storage, separations, catalysis and more. Their crystalline nanoporous structures paired with designability at the molecular level impart unprecedented control over properties. With continued development, MOF technologies may contribute solutions to key challenges in energy, environment and healthcare
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