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 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 CO2 adsorption at 294 and 314 K. Adsorption of CO (another polar gas), N-2 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). Henrys 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.