Kinetic Studies as a Method to Differentiate between Oxygen Species Involved in the Oxidation of Propene

Kinetic equations of propene oxidation along the nondestructive (nucleophilic oxidation to allylic products) and the destructive (electrophilic oxidation to degraded oxygenated and total oxidation products) pathways have been determined for three types of oxide catalysts: Bi2O3/MoO3, giving only allylic products, Co3O4, showing only total oxidation, and SnO2/MoO3, giving both types of products. In all cases the reaction order of the nucleophilic oxidation was 1 with respect to propene and 0 with respect to oxygen, whereas that of electrophilic oxidation was close to 1 with respect to oxygen. The model of surface interactions is discussed in which propene reacts either with surface lattice oxide ions to give nucleophilic oxidation products or with transient surface oxygen species to give electrophilic oxidation. These transients result from the dynamic equilibrium between nonstoichiometric transition metal oxides and gas phase oxygen, so that the two kinetic equations are coupled by the equation expressing the equilibrium of surface oxygen vacancies. The rate constants of the homomolecular isotopic exchange of oxygen may be taken as a measure of the surface concentration of the transient oxygen species.