The selective oxidation and ammoxidation of alkenes by mixed metal oxide catalysts is driven by two necessary chemical mechanisms. One is the surface reaction entailing, in the case of propylene as the reactant, the rate determining α-hydrogen abstraction to form the symmetric allyl intermediate followed by sequential oxygen (or nitrogen from ammonia) insertion and further hydrogen abstraction producing acrolein or acrylonitrile, respectively. This mechanism was initially developed by Adams and Jennings[1] and then further expanded by Burrington and co-workers[2]. The second critical mechanistic requirement involves the lattice oxide ions of the catalyst first described by Mars and van Krevelen[3]. This mechanism is greatly influenced by the solid state structure and redox chemistry of the oxide catalyst. The necessary characteristics for this are: 1) Lattice oxide ions having the optimal metal-oxygen bond strength to effect single insertion of oxygen (or NH from ammonia) into the allylic intermediate. 2) Facile migration of oxide ions from the bulk to the surface active for regeneration of the active site. 3) Separate surface sites for reduction of oxygen, incorporation of the oxide ion into the lattice and transport of this oxide ion through the lattice to be available for regeneration of the surface hydrocarbon activation site. The design an oxide catalysts to effect these steps is achieved by the judicious incorporation of redox active cations and the existence and control of structural defect chemistry such as cation and anion vacancies in order to promote lattice oxygen migration. All this necessarily requires the propitious selection of solid state structures that can integrate both the cations necessary for the hydrocarbon activation and transformation (e.g. bismuth and molybdenum) with the characteristics necessary for redox activity. This presentation will provide examples of how various families of metal oxides are able to be identified and promoted to produce highly active and selective catalysts for the ammoxidation of propylene to acrylonitrile. [1] C. R. Adams, T. J. Jennings, J. Catal. 2 (1963) 63–68; J. Catal. 3 (1964) 549–558. [2] J. D. Burrington, C. T. Kartisek, R. K. Grasselli, J. Catal. 87 (1984) 363–380. [3] P. Mars, D. W. van Krevelen, Chem. Eng. Sci. (Spec. Suppl.) 3 (1954) 41–57.

Solid state mechanisms in selective oxidation by metal oxide catalysts

249th National Meeting (2015)

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