Incorporation of a dioxomolybdenum(VI) complex in a Zr-IV-based Metal-Organic Framework and its application in catalytic olefin epoxidation

Abstract

The complex [MoO2Cl2(bpydc)] (1) (H(2)bpydc = 2,2'-bipyridine-5,5'-dicarboxylic acid) has been incorporated into a Zr-IV-based MetalOrganic Framework (UiO-67) by partial replacement of 4,4'-biphenyldicarboxylic acid (H(2)bpdc) in the solvothermal synthesis by the complex [MoO2Cl2(H(2)bpydc)]. The resultant material, designated as UiO-67-MoO2Cl2(bpydc) (3), was isotypical with the parent UiO-67 framework as shown by powder X-ray diffraction (PXRD). MOF 3 was also characterised by elemental and thermogravimetric analyses, FT-IR spectroscopy and C-13{1H} MAS NMR, N-2 adsorption, and scanning electron microscopy. MOF 3 catalyses the epoxidation of cis-cyclooctene (Cy8) and limonene (Lim) with tert-butylhydroperoxide as oxidant. When using a,a,a-trifluorotoluene as cosolvent at 75 degrees C, very good epoxide selectivity was observed: 100% for Cy8 at 97% conversion, and 90% for Lim at 67% conversion. The catalyst was recovered, characterised, and used in consecutive batch runs. Drops in catalytic activity between runs were attributed to catalyst deactivation (caused by structural breakdown) rather than molybdenum leaching. Stability tests showed that protic media (due to the solvent and/or oxidant) and higher reaction temperatures (5575 degrees C) prompted the loss of crystallinity and structural degradation. The related MOF UiO-67-bpdc (2) is more stable than 3. The reduced stability of 3 may be related to the incorporation of the bpydc(2)- organic linker in the framework, as well the existence of a high number of missing-linker defects. (C) 2014 Elsevier Inc. All rights reserved.