Acta Phys. -Chim. Sin. ›› 2019, Vol. 35 ›› Issue (5): 531-538.doi: 10.3866/PKU.WHXB201805231

• ARTICLE • Previous Articles     Next Articles

Reactivities of VO1–4+ Toward n-CmH2m+2 (m = 3, 5, 7) as Functions of Oxygen Content and Carbon Chain Length

Yue ZHAO1,Jiatong CUI1,Jichuang HU1,2,Jiabi MA1,*()   

  1. 1 Key Laboratory of Cluster Science, The Institute for Chemical Physics, School of Chemistry and Chemical Engineering, Beijing Institute of Technology, Beijing 102488, P. R. China
    2 Shanghai Yuda Industrial Co., LTD., The Institute of Shanghai Spacecraft Equipment, Shanghai 200240, P. R. China
  • Received:2018-05-09 Published:2018-10-19
  • Contact: Jiabi MA E-mail:majiabi@bit.edu.cn
  • Supported by:
    the National Key R & D Program of China(2016YFC0203000);the National Natural Science Foundation of China(21503015)

Abstract:

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 VO1−4+ with alkanes (n-CmH2m+2, m = 3, 5, 7) were systematically investigated by time-of-flight mass spectrometry and the reactions of VO1−3+ 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 VO1−4+. 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.

Key words: Mass spectrometry, Vanadium oxide cations, Alkanes, Density functional theory, Reaction mechanism

MSC2000: 

  • O643