Single-Site Vanadyl Activation, Functionalization, and Reoxidation Reaction Mechanism for Propane Oxidative Dehydrogenation on the Cubic V₄O₁₀Cluster

Abstract

Vanadyl oxide (VO) sites are thought to play a role in a number of industrially important catalysts for activating saturated alkanes, but in no system is the mechanism for the activation, product formation, and reoxidation steps established. In this paper, we use quantum mechanical methods (B3LYP flavor of density functional theory) to examine the detailed mechanism for propane reacting with a V4O10 cluster to model the catalytic oxidative dehydrogenation (ODH) of propane on the V2O5(001) surface. We here report the mechanism of the complete catalytic cycle, including the regeneration of the reduced catalyst using gaseous O2. The rate-determining step is hydrogen abstraction by the vanadyl (VO) group (in agreement with experiment) to form an iso-propyl radical that binds to an adjacent V−O−V site. Subsequently, this bound iso-propyl forms propene product by β-hydride elimination to form bound H2O. We find that this H2O (bound to a VIII site) is too stable to desorb unimolecularly. Instead, the desorption is induced by binding of gaseous O2 to the VIII site, which dramatically decreases the coordination energy of H2O from 37.8 to 12.9 kcal/mol. Further rearrangement of the O2 molecule leads to formation of a cyclic VO2 peroxide, which activates the C−H bond of a second propane to form a second propene (with a lower reaction barrier). Desorption of this propene regenerates the original V4O10 cluster. We find that all reactions involve the single vanadyl oxygen (VO), with the bridging oxygens (V−O−V) serving to stabilize the iso-propyl radical intermediate. We refer to this mechanism as the single-site vanadyl activation, functionalization, and reoxidation mechanism (SS-VAFR). This SS-VAFR mechanism should be applicable to propane ODH on the supported vanadium oxide catalysts where only monovanadate (VO4) species are present.

Synthesis of Three-Dimensionally Ordered Macroporous NiCe Catalysts for Oxidative Dehydrogenation of Propane to Propene

“…However, these catalysts are usually conducted at relatively higher temperatures (above 500 • C), which cause the occurrence of undesirable combustion and then restrain propene formation. In the past few decades, great efforts have been dedicated to the improvement of vanadium and molybdenum-based ODH catalysts for highly preferable alkene yield [6][7][8][9][10], but propane conversion was normally unsatisfactory at the lower temperature of 400 • C using the catalysts above. Further studies are still required to develop highly active catalysts used under the mild reaction conditions in the ODH process.…”

The effect of a silica support: a density functional theory study of the C-H bond activation of ethane on a nickel oxide cluster

“…[21] For both of the unsupported and silica-supported Ni 3 O 3 clusters, several different spin states in terms of spin multiplicity were considered in this paper. Geometry optimizations were performed for both unsupported Ni 3 O 3 and Ni 3 O 3 @12-Si clusters in all of the singlet, triplet, quintet, heptet, and nonet states (i.e., spin multiplicity = 1, 3, 5, 7, and 9, respectively).…”

“…B3LYP is known to provide a good description of the PES for the many transition metal oxide clusters. [20,21] The basis set information is specified after the description of the model of the reactants. All PESs were explored by optimizing the geometries in the energy minimums for the reactants, the intermediates and the products, and the first-order saddle points for transition states using the Gaussian 09 program suite (B.01 [22] ).…”

Inhibition Performance Study of Vanadate on AA2024-T3 at High Temperature by SEM, FIB, Raman and XPS

“…Cheng et al described this catalytic process as involving the insertion of an O 2 molecule at a V 4+ site to form a cyclic VO 2 peroxide, which in turn leads to oxidative dehydrogenation of a hydrocarbon such as propane. 45 A similar scenario could take place between two monovanadate units (rather than a monovanadate unit and propane), leading to the polymerization of vanadate. 45 Furthermore, a similar mechanism was observed by Frankel et al on IMCs in AA2024-T3 with a molybdate inhibitor, wherein molybdate was quickly reduced at the surface of IMCs but was subsequently oxidized to protective Mo 6+ oxide layer.…”

“…45 A similar scenario could take place between two monovanadate units (rather than a monovanadate unit and propane), leading to the polymerization of vanadate. 45 Furthermore, a similar mechanism was observed by Frankel et al on IMCs in AA2024-T3 with a molybdate inhibitor, wherein molybdate was quickly reduced at the surface of IMCs but was subsequently oxidized to protective Mo 6+ oxide layer. 46 Although this scenario is speculative at this point, in addition to similar reactions in vanadate catalysts, this work provides experimental and theoretical evidence to support the occurrence of a similar process to the reactions observed in vanadate catalyst noted above, albeit at the surface of metal electrode exposed to aqueous vanadate rather than at catalytic vanadate surface.…”