Influence of the preparation method on the activity and stability of copper–zirconium catalysts for propene deep oxidation reaction

Copper was introduced to zirconium-based catalysts using three methods before calcination at 600 °C: (i) coprecipitation of copper nitrate and zirconium oxychloride, (ii) wet impregnation of copper nitrate on zirconium oxyhydroxide (ZrO(OH)2), (iii) or on zirconium oxyhydroxide previously calcined at 600 °C (ZrO2). Two atomic ratios (Cu/Zr=0.01 and 0.1) were prepared. The crystallization phase of the catalysts was evidenced by X-ray diffraction (XRD). Copper stabilizes the metastable tetragonal phase of zirconia for the samples prepared by coprecipitation and by impregnation over oxyhydroxide. Electron paramagnetic resonance (EPR) study of Fe3+ isolated ions allowed also to evidence the stabilization of this phase. Moreover, this technique evidenced the presence of two types of paramagnetic copper species that occupy the same sites: the isolated copper ions in octahedral environment, which is lacunal in the case of the coprecipitates and the clusters. The activity of these catalysts for the complete oxidation of propene was investigated. The impregnated samples have shown better catalytic activities than the coprecipitated ones. EPR study and surface area measurements by the BET method, after the catalytic test, were done in order to detect an eventual deactivation of the catalysts. A decrease in the surface area due to water formation during oxidation reaction, accompanied by a structural change on the surface of the catalyst from tetragonal to monoclinic phase, was detected for the coprecipitates and that impregnated over oxyhydroxide samples. The catalysts copper impregnated over ZrO(OH)2 previously calcined at 600 °C have combined the best activity and the best resistance towards deactivation.