Partially fluorinated MIL-47 and Al-MIL-53 frameworks: influence of functionalization on sorption and breathing properties

Abstract

Two perfluorinated metal hydroxo terephthalates [M^III(OH)(BDC-F)]·n(guests) (M^III = V, MIL-47-F-AS or 1-AS; Al, Al-MIL-53-F-AS or 2-AS) (BDC-F = 2-fluoro-1,4-benzenedicarboxylate; AS = as-synthesized) have been synthesized by a hydrothermal method using microwave irradiation (1-AS) or conventional electric heating (2-AS), respectively. The unreacted or occluded H₂BDC-F molecules can be removed under vacuum by direct thermal activation or exchange of guest molecules followed by thermal treatment leading to the empty-pore forms of the title compounds [V^IV(O)(BDC-F)] (MIL-47-F, 1) and [Al^III(OH)(BDC-F)] (Al-MIL-53-F, 2). Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments indicate that the compounds are stable up to 385 and 480 °C, respectively. Both of the thermally activated compounds exhibit significant microporosity, as verified by N₂, CO₂, n-hexane, o- and p-xylene sorption analyses. The structural changes of 2 upon adsorption of CO₂, n-hexane, o- and p-xylene were highly influenced due to functionalization by –F groups, as compared to parent Al-MIL-53. The –F groups also introduce a certain degree of hydrophobicity into the frameworks, as demonstrated by the H₂O sorption analyses.