(100g) Capitalization of Breathing Phenomena of Flexible MIL-53 (Al) Solid to Enhance Its Carbon Dioxide Capacity and Selectivity

Metal organic frameworks (MOFs) are being widely investigated for their potential to efficiently remove of CO₂ from process streams. Some MOFs (DOBDC series for example) have shown good CO₂ uptake and selectivity over other gases like N₂, CH₄ etc. at the desired process conditions. But poor stability towards moisture makes them unrealistic for any practical separation application. On the other hand, some MOFs (like in the MIL-53 series) are readily stable towards moisture, but have poor CO₂ uptake and selectivity at low pressures. MIL-53 MOFs exhibit structural transformation from (large pore) lp domain to (narrow pore) np domain and vice versa (so called breathing phenomena) upon adsorption of certain guest molecules (like CO₂, H₂O etc.) or by change in temperature.    

  Breathing phenomenon of MIL-53(Al) metal organic framework is capitalized to tune its structure for increasing CO₂ loading and selectivity in the sub-atmospheric region. We report a procedure to tune the pores of parent MIL-53(Al) into narrow pore (np) domain at ambient temperature in absence of guest molecules. On one hand, this tuned sample exhibits a significant (greater than four-fold) increase in CO₂ capacity at about 0.17 bar (in flue gases partial pressure of CO₂ is typically between 0.12 - 0.20 bar) and 293 K. On the other hand, it shows almost zero loading for CH₄, N₂, CO and O₂ at the same conditions. In addition we also propose a method to retain the lp form of the sample for adsorption in 1 - 8 bar region, thereby enhancing its CO₂ capacity.