Selective Hydrogenation of Lactic Acid to 1,2-Propanediol over Highly Active Ruthenium-Molybdenum Oxide Catalysts

Modification of Ru/C with a small amount of MoOx (RuMoOx/C) enhanced the catalytic activity in the hydrogenation of L-lactic acid to form 1,2-propanediol and maintained high selectivity. The turnover frequency based on the amount of Ru over the optimized RuMoOx/C catalyst (Mo/Ru molar ratio=1:16) was 114h(-1) at 393K, which was about 4 times higher than that over Ru/C. The same effect of MoOx was obtained over RuMoOx/SiO2, although RuMoOx/SiO2 showed slightly lower activity than that of RuMoOx/C. RuMoOx/C achieved a high yield of 95% in 18h at 393K and was applicable to various carboxylic acids to provide the corresponding alcohols in high yields. Modification with MoOx also brought about suppression of racemization and (S)-1,2-propanediol was obtained in high enantiomeric excess at 353K. Based on kinetic analysis and characterization data, such as XRD, TEM, CO adsorption by a volumetric method, FTIR spectroscopy, and X-ray absorption spectroscopy, for RuMoOx/C and RuMoOx/SiO2, the catalyst structure and reaction mechanism are proposed.