Metal Organic Frameworks (MOFs)

MOFs are short for metal organic framework compounds. They are a kind of crystalline porous material with periodic network structure formed by inorganic metal centers (metal ions or metal clusters) and bridged organic ligands connected to each other through self-assembly. MOFs are organic-inorganic hybrid materials, also known as coordination polymers, which are not only different from inorganic porous materials, but also different from general organic complexes. They combine the rigidity of inorganic materials and the flexibility of organic materials, so they have shown huge development potential and attractive development prospects in modern materials research.

Applications:

MOFs materials have the characteristics of regular pore structure, high porosity, large specific surface area, diverse structures and properties, and adjustability. These characteristics make them have important application prospects in the fields of gas storage, gas separation, catalysis, luminescence, sensing, biomedicine, etc.

• Gas storage: The gas storage application of MOFs is based on the high adsorption capacity of MOFs for certain gas molecules. MOFs can enhance the adsorption capacity of gases such as hydrogen, methane, and ethylene by increasing the specific surface area, increasing the number of open metal sites, and modifying some functional groups on organic ligands. MOFs have important application value in clean energy and other fields. Some researchers have prepared a super-sponge-like substance MOF-177 with a Zn₄O inorganic secondary structural unit using a three-node carboxylic acid ligand (H₃BTB) as a ligand. It can adsorb a variety of gas molecules and organic macromolecules such as bromobenzene, bromonaphthalene, bromoanthracene, etc., and its ability to store CO₂ far exceeds that of other porous materials.

Figure 1. H₃BTB molecular structure (left) and MOF-177 lattice structure (right)

• Gas separation: The pore size of MOFs can be precisely adjusted, and the surface of the pores can also be easily modified by functional groups, making them a great advantage in gas separation. At present, there are two main ways to apply MOFs to gas separation: adsorption separation of MOFs powder and membrane separation of MOFs. The adsorption separation of MOFs mainly utilizes the different adsorption/desorption performance of MOFs materials for different components in the mixture to achieve selective adsorption separation. The membrane separation of MOFs mainly depends on the relationship between the size of the component molecules and the pore size of the MOFs, and the interaction between the component molecules and the MOFs. Since the structure and properties of the pores of MOFs can be flexibly adjusted, membrane separation of MOFs is very promising. For example, currently, the most widely used ZIF-8 membrane has very good separation selectivity for H₂/N₂, C₃H₆/C₃H₈.

Figure 2. Preparation of ZIF-8 membrane by the current driving method

• Catalysis: The excellent properties of MOFs make them excellent catalytic materials or carrier materials for catalysts. The metal ions with unsaturated coordination sites in the structure of MOFs can be used as the active center of the catalytic reaction. A variety of organic ligands with catalytic functions can also be introduced on the MOFs framework, for example, chiral ligands that can realize asymmetric catalytic reactions. MOFs have special structures, such as chiral spiral shaft channels, to provide an asymmetric catalytic microenvironment. Besides, because MOFs materials have the advantages of large specific surface area, high porosity and wide pore size, so highly dispersed nano metal particles can be loaded in the pores of MOFs in the form of guests. These characteristics make MOFs materials have unique advantages in the field of catalysis. For example, the Au/MOF-5 catalyst prepared by impregnation has good catalytic activity in the three-component coupling reaction of aldehyde, alkyne and amine.

Figure 3. Synthesis of MOF-5

• Sustained drug release: MOFs have high drug loading, biocompatibility and functional diversity, and can be widely used in drug carriers. For example, MIL-100 and MIL-101 have good drug loading and release effects on ibuprofen.

References:

1. S. Robinson, R. Jubin, B. Choate. (2005). "Materials for Separation Technology: Energy and Emission Reduction Opportunities." Nature. 532, 435-438
2. J. E. Bachman, Z. P. Smith. (2016). "Enhanced ethylene separation and plasticization resistance in polymer membranes incorporating metal-organic framework nanocrystals." Nat. Mater. 15, 845-849.