Robust Ru(II)–Bisbenzimidazole Catalyst Enables Sustainable and Scalable Valorization of Glycerol to Lactic Acid

Abstract

Sustainable valorization of glycerol to lactic acid (LA) through catalytic dehydrogenation addresses both waste minimization and value addition. Yet, the efficient isolation of lactic acid (LA) and its conversion into valuable products such as calcium lactate (CaL2) remains a significant challenge. Herein, we demonstrate the catalytic dehydrogenation of glycerol to lactic acid (LA) using a well-defined Ru-1 complex, derived from Ru(II)–p-cymene and 2,2′-bisbenzimidazole (BiBzImH₂), in a basic aqueous medium at 160 ℃ with efficient conversion of glycerol with high selectivity toward lactic acid (LA). Remarkably, Ru-1 is 7–17 times more cost-effective than previously reported ruthenium catalysts for glycerol dehydrogenation. Furthermore, crude lactic acid (LA) can be efficiently isolated from the reaction mixture through neutralization, filtration, and evaporation, and subsequently converted into calcium lactate (CaL2). Reusability and scalability studies confirm the practicality of this process, while green chemistry assessments (E-factor, CHEM21 toolkit, and EcoScale analysis) highlight its environmental sustainability. Mechanistic investigations using DFT calculations, kinetics, spectroscopy, and control experiments reveal that Ru-1 promotes glycerol dehydrogenation to glyceraldehyde/dihydroxyacetone through a base-assisted metal–ligand cooperative pathway, in which H2 elimination is the rate-determining step. The catalyst remains molecular and proceeds through a homogeneous Ru–H intermediate, while the resulting carbonyl intermediates undergo spontaneous base-promoted Cannizzaro rearrangement to lactic acid.