Synthesis of an efficient VPO catalyst for the selective oxidation of n-butane to the MA product: Mechanism of crystal transformation
Serial VPO catalysts with different crystal structures were prepared by adjusting the ratios of air and N 2 in the calcination process. The precursors and vanadium phosphorus oxide (VPO) catalysts were characterized by various measurements. As the air and N 2 ratios were lower than 1:1, the ordered VOHPO 4 ·0.5H 2 O phase was mainly formed in the precursors during calcination. However, as the ratio of air to N 2 reached 3:1, amorphous VOPO 4 phases were formed over the surface of the precursor during calcination, which prevented the formation of ordered V5+ species over the surface of the VPO catalyst after activation. Meanwhile, a high proportion of VOHPO 4 ·0.5H 2 O in the precursor could facilitate more formation of the ordered α 1 -VOPO 4 phase over the surface of the VPO catalyst. Interestingly, the crystal structure over the surface of active VPO catalysts was totally different from the core of VPO catalysts. There were more P species and V species with 5+ existing over the surface of the VPO catalysts. The evaluation results of n-butane oxidation showed that when the ratio of air to N 2 was 1:3, the corresponding VPO-1A catalyst presented the highest conversion of n-butane and yield of the MA product (57.6 m% at 412 °C). The reason should be related to the fact that the VPO-1A catalyst possessed the highest acidity and proportions of surface V5+ species and Sur-O species. As the ratio of air to N 2 reached 3:1, the VPO-3A catalyst showed the poorest performance in the oxidation of n-butane, which could be due to the formation of inactive and amorphous VOPO 4 phases over the surface of the VPO catalyst. Finally, kinetic and thermodynamic models were established and calculated to investigate the characteristics of n-butane oxidation reactions for various VPO catalysts.
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