Oxidative Dehydrogenation of Propane by Monomeric Vanadium Oxide Sites on Silica Support

Abstract

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 (OdV V ) 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 OdV V sites. Therefore the rate of propene formation after readsorption on OdV V 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 OdV V sites, while the second hydrogen abstraction can also occur on V-O-Si bridging oxygen sites.

Active Sites for Propane Oxidation: Some Generic Considerations

“…4). The statement [7,8] about the rate-determining nature [55] of alkane activation is not inconsistent with this notion if we consider the first intermediate as a rare species limiting the equilibration of all consecutive steps, much in the way as a conventional Langmuir-Hinshelwood reaction may be limited by a pre-equilibrium.…”

“…1, 2 without making any statement about the atomistic nature of the atom transfer which is likely to occur via radicalic [8] activation. It is obvious that we need twice the reaction (2) in order to complete Eq.…”

“…Surface-science approaches work only with activated molecules such as methanol [5,6] and theory [5,7,8] is confined to simplified yet highly valuable model systems [6,9,10]. It is still a great challenge to construct concepts of active sites [11,12] for selective oxidation that are based upon experimental observations [13] of reacting surfaces.…”

Models in Catalysis

“…This has been also demonstrated for vanadium oxide supported on oxides, a catalyst for selective oxidations. Vanadyl groups, O = V(O-) 3 have been identified as active sites [30], and two different types of cationic clusters have been used for gas phase studies. The V 4 O 10 Á?…”

MCF Material as an Attractive Support for Vanadium Oxide Applied as a Catalyst for Propene Epoxidation with N2O

“…Previous investigations concerning the use of vanadia-based silica supported catalysts for propene epoxidation have shown that various parameters of the catalysts, including aggregation state and reducibility of vanadium species, acid/base properties, and vanadium content, need to be considered to account for the catalytic behavior noted for the selective oxidation of propene [15,19]. It has been shown that catalysts with high vanadium dispersion show better performance in a number of oxidation reactions such as propane oxydehydrogenation [31], methanol partial oxidation [32], and also in selective propene oxidation [15,19,33]. The presence of an isolated tetrahedral vanadium oxide species is essential as it facilitates the interaction with -N δ− =N δ+ =O species, a mezomeric form of N 2 O, with the formation of mildly electrophilic oxygen forms.…”