The Kinetics and Model of Selective Oxidation of Propylene On Bismuth Vanadium Molybdenum Oxide Catalysts

The selective catalytic oxidation of lower olefins to produce α,β-unsaturated aldehydes is widely used industrially. One of the largest scale processes is the selective oxidation of propylene to acrolein.[1] Since the discovery of bismuth molybdate-based catalysts in the 1950s, a large number of multi-component bismuth molybdates have been developed. However, it is still difficult to explain why multicomponent bismuth molybdate catalysts demonstrate superior activity and selectivity relative to bismuth molybdate. One of the scheelite-type multicomponent bismuth molybdate catalysts, Bi_1-x/3V_1-xMo_xO₄, is more active and selective for conversion of propylene to acrolein than bismuth molybdate or bismuth vanadate. In this work, a series of Bi_1-x/3V_1-xMo_xO₄ catalysts were prepared and investigated with the aim of understanding how composition affects catalyst activity and selectivity. XRD data showed that solid solution was formed for all compositions. X-ray absorption near edge spectroscopy (XANES) was used to characterize metal oxidation state. XANES data showed that Bi³⁺ was not reduced but Mo⁶⁺ and V⁵⁺ were reduced during the reaction.  Measurements of reaction kinetics were performed on Bi_1-x/3V_1-xMo_xO₄ catalysts. For Bi₂Mo₃O₁₂ and BiVO₄, acrolein production follows first order dependence of C₃H₆ and zero dependence of O₂. For intermediate compositions, partial pressure dependence of C₃H₆ and O₂ were found to be substantially dependent on temperature. The activation energies measured at low and high temperature were also different. The difference may be attributed to the change of the rate-determining step (RDS). At low temperature, the RDS is hypothesized to be re-oxidation of lattice oxygen; however, at high temperature an initial abstraction of a hydrogen atom becomes the RDS.  A model for the catalyst activity was developed that accounts for changes in catalyst composition. According to the model, vanadium and molybdenum are randomly distributed to form three types of sites with different reaction properties. The model matches the experimental results very well. Mo-V site are found to exhibit the highest activity. Since vanadium affects the ease of H abstraction, V-V and Mo-V sites have lower values of E_app compared with Mo-Mo site. The manner in which the electronic and hence, the chemical properties of Bi_1-x/3V_1-xMo_xO₄are influenced by the properties of molybdenum and vanadium will be discussed.

[1] Keulks, G.W., Krenzke L. D., Notermann, T. M. Advances in Catalysis, 1979, 27, 183.