Vanadium species: Sm-V-O catalytic system for oxidative dehydrogenation of propane

Abstract

This work deals with the catalytic behavior of SmVO 4 impregnated with vanadium in the oxidative dehydrogenation of propane. SmVO 4 prepared by the citrate method was impregnated using NH 4VO 3 as precursor of vanadium species. The vanadium contents have been selected to obtain surface coverages as follows: (a) below the theoretical monolayer; (b) slightly above the theoretical monolayer; and (c) equivalent to the double of the theoretical monolayer. Catalysts were investigated by several physicochemical characterization techniques, i.e. BET specific surface area (SSA), X-ray diffraction (XRD), temperature programmed reduction (TPR), Fourier transform infrared spectroscopy (FT-IR), laser Raman spectroscopy (LRS), and electron paramagnetic resonance (EPR). A slight excess of samarium on SmVO 4 was found and it was responsible for the direct combustion of propane. Part of the vanadium added by impregnation reacted with the excess of samarium towards to the formation of SmVO 4. Then on the impregnated catalysts, the direct combustion of propane was controlled and higher selectivity to propene at low conversion levels was obtained. At vanadium loading below the theoretical monolayer, surface VO x species were formed. They were easily reducible and the bridging oxygen atoms (VOV) would increase the catalytic activity at low reaction temperature. At vanadium loading above the theoretical monolayer, the appearance of V 2O 5 crystals was favored. The terminal oxygen atoms (VO) existing in V 2O 5 cause the consecutive combustion of propene. From the Raman and EPR results on the catalysts before and after the catalytic test, the effect of the reaction on the surface definition of the catalysts can be inferred.