XRD and IR structural investigations of a particular breathing effect in the MOF-type gallium terephthalate MIL-53(Ga)

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Abstract
The gallium terephthalate Ga(OH)[O2C-C6H4-CO2]·xA (A = HO2C–C6H4–CO2H) was hydrothermally synthesized in water under mild conditions (210 °C, 3½ h) in the presence of terephthalic acid. The compound was characterized by powder X-ray diffraction, TGA, IR and BET method. This compound is isostructural to the MIL-53 type, previously observed with the trivalent cations Cr, Fe, Al, In. It exhibits a three-dimensional metal–organic framework built up from infinite chains of trans corner-sharing GaO4(OH)2 octahedra (viaμ2-hydroxo bonds) linked to each other through the terephthalate linkers. It results in the formation of lozenge-shape channels structure running parallel to the infinite files of gallium-centered octahedra. After activation, the compound is able to adsorb one molar equivalent of water at room temperature under ambient air (MIL-53(Ga){H2O}). Different hydrogen bond interactions are observed for the encapsulated water within the channels. In one tunnel, pairs of water species with strong hydrogen-bond interactions were observed whereas in the adjacent tunnel, only a continuous linear and weakly hydrogen bonded network occurs. The dehydrated form is obtained upon heating the MIL-53(Ga) solid at 80 °C together with the shrinkage of the channels (MIL-53(Ga)_lt. This form is stable up to 220 °C and then the open structure MIL-53(Ga)_ht is visible, but starts to decompose from 350 °C. Such a breathing effect was previously reported with cations such as Cr or Al but in the case of Ga, the stability domain of the narrow pore structure MIL-53(Ga)_lt is larger (160 °C instead of 20–30 °C for Al, for instance). The BET surface area was 1140 ± 114 m2.g−1. The phase transitions were characterized by IR spectroscopy at different temperatures, which confirms the stability domain of the narrow close form (specific band at 1016 cm−1) of MIL-53(Ga) and then the pore opening (shifted band toward 1024 cm−1) together with the structure collapse. An identical behaviour is also discussed for the aluminum MIL-53 analogue. A comparison between the behaviour of the Al, Ga and Fe samples is presented.