Computational Studies On Heat Capacities And Gas Separation Properties Of Metal-Organic Frameworks

Metal-organic frameworks (MOFs) have exhibited good application prospects in the fields of gas adsorption separation due to their many unique structureal features, such as complicated structure, large specific surface area and large pore volume. Many experimental and simulation studies have shown that, the thermodynamic effects play a very important role during the processes of adsorption separation, thus becoming one of factors that need to be taken into account in the related studies. At the moment, little information is available for the heat capacities of MOFs from both experimental and theoretical studies. Moreover, it is not only difficult to get the heat capacities experimentally but also is a waste of resources. Considering this issue, we provided a fast estimation method for calculating the heat capacities of MOFs, with the aid of quantum chemistry calculations, and then built the heat capacity database for MOF materials.MOFs have a good industrial application prospect in terms of gas adsorption separation. By systematically modulating the structures, porevolumes, specific surface areas and chemical functionalities of such types of materials, the interaction forces between adsorbate molecules and their frameworks can be regulated and thus the separation capabilities of MOFs can be greatly enhanced. As a representative of MOFs, UiO-66 (Zr) possesses high thermal stability, chemical stability, and excellent property on gas separation. The existing studies found that the crystal structure of this MOF synthesized under certain conditions is not perfect and has some certain defects. Therefore, according to the available experimental information, we built plausible structure models of UiO-66 (Zr) with ligands defect in this work. We studied the microscopic mechanism of gas adsorption in the defect UiO-66 by using the method of molecular simulation, and studied the effect of defects on the adsorption heat and adsorption selectivity. The detail contents are as follows:1. Using the idea of group contribution, we studied the heat capacities of 7 different MOFs by the method of computational chemistry, and built the heat capacity database of secondary building units (SBUs) related to these MOFs. This work provides a basis for calculating the heat capacities of other MOFs containing these SBUs.2. On the basis of the built structure model of UiO-66 with ligand defect, and removed the coordination water, we explored the separation performance of this material for CO2/N2 and CO2/CH4 mixtures by GCMC simulation. The results show that the defect UiO-66 has a higher adsorption capacity toward CO2, The underlying microcosmic separation mechanisms were further analyzed so as to provide a theoretical basis for the design of new materials.3. We studied the influence of coordination water on the separation properties of defect UiO-66. The results indicate that the existence of coordination water molecules will greatly reduce the gas adsorption capacity and adsorption selectivity compared to those of the material with perfect structure.