Computational Study of Metal-Organic Frameworks in Natural Gas Storage, Water Adsorption, and Ethane/Ethylene Separation

Metal-organic frameworks (MOFs) are crystalline, porous materials synthesized through self-assembly from metal nodes and organic linkers. The great variety of building blocks, along with the potential to combine them in many ways, makes MOFs highly tunable. Furthermore, by functionalization one can optimize these materials for different applications including gas storage, separation, and catalysis. The goal of this work was to use computational techniques to search for good MOF candidates for different gas storage and separation systems and to discover new insights into the physical nature of the adsorption process that can be used to design MOFs with improved performance. Advanced Monte Carlo algorithms that accelerate the simulation speed played an essential role in this work.;Adsorbed natural gas (ANG) has many advantages for vehicular applications, including higher safety and lower cost, compared with traditional compressed natural gas storage. However, in addition to methane, commercial natural gas always contains small amounts of impurities including ethane and propane. These higher hydrocarbons are more easily adsorbed by the adsorbents due to their stronger interactions with the adsorbent framework atoms. In order to study the effect of these impurities on the performance of an ANG tank containing a MOF adsorbent, we combined grand canonical Monte Carlo (GCMC) simulations and macroscopic thermodynamics to develop a model for an ANG tank. With this model, the performance, especially the deliverable energy, of the natural gas storage system with different MOFs was tested over many operation (adsorption/desorption) cycles. Furthermore, screening of a small MOF database containing 120 structures was carried out. Based on the screening result, good MOF candidates were identified, and some interesting trends were observed.;Water is one of the most common components in industrial gas or liquid systems. It often has an important effect on chemical and physical processes including gas or liquid adsorption in porous materials, such as zeolites and MOFs. However, the molecular simulation of water adsorption brings many challenges, especially the slow simulation speed, which is mainly due to the clustering of water molecules through hydrogen bonds. In this study, we examined the hydrophobic MOF ZIF-8 as a representative adsorbent to discover the adsorption mechanism of water in hydrophobic MOFs. In addition, we proposed and investigated several advanced Monte Carlo algorithms including the energy-bias moves and continuous fractional component Monte Carlo (CFC MC) moves and successfully accelerated the simulation speed.;Finally, we explored ethane-ethylene separation in MOFs. Separation of olefin and paraffin mixtures is one of the most energy-intensive and challenging separations in the petrochemical industry due to the very similar volatilities and close molecular sizes of these molecules. Selective physisorption of olefin and paraffin molecules in porous materials has been proposed as a promising alternative separation strategy, which would benefit from its lower energy cost compared to cryogenic distillation. In this work, we designed a new adsorbent material by grafting ligands into Co-MOF-74 to improve the ethylene over ethane selectivity. We first validated this idea via modeling, and subsequent experiments by collaborators successfully supported our results. By this grafting strategy, the selectivity of ethylene over ethane in this MOF under high pressures was improved, and some interesting insights have been discovered.