Selective oxidation of benzyl alcohol to benzaldehyde in “supercritical” carbon dioxide

The selective oxidation of benzyl alcohol to benzaldehyde with molecular oxygen in “supercritical” carbon dioxide over a commercial 0.5 wt.% Pd/Al2O3 catalyst has been studied in a continuous flow fixed-bed reactor. Optimization of the reaction conditions afforded a high turnover frequency (TOF) of 1585 h−1 at 80 °C and 150 bar, with a rather constant selectivity around 95%. Parameter studies showed a strong dependence of the catalyst activity on temperature and oxygen concentration. Phase behaviour studies indicated that the increase in activity, which resulted in the high TOF, could be traced back to the fact that the reactor was operated under single phase conditions. High pressure in situ XAS uncovered that the palladium constituent was mainly in a reduced (metallic) state during alcohol oxidation, and that it was quite easily reduced and re-oxidized at 80 °C by the reactants, alcohol and oxygen, respectively.