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. CO<sub>2</sub>, CO, CH<sub>4</sub>, C<sub>2</sub>H<sub>6</sub>, N<sub>2</sub>, 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 CO<sub>2</sub> and CO, it has a negligible effect on adsorption of other relatively nonpolar gases. On one hand, Henry’s constant of CO<sub>2</sub> 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 CO<sub>2</sub> 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 CO<sub>2</sub>.