Preparation of coimpregnated Cu-Zn/SiO2 catalysts:: Influence of the drying step on metallic particle size and on Cu0-ZnII interactions

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 degrees C, the nitrates are still present as an amorphous compound, whereas the drying step at 90 degrees 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 [Cu2-xZnx(OH)(3)(NO3)] in which the two cations are closely associated. After reduction at 350 degrees 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.