CO2 Open Metal Site Selectivity in MIL-100 (Cr): A Computational Study

Metal-organic frameworks (MOFs) are porous organometallic compounds that are of high interest due to their capability to trap industrial greenhouse gases, such as CO2, that contribute to anthropogenic global warming. A promising MOF that has gained the attention of the research community in recent years is the CO2 adsorbent MIL-100 (Cr). This compound consists of two primary structural components: a set of Cr3-?3-oxo clusters referred to as secondary building units (SBU) and organic linkers derived from trimesic acid. Thermally activated SBUs possess coordinatively unsaturated sites Cr sites -- or open metal sites (OMS) -- that can have oxidation states of +2 or +3. Previous experimental work indicated that CO2 molecules bind more strongly to Cr2+ OMS than to Cr3+ OMS at low adsorptive pressures. In this thesis project, two central questions were addressed. Firstly, can the experimentally observed OMS selectivity be verified through density functional theory (DFT) simulations? Secondly, what electronic processes are responsible for OMS selectivity? DFT computations of the binding energy, enthalpy, and Gibbs free energy of CO2 adsorption onto Cr2+ and Cr3+ OMS verify that CO2 exhibits a significantly greater affinity for the +2 OMS. In addition, a comparison of the adsorption charge transfer and optimized binding geometries reveal that this selectivity arises from the energetically favorable chemisorption of CO2 onto +2 OMS – relative to the weaker physisorption of the greenhouse gas onto +3 OMS. The novel methodology utilized for this study -- which addresses the issue of charge delocalization in DFT simulations -- can be implemented in computational investigations of CO2 OMS selectivity for other promising MOFs being considered for carbon capture applications.