Introduction of Sb into Trigonal Mo3VOx Oxide and Its Crystal Transformation to Orthorhombic Mo3VOx By Heat Treatment

Abstract

Sb was introduced into trigonal Mo₃VO_x (Tri-MoVO) by an ion exchange under acidic condition and the obtained solid was heat-treated. Sb was located mainly at heptagonal channels and partly at hexagonal channels in the Tri-MoVO structure after the ion exchange. Without the introduction of Sb, Tri-MoVO was collapsed by the heat-treatment at 550 °C under N₂ flow, resulting in the formation of MoO₃, (V_0.07Mo_0.93)₅O₁₄ and (V_0.97Mo_0.95)O₅. On the other hand, Tri-MoVO containing Sb was transformed to orthorhombic Mo₃VO_x (Orth-MoVO) structure by the above heat-treatment. Since Sb was located mainly at the hexagonal channel of the Orth-MoVO structure after the phase transformation, we concluded that the phase transformation proceeds as to transform the heptagonal channel of Tri-MoVO structure to the hexagonal channel of Orth-MoVO structure. Thus obtained orthorhombic MoVSbO showed substantial catalytic activity for ammoxidation of propane toward acrylonitrile.

Introduction

Mo-V-Te(Sb)-Nb oxide (MoVTe(Sb)NbO) is well known as an extremely active catalyst for selective (amm)oxidation of light alkanes¹. MoVTe(Sb)NbO is formed by an appropriate heat-treatment of an amorphous solid comprised of the constituent elements. Much effort has been devoted for a long time in order to clarify this crystal formation process since the clear understanding of the transformation process may allow the rational crystal design of MoVTe(Sb)NbO for further improvement. However, no clear conclusion has been obtained up to the date, because the amorphous nature of the MoVTe(Sb)NbO precursor is not allowed to provide clear characterization results during this crystal transformation process. Crystalline Mo₃VO_x (MoVO) has several crystal phases, including orthorhombic phase (Orth-MoVO) and trigonal phase (Tri-MoVO). Among the crystal phases, Orth-MoVO has identical crystal framework with MoVTe(Sb)NbO. Recently, we found that the crystal structure of Tri-MoVO can transform into the Orth-MoVO structure by the heat-treatment under the heat-treatment condition normally employed to form MoVTe(Sb)NbO when Sb was introduced into Tri-MoVO structure. Since the phase transformation from the Tri-MoVO structure to the Orth-MoVO structure proceeded in the crystal to crystal manner, detail investigations would be possible to clarify this phase transformation mechanism. Due to the similarity of the phase transformation between MoVTe(Sb)NbO precursor to MoVTe(Sb)NbO and Tri-MoVO to Orth-MoVO in terms of the crystal structure and the heat treatment condition, some insights would be obtainable for the crystal formation of MoVTe(Sb)NbO through the investigation of the phase transformation mechanism from Tri-MoVO to Orth-MoVO. Here, we deeply investigated the above phase transformation mechanism in order to gain insights about the crystal formation process of MoVTe(Sb)NbO for further improvement of the catalytic activity.

Experimental

Tri-MoVO was prepared by the reported procedure². Sb was introduced into Tri-MoVO by ion exchange method under acidic solution. First, the desired amount of Sb₂O₃ (Sb:0.144 mmol) was completely dissolved in 80 mL of diluted HCl solution (1.2 M) at 60 °C. Then, 0.5 g of Tri-MoVO was dispersed in the Sb containing solution, followed by the stirring for 3 h at the same temperature. After the filtration, obtained solids were abbreviated as xSb, where x indicates 100×Sb/Mo ratio measured by ICP. Heat-treatment of xSb was conducted at 550 °C for 2 h under 50 mL min^-1 of N₂ flow

Results and discussion

First, we investigated the relationship between the preparative Sb/Mo ratio and the Sb/Mo ratio of the obtained solid measured by ICP. The amount of Sb in the solid was increased with the increase of the preparative Sb/Mo ratio. However, no further increase in the solid Sb/Mo ratio was observed when preparative Sb/Mo ratio reached to 0.065. This value corresponds one Sb atom upon a unit cell of the Tri-MoVO structure. Then, in situ XRD experiments were carried out for xSb. Before the heat-treatment, all of the xSb showed the XRD peaks at 2θ = 4.7°, 9.0° attributable to the diffraction of (100) and (110) planes of the Tri-MoVO structure. With increasing the temperature under N₂ flow, the XRD peak intensity attributable to the (100) diffraction decreased which implied the increased electron density around the center of the three heptagonal channel sites of the Tri-MoVO structure³. 0Sb (original Tri-MoVO) transformed into MoO₃ or (Mo_0.93V­_0.07)₅O₁₄ by the further heating. On the other hand, additional diffraction peaks were observed at 2θ = 6.7°, 7.9° and 9.0° when Sb was introduced into the Tri-MoVO structure. These peaks were attributable to the diffraction of (010), (120) and (210) planes of the Orth-MoVO structure. The solid Sb/Mo ratio significantly affected this phase transformation since (1) the purity of the Orth-MoVO structure was improved and (2) phase transformation temperature was decreased with the Sb/Mo ratio. The introduction of Sb was found to be crucial for the phase transformation to the Orth-MoVO structure. Through the adsorption experiments, Rietveld analysis and DFT calculation, it was found that Sb was located mainly at the heptagonal channel and partly at the hexagonal channel of the Tri-MoVO structure, while Sb in the Orth-MoVO structure was placed mainly at the hexagonal channel and partly at the heptagonal channel. In order to further make sure this phase transformation mechanism, HAADF-STEM analysis was carried out for 6.2Sb with or without the heat treatment. During the phase transformation, the increased electron density at the center of the three heptagonal channels of the Tri-MoVO structure was observed possibly due to the location of Sb, while the location of Sb at the hexagonal channel was observed after the phase transformation to the Orth-MoVO structure. Based on the above facts, we concluded that the phase transformation proceeds as to transform the heptagonal channel of the Tri-MoVO structure into the hexagonal channel of the Orth-MoVO structure. Then, ammoxidation of propane was carried out over Tri-MoVO and 6.2Sb heat treated either at 400 °C or 550 °C (6.2Sb-NT400 or 6.2Sb-NT550, respectively). After the catalytic reaction, Tri-MoVO and 6.2Sb-NT400 had the Tri-MoVO structure, while 6.2Sb-NT550 had the Orth-MoVO structure. By the comparison between Tri-MoVO and 6.2Sb-NT400, it was clear that the introduction of Sb significantly improved the acrylonitrile (AN) selectivity. The phase transformation from the Tri-MoVO structure to the Orth-MoVO structure not only enhanced the AN selectivity but also substrates conversion.

Conclusions

Tri-MoVO was collapsed to MoO₃, (V_0.07Mo_0.93)₅O₁₄ and V_0.95Mo_0.97O₅ by the heat-treatment at 550 °C under N₂ flow. However, Tri-MoVO containing Sb was transformed to Orth-MoVO structure by the same heat-treatment. The phase transformation was found to proceed as to transform the heptagonal channel of Tri-MoVO structure into hexagonal channel of Orth-MoVO structure. Due to the similarity of the phase transformation between the MoVTe(Sb)NbO precursor to the MoVTe(Sb)NbO and the Tri-MoVO structure to the Orth-MoVO structure, we suggest that Te or Sb being located at the heptagonal channel moiety in the MoVTe(Sb)NbO precursor plays crucial role for the crystal formation of the MoVTe(Sb)NbO as to stabilize the crystal structure during the heating condition.

References

1. R. K. Grasselli, Catal. Today 99 (2005) 23.
2. M. Sadakane, N. Watanabe, T. Katou, Y. Nodasaka, W. Ueda, Angew. Chem. Int. Ed. 46 (2007) 1493.
3. Ishikawa, S.; Murayama, T.; Kumaki, M.; Tashiro, M.; Zhang, Z.; Yoshida, A.; Ueda, W. Top. Catal. 2016, 59, 1477–1488.