Theoretical and experimental consideration of the reactions between VxOy+ and ethylene

Article abstract of DOI:10.1021/ja021349k

We present joint theoretical and experimental results which provide evidence for the selectivity of V_xO_y⁺ clusters in reactions toward ethylene due to the charge and different oxidation states of vanadium for different cluster sizes. Density functional calculations were performed on the reactions between V_xO_y⁺ and ethylene, allowing us to identify the structure-reactivity relationship and to corroborate the experimental results obtained by Castleman and co-workers (Zemski, K. A.; Justes, D. R.; Castleman, A. W., Jr. J. Phys. Chem. A 2001, 105, 10237). The lowest-energy structures for the V₂O_2-6⁺ and V₄O_8-10⁺ clusters and the V₂O_3-6⁺-C₂H₄ and V₄O₁₀⁺-C₂H₄ complexes, as well as the energetics for reactions between ethylene and V₂O_-6⁺ and V₄O₁₀⁺ are presented here. The oxygen transfer reaction pathway was determined to be the most energetically favorable one available to V₂O₅⁺ and V₄O₁₀⁺ via a radical-cation mechanism. The association and replacement reaction pathways were found to be the optimal channels for V₂O₄⁺ and V₂O₆⁺, respectively. These results are in agreement with the experimental results reported previously. Experiments were also conducted for the reactions between V₂O₅⁺ and ethylene to include an energetic analysis at increasing pressures. It was found that the addition of energy depleted the production of V₂O₄⁺, confirming that a more involved reaction rather than a collisional process is responsible for the observed phenomenon. In this contribution we show that investigation of reactions involving gas-phase cationic vanadium oxide clusters with small hydrocarbons is suitable for the identification of reactive centers responsible for selectivity in heterogeneous catalysis.