Adsorption Characteristics of Metal–Organic Frameworks Containing Coordinatively Unsaturated Metal Sites: Effect of Metal Cations and Adsorbate Properties

Metal–organic frameworks in the M/DOBDC series are known to contain a large number of coordinatively unsaturated metal (M) sites. In this work, we study the influence of various metal cations (M = Mg, Mn, Co, and Ni) in the framework on its gas adsorption characteristics. The probe gases (viz. CO2, CO, CH4, C2H6, N2, and Ar) were carefully chosen to cover a wider range of polarity and polarizability. While a significant impact of metal atom in the framework is observed on adsorption of polar gases such as CO2 and CO, it has a negligible effect on adsorption of other relatively nonpolar gases. On one hand, Henry’s constant of CO2 for Mg/DOBDC is about 4–10 times higher than that for other frameworks; on the other, Henry’s constant for CO on Ni/DOBDC is about 100 times larger than that on Mn/DOBDC. The pore volume of the framework governs adsorption capacity at higher pressures. Each of the frameworks exhibits widely different adsorption enthalpies for polar gases such as CO2 and CO. At pressures below 15 bar, the Ideal Adsorbed Solution Theory predicts very good selectivity for CO over all other studied gases on Ni and Co/DOBDC frameworks, while Mg and Mn/DOBDC frameworks exhibit preferential selectivity for CO2.