Liquid-phase oxidation of alcohols by oxygen and nitrous oxide catalysed by Ru-Co oxide

Chemoselective catalysts in bulk or supported on γ-Al2O3 binary oxides RuIV-CoIII (Ru/Co = 1:1-1:2), prepared by co-precipitation, were used for liquid-phase oxidation of saturated and unsaturated primary and secondary alcohols to aldehydes and ketones with O2 or N2O. The catalysts can be separated by filtration and reused. No leaching of Ru or Co in solution was observed. The oxidation is enhanced by the presence of hydration water in the Ru-Co catalyst, which indicates the participation of active RuIV hydroxo species in the reaction. From XRD and TGA, the Ru-Co oxide can be approximated as a hydrous binary oxide comprising the amorphous RuO2 and heterogenite-3R cobaltic acid CoO(OH). The alcohol oxidation appears to occur by a nonradical mechanism, which may be viewed as an oxidative dehydrogenation of alcohols to form an aldehyde or ketone. H2-TPR shows that CoIII practically does not affect the oxidising ability of RuO2. This suggests that the cobalt is likely to enhance catalyst reoxidation by O2 rather than to play a significant role in the alcohol dehydrogenation. The alcohol oxidation by N2O exhibits a close similarity to the oxidation by O2 but is much less efficient. Much more active catalysts are required to make the oxidation with N2O synthetically useful.