Investigation of acid sites in a zeotypic giant pores chromium(III) carboxylate.

A study of the zeotypic giant pores chromium(III) tricarboxylate Cr(III)3OF(x)(OH)(1-x)(H2O)2 x {C6H3-(CO2)3}2 x nH2O (MIL-100) has been performed. First, its thermal behavior, studied by X-ray thermodiffractometry and infrared spectroscopy, indicates that the departure of water occurs without any pore contraction and no loss in crystallinity, which confirms the robustness of the framework. In a second step, IR spectroscopy has shown the presence of three distinct types of hydroxy groups depending on the outgassing conditions; first, at high temperatures (573 K), only Cr-OH groups with a medium Brønsted acidity are present; at lower temperatures, two types of Cr-H2O terminal groups are observed; and at room temperature, their relatively high Brønsted acidity allows them to combine with H-bonded water molecules. Finally, a CO sorption study has revealed that at least three Lewis acid sites are present in MIL-100 and that fluorine atoms are located on a terminal position on the trimers of octahedra. A first result of grafting of methanol molecules acting as basic organic molecules on the chromium sites has also been shown, opening the way for a postsynthesis functionalization of MIL-100.