Molybdenum-Based Polyoxometalates as Highly Active and Selective Catalysts for the Epimerization of Aldoses

摘要

In this contribution, we report on the high activity and selectivity of Keggin structure molybdenum-based polyoxometalates (POMs) in catalyzing the epimerization of aldoses. Near-equilibrium conversions and selectivities were obtained within the first hour of operation under aqueous conditions at relatively low temperatures and a wide range of pHs. Characterization of the molybdenum-based POM catalysts using X-ray diffraction and FTIR studies before and after the reaction showed no evidence of their decomposition. Our studies suggest that the active sites for the reaction are the molybdenum oxide octahedra on the surface of the Keggin structure of the molybdenum-based POMs (H_3PMo_(12)O_(40), Ag_3PMo_(12)O_(40), Sn_(0.75)PMo_(12)O_(40)). Further characterization of the system using ^(31)P NMR and X-ray photoelectron spectroscopy experiments showed that the interaction between the aldose (e.g., glucose) and the molybdenum oxide octahedra in the POM results in electron transfer from the aldose to molybdenum, leading to the formation of the reduced form of the POM (also known as heteropoly blue). Isotope labeling experiments demonstrated that the epimerization of glucose using molybdenum-based POMs proceeds via an intramolecular C1–C2 shift mechanism with an activation barrier of as low as 96 kJ/mol, obtained using controlled kinetic experiments. These findings open up avenues for the implementation of molybdenum-based POMs as single, selective, and stable catalytic systems for the efficient epimerization of aldoses under aqueous conditions at relatively low temperatures and a wide range of pHs.