Highly Selective Adsorption of Carbon Dioxide over Acetylene in an Ultramicroporous Metal–Organic Framework

Abstract Separating carbon dioxide from fuel gases like hydrocarbons by physical adsorbents is industrially important and more energy‐efficient than traditional liquid extraction or cryogenic distillation methods. It is very important while very challenging to develop CO₂‐selective adsorbents, considering CO₂ is less polarizable than light hydrocarbon molecules, particularly those simultaneously with almost identical molecular dimensions and physical properties, such as acetylene. Herein, an ultramicroporous metal–organic framework constructed from copper(II) and 5‐fluoropyrimidin‐2‐olate, termed Cu‐F‐pymo, is carefully studied under different activations for inverse separation of CO₂ from C₂H₂. The partially desolvated Cu‐F‐pymo can exclusively capture CO₂ over C₂H₂ with very high selectivity exceeding 10⁵ under ambient conditions, the highest ever reported. Sorption experiments and modeling studies reveal that such molecular sieving effect is attributed to the suppression of C₂H₂ adsorption from the blockage of the preferential sites for C₂H₂ by residual water molecules. The inverse separation is further confirmed by column breakthrough studies given that highly pure acetylene (>99.9%) can be directly harvested from the gas mixture. Cu‐F‐pymo also shows remarkable stability under harsh conditions.