264942 The Kinetics and Model of Selective Oxidation of Propylene On Bismuth Vanadium Molybdenum Oxide Catalysts

Thursday, November 1, 2012: 8:30 AM
321 (Convention Center )
Zheng Zhai, Andrew (Bean) Getsoian and Alexis T. Bell, Department of Chemical and Biomolecular Engineering, University of California - Berkeley, Berkeley, CA

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, Bi1-x/3V1-xMoxO4, is more active and selective for conversion of propylene to acrolein than bismuth molybdate or bismuth vanadate. In this work, a series of Bi1-x/3V1-xMoxO4 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 Bi1-x/3V1-xMoxO4 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 Eapp compared with Mo-Mo site. The manner in which the electronic and hence, the chemical properties of Bi1-x/3V1-xMoxO4are 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.

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