The fabrication of trifunctional polyoxometalate hybrids for the cascade conversion of glycerol to lactic acid

Abstract

Lactic acid (LA) has been produced with cascade reactions under non-noble metal and base-free conditions. In this work, amino acid-functionalized polyoxometalates (POMs) Ly_xH_3−xPMo₁₂O₄₀ (abbreviated as Ly_xH_3−xPMo, x = 1–3, Ly = C₆H₁₅O₂N₂, lysine) with triple active sites were fabricated, which presented a 87% LA yield at 96% glycerol conversion under mild reaction conditions attributed to the combination of basic sites, Brønsted acidic sites, suitable redox ability, and special microenvironments. The mechanism study showed that with Ly_xH_3−xPMo, glycerol was oxidized first to glyceraldehyde (GCA) and dihydroxyacetone (DHA) by the redox sites; then, the existence of basic sites led to the dehydration of DHA and GCA to pyruvaldehyde (PRA) and finally, the Brønsted acid sites catalyzed the hydration of PRA to LA. Meanwhile, the activation energies for different POMs were calculated using Arrhenius plots to confirm the effect of Ly₂H₁PMo for its highest efficiency. These hybrids acted as heterogeneous catalysts with long duration and high stability.