Carbon dioxide–sulphur hexafluoride adsorption and separation with zirconium metal–organic frameworks bearing basic and fluorinated linkers

This study describes the synthesis and characterization of two novel zirconium-based mixed-linker metal–organic frameworks (MIXMOFs) of the UiO-6x family (fcu topology) containing basic and fluorinated linkers together in the same lattice: Zr_TFA_NH2, built with 2-aminoterephthalic acid (H2BDC-NH2) and trifluoroacetic acid (HTFA) with minimal formula [Zr6O4(OH)4(TFA)4.2(BDC-NH2)3.9] and Zr_TFA_PyPy, containing 2,2′-bipyridyl-4,4′-dicarboxylic acid (H2PyPy) and HTFA with minimal formula [Zr6O4(OH)4(TFA)1.8(PyPy)5.1]. The linkers are cheap and commercially available; they were selected to control pore size and functional group distribution: H2BDC-NH2 is a short linker and provides UiO-66-like smaller pores, while H2PyPy is a longer linker and generates larger UiO-67-like pores. TFA− acts both as modulator and fluorinated functional group source. The two MIXMOFs have been exploited for the adsorption and separation of carbon dioxide (CO2) and sulfur hexafluoride (SF6), two potent greenhouse gases. Isosteric heats of CO2 adsorption (Qst) are higher than those found in their non-fluorinated analogues, proving the synergistic adsorption enhancement from combining basic and fluorinated linkers in the same solid. SF6/CO2 selectivity values derived from competitive adsorption experiments of equimolar mixtures at ambient temperature and pressure indicated that Zr_TFA_NH2 is more efficient than Zr_TFA_PyPy in separating the two gases. Density functional theory (DFT) calculations were performed to identify primary adsorption sites and estimate binding energies; the calculated adsorption energies agree well with the experimental Qst values.