Computational Study of Porous Materials for Gas Separations

Nanoporous materials such as zeolites, zeolitic imidazolate frameworks (ZIFs), and metal-organic frameworks (MOFs) are used as sorbents or membranes for gas separations such as carbon dioxide capture, methane capture, paraffin/olefin separations, etc. The total number of nanoporous materials is large; by changing the chemical composition and/or the structural topologies we can envision an infinite number of possible materials. In practice one can synthesize and fully characterize only a small subset of these materials. Hence, computational study can play an important role by utilizing various techniques in molecular simulations as well as quantum chemical calculations to accelerate the search for optimal materials for various energy-related separations. Accordingly, several large-scale computational screenings of over one hundred thousand materials have been performed to find the best materials for carbon capture, methane capture, and ethane/ethene separation. These large-scale screenings identified a number of promising materials for different applications. Moreover, the analysis of these screening studies yielded insights into those molecular characteristics of a material that contribute to an optimal performance for a given application. These insights provided useful guidelines for future structural design and synthesis. For instance, one of the screening studies indicated that some zeolite structures can potentially reduce the energy penalty imposed on a coal-fired power plant by as much as 35% compared to the near-term MEA technology for carbon capture application. These optimal structures have topologies with a maximized density of pockets and they capture and release CO2 molecules with an optimal energy. These screening studies also pointed to some systems, for which conventional force fields were unable to make sufficiently reliable predictions of the adsorption isotherms of different gasses, e.g., CO2 in MOFs with open-metal sites. For these systems, we developed a systematic, transferable, and efficient methodology to generate force fields by using high-level quantum chemical calculations for accurate predictions of properties. The method was first applied to study the adsorption of CO2 and N2 in Mg-MOF-74, an open-metal site MOF. Two different approaches were developed: one approach based on MP2 calculations on a representative cluster and a second approach based on DFT calculations on a fully periodic MOF. Both approaches gave significantly better predictions of the experimental adsorption isotherms compared to conventional force fields. In addition, we extended the DFT approach to study water adsorption in these materials. Moreover, instead of deriving detailed force fields, we have also proposed an alternative method to efficiently correct initial trial force fields with little information obtained from quantum chemical calculations. Finally, we studied the dynamics of CO2 in Mg-MOF-74 using molecular simulations. This study addressed the dynamic behaviors of CO2 adsorbed in Mg-MOF-74, and provided an alternative explanation to the experimentally measured chemical shifts of 13C labeled CO2 adsorbed in a powder Mg-MOF-74 sample. Our results further illustrated that subtle changes in the topology of frameworks greatly influence CO2 dynamics