The dilution of vanadyl pyrophosphate, catalyst for n-butane oxidation to maleic anhydride, with aluminum phosphate: unexpected reactivity due to the contribution of the diluting agent

This paper describes the preparation, characterization and reactivity in n-butane oxidation of catalysts made of vanadyl pyrophosphate (VPP) diluted in aluminum phosphate. Catalysts were prepared by synthesizing at the same time the VPP precursor and AlPO₄, in order to develop mixed systems, suitable for the conglomeration into fluidizable particles by spray-drying. Al phosphate was chosen due to its presumed chemical inertness towards the active phase and towards n-butane. No evidence was obtained for the formation of mixed V/Al phosphates; an homogeneous reciprocal dispersion of the two compounds developed. However, Al phosphate showed an unexpected activity in n-butane oxidation; specifically, in the temperature range comprised between 340 and 400 °C, the hydrocarbon was converted to formaldehyde and carbon monoxide by radical-like reactions. The activity of Al phosphate became negligible above 400 °C, in the temperature range where instead VPP was active in hydrocarbon oxidation and selective to maleic anhydride. Moreover, Al phosphate was also responsible for the consecutive oxidative degradation of maleic anhydride, at high temperature.