Increased CO(2) Affinity and Adsorption Selectivity in MOF-801 Fluorinated Analogues

[Image: see text] The novel Zr(IV)-based perfluorinated metal–organic framework (PF-MOF) [Zr(6)O(4)(OH)(4)(TFS)(6)] (ZrTFS) was prepared under solvent-free conditions using the commercially available tetrafluorosuccinic acid (H(2)TFS) as a bridging ditopic linker. Since H(2)TFS can be seen as the fully aliphatic and perfluorinated C(4) analogue of fumaric acid, ZrTFS was found to be isoreticular to zirconium fumarate (MOF-801). The structure of ZrTFS was solved and refined from X-ray powder diffraction data. Despite this analogy, the gas adsorption capacity of ZrTFS is much lower than that of MOF-801; in the former, the presence of bulky fluorine atoms causes a considerable window size reduction. To have PF-MOFs with more accessible porosity, postsynthetic exchange (PSE) reactions on (defective) MOF-801 suspended in H(2)TFS aqueous solutions were carried out. Despite the different H(2)TFS concentrations used in the PSE process, the exchanges yielded two mixed-linker materials of similar minimal formulae [Zr(6)O(4)(μ(3)-OH)(4)(μ(1)-OH)(2.08)(H(2)O)(2.08)(FUM)(4.04)(HTFS)(1.84)] (PF-MOF1) and [Zr(6)O(4)(μ(3)-OH)(4)(μ(1)-OH)(1.83)(H(2)O)(1.83)(FUM)(4.04)(HTFS)(2.09)] (PF-MOF2) (FUM(2–) = fumarate), where the perfluorinated linker was found to fully replace the capping acetate in the defective sites of pristine MOF-801. CO(2) and N(2) adsorption isotherms collected on all samples reveal that both CO(2) thermodynamic affinity (isosteric heat of adsorption at zero coverage, Q(st)) and CO(2)/N(2) adsorption selectivity increase with the amount of incorporated TFS(2–), reaching the maximum values of 30 kJ mol(–1) and 41 (IAST), respectively, in PF-MOF2. This confirms the beneficial effect coming from the introduction of fluorinated linkers in MOFs on their CO(2) adsorption ability. Finally, solid-state density functional theory calculations were carried out to cast light on the structural features and on the thermodynamics of CO(2) adsorption in MOF-801 and ZrTFS. Due to the difficulties in modeling a defective MOF, an intermediate structure containing both linkers in the framework was also designed. In this structure, the preferential CO(2) adsorption site is the tetrahedral pore in the “UiO-66-like” structure. The extra energy stabilization stems from a hydrogen bond interaction between CO(2) and a hydroxyl group on the inorganic cluster.