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. 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.