Abstract: Vanadyl pyrophosphate oxidation catalyst precursors comprise vanadium phosphate esters of the formula(VO)(((R.sub.1-x /H.sub.x)OPO.sub.3).multidot.((ROH).sub.y /(H.sub.2 O).sub.z)wherein, R is an alkyl group capable of bonding, through oxygen, to phosphorus to form a phosphate ester; x is 0 to about 0.5; y is 0-1.0; z is 0-1.25; and y+z is 0-1.25 are particularly useful for the selective oxidation of butane to maleic anhydride and readily thermally decompose to form the vanadyl pyrophosphate oxidation catalysts.

Abstract: An improved process for calcination and activation of a catalyst containing mixed oxides of vanadium and phosphorus (V/P/O) involving the addition of spray dried catalyst precursor to a fluidizing oxygen containing gas at a controlled rate such that the temperature is maintained at a range of 370 to 410.degree. C. (preferably 390.+-.5.degree. C.). The catalyst precursor experiences, in addition to rapid temperature rise, dehydration/calcination at this elevated temperature in the fluidized state for an average range of residence time of less than about 4 hours at 3.5 atm of air or above, and up to 20 hours at 1 atm of air while simultaneously and continuously maintaining an average V.sub.ox of 4.55 or less. Such a process is useful in preparing a V/P/O catalyst that exhibits enhanced activity for the oxidation of n-butane to maleic anhydride. The novel process also provides an improved method for start-up of a commercial scale recirculating solids reactor.