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Zeolites were the go-to porous material for many years. Even though they had great benefits, the development of other, more efficient, porous materials has minimized the usage of zeolites. Materials such as metal-organic frameworks (MOFs) and covalent organic frameworks (COFs) are two porous materials that have gained a lot of traction in the chemical, nanoscience, and material science fields. In this article, we look at both these materials and the main differences between them.
Metal-Organic Frameworks (MOFs)
Metal-organic frameworks, commonly denoted as MOFs, are a hybrid porous material composed of both organic and inorganic groups. The structure of a MOF is crystalline and three-dimensional in nature and utilizes a combination of rigid inorganic groups (often metal ions or metallic clusters) and flexible organic linker ligands. The utilization of both rigid and flexible groups enables MOFs to possess long-range tunable pores that can accommodate a wide range of molecules and can be tuned to be selective in which types of molecules they allow into their pores.
If you look at the structure more closely, there are specific ways in which the inorganic and organic groups are utilized to form pores. The structure is a coordination network of inorganic nodes, which form the ‘corners’ of the pores and provide geometrical stability and structural regularity, and organic linkers that connect these nodes together (often referred to as a strut) and provide a synthetic versatility and modular functionality. The same structure is repeated in three dimensions and this creates a bulk material with ‘long’ pores in one dimension (and the other two dimensions form the entrance/exit of the pore, which can be tailored to be a defined size). In general, the geometry of the node will determine the geometric shape of the pore, for example, an octahedrally coordinated node will form a square-like pore, but this is just one example out of many.
MOFs are often synthesized through hydrothermal and solvothermal methods. Some of the key properties of MOFs include a high external surface area, a high internal surface area, that they are lightweight, possess a highly regular porous structure and a structure that is easily tuneable. Given their large amount of properties, MOFs are used for a wide range of separation, absorption, catalysis and storage applications, such as separating branched and linear hydrocarbons through size exclusion mechanisms so that only the linear (i.e. the most widely used) hydrocarbons are isolated for use (e.g. for fuels where linear hydrocarbons are more efficient than branched hydrocarbons).
Covalent Organic Frameworks (COFs)
Covalent organic frameworks (COFs) are similar to MOFs, but instead of being composed of metal-based nodes and organic structs, the structure is composed of light elements, such as hydrogen, carbon, boron, nitrogen, and oxygen. Moreover, COFs can form either 2D or 3D constructs, and unlike MOFs, they are composed wholly of covalent bonds, where the pores are formed by covalently linking multiple groups together in a cyclic manner. Just like MOFs, COFs have tuneable pores that can be exploited for a wide range of separation applications.
Many covalent reactions, and therefore the formation of covalent linkages that make up these frameworks, can be reversible. In the early days of COFs, it was believed that reversible bond mechanisms could lower the stability of the COFs. Whilst it has taken a lot of method development and research to increase the stability of COFs, they are now at the point where they have a high thermal and solvent stability, and many are now more stable than most MOFs. In recent years, this has been achieved by kinetically controlling the reaction process with linkages that do not undergo a reversible reaction and by thermally degrading boronic acids into cyclic structures.
It has been found that COFs with larger pores are a lot more stable than those with small pores. The issue with small pore COFs is that there is a weak interlayer dispersion, steric hindrance and a susceptibility for each ‘pore unit’ to template with the solvent. By comparison, larger pore COFs have stronger dispersion forces and don’t suffer from steric or templating effects.
Overall, COFs will tend to form two three dimensional geometries—eclipsed and staggered—where eclipsed structures are more akin to MOFs with a continuous and extended pore, whereas staggered layering will create overlapped (i.e. non-continuous) and short pores. This staggered formation is less favorable and is usually adopted by smaller COFs, whereas the MOF-like pore structures are adopted by COFs with larger pore units.
Even though MOFs are lightweight, they still use metallic elements which are inherently heavier than many non-metal elements. Therefore, COFs are often lighter than MOFs. However, because there are only certain conditions and materials that can be used to create COFs, the number of MOFs that have been created far outweighs the amount of COF structures created so far. Additionally, because there are more MOFs in use, MOFs are used in a lot more applications than COFs are, even though there are some overlapping applications areas. Just like MOFs, COFs also have a high surface area, a permanent porosity and can form extended structures (and extended pores) just like MOFs if synthesized in three dimensions.
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