A Computational Study Of Adsorption, Separation And Diffusion Of Natural Gas In Metal-Organic Frameworks

Metal-Organic Frameworks (MOFs), analogues of zeolites, is a new class of nanoporous materials. MOFs having extremely high porosities, chemical diversity and tailored materials as robust solids with well-defined pore sizes are promising materials for gas storage, separation and catalyst. However, it is difficult to symmetrically study the adsorption mechanisms and diffusion behavior of confined fluids by experimental methods because of the many influencing factors in MOFs such as pore sizes and the chemical environment. Computational chemistry, including quantum chemistry calculation and molecular simulation, can not only overcome the limitations of traditional methods for understanding the microcosmic movement orderliness of fluids in MOFs, but also provide theoretical guidance for the design of optimal adsorbents. It saves a lot of time for complicated experimental work. Quantum chemical calculations can be used for the estimation of the stability and to get the electrostatic potentials in MOFs. On the other hand, molecular simulation that is useful for the study of adsorption mechanisms and diffusion behavior is classified into two methods, Monte Carlo and molecular dynamics. The former is mainly employed to study the equilibrium properties of fluids in materials, while, the latter, apart from those equilibrium properties, is also used to obtain the dynamic properties. Employing the methods above, this work has studied the adsorption and diffusion of fluids in MOFs. The main contents and findings are summarized as follows.1. Using quantum chemical calculations and molecular simulations, we study the effect of the chemical properties of nine organic linkers on CO2/CH4 mixture separation in MOFs. The computational results show that the organic linkers decorated with the electron-donating groups can strengthen the distribution of the electrostatic field in the pores of MOFs, and greatly enhance the adsorption selectivity of CO2/CH4 mixture in MOFs. This enhancement becomes stronger with the increase of the electron-donating ability of groups. In addition, this work also demonstrates that the negative steric hindrance effects on the separation behavior should be considered when the organic linkers are modified with multiple substitutions in designing new materials. The knowledge obtained is expected to provide useful information for tailoring the electrostatic properties of MOFs for separation of various gas mixture systems of practical importance.2. Quantum chemical calculations and molecular simulations were used to study the effect of doping different metal atoms on CO2/CH4 mixture separation in MOFs. The computational results show that the MOFs doped with alkali metals can greatly enhance the adsorption selectivity of CO2/CH4 mixture. However, this enhancement becomes weaker as the atomic number of doped metal atoms increases. In addition, this work also demonstrates that LJ potential parameters of metals should be considered combined with the electrostatic interactions, while using metal-doping strategy to improve the separation performance of MOFs. The knowledge obtained is expected to provide useful information for designing new MOFs with improved separation performance in various practical important gas mixture systems.3. A systematic equilibrium molecular dynamics study was performed to investigate the diffusion rates of gas molecules as a function of the pressure in MOFs with different structures. Methane was chosen as the probe molecule. The self-diffusion coefficients in eight typical MOFs were calculated at room temperature. Combined self-diffusion coefficients with the contour plots of the center of mass (COM) probability densities of methane, the relationship between the diffusion rates of gas molecules and the structure of the pores in the MOFs is discussed. The results show that methane tends to adsorb in the pockets of the MOFs with pocket and channel pores (P-C materials) at low pressure. With an increase in pressure, the gas molecules move to the channel and the self-diffusion coefficient increases. However, the diffusion coefficient of methane changes a little in the low and middle pressure range in the IRMOFs (Isoreticular MOFs) with only one kind of pore. With a further increase in pressure, the self-diffusion coefficient of methane decreases in all the studied MOFs. Therefore, the difference in diffusion rates of methane in the different MOFs may be mainly attributed to the pore structures of the materials. In addition, diffusion rates of the gas molecules in the P-C materials could be controlled in a wide range by varying the pressure, providing useful information for the application of MOFs in gas storage and separation.