Oxidative dehydrogenation of propane by monomeric vanadium oxide sites on silica support

We investigate possible mechanisms of oxidative dehydrogenation of propane using density functional theory. Monomeric vanadium oxide species supported on silica are modeled by vanadyl-substituted silsesquioxane. Similarly to other catalysts with transition metal oxo bonds, the initial C-H bond activation step is hydrogen abstraction by the vanadyl (OVV) group yielding a diradical intermediate in which a propyl radical is bound to a HO-V-IV site. This is followed by a propyl rebound mechanism yielding alkoxide or alcohol attached to a V-III(OSi)(3) surface site from which propene can be formed. Propene is also directly obtained by a second hydrogen abstraction from the diradical intermediate. Desorption of propyl radicals leads to a stationary concentration of propyl in the gas phase and leaves reduced HO-V-IV sites on the surface. Due to fast reoxidation their concentration is much smaller than the concentration of OVV sites. Therefore the rate of propene formation after readsorption on OVV sites is much larger than the rate of isopropyl alcohol (or propene) formation after readsorption on HO-V-IV sites. Generation of surface propyl radicals by the first hydrogen abstraction becomes rate limiting. We predict that at 750 K the apparent activation energy is 123 +/- 5 kJ/mol and the rate constant is about 0.26 s(-1), in close agreement with experiments. The first hydrogen abstraction occurs exclusively on OVV sites, while the second hydrogen abstraction can also occur on V-O-Si bridging oxygen sites.