Influence of calcination conditions on the phase composition of vanadium-phosphorus oxide catalysts

The influence of calcination time and temperature on the formation of phases related to VPO catalysts with P/V ratios in the range 0.90 to 1.10 has been studied. In the preparation, lactic acid was used for the reduction of V₂O₅ and the reduced vanadium complex thus formed was reacted with o-H₃PO₄. The evolution of phases was followed by XRD. For calcinations at 773 K, low P/V ratios favour the formation of βVPO₅; high ratios (>1) favour the formation of a reduced phase labelled β^* (d = 4.67, 4.07, 3.14 and 2.59 Å). For all P/V ratios studied, this phase was not stable in air at 773 K but transformed slowly into βVPO₅. The β^*/βVPO₅ ratio did not influence the surface area. The β^* phase was not observed after calcination above 873 K. For calcination of catalysts with a P/V ratio of 1.10, carried out at 923 K, a reduced phase with principal d spacings at 3.90, 3.13 and 2.99 Å and variously labelled B, β and (VO)₂P₂O₇ in the literature, was observed.