Porous materials with regions of empty space are widely used in both industrial and domestic applications, ranging from gas separation to heterogeneous catalysis and green chemistry. Although a variety of porous materials have been synthesized by various ways, it has proven difficult to prepare organic polymer networks with discrete pores. However, the appearance of reticular chemistry makes it possible to construct these porous materials, which can be synthesized by using topologically designed building blocks.
The first family of such porous materials synthesized based on reticular chemistry is metal-organic frameworks (MOFs), which consist of a regular array of positively charged metal ions surrounded by MOFs ligands (also known as MOFs linkers ) molecules. The metal ions form nodes that bind the arms of the linkers together to form a repeating, cage-like one-, two-, or three-dimensional network structures. Another family of porous materials to be synthesized on the base of reticular chemistry is covalent organic frameworks (COFs). COFs are formed by organic monomer (also known as COFs linkers ) through strong covalent bonds of light atoms such as carbon, nitrogen, and oxygen organic to form a robust two-dimensional (2D) or three-dimensional structures. Although structurally reminiscent between MOFs and COFs, one crucial difference between the two frameworks is the absence of metal cations or connecting nodes in COFs, which leads to porous and crystalline materials that generally have relatively lower densities.
Fig.1 The simulated molecular structure of MOFs(left), and COFs(right)
As the excellent members of porous materials, MOFs and COFs possess the a lot of properties, thereinto the mainly properties include inherent porosity and high surface areas, high crystallinity, tunable pores and functional tunability.
- Porosity and high surface areas: MOFs and COFs are porous materials which enclose void spaces into their bulk structures, thus offering extensive surface areas and high capacities for interaction with guest atoms, ions, or molecules. For examples, MOFs possess high surface areas with a range from 1000 to 10,000 m2/g, and COFs possess comparable surface areas compared with MOFs.
- High crystallinity: The high crystallinity of MOFs and COFs allows obtaining highly ordered and usually conjugated polymers both in 2D and/or 3D structure with well-defined pore aperture and ordered channel structure.
- Tunable pores: The shapes and sizes of the nanopores in MOFs and COFs can be well controlled by sophisticated selection of the building blocks, as well as the underlying network topology, making MOFs and COFs promising materials in various applications such as gas separation and storage, energy conversion, biomedicine, and catalysis.
- Functional tunability: Their versatile structural and chemical composition of MOFs and COFs can be realized by judicious selection of molecular building blocks, and a variety of functional sites/group, thus making the rational design of many interesting properties to realize different functions.
Over the years, interest in the field of porous materials has grown extremely due to their good performances. MOFs and COFs are extensively investigated in potential application such as storage and separation, catalysis, sensing and detection, energy storage, and drug delivery among the numerous porous materials.
Fig.2 The applications of MOFs and COFs
- Storage and separations: Selective storage and adsorption of gas and organic molecules is a major application direction of MOF and COF materials attributing to their intrinsic porous nature, high surface area and high gas uptake capacity. Among these, using MOFs and COFs to achieve selective gas storage and separation (e.g. H2, CO2, CH4 ect.) is a research focus, and MOFs are frequently employed for selective storage and separation of CO2. In addition, the pore size and functionality of MOFs and COFs materials can be designed to host specific chemical interactions and to impose specific constraints ideally suited for selective gas and liquid separation, enantiomer separation.
- Catalysts: The eminent virtues of MOFs and COFs like chemical tunability, large and accessible surface area, and bespoke pore architecture have prompted considerable application in catalysts. Usually, MOF and COF materials are candidate heterogenous catalysts for the hydrogen and oxygen evolution reactions and for small molecule activation. Catalysis using MOFs is normally achieved through two types of approaches. Coordinated metal sites or accessible organic units in some MOFs could be directly used as catalytic centers for catalytic reactions.
- Energy storage:MOFs and COFs have generated great interest in energy storage fields such as batteries, supercapacitors and others, because the ordered porous frameworks can allow a fast-ionic transportation and storage without large volume variation and the structural versatility, and functional tunability of MOFs and COFs from the molecular level will contribute to an in-depth understanding of the mechanism of energy storage. Thereinto, MOFs and COFs are mainly used in battery, including electrode materials or host materials, solid-state electrolyte and separator.
Fig.3 Schematic illustration for the architecture of the solid-state battery by using the MOFs
- Sensing and detection: MOFs and COFs are attractive candidate sensor materials due to their highly exposed pore and surface areas and the inherent sensitivity of their electronic structures to gating through adsorption or binding of analytes. MOFs and COFs materials have the ability to detect gases, solvents, metal ions, toxic species and contaminants, because the interaction of these analytes with the framework of MOFs and COFs induces the changes of optical, electronic, colorimetric or crystallographic.
- Drug delivery: Reticular frameworks of MOFs and COFs can exhibit extraordinarily high porosity and capacity for the storage of guest molecules. In the field of drug delivery, a high drug loading capacity is a generally aspired parameter of drug carriers. The first utilization of highly porous reticular frameworks as drug carriers was demonstrated with MIL-100(Cr) and MIL-101(Cr). The mesoporous MOFs with huge pores (25-34 Å) and high surface areas (3100-5900 m2/g ) were loaded with the analgesic model drug ibuprofen (IBU).
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- Gao X.; et al. MOFs and COFs for Batteries and Supercapacitors [J]. Electrochemical Energy Reviews. 2020, 3, 81-126.
- Horcajada P.; et al. Metal–organic frameworks as efficient materials for drug delivery[J]. Angewandte Chemie International Edition, 2006, 45, 5974-5978.