αII-(V1-xWx)OPO4 catalysts for the selective oxidation of n-butane to maleic anhydride.

α II -V 1- x M x OPO 4 (M = W,Mo). • Solution combustion synthesis (SCS) leads to a variety of phase pure catalysts. • Doping with Mo and W tunes the oxidation state of V in VOPO 4 between 4+ and 5+. • α II -(V 1- x W x)OPO 4 reaches selectivities of maleic anhydride of more than 30%. • α II type solid solutions are a flexible platform for further research. The vanadyl pyrophosphate (VPP) based catalyst is unique in converting n -butane selectively (60–70%) into maleic anhydride (MAN), whereas a MAN selectivity of 20% may be regarded as high for structurally different catalyst systems. We present novel vanadium phosphorus oxides and mixed metal phosphate solid solutions tested for n -butane oxidation to MAN with a selectivity of >30%. The majority of the catalysts were prepared by solution combustion synthesis. (V 1- x W x)OPO 4 with α II structure was found to be more active and selective in the oxidation of n -butane compared to β -VOPO 4. By adjusting the tungsten content the oxidation state of vanadium in (V 1- x W x)OPO 4 can be tuned between 4.74 and 4.99, which is regarded as a key factor for MAN production. All catalysts were structurally stable, but the specific surface area increased during the reaction, as detected by X-ray diffraction and N 2 physisorption, respectively. (V 1- x Mo x)OPO 4 was also stable, but the MAN selectivity was lower compared to β -VOPO 4. Low conversions result from the low surface area of the screening samples, however, could be overcome by advanced synthesis protocols. [ABSTRACT FROM AUTHOR]