论文标题:超临界反应体系在MCM-22中及苯在碳纳米管中的吸附与扩散的分子模拟
Adsorption and Diffusion of Benzene-Propylene System in Supercritical Condition in MCM-22 and Benzene in Swnts with Method of Molecular Simulation
论文作者 孙书勇
论文导师 汪文川,论文学位 硕士,论文专业 化学工程
论文单位 北京化工大学,点击次数 684,论文页数 84页File Size3576k
2003-02-01论文网 http://www.lw23.com/lunwen_134612/ Monte Carlo 模拟;分子动力学;MCM-22分子筛;纳米碳管;烷基化反应
Monte Carlo simulation, Molecular dynamic, MCM-22, Nano-tubes, Alkylation
本论文由两部分内容组成,即超临界烷基化反应体系和轻烃在MCM-22型分子筛中吸附和扩散行为的分子模拟,以及纳米碳管中的芳香烃化合物吸附和动力学行为的分子模拟。纳米介孔材料及其限定空间内的流体构成了化工中的复杂体系。该复杂体系内流体分子表现出的丰富的相态和奇异的行为是纳米介孔材料在化学工业及其他相关工业中广泛应用的重要原因。本文主要选择两种介孔材料作为研究对象,即在化学工业中被广泛应用的分子筛MCM-22以及准一维材料单壁碳纳米管(SWNTs)。分子筛方面工作主要针对烷基化反应相关内容进行展开;碳纳米管方面工作主要研究其在不同管径条件下对内部流体的限制作用,以及由此而引起的吸附和扩散现象。低碳芳构化的工艺已引起了越来越多化学家的关注。为了更充分地利用天然气,已经相继出现了不同的方法。自ZSM-5型高硅沸石问世以来,低碳的芳构化反应的研究基本上都是以其为催化剂,而且主要研究的是C3~C6烷烃。虽然对乙烷及甲烷的研究报道很少,但它们直接芳构化亦有望成为芳烃生产的新途径。本工作在MCM-22型分子筛的模拟计算中,用巨正则Monte Carlo 方法,采用全原子力场,模拟了纯的以及混合的轻烃化合物在其内部的吸附量、吸附热、以及选择性情况。得到的结果与本文通过理想吸附溶液理论(Ideal Adsorbed Solution)所计算的结果进行了比较,同时还与其他纯硅材料的实验及模拟数据进行了比较。结果表明,轻烃的双组分混合物在MCM-22型分子筛中的选择性普遍优于其他纯硅材料;甲烷-乙烷体系、甲烷-丙烷体系在MCM-22型分子筛中基本上属于理想吸附行为。从技术和经济考虑,异丙苯法是当今新建苯酚、丙酮生产装置的唯一可选方法。因此,异丙苯合成技术的发展和进步具有非常重要的意义。传统的生产工艺以固体磷酸或三氯化铝为催化剂,设备腐蚀和环境污染严重。发展趋势是以固体酸分子筛催化剂来取代。由于苯的深度烷基化,生成的碳占据了酸性中心,容易导致分子筛催化剂的失活,这是分子筛催化苯烷基化新工艺亟待解决的问题。而超临界流体具有优良的传热、传质及溶解萃取性能,能有效萃取焦前驱体,可望延缓催化剂结焦失活。本文针对这一问题,应用巨正则Monte Carlo方法以及分子动力学方法在超临界条件下模拟了不同组成的苯-丙烯体系在MCM-22型分子筛中的吸附和扩散,主要考察了二者在分子筛内的吸附位置以及扩散情况。从而为确定比较适合的反应条件以及相应的组分的混合比例提供理论依据。结果发现苯主要吸附在十二元环超笼内部,而丙烯主要吸附在十元环通道内。在4/1和5/1的组成条件下,吸附相的摩尔分率达到~1/1。在三种不同组成的体系的超临界条件下,当对分子筛作为刚性处理的时候,体系的组成的变化,对吸附相分子的扩散系数不会产生很大的影响。但是作为柔性处理的时候,丙烯的扩散系数要增加一个数量级。对三角形排列的准一维介孔材料进行了系列的模拟研究,分别在四种孔径条件下(0.95nm, 1.42nm, 1.89nm, 3.77nm),在300K,100kPa的条件下,研究了纳米碳管对内部流体(苯)的限制作用。结果发现,在孔径较小的情况下(0.95nm),苯主要吸附在碳管的中心位置,而且苯环的法向方向与碳管的中心轴垂直。其显示出碳管对苯环的强烈的限制作用。当管径增加到1.42nm的时候,苯环在管内部只能形成一层六元环柱形壳。当管径再大一点(1.89nm),苯分子不但会形成单层六元环柱形壳,而且在壳内部,苯环沿着中心轴方向成对出现。在管径很大的情况下(3.77nm),苯分子不但会填充整个管的内部,而且还会进入管间的空间。
This thesis is made up of two relatively independent parts, i.e. adsorption and diffusion of light hydrocarbons and benzene in MCM-22 and benzene in nanotubes.Porous nano-materials and confined fluids constitute complicated systems in chemical engineering field. The fantastic phase behavior of confined fluids is an important factor to the application of porous nano-materials in chemical engeneering and other fields. In this thesis, two kinds of materials were chosen to study (i.e. MCM-22 and quasi-one dimension SWNTs). Among the microporous materials, MCM-22 zeolites are outstanding for their special characteristics and widespread used. The use of microporous materials for the alkylation of benzene with propylene to produce cumene is studied in this thesis. The strong confinement of small-diameter SWNT to benzene molecules inside and the diffusion behavior of benzene molecules within the SWNT are also studied.In the simulation of light hydrocarbons in MCM-22 zeolite, the grand canonical Monte Carlo (GCMC) method and an all-atom forcfield model are used. Pure and binary component sorption equilibria are discussed. The results show good agreement with predictions made using the Ideal Adsorbed Solution (IAS) theory. Compared with other zeolites, the selectivity of MCM-22 in binary component is higher. The systems of methane-ethane and methane-propane behave ideally within MCM-22 zeolite.The methods of grand canonical Monte Carlo and molecular dynamics are used for investing the positions of benzene and propylene adsorbed in the zeolite MCM-22 and their diffusion behavior. The energies distributions and mass distributions show that propylene molecules are adsorbed in 10-MR channels of MCM-22 by preference and most benzene molecules are in 12-MR supercages. But with increase of the mole fraction of benzene molecules in the mixture, the number of benzene molecules in 10-MR channels also increase obviously. The simulations are performed at the super critical condition of the mixture, while the mole fraction of the mixture component varies. The critical points of mixture are determined using PR equation. The adsorption results show that the mole fraction of benzene molecules is ~0.5 in MCM-22 when NC6H6/NC3H6=5/1 or 4/1 in bulk phase, indicating that the two mole fractions in mixture are better than others for alkylation of benzene with propylene to produce cumene. Then the MD simulations are employed to analyze the dynamic diffusion process of benzene and propylene in zeolite. The results show that the diffusion coefficients of benzene and propylene molecules in MCM-22 would not change significantly with the varying of the mole fraction. The grand canonical Monte Carlo method is used to adsorption of benzene molecules in single walled carbon nanotubes(SWNT) in triangle array of different diameter(0.95nm, 1.42nm, 1.89nm, 3.77nm). Benzene preferably load in the center and parallel the centre shaft of SWNT when σ=0.95nm, showing strong confinement of small-diameter SWNT. With the increase of diameter of SWNT, monolayer or multilayer benzene molecules cylindrical shell will form within it. When the diameter is 3.77nm, benzene molecules not only load in SWNT but can reach the space formed by three neighboring SWNT neighbor each other.
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