Influence of small amounts of tungsten on the physico-chemical and catalytic properties of PMo11–xWxVO 4–40 heteropolyanions

Abstract

The preparation, characterization and catalytic properties of acids with the general formula H₄PMo_11–xW_xVO₄₀, x= 0–6, and their pyridinium salts are described. The ³¹P NMR spectra reveal that the solids synthesized are mixtures of species with different x values. Three distinct stoichiometric ratios of W : Mo (1 : 10, 2 : 9, 3 : 8) have been used to vary the proportion of each species in these solids (labelled y= 1, 2 and 3). Their thermal stabilities and redox properties in solution are close to those of pure H₄PMo₁₁ VO₄₀. Their Brønsted acidity increases in the order y= 1 < y= 2 < y= 3. Their catalytic properties for the oxydehydrogenation of isoubtyric acid methacrylic acid are modified drastically. An important decrease in oxidative activity and an increase in acid activity are observed even with small amounts of tungsten. Tungsten may modify either the electronic transfer between the Keggin units or the oxygen lability.