FTIR STUDIES ON THE SELECTIVE OXIDATION AND COMBUSTION OF LIGHT-HYDROCARBONS AT METAL-OXIDE SURFACES - PROPANE AND PROPENE OXIDATION ON MGCR2O4

The interaction of propene and propane with the surface of the oxidize d spinel MgCr2O4+x has been studied in the temperature range 300-773 K by FTIR spectroscopy. This solid is reduced reversibly by reaction wi th 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 oxi dation of C3 oxygenates (propan-1-ol, propan-2-ol, allyl alcohol, prop ionaldehyde, acetone, acrolein, propionic acid and acrylic acid) as we ll as of C2 and C1 oxygenates showed that the predominant oxidation pa thways for the two molecules are different. Oxidation of propene occur s predominantly through its previous activation at C1 to give strongly adsorbed acrolein and acrylate species. These species later burn. Ace tone is the primary oxidation product of propane at the surface, at 42 3 K. Acetone is later oxidized to formate species (which rapidly decom pose) and to acetate species that burn at higher temperatures (573-773 K). The different observed paths are rationalized by taking into acco unt the lower C-H dissociation energy at the methylene group in the ca se of propane and at the allylic methyl group in the case of propene. The data reported here are consistent with the data available on catal ytic alkane oxidation over this and similar catalysts. A comparison is made with the behaviour observed with more selective oxidation cataly sts like Mg3(VO4)2, V2O5-TiO2 and MoO3-TiO2. A mechanism for propene a nd propane catalytic combustion is proposed.