Selective Hydrogenation of Cinnamaldehyde over Salt-Promoted Pd-Based Catalysts

The effect of salt additives (AlCl3, SnCl2, FeCl3) on the catalytic behavior of Pd3%/Al2O3 catalyst in the selective liquid-phase hydrogenation of cinnamaldehyde in i- propanol was investigated at 25ºC and 4 bar H2 for salt concentrations in the range 4.5×10^-5-5.0×10^-3 M. The effect of added metal salts on the catalytic behavior of Pd/Al2O3 is rather complex. In the case of AlCl3, a significant enhancement of the C=O bond hydrogenation could be achieved, accompanied by a strong suppression of the C=C bond hydrogenation. The selectivity towards cinnamyl alcohol formation could be raised from 26% to 70% by introducing small concentrations of AlCl3 (~2.0×10-3 M) into the reaction mixture. Selectivity improvement was also observed upon the addition of SnCl2 (up to 67%) and FeCl3 (up to 70%), however, at the expense of activity which is believed to be due to poisoning of the Pd surface by metallic Fe and Sn. The mecha- nisms of the additives effects on the catalytic behavior of Pd/Al2O3 are discussed.