Oxidative thermolysis of Mn(acac)3 on the surface of γ-alumina support

Precipitated γ-alumina support was decorated with Mn(acac)₃ by incipient wetness impregnation with toluene solutions containing Mn(acac)₃ in amount equivalent to loading of 0.35, 0.74, 1.38, 2.38 and 3.50 Mn(acac)₃ moleculs per nm² of the support. In order to evaluate the mechanism of Mn(acac)₃ interaction with the surface of γ-alumina support and subsequent transformations of the supported Mn(acac)₃ species, oxidative thermolysis of Mn(acac)₃/Al₂O₃ samples in air was studied by diffuse reflectance FTIR, thermogravimetric analysis (TG/DTG), differential thermal analysis (DTA) and XRD. It has been found out that decoration of γ-Al₂O₃ support with Mn(acac)₃ results in the formation of surface bound Mn(acac)_3−x species when Mn(acac)₃ loading does not exceed 1.38 Mn(acac)₃/nm². At higher Mn(acac)₃ loading the formation of the supported bulk-like Mn(acac)₃ species also occurs. The interaction of Mn(acac)₃ molecules with the support surface occurs via substitution of acetylacetonate ligand(s) with the oxygen atom of surface hydroxyl group(s) accompanied by elimination of acetylacetone molecules. The evolved acetylacetone reacts with the alumina surface that results in the formation of surface Al(acac)_3−x species. The oxidative thermolysis of Mn(acac)_3−x species on the surface of γ-alumina proceeds via partial elimination of acetylacetonate ligands and partial oxidation of the remaining ligands without destruction of their cyclic structure within 425-550 K. The complete oxidative destruction of acetylacetonate ligands takes place within 600-700 K and results in the formation of manganese oxide species on the alumina surface. The dispersed surface manganese oxide species originate upon the oxidative thermolysis of the surface bound Mn(acac)_3−x species while crystalline Mn₂O₃ phase results from the supported bulk-like Mn(acac)₃ species.