Molecular Simulation And Experimental Study Of Carbon Dioxide Adsorption And Separation In Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs), commonly recognized as "soft" analogues of zeolites, is a new class of nanoporous materials. Due to their large surface areas, adjustable pore sizes and controllable properties, as well as acceptable thermal stability, MOFs are promising candidates for a wide range of applications in gas storage, separation, catalyst, biochemistry, and pharmacy, etc. The study of MOFs has become a research frontier area and hotspots of materials. However, due to the complexity and diversity of MOFs only using experimental method is not adequate for the systematic studies. With the development of computational theory and method, molecular modeling can provide a great opportunity for studying MOFs deeply and systemically, leading to deep insights into the underlying mechanisms.In this research, gases adsorption and separation in MOFs have been studied though both experiment and molecular simulation, which fully yields the greatest benefits of molecular simulation in material design, preparation and performance prediction. It also covers the shortage of experiment in the research of microscopic mechanism and realizes the transformation from experimental to quantification and directivity. The main contents and findings are listed as follows:1. A systematic Grand canonical Monte Carlo simulations was performed to investigate the "stepped behaviors" of carbon dioxide adsorption in five isoreticular metal-organic frameworks (IRMOFs).The results show that the stepped phenomenon occurs easily when the temperature is low and the pore size is large for these IRMOFs. The critical pressure and temperature where the stepped behavior occurs shows a linear relationship with the pore size. The results also further indicate that the electrostatic interaction between CO2-CO2 molecules plays a dominant role on stepped behavior. A new MOF named IRMOF-1-4Li was also designed. In addition, we apply the stepped behaviors to the separation of gas mixtures such as CO2/CH4 and find that the stepped behaviors can significantly enhance the adsorption selectivity of CO2 from the mixture.2. The adsorption and separation properties of two zeolite-like metal-organic frameworks (ZMOFs) named usf-ZMOF and sod-ZMOF were studied by Grand canonical Monte Carlo simulations and experiment in detail. Molecular simulation predicted that the adsorptive capacity of CO2 in ion-exchanged sod-ZMOF is superior to the as synthesized samples and the selectivity of CO2 from CO2/CH4 was higher in ion exchanged ZMOFs than some published MOFs. So we synthesized the two ZMOFs according to the solvthermal method, then ion exchanged and detected. The results showed that the trend of experimental results was in accordance with that of molecular simulation.3. One of the MOFs called Cu-BTC was synthesized under hydrothermal conditions. The effect of supercritical carbon dioxide desiccation on the structural properties was investigated in Cu-BTC. The BET specific surface area of the samples with supercritical carbon dioxide desiccation and ordinary desiccation were 1679.082,1545.171 m2·g-1, and the pore volume were 0.796,0.559 cm3·g-1, respectively. The results indicated that supercritical carbon dioxide desiccation can improve the pore properties but the effect is not evident. Supercritical carbon dioxide desiccation may be possibly more useful for the preparation of MOFs with flexible frameworks.