FTIR studies on the selective oxidation and combustion of light hydrocarbns at metal oxide surfaces. Propane and propene oxidation on MgCr2O4

Abstract

The interaction of propene and propane with the surface of the oxidized spinel MgCr₂O_4+x has been studied in the temperature range 300–773 K by FTIR spectroscopy. This solid is reduced reversibly by reaction with these organic compounds in the temperature range 300–673 K, giving rise to stoichiometric MgCr₂O₄ and more oxidized organic species that finally produce CO₂. Comparison with the results of adsorption and oxidation of C₃ oxygenates (propan-1-ol, propan-2-ol, allyl alcohol, propionaldehyde, acetone, acrolein, propionic acid and acrylic acid) as well as of C₂ and C₁ oxygenates showed that the predominant oxidation pathways for the two molecules are different. Oxidation of propene occurs predominantly through its previous activation at C₁ to give strongly adsorbed acrolein and acrylate species. These species later burn. Acetone is the primary oxidation product of propane at the surface, at 423 K. Acetone is later oxidized to formate species (which rapidly decompose) and to acetate species that burn at higher temperatures (573–773 K). The different observed paths are rationalized by taking into account the lower C—H dissociation energy at the methylene group in the case of propane and at the allylic methyl group in the case of propene. The data reported here are consistent with the data available on catalytic alkane oxidation over this and similar catalysts. A comparison is made with the behaviour observed with more selective catalysts like Mg₃(VO₄)₂, V₂O₅–TiO₂ and MoO₃–TiO₂. A mechanism for propene and propane catalytic combustion is proposed.