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

Abstract

The transformation of nano-sized VOHPO 4⋅0.5H 2O crystallites (340 nm × 40 nm) to (VO) 2P 2O 7 was examined. The crystalline structure of VOHPO 4⋅0.5H 2O quickly collapsed to form an oxidized amorphous phase within 1 h (reaction conditions: 1.5% n-butane, 17% O 2, and 81.5% He at 663 K), followed by gradual crystallization to (VO) 2P 2O 7, accompanied by the formation of sharply angular nano-sized crystallites (about 50 nm). No crystalline phases other than (VO) 2P 2O 7 were formed during this transformation. This is quite different from the transformation of large and thick VOHPO 4⋅0.5H 2O crystallites, in which α II-VOPO 4 and δ-VOPO 4 have been detected in the resulting catalyst. The selectivity to maleic anhydride increases during the transformation as a result of the formation of (VO) 2P 2O 7. 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.