The effect of oxygen and the reduction temperature of the Pt/Al2O3 catalyst in enantioselective hydrogenation of 1-phenyl-1,2-propanedione

The effect of oxygen and catalyst reduction temperature in enantioselective hydrogenation of 1-phenyl-1,2-propanedione over commercial Pt/Al₂O₃ catalyst was investigated. Dichloromethane was used as solvent. The catalyst was modified in situ with (−)-cinchonidine. Relatively high enantiomeric excesses (65%) of (R)-1-hydroxy-1-phenyl-2-propanone were obtained with the solvent used as received, i.e. containing traces of dissolved oxygen and other impurities. Dichloromethane dissociated partially on the Pt/Al₂O₃ surface causing desorption of methane, ethene and HCl from the catalyst during TPD according to mass spectrometric analysis. Under anaerobic conditions the reaction rate was low giving only about 40% enantiomeric excesses of (R)-1-hydroxy-1-phenyl-2-propanone. When injecting 5 mm³ of oxygen into the reactor a beneficial effect was observed (i.e. higher reaction rate and enantiomeric excess) in comparison with anaerobic conditions. Poisoning effect of oxygen was observed when injecting 500 mm³ of oxygen into the reactor. Effect of catalyst reduction temperature was studied at three different temperatures (170, 400 and 455°C). Highest reaction rates and enantiomeric excesses were obtained with the catalyst reduced at 400°C. Methane was desorbed from the catalyst at temperatures between 263 and 383°C which could be the explanation for the lower activity of the catalyst reduced at 170°C. It was demonstrated that small amounts of oxygen can have a beneficial effect in enantioselective hydrogenation of 1-phenyl-1,2-propanedione and also that catalyst reduction temperature plays an important role in obtaining high enantiomeric excesses.