Catalysis of the oxidation of isobutyric acid by vanadyl, copper and mixed vanadyl-copper salts of H-3[PMo12O40] and H-4[PMo11VO40]

Effects of variable amounts of vanadyl or/and copper counter-ions of the heteropolyanions [PMo12O40](3-) and [PMo11VO40](4-) on the catalytic oxidative dehydrogenation of isobutyric acid are reported. For the molybdophosphate. selectivity to methacrylic acid increases continuously with respect to the quantity of vanadyl at the expense of both acetone and propene. On the opposite, copper cations does not modify the selectivity to methacrylic acid. A synergy between the two cations is observed, which is maximal when they are in equal quantities. For the vanadomolybdophosphate, vanadyl alone or vanadyl and copper have a positive effect on the selectivity to methacrylic acid only if there is less than one metal cation per heteropolyanion. This is related to the transformation of the catalyst during the first stage of the reaction: vanadium is reduced by isobutyric acid and expelled from the Keggin unit. For high amounts of vanadyl and/or copper cations, decrease of the activity and selectivity to methacrylic acid could be related to the formation of oxides.