Adsorbate-Selective Induced-Fit in an Ultramicroporous Metal-Organic Framework for Efficient CO₂ Sieving

Capturing CO₂ from flue and natural gas is very critical for environmental and industrial progress. However, achieving high CO₂ selectivity over nitrogen (N₂) and methane (CH₄) via the molecular sieving mechanism remains a significant challenge. Flexible metal-organic frameworks (MOFs) with specific CO₂-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 CO₂ sieving from N₂ and CH₄. 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 CO₂ to enter the channels, while repelling larger N₂ and CH₄ molecules. Therefore, NUM-19a displays ultrahigh selectivities of 3226 and 7010 for CO₂/N₂ and CO₂/CH₄ 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 CO₂ adsorption in NUM-19a is facilitated by an induced-fit mechanism between CO₂ and the host framework. This work provides some valuable insights for developing highly efficient CO₂-selective sorbents.