Effect of ultrasound in enantioselective hydrogenation of 1-phenyl-1,2-propanedione: comparison of catalyst activation, solvents and supports.

The enantioselective hydrogenation of 1-phenyl-1,2-propanedione was carried out over Pt/Al2O3, Pt/SiO2, Pt/SF (silica fiber), Pt/C catalysts modified with cinchonidine under ultrasonic irradiation. The initial rate, regioselectivity and enantioselectivity were investigated for different catalyst pretreatments, solvents and ultrasonic powers. The ultrasound effects were very catalyst dependent. The sonication significantly enhanced enantioselectivity and activity of the Pt/SF (silica fiber) catalyst. For the other Pt supported catalysts the reaction rate, enantioselectivity and regioselectivity increased moderately. The choice of solvent influenced the impact of ultrasound effect, namely in mesitylene, which has the lowest vapor pressure, the highest ultrasound enhancement was observed. The effect of sonication on catalysts surface was studied by transmission electron microscopy and scanning electron microscopy (SEM). No significant change in the metal particle size distribution due to sonication was observed. However, in the case of the Pt/SF catalyst, acoustic irradiation induced morphological changes on the catalyst particle surface (SEM), which might be the cause for enhancement of the initial reaction rate and enantioselectivity.