Sorption and breathing properties of difluorinated MIL-47 and Al-MIL-53 frameworks

• Difluorinated MIL-47 and Al-MIL-53 have been synthesized and characterized.
• Al-MIL-53-F2 shows an ideal CO2/N2 adsorption selectivity value of 11.
• Fluorination affects breathing of Al-MIL-53-F2 upon sorption of CO2 and n-hexane.
• Fluorination induces noticeable hydrophobicity to both difluorinated compounds.
• n-Hexane uptake of MIL-47-F2 exceeds that of un-functionalized MIL-47 at 50 °C.

Synthesis of two difluorinated metal hydroxo terephthalates [MIII(OH)(BDC-F2)]·n(guests) (MIII = V, MIL-47-F2-AS or 1-AS; Al, Al-MIL-53-F2-AS or2-AS) (BDC-F2 = 2,5-difluoro-1,4-benzenedicarboxylate; AS = as-synthesized) has been accomplished by a hydrothermal method using microwave irradiation (1-AS) or electric heating (2-AS), respectively. The empty-pore forms of the title compounds [VIV(O)(BDC-F2] (MIL-47-F2, 1) and [AlIII(OH)(BDC-F2)] (Al-MIL-53-F2, 2) have been prepared after removal of the unreacted or guest H2BDC-F2 molecules under vacuum by direct thermal treatment or solvent-exchange of occluded molecules followed by thermal activation. The high thermal stability of the compounds (340 °C, 1; 480 °C, 2) has been revealed by thermogravimetric analysis. N2, CO2 and n-hexane sorption analyses confirm that the thermally activated compounds possess noticeable microporosity. An ideal CO2/N2 selectivity value of 11 has been deduced for 2 from the single component adsorption isotherms. The breathing behavior of 2 upon adsorption of CO2 and n-hexane are similar to monofluorinated Al-MIL-53, but different from parent Al-MIL-53. H2O sorption analyses demonstrate that the fluorination induce a certain extent of hydrophobicity in the frameworks, similar to their monofluorinated counterparts. Remarkably, the n-hexane adsorption capacity of both mono- and difluorinated MIL-47 at 50 °C and p/p0 = 0.5 exceeds that of parent MIL-47 owing to the enhanced hydrophobicity achieved through fluorination of their frameworks.

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