Reactivities of VO⁺ _1–4 toward n-C_mH_2m+2 (m = 3, 5, 7) as functions of oxygen content and carbon chain length

Vanadium oxides are one of the most important heterogeneous catalysts that are widely used to oxidize hydrocarbon molecules into value-added chemicals. In order to reveal the mechanisms and the nature of active sites, numerous experimental and theoretical studies have been reported on the reactivities of gas-phase vanadium oxide clusters toward small molecules. However, there has been very limited research on the chemical reactivity changes associated with the oxygen contents of vanadium oxides and the carbon chain lengths of alkanes. In this work, the reactions of vanadium oxide ions VO⁺ ₁₋₄ with alkanes (n-C_mH_2m+2, m = 3, 5, 7) were systematically investigated by time-of-flight mass spectrometry and the reactions of VO⁺ ₁₋₃ with pentane were further studied by density functional theory calculations. Experimental results show that in the reactions of VO⁺, VO₃ ⁺, and VO₄ ⁺ with n-C₅H₁₂, in addition to the major adsorption processes, the activation of the C―H and C―C bonds of n-C₅H₁₂ was observed. The activation of both the bonds was observed experimentally during the reaction of VO₂ ⁺ with n-C₅H₁₂ with large branching ratios. Among the vanadium oxide cations studied, VO₂ ⁺ shows the strongest oxidizability and the generation of lighter alkanes and alkenes dominates the reactions; VO⁺ is more reactive than VO₃ ⁺. VO₄ ⁺ pocesses only one η²-O₂ unit. Due to the weak bond between VO₂ ⁺ and η²-O₂, the η²-O₂ unit is released in VO₄ ⁺/n-C₅H₁₂ system leading to the formation of VO₂ ⁺; thus VO₄ ⁺ cations reflect some reactivity of VO₂ ⁺. Although the oxidation states in the vanadium oxide clusters increase from +III in VO⁺ to +V in VO₂ ⁺ and +IV in VO₃ ⁺, the reactivity does not gradually increase. Moreover, the reactivity of the mononuclear vanadium oxide cations also does not exhibit a gradually increasing trend with the increase in oxygen content. Based on the observed reactivity trend, the adsorption channels gradually become weak as the carbon chain lengths increase; meanwhile, the dehydrogenation and C―C bond activation channels gradually become obvious and some oxygen transfer products appear. Therefore, much lighter fragments of alkanes/alkenes will be obtained if linear alkanes with more carbon atoms were reacted with VO⁺ ₁₋₄. The theoretical results are generally consistent with those obtained from the experiments. The various reaction channels and versatile reactivity of the mononuclear vanadium oxide cations investigated in this study not only offer new insights into gas-phase reactions but also shed light on the processes occurring on the surfaces of the corresponding condensed-phase catalysts.