Computational study of point defects in metal-organic frameworks

Abstract

Metal-Organic Frameworks (MOFs) are a class of porous materials composed of metal clusters connecting by organic ligands and forming in one-, two-, or three- dimensional structures. The tunable pore sizes, ultrahigh surface areas and pore volumes, together with the versatile functionalization of ligands make MOFs ideal candidates for applications including gas storage and separations, catalysis, and drug delivery. However, many MOFs have been found to degrade upon exposure to humid conditions or humid acid-gases. High chemical stability is required for MOFs to be practical applications as their working environments may be humid or acidic. Thus, it is of great importance to understand the degradation mechanisms of MOFs under related conditions. In my thesis work, I adopt ZIFs, an important subclass of MOFs, as prototypical models to investigate the potential degradation reactions occurring by the attack of water and acid gases. I utilized density functional theory methods and developed atomistic models to explore the energetic properties of point defects resulted by these reactions. Diffusion-based gas separations in MOFs has promising applications for chemical mixture separations. Extensive experimental and computational investigations have been conducted for the screening of MOFs for separating specific components with high selectivity. However, the impact of defective structures on molecular diffusion has not been widely considered. This motivated me to perform molecular dynamics simulations using transition state theory method to explore the change in hopping rates of adsorbates caused by defective structures in MOFs. In general, the point defects I have examined in ZIF-8 increase the local hopping rate for molecular diffusion, suggesting that low concentrations of these defects will not dominate long range molecular diffusion in ZIF-8.