Diamine-functionalized metal-organic framework: Exceptionally high CO ₂ capacities from ambient air and flue gas, ultrafast CO₂ uptake rate, and adsorption mechanism

A framework en-Mg₂(dobpdc) (1-en; en = ethylenediamine) functionalized with the primary amine en was prepared via postmodification. From synchrotron PXRD data, it is revealed that the cell parameters change upon grafting of en and CO₂ uptake. The adsorbed CO₂ amount of 1-en is 4.57 mmol g^-1 (16.7 wt%) at 25 °C and 1 bar and decreases to 3.00 mmol g^-1 (11.7 wt%) at 150 °C. Noticeably, 1-en shows a significant CO₂ uptake (3.62 mmol g^-1, 13.7 wt%) at 0.15 bar, which is comparable to the CO₂ partial pressure of a post-combustion flue gas. The CO₂ capacity of 1-en at 0.39 mbar, close to atmospheric CO₂ concentration, is 2.83 mmol g^-1 (11.1 wt%), which marks the highest amount among MOFs. The isosteric heat of adsorption (-Q_st) of 1-en in CO₂ capture corresponds to 49-51 kJ mol^-1, which is supported by DFT calculations (-52.8 kJ mol^-1). These results suggest that the adsorption of CO₂ onto the free amines of en leads to the formation of a carbamic acid. Adsorption-desorption cyclings of CO₂ at the real dilute concentrations of air and flue gas are established with almost retaining CO ₂ capacities, which could provide superior potential for practical application in CO₂ capture. The adsorption rate of CO₂ in 1-en exceeds that in some other tested porous materials. The recyclability in CO₂ uptake for 1-en is maintained even after exposure to humidity.