Preparation of Coimpregnated Cu−Zn/SiO₂Catalysts:  Influence of the Drying Step on Metallic Particle Size and on Cu⁰−Zn^IIInteractions

Abstract

Mixed copper−zinc catalysts supported on silica were prepared by incipient wetness impregnation of nitrate salt solution. The influence of the drying temperature on the properties of the final calcined and/or reduced material was investigated. It was shown that, as in the case of monometallic Cu or Zn systems, after a drying step performed at 25 °C, the nitrates are still present as an amorphous compound, whereas the drying step at 90 °C led to the formation of a poorly crystallized hydroxynitrate compound. From XRD and DTG measurements, it was concluded that the hydroxynitrate compound in the mixed systems is a copper−zinc hydroxynitrate [Cu 2− x Zn x (OH) 3 (NO 3 )] in which the two cations are closely associated. After reduction at 350 °C, a “memory effect” of the drying step was observed, and the intermediate formation of the mixed hydroxynitrate led to a better interdispersion of Cu 0 and Zn II in the reduced material than when drying was performed at room temperature. This is attested by the higher selectivity to crotyl alcohol in the reaction of hydrogenation of crotonaldehyde. When a calcination step was performed before reduction, the formation of CuO particles partially destroyed the interaction between Cu II and Zn II created during the drying step.