Development Of Flexible Force Fields For MOFs And The Study Of Their Dynamic Properties Using Molecular Simulations

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.