First-principles studies of small-molecule adsorption and mechanical properties of amine-appended metal-organic frameworks

Metal-organic frameworks (MOFs) are a class of highly-porous solids, consisting of metal centers (M) connected by organic ligands, that are of interest for gaseous storage and carbon capture. Recently, a new class of amine-appended MOFs, mmen-M$_{\mathrm{2}}$(dobpdc), have been shown to have exceptional CO2 adsorption properties via a novel phase transition with CO2 concentration. Here, we study this cooperative effect, as well as the mechanical properties of such MOFs with M$=$Mg, Mn, and Zn, using first-principles density functional theory (DFT) calculations with van der Waals corrections. We find excellent agreement with measured CO$_{\mathrm{2}}$ heats of adsorption, and demonstrate that amine ligands enhance CO$_{\mathrm{2}}$ binding energies (by about 30 kJ/mol) and selectivity under humid conditions, in agreement with multicomponent adsorption measurements. We further calculate that the polycrystalline Young's modulus of mmen-Mn$_{\mathrm{2}}$(dobpdc) increases by 70{\%} due to the amine ligands, a dramatic enhancement. All together, our calculations suggest that amine-based ligands can be used to optimize both small gas separation and mechanical properties of MOFs.