EPR of vanadium in Ce0.25H3PVMo11O40 catalyst. Formation of vanadyl dimers during the oxidative dehydrogenation of isobutyric acid

Abstract

The catalytic oxidative dehydrogenation of isobutyric acid (IBA) to methacrylic acid (MAA) over the heteropolyacid Ce_0.25H₃PVMo₁₁O₄₀ has been studied. After catalysis, the vanadium ions are ejected from the Keggin structure as vanadyl dimers. Such species remain stable at room temperature in the presence of water. The presence of cerium as a counter-cation appears to promote dimer formation. The distance between the two VO²⁺ ions in the dimer has been estimated to be 4.31 Å. The dimers are located on oxygen vacancies near Ce⁴⁺ ions. The hyperfine structure of the half magnetic field signal obtained was well resolved. 15 lines with relative intensities: 1, 2, 3, 4, 5, 6, 7, 8, 7, 6, 5, 4, 3, 2, 1 have been observed.