On the nature and reactivity of active oxygen species formed from O2 and N2O on VOx/MCM-41 used for oxidative dehydrogenation of propane

Electron paramagnetic resonance (EPR) spectroscopy under controlled atmosphere conditions has been applied to elucidate the nature and reactivity of oxygen species formed upon reoxidation of a reduced VOx(2.7 wt.%)/MCM-41 catalyst by gas-phase O2 or N2O. An electrophilic Vn+⋯O− (n = 4, 5) radical anion intermediate is formed from O2 probably via dissociation of a bi-atomic adsorbed oxygen species. This radical is remarkably stable in inert atmosphere up to elevated temperatures but reacts partly with C3H8 and almost completely with C3H6 and CO already at room temperature. In contrast, no such species could be observed upon reoxidation of reduced VOx species with N2O. This might be due to the rapid formation of nucleophilic O2− oxide ions, which are not EPR-active. The different electronic nature of these oxygen intermediates is discussed as a reason for the higher propene selectivity obtained in propane oxidative dehydrogenation over VOx/MCM-41 with N2O.

Vn+⋯O− (n = 4, 5) species, able to oxidize CO, C3H6, and C3H8 even at room temperature, are formed on prereduced highly dispersed VOx/MCM-41 upon reaction with O2 but not with N2O. They are considered to be responsible for the higher activity but lower selectivity of VOx/MCM-41 in the oxidative dehydrogenation of propane by O2 in comparison with N2O.Figure optionsDownload high-quality image (78 K)Download as PowerPoint slide