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

Hydrogen Adsorption on Zn-BDC, Cr-BDC, Ni-DABCO, and Mg-DOBDC Metal–Organic Frameworks

This work reports hydrogen adsorption properties of four different metal–organic frameworks (MOFs) namely Zn-BDC, Cr-BDC, Ni-DABCO, and Mg-DOBDC. Gravimetric hydrogen adsorption measurements are performed over a wide range of temperature (90 K to 298 K) and pressure (0 bar to 100 bar). At the lowest experimental temperature (90 K to 100 K) all the isotherms are saturated and the adsorption capacity is governed by pore volume. On the other hand, at room temperature the isotherms closely follow Henry’s law. Modeling of the excess isotherms is also done. Net adsorption isotherms, which can directly indicate the efficiency of porous adsorbent for storage, are also presented. In terms of volumetric efficiency, Mg-DOBDC MOF exhibits best storage capacity out of all the MOFs considered in this study

Effect of Adsorbent History on Adsorption Characteristics of MIL-53(Al) Metal Organic Framework

Structural transformation of MIL-53­(Al) metal organic framework from large pore to narrow pore form (lp → np) or vice versa is known to occur by adsorption of certain guest molecules, by temperature change or by applying mechanical pressure. In this work, we perform a systematic investigation to demonstrate that adsorbent history also plays a decisive role in the structural transitions of this material (and hence on its adsorption characteristics). By changing the adsorbent history, parent MIL-53­(Al) is tuned into its np domain at ambient temperature such that it not only exhibits a significant increase in CO₂ capacity, but also shows negligible uptake for CH₄, N₂, CO, and O₂ at subatmospheric pressure. In addition, for the high pressure region (1–8 bar), we propose a method to retain the lp form of the sample to enhance its CO₂ uptake