Trace CO₂ capture by an ultramicroporous physisorbent with low water affinity

CO₂ accumulation in confined spaces represents an increasing environmental and health problem. Trace CO₂ capture remains an unmet challenge because human health risks can occur at 1000 parts per million (ppm), a level that challenges current generations of chemisorbents (high energy footprint and slow kinetics) and physisorbents (poor selectivity for CO₂, especially versus water vapor, and/or poor hydrolytic stability). Here, dynamic breakthrough gas experiments conducted upon the ultramicroporous material SIFSIX-18-Ni-β reveal trace (1000 to 10,000 ppm) CO₂ removal from humid air. We attribute the performance of SIFSIX-18-Ni-β to two factors that are usually mutually exclusive: a new type of strong CO₂ binding site and hydrophobicity similar to ZIF-8. SIFSIX-18-Ni-β also offers fast sorption kinetics to enable selective capture of CO₂ over both N₂ (S _CN) and H₂O (S _CW), making it prototypal for a previously unknown class of physisorbents that exhibit effective trace CO₂ capture under both dry and humid conditions.