Generation of gaseous radicals by a VMgO catalyst during oxidative dehydrogenation of propane

The oxidative dehydrogenation of propane was studied on a VMgO catalyst for possible gas-phase reactions initiated by the desorption of reactive intermediates from the catalyst surface. It was found that the conversions of propane at 556 and 570°C in the void volume positioned immediately downstream of the catalyst were higher than those in the same void volume positioned upstream of the catalyst, and much higher than those in the absence of the catalyst. The propane conversion due to postcatalytic reactions increased with the weight of the catalyst wafer, the propane/oxygen partial pressure, and the propane/oxygen ratio. That this conversion was much higher in the presence than in the absence of a catalyst was due to the heats of reaction generated on the catalyst surface and gas-phase reactions of propane initiated by the desorption of free radicals or reactive intermediates from the catalyst surface. It was also established that at 585°C there were few reactions due to the oxidative pyrolysis of methane, ethene, or propene. These results strongly suggested that the overall reaction mechanism consisted of a heterogeneous, a heterogeneous-homogeneous, and a homogeneous reaction pathway.