Optimizing Sieving Effect for CO₂ Capture from Humid Air Using an Adaptive Ultramicroporous Framework

Abstract Excessive CO₂ in the air can not only lead to serious climate problems but also cause serious damage to humans in confined spaces. Here, a novel metal–organic framework (FJI‐H38) with adaptive ultramicropores and multiple active sites is prepared. It can sieve CO₂ from air with the very high adsorption capacity/selectivity but the lowest adsorption enthalpy among the reported physical adsorbents. Such excellent adsorption performances can be retained even at high humidity. Mechanistic studies show that the polar ultramicropore is very suitable for molecular sieving of CO₂ from N₂, and the distinguishable adsorption sites for H₂O and CO₂ enable them to be co‐adsorbed. Notably, the adsorbed‐CO₂‐driven pore shrinkage can further promote CO₂ capture while the adsorbed‐H₂O‐induced phase transitions in turn inhibit H₂O adsorption. Moreover, FJI‐H38 has excellent stability and recyclability and can be synthesized on a large scale, making it a practical trace CO₂ adsorbent. This will provide a new strategy for developing practical adsorbents for CO₂ capture from the air.