VO_(1-4)^+阳离子与n-C_mH_(2m+2)(m=3,5,7)烷烃的反应性研究:氧含量和碳链长度的影响（英文）

Reactivities of VO_(1–4)^+ Toward n-C_mH_(2m+2)(m = 3, 5, 7) as Functions of Oxygen Content and Carbon Chain Length

本论文利用飞行时间质谱的实验方法对钒氧阳离子VO^+_(1–4)与直链烷烃(n-C_m H_(2m+2),m=3, 5, 7)的反应性进行了系统研究,并且利用密度泛函理论方法对VO^+_(1–3)与戊烷的反应进行了理论计算。实验结果表明,在VO^+,VO_3^+和VO_4^+与n-C_5H_(12)反应中,吸附反应为主要的反应通道,同时还伴随着C―H键和C―C键的活化反应。在本实验体系中,VO^+的反应活性要优于VO_3^+,而VO_2^+具有最优的反应活性,可以有效的活化烷烃的C―H键和C―C键生成较轻的烷烃和烯烃。VO_4^+含有一个η~2-O_2单元,由于VO_2^+和η~2-O_2之间的弱相互作用力,在实验中该阳离子经过一定时间的冷却碰撞后会释放η~2-O_2单元,并以吸附为主要通道。尽管钒氧阳离子中氧原子的氧化态从VO^+中的+Ⅲ增加到VO_2^+中的+V和VO_3^+中的+IV,其反应性却没有逐渐增加。基于目前的反应性趋势,我们做出推测如果具有更多碳原子的直链烷烃与VO^+_(1–4)反应,将会生成更轻的烷烃/烯烃片段。本文中理论计算结果与实验观测结果基本一致。同时,我们的研究可以帮助研究者们理解在凝聚相催化剂表面发生的微观反应过程,从而建立更为完善的催化反应机理。

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 η^2-O2 unit. Due to the weak bond between VO2^+ and η^2-O2, the η^2-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.