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

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.