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

Abstract

The gallium terephthalate Ga(OH)[O(2)C-C(6)H(4)-CO(2)].xA (A = HO(2)C-C(6)H(4)-CO(2)H) was hydrothermally synthesized in water under mild conditions (210 degrees C, 3(1/2) 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 GaO(4)(OH)(2) octahedra (viamu(2)-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){H(2)O}). 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 degrees C together with the shrinkage of the channels (MIL-53(Ga)_lt. This form is stable up to 220 degrees C and then the open structure MIL-53(Ga)_ht is visible, but starts to decompose from 350 degrees 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 degrees C instead of 20-30 degrees C for Al, for instance). The BET surface area was 1140 +/- 114 m(2).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.