INFLUENCE OF THE ACTIVATION TEMPERATURE ON THE ACTIVITY OF COPPER-ZINC OXIDE CATALYSTS IN THE HYDROGENATION OF PROPENE

The behaviour of Cu-ZnO catalysts in propene hydrogenation at 323 K has been investigated in order to gain information as to whether or not a synergic effect due to ZnO on the activity of copper is present. With this aim, two different series of catalysts were prepared by coprecipitation at (A) variable or (B) constant (almost-equal-to 8) pH. The whole composition range from CuO/ZnO = 100:0 to 0:100 was covered in preparation A, while only copper-rich samples (CuO/ZnO greater-than-or-equal-to 67:33) were prepared from method B. Chemisorption experiments of hydrogen and propene on samples reduced with hydrogen at 473 K point to the presence of adsorption sites in binary samples different from those existing in single components, that are influenced by the outgassing temperature. The last parameter has also a great effect on the catalytic activity of Cu-ZnO dilute samples. In fact, while the activity of all samples outgassed at the temperature of reduction (473 K) is practically correlated with the exposed area of copper metal, outgassing at 723 K leads to an increase in the activity of dilute catalysts. The effect is greater the more dilute the catalyst. By also considering that a solid solution of CuII in the surface layer of ZnO has been observed on dilute samples before reduction, the hypothesis is made that after activating at 723 K extra activity develops due to sites at the contact region between copper metal and ZnO. In this view importance is given to the degree of hydroxylation of the oxide. The influence of the latter parameter under methanol synthesis on Cu-ZnO catalysts is also discussed.