Ma Yan-Ping, Xue Wei, Wang Zhe-Chen, Ge Mao-Fa, He Sheng-Gui
Beijing National Laboratory for Molecular Sciences, State Key Laboratory for Structural Chemistry of Unstable and Stable Species, Institute of Chemistry, Chinese Academy of Sciences, Beijing 100080, People's Republic of China.
J Phys Chem A. 2008 Apr 24;112(16):3731-41. doi: 10.1021/jp711027z. Epub 2008 Mar 19.
Density functional theory (DFT) calculations have been used to investigate acetylene cyclotrimerization catalyzed by titanium and vanadium dioxides. The calculated results illustrate that the overall process is highly favorable at room temperature from both thermodynamic and kinetic points of view. The mechanism of C2H2 cyclotrimerization over MO2 (M = Ti, V) can be understood as four steps: (1) a four-membered ring (-O-M-C=C-) formation that coordinates and activates the first C2H2 molecule; (2) the second C2H2 insertion into the M-C bond to form a six-membered ring (-O-M-C=C-C=C-); (3) the third C2H2 insertion into the M-C bond to form an eight-membered ring (-O-M-C=C-C=C-C=C-); and (4) contraction of the eight-membered ring and benzene formation and desorption. All of the reaction steps are overall barrierless with respect to the separated reactants (MO2C2xH2x + C2H2, x = 0, 1, 2). This theoretical study predicts that the M=O double bond in MO2 is very catalytic toward the C2H2 cyclotrimerization. The metal center in this study can be considered always in the same +4 oxidation state (Ti4+ and V4+). In contrast, two-electron cycling of the metal center is present in the documented mechanism for the C2H2 cyclotrimerization. The C2H2 cyclotrimerization over the Ti atom and TiO molecule is also studied, and the documented mechanism applies in this case. The new mechanism is suggested to apply to reactions using titanium and vanadium oxides as catalysts.
密度泛函理论（DFT）计算已被用于研究二氧化钛和二氧化钒催化的乙炔环三聚反应。计算结果表明，从热力学和动力学角度来看，整个过程在室温下都是非常有利的。C2H2在MO2（M = Ti，V）上的环三聚反应机理可理解为四个步骤：（1）形成一个四元环（-O-M-C=C-），该环配位并活化第一个C2H2分子；（2）第二个C2H2插入M-C键形成六元环（-O-M-C=C-C=C-）；（3）第三个C2H2插入M-C键形成八元环（-O-M-C=C-C=C-C=C-）；（4）八元环收缩并形成苯并解吸。相对于分离的反应物（MO2C2xH2x + C2H2，x = 0, 1, 2），所有反应步骤总体上都是无势垒的。该理论研究预测，MO2中的M=O双键对C2H2环三聚反应具有很强的催化作用。本研究中的金属中心可始终被认为处于相同的+4氧化态（Ti4+和V4+）。相比之下，在已报道的C2H2环三聚反应机理中存在金属中心的双电子循环。还研究了C2H2在Ti原子和TiO分子上的环三聚反应，在这种情况下适用已报道的机理。建议新机理适用于以钛和钒的氧化物为催化剂的反应。
