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

Abstract

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 (H3PMo12O40, Ag3PMo12O40, Sn0.75PMo12O40). Further characterization of the system using 31P 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...