NMR Spectroscopic Studies of Interactions in Solution during the Synthesis of MoVTeNb Oxide Catalysts

摘要

Aqueous solutions used for the synthesis of MoVTeNb oxide Catalysts have been studied with Te-125, Mo-95, V-51, and O-17 NMR spectroscopy. Polyoxometalate (POM) anion V9TeO285- (I), which has the decavanadate structure, containing Te6+ in one of its two central positions, has been identified for the first time in V-v-Te-vI solutions. In ternary Mo-vI-V-v-Te-vI solutions, the POMs TeMO6O246- (II) (pH ca. 7.8-2) as well as previously unknown complexes of the proposed compositions TeV3MO3O249- (III) (pH ca. 8-5) and TeMO3V5O275- (IV) (pH ca. 6-1) were observed. In the structure of IV, halt of the decavanadate anion is combined with half of the octamolybdate anion, in which the central Mo atom is replaced with Te6+. There is an equilibrium, IV reversible arrow I + II, in the ternary solutions. Despite its stability in Solution, polyanion IV could not be isolated in the solid state. During concentration under ambient conditions, less soluble polyanion II begins to crystallize first, and the equilibrium in solution gradually shifts to I. Upon dissolving the precipitate, complex IV forms again. The distribution diagrams have. been derived from the NMR spectra for a solution of the cationic composition Mo1V0.3Te0.23 with NH4+ as a counterion, acidified with HClO4 in the pH range 7.25-1. In addition to the above-mentioned complexes, VO2+ and MoO22+ oxalates form in the presence of oxalic acid. Upon adding niobium oxalate, Nb2O5 gel forms without. any Nb-containing species in solution. (C) Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008)