Transformation of nano-sized vanadyl hydrogen phosphate hemihydrate crystallites to vanadyl pyrophosphate during activation in the presence of n-butane and oxygen

The transformation of nano-sized VOHPO4⋅0.5H2O crystallites (340 nm × 40 nm) to (VO)2P2O7 was examined. The crystalline structure of VOHPO4⋅0.5H2O quickly collapsed to form an oxidized amorphous phase within 1 h (reaction conditions: 1.5% n-butane, 17% O2, and 81.5% He at 663 K), followed by gradual crystallization to (VO)2P2O7, accompanied by the formation of sharply angular nano-sized crystallites (about 50 nm). No crystalline phases other than (VO)2P2O7 were formed during this transformation. This is quite different from the transformation of large and thick VOHPO4⋅0.5H2O crystallites, in which αII-VOPO4 and δ-VOPO4 have been detected in the resulting catalyst. The selectivity to maleic anhydride increases during the transformation as a result of the formation of (VO)2P2O7. The activity increases in the early stages of the transformation, then decreases with time. The decrease in activity is caused by gradual reduction of the catalyst surface during the transformation.