The kinetics of selective oxidation of propene on bismuth vanadium molybdenum oxide catalysts

• The activity Bi1−x/3V1−xMoxO4 for propene oxidation to acrolein exhibits a maximum at x = 0.45.
• XANES measurements reveal that only Mo and V undergo reduction during reaction, but not Bi.
• The observed pattern of activity with catalyst composition can be interpreted in terms of three types of active sites.
• Rate parameters for each type of active site are determined and then used to describe the overall activity of Bi1−x/3V1−xMoxO4.

We report the results of a systematic investigation of the kinetics of propene oxidation to acrolein over Bi1−x/3V1−xMoxO4. BET isotherms were measured to determine catalyst surface area, and powder X-ray diffraction was used to characterize the bulk structure. Further characterization by X-ray absorption near-edge spectroscopy (XANES) was used to determine the oxidation states of Bi, Mo, and V before and after exposure of the catalyst to propene at 713 K. We find that, contrary to previous discussions of the mechanism of propene oxidation on Bi1−x/3V1−xMoxO4, Bi remains in the 3+ state and only V and Mo undergo reduction and oxidation during reaction. The kinetics of propene oxidation were examined to establish the activation barrier for acrolein formation, and how the partial pressure dependences on propene and oxygen change with the value of x. The data obtained from this study were then used to propose a generalized model for the kinetics of propene oxidation over Bi1−x/3V1−xMoxO4 that is consistent with our findings about the reducibility of the three metallic elements in the oxide. According to this model, vanadium and molybdenum are randomly distributed to form three types of sites each associated with its own rate parameters. MoV sites are found to exhibit the highest activity. The proposed model provides a good description of the experimental data for all catalyst formulations examined, for a range of propene and oxygen partial pressures, and for temperatures above 653 K.

The oxidation of propene to acrolein on Bi1−x/3V1−xMoxO4 was investigated in a systematic manner to identify the role of composition on catalyst activity and selectivity. The apparent rate coefficient kapp exhibits a maximum for 0 < x < 1. This pattern is very well described by a model involving three types of active sites.Figure optionsDownload high-quality image (86 K)Download as PowerPoint slide