Explaining the role of vanadium in homogeneous glucose transformation reactions using NMR and EPR spectroscopy

Abstract

Our contribution investigates the formation of various catalytically active vanadium species for different glucose transformation reactions. By applying both 51V-NMR (nuclear magnetic resonance) and continuous wave EPR (electron paramagnetic resonance) spectroscopy, we were able to identify the different vanadium species that catalyse glucose transformation to several organic acids in aqueous solution depending on the reaction atmosphere and type of vanadium precursor. Under aerobic conditions (20 bar oxygen atmosphere) we could prove that only the higher-substituted vanadium containing polyoxometalate HPA-5 (H8PV5Mo7O40) catalyses the desired glucose oxidation to formic acid at 90 °C. Moreover, our results suggest that substituted V5+ species are the predominantly catalytic active species in the production of formic acid. Using anaerobic conditions (20 bar nitrogen atmosphere), we could show that different vanadium species are formed depending on the nature of the precursor. Hereby, paramagnetic acid-bound vanadyl species formed from the VOSO4 and NH4VO3 as well as the HPA-5 precursor seem to be predominantly responsible for lactic acid formation from glucose under anaerobic conditions.