Adsorption and Separation of Carbon Dioxide Using MIL-53(Al) Metal-Organic Framework

In this work, we report adsorption isotherms of various industrially important gases, viz. CO₂, CO, CH₄, and N₂ 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 CO₂ adsorption at 294 and 314 K. Adsorption of CO (another polar gas), N₂ and CH₄ did not induce any structural transformation in this adsorbent for the experimental conditions considered in this work. Since the CO₂ 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 CO₂ on MIL-53­(Al). Henry’s constants and adsorption enthalpies of CO₂ on the two structural forms were calculated using this model. The Ideal Adsorbed Solution Theory (IAST) was used to predict the selectivity of CO₂ at 350 K over other gases studied in this work

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