Quantum Chemical Calculation And Molecular Simulation Study Of Fluid Adsorption In Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs), "soft" analogues of zeolites, is a new class of nanoporous materials. MOFs having extremely high porosities, chemical diversity and tailored materials as robust solids with high thermal stability and well-defined pore sizes are promising materials for gases storage, separation, and catalyst. Computational chemistry, including molecular simulation and quantum chemistry, can not only overcome the limitations of traditional methods, but also provide theoretical guidance for the design of optimal adsorbents and the determinations of optimal industrial operations. It saves a lot of time for complicated experimental work and realizes the transformation from the experimental to quantification. So, theoretical studies of fluid behaviors in MOFs will be very useful for the application of these materials.In this work, gas storage, separation and diffusion in MOFs have been carried out using quantum chemical calculations and molecular simulations. The main contents and findings are summarized as follows.1. Quantum chemical calculations were used to study the adsorption of methane in IRMOF-1 and IRMOF-6. The results show that there are four adsorption sites including the corners of cell and the sides or upsides of the linkers in MOFs, and the OZn₄ clusters are the preferential adsorption sites. It could be concluded that the adsorption energy between methane and frameworks could be increased by increasing the length of the linkers and introducing electron-donor functional or polar functional groups to the organic linkers.2. A systematic Monte Carlo simulation study has been performed on the adsorption of CH₄ in a series of MOFs to confirm the desired characteristics of an optimal adsorbent for methane storage. The simulations show that isosteric heat of adsorption, accessible special area, free volume are all influence properties for a material with adsorption capacity. However, not all these properties are compatible. The same series MOFs have the similar characteristics of gas adsorption from the simulations. The results reveal the existence of three adsorption regimes: at low pressure, methane uptake correlates with the heat of adsorption; at intermediate pressure, methane uptake correlates with the surface area; and at the highest pressures, methane uptake correlates with the free volume.3. GCMC simulations were conducted to systematically evaluate the gas adsorption separation in Cu-BTC. The results show that Cu-BTC could be potentially used for the gas purification and separation. Pore sizes in Cu-BTC and the electrostatic actions between adsorbent and absorbate improve the selective adsorption behaviors.4. Quantum chemical calculations and GCMC simulations were performed to investigate the effects of catenation on gas storage and separation in IRMOF-9 and IRMOF-10. The results show that the interpenetrating MOFs have higher capacity and selective factor than those non-interpenetrating MOFs at lower pressure or loading.5. Adsorption and diffusion behaviors of methanol in MOF-E were obtained with flexible and rigid models by using molecular mechanics and molecular dynamics. The results indicate that per Ni₂ unit can sorb two methanol molecules stably and the structure undergo obvious deformation when the loading is above two molecules, which resulted in methanol adsorption isotherm step. The calculated adsorption energy and diffusion barrier are agreement with the experiment well by flexible model. It could be concluded that flexible model should be considered to study the adsorption and diffusion characterizations of adsorbate in such MOFs.