| 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 of materials, and hotspots. Due to the chemical diversity and complex structure of MOFs, purely experimental approach is insufficient to conduct systematic studies. With the development of chemical theory, efficient calculation method and computer technology, computational chemistry can provides theoretical guidance for the design of optimal adsorbents and the determination of optimal industrial operation conditions, which also saves a lot of time for complicated experimental works. In this work, a systematic study for gas adsorption, separation and material design in MOFs was carried out using a combination of quantum chemistry calculation and molecular simulation. The main contents and findings are summarized as follows. 1. A systematic molecular simulation study was performed to investigate the influence of pore size and temperature on the quantum effects of hydrogen adsorption in MOFs. This may provide a theoretical guideline for the hydrogen storage applications and design of new materials. The results show that quantum effects diminish with increasing pore size of the isoreticular metal-organic frameworks (IRMOFs) at lower pressure (loading), while the opposite trend appears at higher pressure (loading).Through the simulations it is also found that the quantum effects may be dominantly determined by the adsorbate-adsorbate or adsorbate-MOFs interactions with the varying of pressure (loading). In addition, the results also indicate how the temperature influences the quantum effects of H2 adsorption in MOFs within the pressure range considered.2. Grand canonical Monte Carlo simulations were performed to study CO2/H2 mixture separation in three pairs of IRMOFs with and without catenation at room temperature. The results show that CO2 selectivity in catenated MOFs is much higher than their non-catenated counterparts. The simulations also show that the electrostatic interactions are very important for the selectivity, and the contributions of different electrostatic interactions are different, depending on pore size, pressure and mixture composition. A general conclusion is that the electrostatic interactions between adsorbate molecules and the framework atoms play a dominant role at low pressures, and these interactions in catenated MOFs have much more pronounced effects than those in their non-catenated counterparts, while the electrostatic interactions between adsorbate molecules become evident with increasing pressure, and eventually dominant.3. This work provides a strategy for estimating framework charges in MOFs. We developed a so-called connectivity-based atom contribution method (CBAC), in which it is assumed that the atoms with same bonding connectivity have identical charges in different MOFs. The results for 43 MOFs including a training set of 30 MOFs and a test set of 13 MOFs show that the CBAC charges give nearly identical results to those from quantum mechanical (QM) calculations for adsorption isotherms of CO2, CO and N2 in them. Since the method is readily to include new atom types, it is applicable to any MOF as long as its structure is known. The strategy, which is applicable to other porous materials, paves a way for large-scale computational screening of MOFs for specific applications as well as contributes to a better understanding of the structure-property relationships for MOFs, and eventually contributes to the development and application of MOFs.4. Three Li-modified MOFs were constructed, deduced from MOF-5 by substituting the H atoms by O-Li groups in the organic linkers, and a multiscale approach combining grand canonical Monte Carlo simulation and density functional theory calculation was adopted to investigate the separation of CO2/CH4 mixtures in these new Li-modified MOFs as well as in a previously proposed Li-doped MOF-5 for hydrogen storage and the original MOF-5. The results show that the selectivity of CO2 from CH4/CO2 mixtures in Li-modified MOFs is greatly improved, due to the enhancement of electrostatic potential in the materials by the presence of the metals, and one of the new Li-modified MOFs, chem-4Li, shows highest CO2 selectivity that is much higher than any other MOFs known. Therefore, this work provides a route to improve the separation performance of MOFs for gas mixtures with components having large differences in dipole and/or quadrupole moments. In addition, the mechanisms for selectivity enhancement in the Li-modified MOFs were elucidated at the molecular level, and we found the location of doped metals can change the adsorption sites for CO2, and in turn may change the active sites in MOFs when used as catalyst.5. The newly designed Li-modified MOFs were applied to CO2 capture. The results show that the selectivities of CO2 from various gas mixtures in the Li-modified MOFs are greatly improved. In addition, the mechanisms for selectivity enhancement were elucidated.
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