• carbon capture
• co2 adsorption
• mof
• multivariate mofs

The Paris Agreement has long-term goal of limiting the temperature increase to 2 °C above preindustrial levels, and net-zero emissions in the second half of this century. The world is seeking deep reductions in emissions, for this the development of efficient and cost-effective CO2 separation technologies is imperative. In this context, an appealing candidate for a real life application in carbon capture is F4_MIL-140A(Ce), an ultramicroporous metal-organic framework (MOF) based on Ce(IV) and tetrafluoroterephthalic acid as organic linker. This system exhibits a non-hysteretic step-shaped CO2 adsorption isotherm below 1 bar, due to a phase transition occurring upon CO2 adsorption, whose origin is attributed to a cooperative CO2 mechanism that involves the concerted rotation of fluorinated aromatic rings. Isoreticular MOFs based on linkers with lower fluorination degree and Zr as the metal do not display this cooperative mechanism, suggesting that there is a peculiar synergy between metal and linker in F4_MIL-140A(Ce).
In this work, we investigated the influence of the fluorination of ligands and the nature of the metal on the step-shaped adsorption isotherm of mixed-component analogues of F4_MIL-140A(Ce). To this end, we report new synthetic methods to obtain novel mixed-linker F4:Fx_MIL-140A(Ce) (where x = 3, 2, 1, 0) and mixed-metal F4_MIL-140A(Ce/Zr) MOFs. Mixed-linker MOFs were obtained by means of both methanol/water mixed solvent and pure water solvent approach: the latter allowed cleaner, phase-pure and higher crystalline materials to be obtained. Each MOF was characterized by Powder X-Ray Diffraction, while the F4/Fx ligand ratio was determined by 1H and 19F liquid-state nuclear magnetic resonance upon MOF basic digestion. The CO₂ adsorption measurement displays that the characteristic cooperative adsorption mechanism observed in F4_MIL-140A(Ce) is also present in mixed-ligand materials, provided the overall fluorination degree of the structure exceeds 80%. This highlights the critical role of the fluorination degree in enabling the cooperative mechanism, independent of both the number of fluorine atoms on the aromatic ring and their specific positions, relying solely on the overall fluorination level within the framework.
Many direct synthesis and post-synthetic approaches were tried to obtain mixed-metal F4_MIL-140A(Ce/Zr) MOFs. The most promising method was a post-synthetic approach, by exposing the pristine F4_MIL-140A(Ce) to a zirconium methanolic solution for some days. The Ce/Zr ratio into the materials was determined by plasma-optical emission spectrometry (ICP-OES) analysis that detected an amount of Zr about 20%. Further analysis is required to thoroughly characterize the mixed-metal materials. For example, CO2 adsorption isotherm will be useful to investigate the role of cerium for the cooperative CO2 adsorption mechanism, characteristic of F4_MIL-140A(Ce). Furthermore, techniques such as Pair Distribution Function (PDF) analysis or X-ray Absorption Spectroscopy (XAS) can provide valuable insights into the distribution of zirconium and the Ce-Zr distance in the inorganic unit. These analyses will help clarify whether zirconium is randomly incorporated, follows an ordered arrangement, or segregates separately from cerium.
Understanding the factors that contribute to this cooperative mechanism can enhance the comprehension of the chemistry of this MOF. Moreover, it may enable to replicate these factors in other materials, thereby improving their practical applications.