Partially fluorinated MIL-101(Cr): from a miniscule structure modification to a huge chemical environment transformation inspected by 129Xe NMR

Abstract

The partial functionalisation of MIL-101(Cr) with fluorine was successfully achieved, for the first time, with 2,3,5,6-tetrafluoro-1,4-benzenedicarboxylate (BDC-4F) affording MIL-101(Cr)-4F(1%). As a result of the functionalisation the acidity of the metal centres for MIL-101(Cr)-4F(1%) was enhanced, which was measured by cyclic voltammetry. The resulting adsorption properties of the new material were explored and compared to those of MIL-101(Cr). H₂O sorption experiments demonstrated an augmented hydrophobicity of the material due to the incorporation of fluorine atoms. CO₂ adsorption experiments exhibited an augmented CO₂ interaction energy (ΔH) for MIL-101(Cr)-4F(1%). The diffusion time constant for CO₂ showed a higher CO₂ mobility inside the pores of MIL-101(Cr) than in MIL-101(Cr)-4F(1%), which was also associated with a higher selectivity (α) of MIL-101(Cr)-4F(1%) towards CO₂. O₂ sorption isotherms of MIL-101(Cr)-4F(1%), at a high pressure (up to 90 bar), demonstrated O₂ uptakes among the highest values reported for a MOF material. H₂ adsorption isotherms exhibited, at low pressures, higher H₂ uptakes for the fluorine functionalised material. This was attributed to dipolar interactions of H₂ molecules with the fluorine atoms and corroborated by the calculation of ΔH for H₂. ¹²⁹Xe NMR experiments were used to investigate the atypical pore-surface electron density of MIL-101(Cr)-4F(1%), demonstrating a huge effect of the polarising power of the fluorine atoms on the highly polarisable Xe atoms. Finally, the capture of I₂ and H₂S on MIL-101(Cr)-4F(1%) showed considerably higher values in comparison to MIL-101(Cr). This was attributed to the high dipolarity and polarisability of the fluorinated pores.