Manganese-catalysed dehydrogenative oxidation of glycerol to lactic acid

Herein, we report a well-defined pincer manganese complex supported by PNP ligands for the conversion of glycerol to sodium lactate with dihydrogen liberation. Sodium lactate could be obtained in 96% yield with 96% selectivity under mild reaction conditions, with the Mn-3 complex as the catalyst. In this transformation, catalyst loadings even as low as 0.025 mol% could also result in high reactivity and excellent selectivity (up to 98%) at 180 °C in 36 hours. Mechanistic studies demonstrated that the manganese catalyst was mainly responsible for glycerol dehydrogenation, while the amount of sodium hydroxide had a crucial effect on the subsequent Cannizzaro reaction. In addition, the catalytically active species in the reaction were investigated by using in situ 31 P NMR spectroscopy. This catalytic system provides a green, efficient, and atom-economical approach for upgrading biodiesel waste into high-value chemicals. An efficient and robust pincer manganese complex for the dehydrogenative oxidation of glycerol to lactic acid with dihydrogen liberation is presented.