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

摘要

We report the results of a systematic investigation of the kinetics of propene oxidation to acrolein over Bi_(1-x/3)V_(1-x)Mo_xO_4. 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 Bi_(1-x/3)V_(1-x)Mo_xO_4, 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 Bi_(1-x/3)V_(1-x)Mo_xO_4 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. MoAV 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.