On the role of CO formation during the aerobic oxidation of alcohols on Pd/Al2O3: an in situ attenuated total reflection infrared study

Dehydrogenation and oxidative dehydrogenation of six different primary and secondary, aliphatic and aromatic alcohols to the corresponding carbonyl compounds were studied in cyclohexane, in the presence and absence of air. The reaction network was examined by attenuated total reflection infrared (ATR-IR) spectroscopy of the catalytic solid-liquid interface and by GC analysis of the effluent in a continuous-flow reactor. The 5 wt% Pd/Al2O3 catalyst was located in an ATR-IR cell that served as a tiny reactor. The studies revealed that all aldehydes formed from primary alcohols (1-octanol, benzyl and cinnamyl alcohol) decarbonylated on Pd, whereas the ketones (2-octanone, cyclohexanone, and acetophenone) were stable. The decarbonylation reactions indicated by IR and the hydrogenation and hydrogenolysis-type side reactions detected by GC even in the presence of molecular oxygen corroborated the presence of surface Pd0 sites and the classic dehydrogenation mechanism of alcohol oxidation. Moreover, the facile removal of CO and the phenyl radical formed from decarbonylation of benzaldehyde during benzyl alcohol oxidation unambiguously proved the simultaneous presence of adsorbed oxygen and hydrogen on the Pd surface. The relatively high rate of benzyl alcohol oxidation is attributed to the faster removal of strongly adsorbed by-products from the metal surface and partly to the highest polarity of this alcohol among all of the reactants investigated. Benzyl alcohol increases the solubility of water in the apolar solvent and thus minimizes the negative effect of water coproduct that accumulates on the catalyst surface.