Molecular Simulation Studies On Gas Adsorption, Diffusion And Catalytic Conversion Process On Micro-Mesoporous Materials

In this thesis, we have studied the gas adsorption, diffusion and catalytic conversion process in micro-mesoporous materials with the force field as the core. The involved materials include zeolites, MOFs (Metal Organic Frameworks) and COFs (Covalent Organic Frameworks). The employed computational methods include ab initio quantum chemistry theory, Quantum Mechanics/Molecular Mechanics (QM/MM) method and Grand Canonical ensemble Monte Carlo (GCMC) simulation. The research topics involve zeolitic catalysis for producing propene and ethene, how to improve the hydrogen uptake of COF materials by GCMC simulation, and the complete-freedom all-atom force field development and application of IRMOFs crystals. These works include the following results and conclusions.1. Firstly, we have studied the catalytic cracking of 1-butene to produce propene and ethene over acidic HZSM-5 and HFAU zeolites by QM/MM scheme from two-layer ONIOM method. Two reaction mechanisms, direct cracking and dimerization cracking, are proposed. The calculated results show that the direct cracking mechanism is endothermic but the dimerization cracking is exothermic, which indicates that dimerization cracking is more favorable on both zeolites. Through the analysis for the activation barrier of rate-limiting step for dimerization cracking on both zeolites, we have found that the activation barrier on HZSM-5 is lower and the reactions occur more easily. The HZSM-5 zeolite is more catalytically effective. These results are in agreement with experimental works and could explain the experimental findings. The main reason for these results is that the pore size of HZSM-5 is smaller and the interactions between zeolitic framework and organic molecules are larger. The stabilization ability of framework for organic molecules is stronger, which makes the transition states more stable and the activation barriers height lower.2. In the study on hydrogen storage materials by quantum chemistry method, we have investigated the interactions of H₂ molecules with Cn_HnN_5-n- and Cn_HnN_5-n-Li systems (n = 1-5). The system is selected mainly because the Li cation could interact with substrate more strongly than the Li atom and so the lithium salt is more stable and the experimental operation is potentially safer. In this work, the binding energies of H₂ with Cn_HnN_5-n- and Cn_HnN_5-n-Li are calculated and the nature of interaction could be mainly attributed to charge-quadrupole and charge-induced-dipole forces by NPA analysis. For the maximum number of adsorbed H₂ molecules, the following rules could be obtained. For the CmHmNn- anions [n = 1-5], (2n+2) H₂ can be adsorbed where the n is the number of nitrogen atoms. Each nitrogen atom may bind to two H₂ molecules. Two additional H₂ can be adsorbed on both sides of the aromatic ring. For the lithium salts, the situation is slightly complicated. If the Li binds to one nitrogen atom, then the number of adsorbed H₂ is five. When the Li binds to two nitrogen atoms, four H₂ can be adsorbed over the Li atom. If the Li cation resides on both sides of aromatic ring, i.e. the vertical configuration, the maximum number of adsorbed H₂ is three. Additional H₂ molecules would lead to coplanar configurations. All these QM calculations provide necessary data for force field parameterization.3. In this thesis, we have also investigated how to dope Li cation to improve the hydrogen storage ability of COF material by GCMC simulation. The proposed method is doping CHN4-Li group into COF material and the resulting crystal is called PAF-4. By fitting the first-principle quantum chemistry data, we have obtained the force field parameters that could describe the interactions of H₂ molecules with CHN4-Li. Based on the first-principle-derived force field, the GCMC simulation has been carried out to predict the hydrogen uptake of Li-cation-doped COF material. The calculated results suggest that at 77K and 10 MPa, the H₂ gravimetric adsorption of PAF-4 is 20.68 wt% and the volumetric adsorption is 68.61 g/L. The isosteric heat at near zero loading is 14.54 kJ/mol. At 233 K, the maximum gravimetric uptake is 5.08 wt%, which reaches the 2010 DOE target of 4.5 wt% (the lowest standard).4. The last section of this thesis is the complete-freedom all-atom force field development and applications of MOF-5 and IRMOFs crystal. The employed functional form is TEAM force field. Through computing the crystal properties, such as the thermal expansion coefficient, crystal bulk modulus and Young's modulus, diffusion coefficient and activation barrier of benzene molecules in MOF-5, and atomic vibrational spectrum, we obtained the results that was good agreement with literature values and validated our computational methods. Furthermore, we developed the TEAM force field for MOF-5 crystal. This force field could predict very well the properties mentioned above. The calculated data are in accord with the results predicted by other force fields, such as CVFF, MM3 and DREIDING and are improved to some extent. From these work, we extended the force field of MOF-5 into that of IRMOFs. Though the progress is preliminary, the predicted lattice constants of many crystals by the complete-freedom force field of IRMOFs are in good agreement with experimental results.