V2O5-SiO2 systems prepared by flame pyrolysis as catalysts for the oxidative dehydrogenation of propane

VOx/SiO2 catalysts were prepared through an innovative flame pyrolysis (FP) technique and tested for the oxidative dehydrogenation of propane. The samples, of different V loadings, were characterized by electron paramagnetic resonance (EPR), micro-Raman, and FT-IR spectroscopy to assess the nature of the vanadium active sites in comparison with a sample prepared by impregnation. The active sites of the FP-prepared catalysts appeared as highly dispersed V5+O groups partly incorporated into the silica matrix and interacting poorly with one another. Lower dispersion can be achieved at the same V loading with the catalyst prepared by impregnation. EPR revealed V4+O groups pointing out normally from the catalyst surface and sitting in the center of a surface array of oxygen atoms. These V4+ groups helped demonstrate the difference in catalytic behavior of the various samples. Catalytic activity measured under both aerobic and anaerobic conditions revealed a promisingly high selectivity with the FP-prepared samples. The best results were obtained with a 10 wt% V loading, which provided the best propylene yield among the samples, especially under anaerobic conditions (up to 80% selectivity at 10% propane conversion). This is likely related to the much higher dispersion of the VOx species in the FP-prepared samples, which also helps reduce the coke formation triggered by exposed surface acid sites. (C) 2008 Elsevier Inc. All rights reserved.