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

Synthesis of two difluorinated metal hydroxo terephthalates [M-III(OH)(BDC-F-2)center dot n(guests) (M-III = V, MIL-47-F-2-AS or 1-AS; Al, Al-MIL-53-F-2-AS or2-AS) (BDC-F-2 = 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 [V-IV(O)(BDC-F-2] (MIL-47-F-2, 1) and [Al-III(OH)(BDC-F-2)] (Al-MIL-53-F-2, 2) have been prepared after removal of the unreacted or guest H2BDC-F-2 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 degrees C, 1; 480 degrees C, 2) has been revealed by thermogravimetric analysis. N-2, CO2 and n-hexane sorption analyses confirm that the thermally activated compounds possess noticeable microporosity. An ideal CO2/N-2 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 degrees C and P/P-0 = 0.5 exceeds that of parent MIL-47 owing to the enhanced hydrophobicity achieved through fluorination of their frameworks. (c) 2013 Elsevier Inc. All rights reserved.