Activity and selectivity in toluene oxidation on well characterized vanadium oxide catalysts

Abstract

Activity and selectivity in the oxidation of toluene on unsupported and supported V₂O₅ catalysts (V₂O₅/TiO₂ and V₂O₅/Al₂O₃) have been investigated in terms of the catalyst structure under the following conditions: total pressure, 1 atm (1 atm ≈ 101.3 kPa); partial pressure of toluene, 0.041 atm; partial pressure of O₂, 0–0.861 atm; temperature, 588–644 K. The results under various contact times indicated that partial oxidation to benzaldehyde, benzoic acid and maleic anhydride proceeds concurrently with the total oxidation to carbon oxides on the vanadium oxide catalyst. It was also found that the surface VO species plays the active oxygen species and that the activity is mainly determined by the number of surface VO species for both unsupported and supported catalysts. Although the selectivity to partial oxidation products (S_p) was independent of the oxidation state of the vanadium oxide, it was greatly affected by the roughness of the catalyst surface and rough V₂O₅ was favourable for the selective oxidation. This provides a new type of structure–selectivity correlation for the vanadium oxide and has never been observed for the oxidation of benzene, but-1-ene, buta-1,3-diene or furan. In agreement with the correlation, the mode of change in the selectivity with the structure of supported catalysts was much different from that for previous reactions and was explained in terms of the change in surface roughness. A mechanism is also proposed to explain the new structure–activity/selectivity correlation for the toluene oxidation.