Enantioselective hydrogenation of α-keto esters: Temperature-programmed reduction study of liquid-phase Pt/Al2O3 hydrogenation catalysts

The effect of thermally pretreating Pt/Al2O3 catalysts in hydrogen on enantioselectivity and activity in the liquid-phase hydrogenation of α-keto esters and the structure and oxidation state of the catalysts was studied. Thermal treatments in hydrogen at 673 K gave 15–20% higher optical yields of the α-hydroxy esters produced. In addition, the activity of the catalysts was up to three times higher for the activated catalysts. The processes taking place during the pretreatment of commercial and laboratory-made catalysts were characterized by temperature-programmed reduction (TPR), carbon monoxide and nitrogen adsorption and X-ray diffraction. The TPR profiles showed three well resolved peaks which were assigned to a platinum oxide shell on a platinum core (200–400 K), residual platinum salts (400–550 K) and carbon deposits that originate from the liquid-phase preparation processes (600–700 K). The amount and reducibility of the platinum oxide was determined by TPR and correlated with the activity of the catalysts in the sense that strongly oxidized catalysts show much lower activities. There is no simple correlation between the improvement in selectivity and the processes taking place during thermal activation, but there are indications that residual platinum salts have a negative influence on enantioselectivity.