Amine-Functionalized Quasi-MOF for Direct Air Capture of CO₂

Direct air capture of CO2 from ambient air conditions remains a huge challenge due to the strongly competitive adsorption of water and low CO2 pressure. Here, we successfully developed a class of amine-functionalized quasi-MOF (X-UIO-EDA) with high CO2 capture capacity and found a water-assisted enhancement effect under 400 ppm of CO2 in the presence of humidity. In the quasi X-UIO-EDA series, 2-UIO-EDA displays a CO2 capture capacity of 0.44 mmol/g under humid air conditions, which is 10-fold that under dry air conditions, and exhibits lower CO2 desorption energy under humid air conditions. The comparative experiments also demonstrated that quasi-UIO-66 provided more amine modification sites, leading to higher CO2 capture capacity. The in situ FT-IR analysis demonstrated that water plays a crucial role in facilitating the transformation of −NHCOO– to –OCOOH species due to the steric hindrance and micropore confinement effects. Combining solid-state 13C CP-MAS NMR and XPS measurements, we concluded that the observed enhancement of the CO2 capture capacity under humid air conditions is due to the formation of bicarbonate and hydronium carbamate species, along with the formation of weak hydrogen bonds between these species.