Role of dispersed vanadia and of cationic vacancies for light hydrocarbon ammoxidation and oxidative dehydrogenation on VSbO4 , theoretical and operando insight on structure-activity relationships

Abstract

We summarize our investigation on structure-activity relationships for ethane and propane ammoxidation and oxidative dehydrogenation (ODH) on VSbO4 catalysts standing on preparation of model systems, operando Raman-GC studies and Density Functional Theory (DFT). To do so, we combine the use of support-stabilized nanoscaled-VSbO4 with operando Raman-GC studies during reaction. Alumina-supported nanoscaled VSbO4 exhibits the same performance than conventional bulk VSbO4. It hardly converts ethane but it does convert propane. DFT shows that ammonia, ethane and ethylene compete for the same active sites: the very strong adsorption of NH3 prevents ethane conversion. For C3 molecules, ammonia does not block propylene adsorption, but prevents that of propane. Thus, during ammoxidation, molecularly dispersed VOx species are necessary to transform the propane molecule into propylene. Then, intermediate propylene can co-adsorb along with ammonia on VSbO4 , inserting the nitrogen atom that forms acrylonitrile. Calculations show that the adsorptions studied are more favored when the rutile structure presents a cationic vacancy.