Precise Construction of Flexible Two‐Dimensional Metal‐Organic Frameworks Through Pillar‐Ligand Tailoring for Efficient C ₂ H ₂ /CO ₂ /C ₂ H ₄ Separation

The key to achieving efficient separation of C₂H₂/CO₂/C₂H₄ is to prepare materials with both high adsorption capacity and high selectivity. Flexible two-dimensional metal-organic frameworks (2D MOFs) exhibit excellent performance in difficult separation applications due to their high structural toughness within the layers and strong stimulus responsiveness between layers, but their controllable preparation is a challenge. Herein, under the guidance of crystal engineering, we develop a framework dimensionality reduction strategy (3D → 2D) for tailoring of pillaring ligands to directionally construct flexible 2D MOFs, based on a 3D MOF (UPC-191) constructed by terpyridine pillaring ligands and organic carboxylic acid ligands. The interlayer and intralayer pore environments are synergistically optimized by co-modification of semi-pillared ligands and carboxylic acid ligands as well as metal ion regulation. Remarkably, the optimal structure of difluoro-functionalized MOF (UPC-197) exhibits temperature-responsive framework flexibility with a unique "gate-opening effect" for C₂H₂ and reverse adsorption behavior for CO₂/C₂H₄ at 195 K, achieving efficient separation of C₂H₂/C₂H₄ and CO₂/C₂H₄ as well as C₂H₂/CO₂/C₂H₄, which has been confirmed by adsorption isotherms, experimental breakthrough curves, and density functional theory calculations. This work provides a guidance for the precise construction of flexible 2D MOFs for the challenging separation of multi-component low-carbon hydrocarbons.