【作者】 向辉；

【导师】 仲崇立；

【作者基本信息】 北京化工大学 ， 化学工程与技术， 2011， 硕士

【摘要】 金属-有机骨架材料(Metal-Organic Frameworks, MOFs)是一类新型的多孔纳米材料,它具有很多优点：具有较好的热稳定性、具有多样性的组织结构、具有较大的比表面积和很高的孔隙率。因此,在分离、催化和储能等领域的应用前景很广阔。在这类材料中,MOF-5材料是很典型的,它具有很规则的孔道结构,形成了很有序的空间结构,能反应出一般MOFs的基本特性。在工业生产中,烷烃又是作为最基础的原料。因此,研究其在多孔材料中的扩散性质,具有很重要的现实意义。本文就是以MOF-5为材料,以C1-C4烷烃作为客体分子,采用分子模拟的方法研究了各个因素对分子扩散的影响。这对这类材料的实际应用和新材料的开发具有指导意义。本文主要采用分子动力学来进行模拟研究,材料骨架采用柔性和刚性的力场作为对比。研究的主要内容如下：(1)验证所选力场参数的可靠性。根据参考文献和资料,选取合适的分子和骨架力场参数,选择所要用的模拟程序,进行分子模拟。在相同的条件下所得结果和文献对比,吻合较好。(2)对温度、分子数、骨架柔性和刚性、烷烃链长四个因素分别进行分析。在相同条件下,MOF-5骨架采用柔性和刚性力场作为模拟对比。各种烷烃都表现出当骨架为柔性时的自扩散系数大于骨架为刚性时的。柔性骨架更有利于分子的扩散。(3)其它因素相同,改变扩散分子的链长进行模拟。发现扩散分子都是随着链的增长自扩散系数都是减小的。分子的链越长,分子之间以及分子和材料之间的相互作用都增加,碰撞加剧,扩散分子之间的相对运动空间减小,扩散减小。(4)温度和分子数这两个单因素是交互作用的。在较低200 K温度时,分子动能较小,材料对分子的吸附作用较强,起主要作用,导致扩散分子自扩散系数Ds会随所吸附的分子数增加而增加,达到一个极大值后又由于分子间相互阻碍开始降低。在较高温度300 K和500 K时,分子的动能随温度的升高而增加,这时扩散分子能够在一定程度上克服材料对其吸附的限制作用,所以此时分子以扩散为主。当分子数增多时,由于分子阻碍作用Ds降低。(5)本工作又从能量方面来分析了自扩散系数的影响。根据Arrhenius方程,我们做出了各类烷烃的活化能。材料骨架柔性时的分子活化能总是小于刚性的情况,即柔性骨架Ds大于刚性骨架Ds。并且可以看出,随着分子链长的增加,分子活化能是逐渐增加的,从而说明分子的Ds是逐渐减小的。更多还原

【Abstract】 Metal-Organic Frameworks (MOFs) is a new type of nanoporous material. It has many advantages:structure diversity, high surface area and porosities. Therefore, in the field of separation, catalysis and gas storage it has very broad prospect. In this kind of materials, MOF-5 material is very typical, which has very regular three-dimensional pore structure. In industry, as the basic raw materials, it is important to study alkanes’ diffusion properties in porous materials. In this work, MOF-5 and the C1-C4 alkanes are adopted to study the factors that affect molecular diffusion.In this paper, molecular dynamic simulation was used to study the diffusion properties, and the framework of MOF-5 was treated both flexible and rigid for comparison. The main contents and findings are summarized as follows:(1) Testing of the reliability of the force field parameter. The force fields and the simulation program were tested by comparing with literature, and good agreement was obtained.(2) Analysis of the influencing factors. The effects of the four factors: temperature, the number of molecules, flexibility of the framework and the chain length of alkanes, were analysized. The self-diffusion coefficients of the alkanes are larger in flexible framework than that in the rigid one, illustrating that flexible framework is more convenient to diffuse for molecular.(3) When the other factors are the same, the longer the chain, the slower the diffusion.(4) Temperature and the number of the molecules are two factors that interplay. At lower temperature of 200 K, the kinetic energy is smaller, but the adsorption force between the material and molecules is stronger, so it is a major role, which causes the self-diffusion of molecules increase with the number of molecules increase. And after reached a peak the self-diffusion begin to reduce because of inhibitions between molecules. At higher temperatures like 300 K and 500 K, the kinetic energy of the molecules rises with the increase of temperature, and the diffusion is the main role.(5) To understand better the diffusion mechanism, we further calculated the activated energy. The activation energy in the flexible framework is smaller than that in the rigid one, and thus the Ds in the former is bigger than that in the later. Furthermore, the activation energy increases with increasing the length of chain, resulting a decrease of diffusivity with increasing chain length.更多还原