(VO) 2 P 2 O 7 is oxidized to β-VOPO 4 via β′-phase (d= 4.87, 4.23, 3.19, 2.64 Å) which was probably a phosphate such as VPO 4.75 containing vanadium in the average oxidation state of+ 4.5. However, when excess phosphorus is present in (VO) 2 P 2 O 7, oxidation stops at the stage of β′-phase. This suggests that the new phase, β′-VPO 4.75, is a selective catalyst for n-butane oxidation to maleic anhydride.
