Understanding the Adsorption Mechanism of C2H2, CO2, and CH4 in Isostructural Metal-Organic Frameworks with Coordinatively Unsaturated Metal Sites

An computational study using density functional theory and grand-canonical Monte Carlo simulation that explore the adsorption mechanism of C2H2, CO2, and CH4 to metal organic frameworks (MOFs) with coordinatively unsaturated metal sites (M-MOF-74, M = Mg and Zn) has been carried out. The theoretical studies reveal that open metal sites have important roles in adsorption. The high CO2 adsorption ability of M-MOF-74 is due to the strong Lewis acid and base interactions between metal ions and oxygen atom of CO2, as well as carbon atom of CO2 with oxygen atoms in organic linkers. Meanwhile, the high C2H2 adsorption for M-MOF-74 is contributed by the strong complexation between the metal ions and the pi orbital of C2H2. The different adsorption mechanisms of CO2, C2H2, and CH4 in M-MOF-74 can qualitatively explain the high CO2 selectivity in CO2/CH4 mixture and high C2H2 selectivity in C2H2/CH4 mixture. Energy decomposition analysis reveals that electrostatic energy, exchange energy, and repulsive energy are key factors in the binding strength of gas molecules on M-MOF-74. The preferential adsorption sites are confirmed to be located near the five-coordinate metal ions decorating the edges of the hexagonal channels. The elucidation of the adsorption mechanism at the molecular level provides key information for designing novel MOFs with high capacity and selectivity for CO2 from light hydrocarbon mixtures.