n-Butane oxidation on V-P-O catalysts. Influence of alkali and alkaline-earth metal ions as additions

The mechanism by which the introduction of some elements modifies principal properties of the V-P-O system in oxidation of n-butane to maleic anhydride has been studied. The incorporation of Li, Na, K, Cs, Be, Mg, Ca, Ba at different concentrations which can easily donate electrons to the framework of vanadyl phosphate with P/V ratio = 1.07 and 1.20, leads to an increase of the effective negative charge on oxygen atom and of the rate of butane oxidation. The presence of additives causes an increase of the surface P/V ratio and corresponding changes of acidic properties of the catalysts. Selectivity towards maleic anhydride passes through a maximum when plotted as a function of the amount of acidic centres at the surface. The preparation of a catalyst characterised by high activity in butane oxidation and high selectivity to maleic anhydride requires a fine tuning of the basicity of surface oxygen atoms to accelerate the activation of butane and of the acidity of the surface to secure the appropriate residence time of the reaction intermediates.