【作者】 郗瑞鑫；

【导师】 吕永康；

【作者基本信息】 太原理工大学 ， 化学工艺， 2011， 硕士

【摘要】 氯乙烯是一种重要的化工生产原料,可以用来合成聚氯乙烯等多种化工产品。随着氯乙烯生产规模的不断扩大,它对人类健康的危害、对大气臭氧层的影响也日益严重。氯乙烯选择性环氧化反应是一种不对称分子的环氧化反应。对称分子的环氧化反应,比如对乙烯选择性环氧化反应的研究在过去已经取得较大进展,但对不对称分子环氧化反应的研究并不充分。本文选取氯乙烯分子为研究对象,对其在不同金属表面的选择性环氧化反应进行了研究,希望能够对完善不对称分子环氧化反应的反应机理有所助益。本文应用以平面波基组展开波函数的第一性原理的维也纳从头算软件包(Vienna Ab-initio Simulation Package, VASP),在建立的平板模型基础上,在广义梯度近似(generalized gradient approximation)泛函的PW91交换相关势及PAW原子实势水平上,对氯乙烯在Ag(111)、Pt(111)、Rh(111)、Ag(100)而的选择性环氧化反应进行模拟计算。具体计算过程包括：应用能量梯度法对反应物氯乙烯和氧原子,产物氯代环氧乙烷、氯乙醛和乙酰氯在金属表面吸附态的构型进行优化；应用climbing-nudged(cNEB)方法搜寻基元步骤的一级鞍点——过渡态；应用对角化Hessian矩阵的方法,进行振动频率计算,通过检查正则振动中是否存在唯一虚频方法来确定过渡态；通过对反应体系各物种进行总能量、活化能、反应热等计算和对反应路径坐标跟踪确定反应机理；最终,在分析反应机理和活化能基础上,估算了不同金属表面对氯乙烯氧化反应产物氯代环氧乙烷、氯乙醛和乙酰氯的选择性。在采用上述方法对氯乙烯在所涉及的金属表面的选择性环氧化反应进行模拟计算后,得到了如下结论：(1)氧原子在Ag(111)、Rh(111)、Pt(111)表面的稳定吸附位为fcc位；而且当氧原子吸附于Ag(111)、Rh(111)、Pt(111)表面fcc位时,吸附作用按照Ag、Pt、Rh的顺序逐渐增强。吸附作用的强弱同吸附原子与底物金属表面成键的数目成正比关系,即成键越多,吸附作用会越强；(2)氯乙烯在金属表面的环氧化反应分为两步进行：首先由反应物进行反应生成OMMC中间体；然后中间体进一步反应,生成产物。在氯乙烯环氧化反应中,由于氯乙烯分子是一个不对称分子,所以能够形成三种OMMC中间体,反应最容易通过OMMC (3)中间体进行。在三种金属面上,由OMMC (3)到产物的反应活化能的顺序为Ag<Pt<Rh,但是,在Ag(111)面上的反应选择性要比在Pt(111)、Rh(111)面上的选择性要差。进行态密度分析后,可以发现OMMC在金属表面的吸附稳定性与上述的活化能有直接关系,OMMC中间体越稳定,则通过该中间体进行反应的活化能越高。更多还原

【Abstract】 Vinyl ch]oride(VC) is versatile intermediates for chemical synthesis, especially, poly-vinyl chloride(PVC). With the development of vinyl chloride processing, its damages become more and more serious for both humen and ozonosphere. The partial oxidation of vinyl chloride on different metal surfaces is one of asymmetric molecules oxidation reaction. Although a tremendous amount of work has accumulated in the literature regarding the reaction of ethylene to ethylene oxide, the detailed selective oxidation mechanism for the asymmetric molecules is insufficient. In the present work, we give a systemic density functional calculation of vinyl chloride selectivity oxidation on some oxygen preadsorbed metal surfaces, Ag(111), Pt(111), Rh(111) and Ag(100), intend to understand the mechanism of this reaction.All calculations were carried out using the Vienna ab initio simulation package(VASP) code. In order to model the metal surfaces, a periodical array model containing four atomic layers was used. The project-augment wave (PAW) method was used to describe the inner cores, and the electronic wave functions of the valence electrons were expanded on a plane wave. The exchange-correlation functional used to obtain the energy was the Perdew-Wang (PW91) implementation of the generalized gradient approach, and a climbing-nudged elastic band method (cNEB) was used for locating the transition state (TS), the frequency analysis was performed to confirm the transition state.The main conclusions of this work are summarized as follow:(1) The calculated results indicated that fcc site was the most steady adsorption site of O atom on Ag(111), Pt(111), Rh(111). The order of the adsorption energies is:Ag(111)<Pt(111)<Rh(111), and the more of band between O atom and metal atom, the more steady of O atom adsorption;(2) The results show that the reaction mechanism is a two-step process: first forming OMMC and then the products. Because of the asymmetry of vinyl chloride, there are three competitive reaction pathways, and all the processes are investigated. The results imply that the most possible pathway is pathwaylll. Compared the activation energies of the epoxidation reaction on Ag(111), Pt(111) and Rh(111), it is obviously that the reaction via OMMC(3) on Ag(111) is the most favored process. The activation energies of reactions from OMMC(3) to products are in the order of Ag< Pt< Rh. By analyzing the metal atom d-band center position, the order of the stability of OMMC(3) on different metal surfaces is obtained and the order is the same with that of the activation energies of vinyl chloride epoxidation. Namely, the more stable of OMMC intermediate, the higher of the correlative activation energy is.更多还原