In this work, we report adsorption isotherms of various industrially important gases, viz. CO 2 , CO, CH 4 , and N 2 on MIL-53(Al) metal organic framework (MOF). The isotherms were measured in the range of 0−25 bar over a wide temperature range (294−350 K). The structural transformation of the adsorbent and the resulting breathing phenomenon were observed only in the case of CO 2 adsorption at 294 and 314 K. Adsorption of CO (another polar gas), N 2 and CH 4 did not induce any structural transformation in this adsorbent for the experimental conditions considered in this work. Since the CO 2 isotherms at 294 and 314 K involve structural transformation and show a distinct step, a conventional isotherm model cannot be used to describe such behavior. In order to model these isotherms, a dual-site Langmuir-type equation (one site each for the two structural forms, i.e., large pore phase and narrow pore phase) that includes a normal distribution function to account for structural transformation is proposed. This model successfully mimics the Type-IV isotherm behavior of CO 2 on MIL-53(Al). Henry's constants and adsorption enthalpies of CO 2 on the two structural forms were calculated using this model. The Ideal Adsorbed Solution Theory (IAST) was used to predict the selectivity of CO 2 at 350 K over other gases studied in this work.

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.

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In this work, we report the gas adsorption properties of C2H6, C3H8, i-C4H10, and n-C5H12 on Zn2(bdc)2(dabco)(H2O)0.5(DMF)4 commonly known as ZnDABCO metal organic framework (MOF). Gravimetric adsorption measurements were performed at three different temperatures (294, 314, and 350 K) and a wide pressure range. For the linear alkanes, the isotherms were of Type-I; however, i-C4H10 exhibits a Type-IV isotherm at 294 K. Adsorption capacities on ZnDABCO are found to be higher than that on convention adsorbents like activated carbons and silicalites. The isotherms for linear alkanes were fit to a modified virial model. Enthalpy of adsorption was calculated using model parameters; it increases with loading indicating the role of lateral interactions. As is to be expected, a linear correlation was observed between the polarizability of the adsorbates and the enthalpy at zero loading (vertical interactions).

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