Isomorphous Substitution in a Flexible Metal鈥揙rganic Framework: Mixed-Metal, Mixed-Valent MIL-53 Type Materials

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• 作者：Matthew I. Breeze ; Guillaume Clet ; Betiana C. Campo ; Alexandre Vimont ; Marco Daturi ; Jean-Marc Gren猫che ; Andrew J. Dent ; Franck Millange ; Richard I. Walton
• 刊名：Inorganic Chemistry
• 出版年：2013
• 出版时间：July 15, 2013
• 年：2013
• 卷：52
• 期：14
• 页码：8171-8182
• 全文大小：764K
• 年卷期：v.52,no.14(July 15, 2013)
• ISSN：1520-510X

文摘

Mixed-metal iron鈥搗anadium analogues of the 1,4-benzenedicarboxylate (BDC) metal鈥搊rganic framework MIL-53 have been synthesized solvothermally in N,N鈥?dimethylformamide (DMF) from metal chlorides using initial Fe:V ratios of 2:1 and 1:1. At 200 掳C and short reaction time (1 h), materials (Fe,V)^II/IIIBDC(DMF_1鈥?i>xF_x) crystallize directly, whereas the use of longer reaction times (3 days) at 170 掳C yields phases of composition [(Fe,V)^III_0.5(Fe,V)_0.5^II(BDC)(OH,F)]^{0.5鈥?/sup>路0.5DMA+ (DMA = dimethylammonium). The identity of the materials is confirmed using high-resolution powder X-ray diffraction, with refined unit cell parameters compared to known pure iron analogues of the same phases. The oxidation states of iron and vanadium in all samples are verified using X-ray absorption near edge structure (XANES) spectroscopy at the metal K-edges. This shows that in the two sets of materials each of the vanadium and the iron centers are present in both +2 and +3 oxidation states. The local environment and oxidation state of iron is confirmed by 57Fe M枚ssbauer spectrometry. Infrared and Raman spectroscopies as a function of temperature allowed the conditions for removal of extra-framework species to be identified, and the evolution of 渭₂-hydroxyls to be monitored. Thus calcination of the mixed-valent, mixed-metal phases [(Fe,V)III_0.5(Fe,V)_0.5II(BDC)(OH,F)]0.5鈥?/sup>路0.5DMA+ yields single-phase MIL-53-type materials, (Fe,V)III(BDC)(OH,F). The iron-rich, mixed-metal MIL-53 shows structural flexibility that is distinct from either the pure Fe material or the pure V material, with a thermally induced pore opening upon heating that is reversible upon cooling. In contrast, the material with a Fe:V content of 1:1 shows an irreversible expansion upon heating, akin to the pure vanadium analogue, suggesting the presence of some domains of vanadium-rich regions that can be permanently oxidized to V(IV).}