Magnetic resonance investigation of vanadia and vanadium-molybdenum gels synthesized with peroxovanadate precursors.

Vanadia gels and vanadium-molybdenum oxide gels were investigated using the magnetic resonance techniques, EPR spectroscopy and (51)V MAS NMR spectroscopy. The vanadium oxide gels were derived from the reaction of H(2)O(2) and V(2)O(5), and the vanadium-molybdenum oxide (VMoO) gels were derived from the reaction of peroxovanadates with an ammonium molybdate solution. EPR spectroscopy was utilized to determine quantitative information about the concentration of V(4+) paramagnetic species present in the samples and additional structural information about the V(4+) coordination environment. (51)V MAS NMR spectroscopy was used to elucidate the V(5+) electronic environment and how it changes as a function of molybdenum content. The observed line broadening of the (51)V NMR signal with increasing molybdenum content was correlated with an increase in the concentration of paramagnetic species as monitored by EPR spectroscopy. The evolution of various vanadium sites during thermal treatment was also investigated. This work provides further support for the hypothesis that the selectivity of VMoO catalysts in the oxidation of 1,3-butadiene to maleic anhydride is due to the presence of paramagnetic V(4+) sites.