Preparation and Characterization of Cs_2.8H_1.2PMo₁₁Fe(H₂O)O₃₉·6H₂O and Investigation of Effects of Iron-Substitution on Heterogeneous Oxidative Dehydrogenation of 2-Propanol

Synthesis and characterization of mono-iron(III)-substituted molybdophosphate, the solidification as a heterogeneous catalyst, and the oxidative dehydrogenation of 2-propanol were reported. The catalyst was isolated as Cs_2.8H_1.2PMo₁₁Fe(H₂O)O₃₉·6H₂O, and characterized by elemental analysis and X-ray diffraction, infrared, ³¹P NMR, and ESR spectroscopy. The Fe³⁺ in Cs_2.8H_1.2PMo₁₁Fe(H₂O)O₃₉·6H₂O was incorporated into the molybdophosphate framework while that in Fe³⁺(2.5 wt %)/Cs_3.0PMo₁₂O₄₀ existed as a countercation in a relatively distorted octahedral site. The seven water molecules were desorbed by the thermal treatment at 63 °C and the cesium hydrogen salt was stable below 210 °C. The cesium hydrogen salt was used for heterogeneous oxidative dehydrogenation of 2-propanol to acetone and intrinsically has a higher rate than those for the iron-impregnated Fe³⁺/Cs_3.0PMo₁₂O₄₀ and Cs_3.0PMo₁₂O₄₀ catalysts, showing the effectiveness of isolated iron in the PMo₁₁O₃₉^7- polyoxometalate on the oxidative dehydrogenation. The data for the stop of the supply of the oxygen at the stationary state in the flow experiment showed that acetone was produced by the reaction of 2-propanol with the cesium hydrogen salt. The correlation between intrinsic rates of oxidative dehydrogenation of 2-propanol and reducibility of catalysts, the rate equation of −d[2-PrOH]/dt = k·P_2-PrOH^0.80·PO₂^-0.06, and kinetic isotope effects of 1.6−1.9 showed that the reduction of the catalyst with the β-hydrogen elimination from 2-propanol was the rate-determining step.