Structural Modifications Of V-P Mixed Oxides During Calcination In Air Or In A Mixture Of Butenes-Air

The chemistry of the preparation of vanadium-phosphorous mixed oxides catalysts active in the oxidation of the C 4 fraction to maleic anhydride was investigated. Different compounds can be obtained according to (i) P:V ratio; (ii) calcination temperature; (iii) calcination atmosphere; (iv) calcination rate. The presence of a small excess of phosphorous with respect to P:V ratio = 1.0 and a slow calcination were suitable to form the active and selective phase, a polyphosphate of V(IV) and to prevent its oxidation to ?, ?-VOPO 4. The active phase can be formed under the reaction conditions either from the precursor or from VOPO 4 and its formation does not depend on the V:P ratio.