【作者】 赵雷；

【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， 博士

【摘要】 金属—有机骨架材料(metal—organic frameworks, MOFs)是一种新型的纳米多孔材料,其在储气、催化和分离等领域有很大的潜在应用价值,MOFs已经成为当今在材料领域中的一个研究的前沿与热点。到目前为止,已经合成出来的MOF材料达到成千上万种,再加上此类材料的结构相当复杂多变。所以,如果单纯的采用实验的方法很难对其进行比较系统的研究。随着计算化学理论的发展,计算机模拟计算已经开始用于探索MOF材料的结构和性质。在计算结果指导下,进行材料的合成与筛选,可以节省大量的资源,促进MOF材料的实际应用。到目前为止,大多数工作都是局限在把MOF材料看做不运动的,来研究客体分子在其中的吸附和扩散行为。但是已有的实验和理论研究表明,某些MOF材料表现出明显的可变形性质,MOF材料自身的动力学特性的研究越来越受到国内外研究人员的关注,因此针对MOF材料开发可描述其柔性的力场,并且研究其动力学性质具有非常重要的意义。本文针对MOF材料开发柔性骨架力场,并且研究其动力学性质,主要内容如下：1、针对一种非常有代表性的MOF材料Cu-BTC,开发了其柔性力场,其参数来源主要是三个方面：其它力场,量化计算和根据实验数据拟合,通过与实验的晶体数据以及实验测得的热收缩性质、振动频率、体积模量相比较,证明我们的新力场可以很好的描述材料本身的结构以及运动情况。此外,通过计算CO2吸附等温线并且与实验值和传统的刚性力场结果相比较,证明我们的新力场不但可以很好的描述材料本身的运动性质,还可以描述材料与客体分子之间的作用力。2、基于Cu-BTC的柔性力场,开发出PCN-6’以及MOF-HTB’的柔性力场,此力场能很好的描述晶体的结构性质,证明我们的Cu-BTC柔性力场可以很容易的推广到其它类似材料力场的开发中。首次通过模拟预测出PCN-6’和MOF-HTB’也是具有负热膨胀性质的MOF材料,并研究了其机理。通过计算得到膨胀系数分别为a=-9.2x10-6 K-1和a=-11.5x10-6K-1。比较三种材料的负热膨胀系数,得出MOF材料负的热膨胀系数的绝对值与有机配体的长度有关,配体越长,负的热膨胀系数的绝对值越大。另外,研究了加入CO2分子对Cu-BTC的负热膨胀系数产生的影响,由于C02的膨胀系数为正,两者发生中和,而改变了Cu-BTC膨胀系数。3、通过设计材料,以及针对新材料开发柔性力场,利用分子动力学模拟,对材料的负热膨胀行为进行了系统研究,证明我们所开发的Cu-BTC柔性力场可以推广应用的新材料的性质预测。结果表明,通过改变配体的长度可以控制其膨胀系数,可以为正或为负,这就为以后设计目标膨胀系数的材料提供了理论基础。通过比较,本工作还得出一个结论,除了长度,有机配体的自身的性质也会影响材料的膨胀系数。4、研究了Cu-BTC, PCN-6’和MOF-HTB’的体积模量和杨氏模量。通过分子动力学模拟,发现Cu-BTC, PCN-6’和MOF-HTB’在压力到达一定值的时候会发生形状改变,Cu-BTC的抗压能力要远远的大于后两种MOF材料。通过截取构型文件分析,得到发生突变的机理：主要是克服两种力发生变形,即二面角Cu-O-C(1)-C(2)和有机配体之间的范德华作用。当外部的压力足够大到能克服此能垒的情况下,材料的形状发生突变。通过对比三种材料的结构特点,得出Cu-BTC比另外两种材料硬的原因,主要是有机配体和金属簇的连接方式不同。5、通过模拟预测出COF-102是具有负热膨胀性质的材料,其膨胀系数为-1.51x10-6K-1,比MOF材料的负热膨胀系数要小,比MOF材料的负热膨胀系数小的原因是由于其本身的结构造成的。通过动力学轨迹分析,发现COF-102的负的热膨胀行为是由于苯环的摆动引起的,与MOF材料的机理一致。从机理上可以得到结论,其它的三维COF材料也应该同样具有负热膨胀性质。更多还原

【Abstract】 Metal-organic frameworks (MOFs) have been recognized as a new family of nanoporous materials with a wide range of possible applications in gas storage, separation and catalysis etc. The study of MOFs has become a research frontier area of materials, and hotspots. Up to now, many kinds of MOFs have been synthesized and because of the complex structure of MOFs, it is insufficient to conduct systematic studies by purely experimental approach. With the development of chemical theory, computational chemistry has been used to study the structure and properties of MOFs. It can provides theoretical guidance for the design of MOFs and the determination of optimal industrial operation conditions, which also saves a lot of time for complicated experimental works. Extensive molecular simulations have been performed on the adsorption and diffusion in MOFs. However, most of them used rigid frameworks with the framework atoms in MOFs fixed in their experimentally determined crystallographic positions. Since MOFs are flexible and may exhibit substantial changes in unit cell volume upon external stimulus such as temperature and guest molecules, it is highly needed to develop flexible force fields to study their dynamic properties.In this work, flexible force fields for MOFs have been developed and dynamic properties of MOFs have also been studied. The main contents and findings are summarized as follows.1. A new force field that can describe the flexibility of Cu-BTC was developed in this work. Part of the parameters were obtained using density functional theory calculations or fitting by us to reproduce the experimental values, and the other part were taken from other force fields. The new force field could reproduce well the experimental crystal structure, negative thermal expansion, vibrational properties, and bulk modulus as well as adsorption behavior in Cu-BTC. We believe the new force field is useful in understanding the structure-property relationships for MOFs.2. Base on the new force field of Cu-BTC, force fields can describe the flexibility of PCN-6’and MOF-HTB’were further developed in this work, indicating that the approach can be extended to other MOFs easily. The results of molecular simulations demonstrate that PCN-6’and MOF-HTB’also show negative thermal expansion (NTE), and the origin of the NTE behavior is the motion of the aromatic carbon rings with temperature. The NTE coefficients are-9.2×10-6 K-1 and-11.5×10-6 K-1, respectively. By the comparison of the NTE coefficients of the three MOFs, it is clear that the length of the organic linker has an effect on the NTE coefficients of MOFs. The thermal expansion behavior of Cu-BTC with CO2 adsorption has also been studied. Because CO2 expands with the increase of temperature, the NTE coefficients of Cu-BTC will be changed when adding CO2.3. Three kinds of material are constructed, deduced from Cu-BTC and MOF-HTB’ by changing the organic linkers. Then, the flexible force fields for them were developed, and molecular simulations were performed on their thermal expansion behavior. The results demonstrate that the thermal expansion coefficients could be adjusted by changing the length of the organic linkers. In addition, the property of the organic linker is another factor that influences the thermal expansion coefficient of material.4. The bulk and Young’s modulus of Cu-BTC, PCN-6’and MOF-HTB’were predicted using molecular simulations. The structures of Cu-BTC, PCN-6’and MOF-HTB’will distort when the pressure is up to certain values, and Cu-BTC is less flexible than the other two MOFs. The mechanism is that when the pressure adding to the materials is large enough to overcome the two kinds of force, the torsion of Cu-O-C(1)-C(2) and van der Waals force between the organic linkers, the structure will distort. And the structure of Cu-BTC makes it more rigid than the other two MOFs.5. We performed a computational study on the thermal expansion behavior of covalent organic frameworks (COFs). The results demonstrate that COFs show negative thermal expansion (NTE), and the origin of the NTE behavior is the motion of the aromatic carbon rings with temperature, providing a better understanding of this new family of materials.更多还原