Continuous oxidation of benzyl alcohol in "supercritical" carbon dioxide

"Supercritical" (dense) carbon dioxide has been applied as solvent for the partial oxidation of benzyl alcohol with molecular oxygen in a fixed-bed re actor. High rate and good selectivity to benzaldehyde (93-97%) has been ach ieved with 0.5 wt% Pd/alumina or 0.5 wt% Pd/C, at around 100 degreesC and 1 00 bar, using only moderate excess of oxygen. The by-product benzoic acid h as an autocatalytic effect on the hydration of benzaldehyde, and the subseq uent oxidative dehydrogenation leads to benzoic acid, and benzyl benzoate b y esterification. Promotion of Pd by Pb improves the selectivity, No cataly st deactivation or metal leaching has been observed. The method provides re asonable yields at much lower temperature than that applied in conventional gas phase oxidation. showing a potential for the synthesis of thermolabile , water-insoluble aromatic aldehydes.