Short-Range Electronic Interactions between Vanadium and Molybdenum in Bimetallic SAPO-5 Catalysts Revealed by Hyperfine Spectroscopy.

Abstract

Engineering two cooperative sites into a catalyst implies the onset of synergistic effects related to the existence of short-range electronic interactions between two metal components. However, these interactions and the relative structure-property correlations are often difficult to obtain. Here we show that hyperfine spectroscopy has the potential to reveal the presence of V4+-O-Mo6+ linkages assessing the degree of spin density transfer from paramagnetic V4+ species to proximal oxo-bridged Mo6+ metal ions. The dimer species were prepared by adsorption of Mo(CO)6 in the pores of SAPO-5, followed by thermal decomposition and oxidation and subsequent grafting of anhydrous VCl4(g) followed by hydrolysis and dehydration. The metal species react with SAPO protons during the exchange process and generate new Lewis acid sites, which act as redox centers. X- and Q-band EPR and HYSCORE experiments have been employed to monitor the local environment of V4+ species obtaining direct evidence for spin delocalization over 27Al, 31P, 95Mo, and 97Mo nuclei, demonstrating the presence of bimetallic V-O-Mo well-defined structures.