Molecular Theories of Gas Adsorption in Nanoporous Materials

With ever-increasing computer power and rapid developments in molecular theories, high-throughput computation will be instrumental to future materials design and discovery. This Ph.D. thesis explores the potential applications of the classical density functional theory (DFT) for fast screening of nanostructured porous materials for gas storage and separation. In addition to developing the computational methods, major efforts have been devoted to clarifying two fundamental yet long debated issues in the adsorption field, i.e., accurate prediction of the heat effect from adsorption isotherm and the connection between the BET and geometric surface areas of porous materials. First we have investigated methane adsorption in a large library of metal-organic frameworks (MOFs) using four versions of classical DFT. The molecular thermodynamic model has been used to identify promising MOF materials and possible variations of operation parameters to meet the ARPA-E target set by the U.S. Department of Energy for natural gas storage. The BET surface area is highly sensitive to the selected pressure region to perform the liner fitting and also the definition of the probe molecule used to determine the surface area. In this thesis, we provide a comprehensive analysis of the BET method. Based on extensive simulation data for over 1000 materials with complex structures and heterogeneous interactions, we find that the surface area obtained by standard BET method is not necessarily correlated with its geometrical accessible surface area. Heat of adsorption is of both fundamental and practical importance. Here we present a rigorous theoretical procedure to predict isosteric heat. Quantitative relations between the differential heat and various isosteres have been established with the GCMC simulation for gas adsorption in amorphous and crystalline porous materials. The inconsistencies with conventional methods for the analysis of heat effect have been clarified in the context of the exact results for model systems. By developing new computational methods for rapid prediction of gas adsorption and diffusivity and by resolving a number of controversial issues related to materials characterization and heat analysis, we hope that the theoretical work will have broad impacts on both materials design and industrial applications of gas adsorption and separation processes