Introduction of MOF for CO2 capture

https://doi.org/10.1021/acsami.2c07640

“The outstanding versatility of MOFs in terms of pore size, shape, and chemistry has led to the development of a large number of sorbents that display remarkable CO₂ capture performance, (10−14) recently culminated with the deployment of CALF-20 for capture in the cement industry. (15) Fine tuning of pore size and shape is crucial to maximize the framework–adsorbate contact. (16) This has recently led ultramicroporous MOFs─i.e., with pores smaller than 8 Å─to gain a privileged spot as promising sorbents for CO₂ capture. (17−19) Pore chemistry plays a key role at the low partial pressures relevant for CCS, where specific interactions with the framework dominate the adsorption process. Typical CO₂ adsorption sites in MOFs include coordinatively unsaturated metal ions and functional groups with a basic character on the organic linker. (7,11,12) A promising approach that is gaining momentum is to introduce fluorine atoms in the backbone of ultramicroporous MOFs to prepare new materials labeled as perfluorinated MOFs (PF-MOFs). (20,21) Besides increasing the thermodynamic affinity for CO₂ (as evidenced by a high isosteric heat of adsorption, Q_st), the presence of (per)fluorinated groups can also render the framework hydrophobic, thus preserving the CO₂ capture performance in humid conditions, which is crucial for real-life applications. The family of hybrid ultramicroporous materials containing fluorinated anions such as SiF₆^2–, NbOF₅^2–, and TiF₆^2– represents an excellent example of how the presence of fluorine atoms exposed in narrow pores provides superior affinity for CO₂ while preserving a physisorption-based mechanism and minimizing competitive water adsorption. (20,22−24) We have recently reported on the water-based synthesis of two perfluorinated Ce^IV analogues of the well-known UiO-66 and MIL-140A framework types. (21,25) The ultramicroporous F4-MIL-140A(Ce) displays an S-shaped isotherm and outstanding CO₂/N₂ selectivity, and it was also found to display inverse CO₂/C₂H₂ selectivity. (26)“