Effect of the nature of the support on the enantioselective hydrogenation of 1-phenyl-1,2-propanedione over supported iridium catalysts

Abstract

The enantioselective hydrogenation of 1-phenyl-1,2-propanedione at 298 K and 40 bar over modified supported iridium catalysts has been studied. Cinchonidine has been used as chiral inducer. The catalysts were obtained by impregnation of Ir(acac)3 on three different supports: SiO2, TiO2 and MoO3, followed by calcination in air and reduction under hydrogen at 773 K. All the solids were characterized by nitrogen adsorption-desorption isotherms at 77 K, H2 chemisorption, XRD, TEM, TPR and XPS. It was found that Ir/SiO2 and Ir/TiO2 catalysts reduced at high temperatures, 773 K, possess similar metal particle size, close to 2.0 nm, eventhough the H/Ir ratio obtained from H2 chemisorption showed larger differences, with the H/Ir ratio being lower for titania- and molybdenum-supported iridium catalysts. In these samples, migration of the partially reduced supports, TiOx and MoOx moieties, on the metal crystals induce the creation of Ird+ species. TPR and XPS results confirmed that the metal component was not completely reduced.

The activity was influenced by the nature of the support, being more active those in the SMSI state such as Ir/TiO2 and Ir/MoO3 being more active. This has been attributed to the presence of electron deficient metal species, Ird+, which are responsible for the polarization of the carbonyl bond of the substrates, thus favoring the activity and enantioselectivity of the reaction. The effect of different solvents on the activity and the enantioselectivity of the reaction was also studied. The highest enantiomeric excess (ee) of (R)-1-phenyl-1-hydroxy-2-propanone (20%) was obtained with the Ir/TiO2 catalyst using acetic acid as solvent.