(394d) First Principles Study of Propane Oxidation On Supported and Unsupported V2O5 Catalysts: C-H Bond Activation and Reaction Path Analysis

The oxidation of hydrocarbons is of fundamental and technological interest because it provides a potential route to effectively transform hydrocarbons to value-added and synthetically useful chemicals and to reduce environmental pollution. Examples of such processes are the oxidative dehydrogenation of light alkanes to olefins, the production of maleic anhydride from butane and of phthalic anhydride from o-xylene, and the elimination of volatile organic compounds (VOCs) from exhaust streams. The selection or the development of an appropriate catalyst for each of these reactions is a challenging task. Theoretical studies based on first principles can help to develop an understanding of the reaction mechanisms involved and the parameters responsible for reactivity and selectivity. Moreover, ab initio calculations allow the determination of kinetic (e.g. activation energies) and thermodynamic (e.g. reaction enthalpies) parameters needed for the development of a suitable microkinetic model.

In this study the dehydrogenation of propane on the fully oxidized vanadia surface is analyzed in the absence of gas-phase oxygen using spin-polarized periodic density functional theory (DFT) with the Perdew-Burke-Ernzerhof (PBE) functional and the projector augmented wave (PAW) method. The elementary steps of the oxidation reaction on the vanadia surface are identified and the formation of other possible by-products (propanol, propanal, acetone) is investigated (Figure 1). Particular attention is given to the C-H bond activation, as this is considered to be the rate determining step of the oxidation mechanism [1]. Both methyl and methylene C-H bond activation have been considered. For the construction of the possible competing activation pathways all structurally and electronically distinct oxygen sites, i.e. singly coordinated vanadyl, doubly coordinated bridge and triply coordinated chain oxygen, of the catalyst have been taken into account. Apart from the unsupported vanadia catalyst represented by the (001) V₂O₅ surface, the supported vanadia catalyst is modeled by an epitaxial monolayer of vanadia on (001) TiO₂ anatase [2].

The C-H bond activation occurs via a direct hydrogen abstraction by a lattice oxygen. The vanadyl oxygen is the most reactive site for propane activation, leading always to the lowest activation energies. In agreement with the order of the C-H bond dissociation energies in propane [3], the hydrogen abstraction from the methylene group is more preferable than that from the methyl group. Interestingly enough, the use of a vanadia monolayer supported on titania strongly enhances the C-H bond activation as compared to bulk V₂O₅, yielding a lower activation energy and a more exothermic chemisorption energy (Figure 2). In accordance with experimental observations [4,5], the calculations show that the titania support not only modifies the activity of the vanadia monolayer but it also affects the selectivity of the catalyst, favouring the formation of propene compared to the formation of oxygenated products (i-propanol and acetone).

References:

[1] K. Chen, E. Iglesia, A.T. Bell, J. Catal., 192 (2000) 197

[2] K. Alexopoulos, P. Hejduk, M. Witko, M.-F. Reyniers, G.B. Marin, J. Phys. Chem. C, 114 (2010) 3115

[3] Y.-R. Luo, Handbook of bond dissociation energies in organic compounds, CRC Press: Boca Raton, Fla., 2003

[4] G.C. Bond, Appl. Catal. A, 157 (1997) 91

[5] B. Grzybowska, Top. Catal., 11/12 (2000) 23

Figure 1. Investigated reaction pathways for propane oxidation over the vanadia surface.

Figure 2. Calculated energy diagrams (at 0 K) for propane oxidation on the vanadyl sites of the unsupported (V₂O₅) and supported (V₂O₅/TiO₂) catalyst. Numbering of transition (-/-) and intermediate states (-) corresponds to Figure 1.

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