Mn–Co–Al–Mg mixed oxides by auto-combustion method and their use as catalysts in the total oxidation of toluene

Abstract

Manganese oxides were prepared by the auto-combustion method and while employing variable Mn/Mg ratios from 0.1 to 1.0 with glycine as a combustible and a glycine/nitrate=1 ratio. The Mn/Mg ratio that showed the best activity in the oxidation of toluene was maintained in order to synthesize the hydrotalcite structure by the co-precipitation method, which by calcination, allows the formation of a mixed oxide. Cobalt oxides were prepared by the two synthesis methods while maintaining the best metal charge (corresponding to the best catalytic behavior).The obtained solids were characterized by X-ray diffraction (XRD), X-ray fluorescence (XRF), temperature programmed reduction (H2-TPR) and surface area measurements, and the catalytic activity for the complete toluene oxidation was evaluated. The results revealed that the auto-combustion catalysts are obtained in very short times and that their physico-chemical and catalytic properties are the same or higher than those of the co-precipitation catalysts. The solid with the Mn/Mg=1 ratio is the most active and selective catalyst and shows more activity than the oxide obtained by the co-precipitation method. The redox behavior in the manganese catalysts depends on the preparation method, which, in the case of auto-combustion, avoids the segregation of phases. The manganese reacts faster than the cobalt with the glycine and produces oxides with better properties in oxidation reactions. On the contrary, the cobalt oxides prepared by the two synthesis methods with a Co/Mg=1 ratio presented similar results in toluene oxidation.