Adsorbate‐Selective Induced‐Fit in an Ultramicroporous Metal–Organic Framework for Efficient CO ₂ Sieving

ABSTRACT Capturing CO ₂ from flue and natural gas is very critical for environmental and industrial progress. However, achieving high CO ₂ selectivity over nitrogen (N ₂ ) and methane (CH ₄ ) via the molecular sieving mechanism remains a significant challenge. Flexible metal–organic frameworks (MOFs) with specific CO ₂ ‐responsive adsorption behaviors offer an appealing platform for developing efficient adsorbents. Herein, we present a robust pillar‐layered MOF ( NUM‐19 ), which exhibits an adaptive pore structure for efficient CO ₂ sieving from N ₂ and CH ₄ . Through removing solvent molecules, NUM‐19 underwent a transformation from the large pore phase to the narrow pore phase, attested through single‐crystal X‐ray diffraction studies. The activated NUM‐19a allows small CO ₂ to enter the channels, while repelling larger N ₂ and CH ₄ molecules. Therefore, NUM‐19a displays ultrahigh selectivities of 3226 and 7010 for CO ₂ /N ₂ and CO ₂ /CH ₄ mixtures under ambient conditions. Both static gas adsorption and dynamic breakthrough experiments confirmed its excellent separation performance. Furthermore, in situ powder X‐ray diffraction and in situ infrared spectroscopy, coupled with theoretical calculations, reveal that selective CO ₂ adsorption in NUM‐19a is facilitated by an induced‐fit mechanism between CO ₂ and the host framework. This work provides some valuable insights for developing highly efficient CO ₂ ‐selective sorbents.