Palladium-Catalyzed Oxidation of Octyl Alcohols in “Supercritical” Carbon Dioxide

Noble metal catalyzed aerobic oxidation of alcohols to carbonyl compounds or carboxylic acids is an efficient method, but the frequently observed catalyst deactivation and flammability of organic solvents in the case of water-insoluble alcohols limit the application range. We applied “supercritical” (dense) carbon dioxide as the solvent, a cheap and nonflammable medium with fair and tunable solubility in particular for weakly polar water-insoluble alcohols. The experiments were carried out in a continuous fixed bed reactor, over 0.5 wt% Pd/alumina, at 80–140°C and 75–125 bar. A comparative study of oxidation of 1- and 2-octanols to carbonyl compounds, chosen as model reactions, demonstrated that the method is well suited for the partial oxidation of secondary alcohols with molecular oxygen, whereas for primary alcohols the selectivity is low. In the oxidation of 2-octanol the selectivity was excellent (>99.5%) and independent of conversion. No significant catalyst deactivation was observed and the rate in CO2 was higher by a factor of up to 2–4 compared to the rate of oxidation in nitrogen. The complex effects of pressure and oxygen concentration on the reaction rate have been interpreted by studying the phase behavior in a high-pressure view cell under reaction conditions.