Abstract: Ru-, Rh-, and Pd/WO₃-ZrO₂ catalysts were studied for the direct oxidation of ethylene to acetic acid. The catalytic activity and selectivity depended strongly on the noble metals used. Rh/WO₃-ZrO₂ gave the highest conversion of ethylene, whereas Ru/WO₃-ZrO₂ was almost inactive. The higher catalytic activity was attributed to a better metal dispersion. Pd/WO₃-ZrO₂ gave the highest selectivity for acetic acid (75%), with the selectivity of the other two catalysts being rather low (~10%). Temperature-programmed oxidation and H₂ reduction experiments were used to show that the strength of the noble metal–oxygen bond played an important role in product distribution. A weak metal–oxygen bond favored the formation of acetic acid, while a strong metal–oxygen bond favored the combustion of ethylene to CO_x.

Key words: ethylene, oxidation, acetic acid, tungstated zirconia, palladium, ruthenium, rhodium