Structure, activity and kinetics of supported molybdenum oxide and mixed molybdenum–vanadium oxide catalysts prepared by flame spray pyrolysis for propane OHD

Abstract

A series of molybdenum oxide (2 to 15wt% Mo) and mixed molybdenum–vanadium oxide (4 to 15wt% Mo and 2wt% V) on alumina catalysts have been synthesized by flame spray pyrolysis (FSP). The materials were structurally characterized by BET surface area, X-ray diffraction (XRD), Raman and diffuse reflectance UV–vis spectroscopy and evaluated as catalysts for the oxidative dehydrogenation (ODH) of propane. The results show that samples with high specific surface areas between 122 and 182m2/g were obtained, resulting in apparent MoOx and VOx surface densities from 0.7 to 7.7nm−2 and 1.5 to 1.9nm−2, respectively. Raman spectroscopy, UV–vis spectroscopy and XRD confirmed the high dispersion of molybdenum and vanadia species on γ-Al2O3 as the main crystalline phase. Only at the highest loading of 15wt% Mo, with theoretically more than monolayer coverage, some crystalline molybdenum oxide was observed. For the mixed molybdenum–vanadium oxide catalysts the surface species were separate molybdenum oxide and vanadium oxide monomers at low loadings of molybdenum, but with increasing molybdenum loading interactions between surface molybdenum and vanadium oxide species were observed with Raman spectroscopy.The catalytic experiments showed that the most selective molybdenum oxide catalysts for the ODH reaction were those with high Mo loadings of 7 to 15wt% Mo, while the most selective mixed molybdenum–vanadium oxide catalyst were at 4wt% Mo, where separate surface species of molybdenum and vanadium oxide were observed by Raman spectroscopy. A simple kinetic model based on the propane ODH reaction, parallel combustion of propane and sequential combustion of propene described the experimental results well and could be used to determine the optimal reaction conditions.