Synthesis and Characterization of Metal-organic Framework-74 Containing 2, 4, 6, 8, and 10 Different Metals

Metal-organic frameworks (MOFs) containing more than two kinds of metal ions mixed in one secondary building unit are rare because the synthesis often yields mixed MOF phases rather than a pure phase of a mixed-metal MOF (MM-MOF). In this study, we use a one-pot reaction to make microcrystalline MOF-74 [M2(DOT); DOT = dioxidoterephthalate] with 2 (Mg and Co), 4 (Mg, Co, Ni, and Zn), 6 (Mg, Sr, Mn, Co, Ni, and Zn), 8 (Mg, Ca, Sr, Mn, Fe, Co, Ni, and Zn), and 10 (Mg, Ca, Sr, Ba, Mn, Fe, Co, Ni, Zn, and Cd) different kinds of divalent metals. The powder X-ray diffraction patterns of MM-MOF-74 were identical with those of single-metal MOF-74, and no amorphous phases were found by scanning electron microscopy. The successful preparation of guest-free MM-MOF-74 samples was confirmed by N2 adsorption measurements. Elemental analysis data also support the fact that all metal ions used in the MOF synthesis are incorporated within the same MOF-74 structure. Energy-dispersive X-ray spectroscopies indicate that metal ions are heterogeneously distributed within each of the crystalline particles. This approach is also employed to incorporate metal ions (i.e., Ca, Sr, Ba, and Cd) from which the parent MOF structure could not be made as a single-metal-containing MOF.