Process for the manufacture of maleic anhydride

In a process for manufacturing maleic anhydride by the vapor phase catalytic oxidation of n-butane with molecular oxygen, an improvement which comprises using a catalyst prepared by mixing vanadyl hydrogen orthophosphate represented by the formula VOHPO.sub.4 .multidot.0.5H.sub.2 O (I), and at least one compound selected from the group consisting of a divalent metal hydrogen orthophosphate represented by the formula MHPO.sub.4 .multidot.nH.sub.2 O (II) wherein M is a divalent metal selected from the group consisting of Mg, Ca, Ba, Mn, Fe, Co, Ni, Cu and Zn and n is a number of coordinated water which differs depending on the temperature at which the compound is prepared, a divalent metal ammonium orthophosphate represented by the formula NH.sub.4 MPO.sub.4 .multidot.nH.sub.2 O (III) wherein M and n have the same meanings as defined above and a divalent metal orthophosphate represented by the formula M.sub.3 (PO.sub.4).sub.2 .multidot.nH.sub.2 O (IV) wherein M and n have the same meanings as defined above, with the atomic ratio of M/V (M has the same meaning as defined above) of 0.003-0.5, by calcining the mixture at 300.degree.-400.degree. C., and then pretreating said catalyst in an inert gas or a mixed gas containing n butane and molecular oxygen at 350.degree.-700.degree. C.