Palladium-catalyzed oxidation of octyl alcohols in "supercritical" carbon dioxide

Noble metal catalyzed aerobic oxidation of alcohols to carbonyl compounds o r carboxylic acids is an efficient method, but the frequently observed cata lyst 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 wit h fair and tunable solubility in particular for weakly polar water-insolubl e alcohols. The experiments were carried out in a continuous fixed bed reac tor, over 0.5 wt% Pd/alumina, at 80-140 degreesC and 75-125 bar. A comparat ive study of oxidation of 1- and 2-octanols to carbonyl compounds, chosen a s model reactions, demonstrated that the method is well suited for the part ial oxidation of secondary alcohols with molecular oxygen, whereas for prim ary alcohols the selectivity is low In the oxidation of 2-octanol the selec tivity was excellent (>99.5%) and independent of conversion. No significant catalyst deactivation was observed and the rate in CO2 was higher by a fac tor 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 bee n interpreted by studying the phase behavior in a high-pressure view cell u nder reaction conditions. (C) 2001 Academic Press.