⁵¹V MAS NMR and XAFS Evidences for Redox of Magnesium Pyro- and Ortho-Vanadates on the Oxidative Dehydrogenation of Propane

Abstract

To evaluate the redox nature of magnesium vanadates during the oxidative dehydrogenation of propane, magnesium pyro- and othro-vanadates (Mg₂V₂O₇ and Mg₃V₂O₈, respectively) used for the conversion of propane with and without gaseous oxygen were analyzed by XRD, solid state ⁵¹V MAS NMR and XAFS. After the propane conversion without oxygen, complete conversion of those magnesium vanadates to the corresponding vanadate containing V⁴⁺ or V³⁺ species together with MgO was observed by XRD, while the reduction of V⁵⁺ was detected by ⁵¹V MAS NMR and V K-edge XAFS from those catalysts. However XRD and ⁵¹V MAS NMR revealed that re-oxidation of the reduced Mg₃V₂O₈ resulted in the regeneration of the original phase together with that of a V⁵⁺ species, while the re-oxidation of the reduced Mg₂V₂O₇ afforded a phase (triclinic system) different from the original one (monoclinic system), but, in which the V⁵⁺ species was regenerated from the reduced vanadium.