Synthesis of Mo–Cr Mixed Metal Oxide Catalyst Based on Pentagonal Unit Assembly and Selective Oxidation of Ethanol

Mo–Cr mixed metal oxide consisting of {Mo6O21}6− pentagonal unit and {CrO6} octahedral, namely High-Dimensionally-Structured Mo–Cr mixed metal oxide (HDS-MoCrO), was synthesized by hydrothermal synthesis. Under the hydrothermal conditions, Anderson-type polyoxometalate, (NH4)3H6CrMo6O24 · 7H2O ({CrMo6}), was formed in the early stage of the hydrothermal synthesis. Then, Keplerate-type polyoxometalate, Mo72Cr30O252 ({Mo72Cr30}), was formed, followed by the formation of HDS-MoCrO. Considering that {Mo72Cr30} and HDS-MoCrO consist of the same structural unit, {Mo6O21}6− pentagonal unit, we concluded that {Mo72Cr30} is decomposed under the hydrothermal conditions to supply the pentagonal unit, which connects with {CrO6} octahedral to form HDS-MoCrO. HDS-MoCrO showed catalytic property for selective oxidation of ethanol and its activity was similar to those of the other HDS-oxides composed of {Mo6O21}6− pentagonal unit and {MoO6} (HDS-MoO) or {FeO6} (HDS-MoFeO). The product selectivity was different depending on the constituent elements, and a high acetaldehyde yield of 91 % was achieved over HDS-MoCrO for at least 60 h.