New Functionalized Flexible Al-MIL-53-X (X = -Cl, -Br, -CH3, -NO2, -(OH)2) Solids: Syntheses, Characterization, Sorption, and Breathing Behavior

Five new flexible functionalized aluminum hydroxo terephthalates [Al(OH)(BDC-X)] center dot n(guests) (BDC = 1,4-benzene-dicarboxylate; X = -Cl, 1-Cl; -Br, 2-Br; -CH3, 3-CH3; -NO2, 4-NO2; -(OH2, 5-OH2) were synthesized under solvothermal conditions. The as synthesized (Al-MIL-53-X-AS) as well as the activated compounds were characterized by X-ray powder diffraction (XRPD), IR spectroscopy, thermogravimetric (TG), and elemental analysis. Activation, that is, removal of unreacted H2BDC-X molecules and/or occluded solvent molecules, followed by hydration in air at room temperature, led to the narrow pore (NP) form of the title compounds [Al(OH)(BDC-X)] center dot n(H2O) (Al-MIL-53-X). Thermogravimetric analysis (TGA) and temperature-dependent XRPD (TDXRPD) experiments performed on the NP-form of the compounds indicate high thermal stability in the range 325-500 degrees C. As verified by N-2, CO2, or H2O sorption measurements, most of the thermally activated compounds exhibit significant microporosity. Similar to pristine Al-MIL-53, the present compounds retain their structural flexibility depending on the nature of guest molecules and temperature, as verified by cell parameter determination from XRPD data. The breathing behavior of the functionalized frameworks upon dehydration-rehydration, investigated by temperature and time-dependent XRPD measurements, differs significantly compared to parent Al-MIL-53.