Understanding Trends in CO2 Adsorption in Metal-Organic Frameworks with Open-Metal Sites

Using van der Waals-corrected density functional theory and a local chemical bond analysis, we study and explain trends in the binding between CO2 and open-metal coordination sites within a series of two metal-organic frameworks (MOFs), BTT, and MOF-74 for Ca, Mg, and nine divalent transition-metal cations. We find that Ti and V result in the largest CO2 binding energies and show that for these cations the CO2 binding energies for both structure types are twice the value expected based on pure electrostatics. We associate this behavior with the specific electronic configuration of the divalent cations and symmetry of the metal coordination site upon CO2 binding, which result in empty antibonding orbitals between CO2 and the metal cation. We demonstrate that a chemical bond analysis and electrostatic considerations can be used to predict trends of CO2 binding affinities to MOFs with transition-metal cations.