The catalytic oxidation of propylene: IX. The kinetics and mechanism over β-Bi 2Mo 2O 9

Abstract

The kinetics and mechanism of propylene oxidation over β-Bi 2Mo 2O 9 from 300 to 470 °C have been investigated. By using oxygen-18 and deuterated propylenes under steady-state reaction conditions and temperatures ranging from 350 to 450 °C, it was determined that the selective oxidation of propylene to acrolein over the β-phase occurs via the redox mechanism through the involvement of numerous sublayers of lattice oxygen. From the kinetic and isotopic data it was learned that the kinetics and energetics of propylene oxidation over the β-phase can be completely described in terms of the coupled kinetics of catalyst reduction and reoxidation. At the higher temperatures in which the rate of acrolein formation is limited by catalyst reduction, the apparent activation energy is approximately 20 kcal/mole and is indicative of allyl formation from adsorbed propylene. At lower temperatures the rate of acrolein formation is limited by catalyst reoxidation and the apparent activation energy is approximately 43 kcal/mole. The kinetic dependencies of oxygen and propylene also reflect the changes in the rate-determining step of the reaction. Carbon dioxide is produced from both the consecutive oxidation of acrolein and the oxidation of a hydrocarbon residue which is present on the surface of the catalyst at steady-state conditions; the former pathway predominates at low temperatures (below 400 °C), while the latter pathway contributes significantly at high temperatures to carbon dioxide formation. Both pathways utilize only lattice oxygen; the extent of lattice oxygen participation is approximately the same as acrolein formation.