Comparison between the catalytic and photocatalytic activities of Cu/Al2O3 and TiO2 in the liquid–phase oxidation of methanol–ethanol mixtures: Development of a kinetic model for the preparation of catalyst

Abstract

This work presents the preparation and characterization of copper supported on gamma alumina (γ–Al2O3) as a catalyst and compared with the photocatalytic activity of titania (TiO2) in the oxidation of methanol and ethanol mixtures; the ratio of both alcohols in the mixture was according to the commercial alcoholic beverages. In order to understand the thermal transformations of Cu/Al2O3 catalyst, a kinetic model of its synthesis has been developed and analyzed in terms of the oxidation of the precursor CuCl2∙2H2O impregnated onto Al2O3 to form CuO/Al2O3, and also its reduction to form the metallic copper supported on alumina (Cu/Al2O3) catalyst. The sequence of reactions was considered according to the X–ray diffraction (XRD) at different temperatures and thermal characterizations. The kinetic parameters revealed how the alumina support intervened in some steps to form the catalyst through chemical metal–support interactions. It was also verified that during the chemical changes that the active phase undergoes, its good dispersion is maintained on the support and the activation energy values of the model can predict that these metal–support chemical interactions were also preserved from the salt added to the formation of elementary copper. These kinetic model’s predictions were accurately demonstrated by High–resolution transmission electron microscopy (HRTEM) micrographs and X–ray photoelectron spectra (XPS) techniques, where the appearance of the copper aluminate phase (CuAl2O4) is an evidence of the metal–support interaction, which could have contributed to the elementary copper nanoparticles formation and the catalytic activity as an active phase in the oxidation of alcohols. Cu/Al2O3 catalyst showed a higher yield of acetic acid from ethanol than formic acid from methanol. In comparison, the TiO2 in the photocatalytic oxidation presented a higher yield of formic acid from methanol than acetic acid from ethanol. A similar behavior was observed in the oxidation tests of each alcohol separately, since, the photocatalytic oxidation of methanol, using TiO2, showed nearly the 100% of selectivity in the formation of formic acid from methanol, and the Cu/Al2O3 catalyst exhibited similar results but, in this case, the selectivity towards to acetic acid from ethanol. Stable acetate intermediates detected by FTIR in ethanol oxidation to acetic acid on Cu/Al2O3 showed that this oxidation proceeds via acetaldehyde intermediates.