Hydrothermal transformation of glycerol to lactic acid in alkaline medium using Cu catalysts obtained from hydrotalcite-like precursors.

Abstract

The conversion of glycerol into lactic acid in alkaline medium using heterogeneous catalysts has been highlighted and has proved to be an efficient alternative to the conventional fermentative route. This work investigated the production of lactic acid from glycerol in alkaline medium using copper catalysts obtained from hydrotalcite-like precursors in a continuous flow reaction system and the effect of different copper loading on catalytic behaviors in terms of yield and selectivity to lactic acid. The catalysts were synthesized by the coprecipitation method and characterized by XRF, XRD, N2 adsorption-desorption, H2-TPR, CO2-TPD, and the copper dispersion was determined by N2O oxidation. The reaction was performed for 30 h at 240°C, 35 atm, using space velocity (WHSV) of 2 h− 1, solution of 10 vol% glycerol, and NaOH/glycerol molar ratio of 0.75. Although the 30CuHT catalyst presented higher BET surface areas, Cu dispersion, and basicity than the 20CuHT catalyst, the best results in terms of glycerol conversion (96.5%) and yield (64%) to lactic acid were obtained in the catalytic test performed with the 20CuHT catalyst. This result may be related to the reaction steps that occur in the liquid phase in the presence of the hydroxyl group (OH-). Furthermore, the high content of Cu may favor the hydrogenation of the reaction intermediates (2-hydroxypropenal and pyruvaldehyde), increasing the formation of 1,2-propanediol and consequently reducing the yield of lactic acid. Finally, the catalytic activity of the reduced and non-reduced catalysts was investigated, and the results showed that copper oxide also has catalytic activity. However, the reduction of copper oxide provides better results.