Introduction of MOF for CO2 capture

“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, Qst), 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 SiF62–, NbOF52–, and TiF62– 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 CeIV 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)

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