Spatially Confined π-Complexation within Pore-Space-Partitioned Metal-Organic Frameworks for Enhanced Light...

Ultramicroporous metal-organic frameworks (MOFs) are demonstrated to be advantageous for the separation and purification of light hydrocarbons such as C₂H₂, C₂H₄, and CH₄. The introduction of transition metal sites with strong π-complexation affinity into MOFs is more effective than other adsorption sites for the selective adsorption of π-electron-rich unsaturated hydrocarbon gases from their mixtures. However, lower coordination numbers make it challenging to produce robust MOFs directly utilizing metal ions with π-coordination activity, such as Cu⁺, Ag⁺, and Pd²⁺. Herein, a series of novel π-complexing MOFs (SNNU-33s) with a pore size of 4.6 Å are precisely constructed by cleverly introducing symmetrically matched C₃-type [Cu(pyz)₃] (pyz = pyrazine) coordinated fragments into 1D hexagonal channels of MIL-88 prototype frameworks. Benifit from the spatial confinement combined with π-complex-active Cu⁺ of [Cu(pyz)₃], pore-space-partitioned SNNU-33 MOFs all present excellent C₂H₂/CH₄, C₂H₄/CH₄, and CO₂/CH₄ separation ability. Notably, the optimized SNNU-33b adsorbent demonstrates top-level IAST selectivity values for C₂H₂/CH₄ (597.4) and C₂H₄/CH₄ (69.8), as well as excellent breakthrough performance. Theoretical calculations further reveal that such benchmark light hydrocarbon separation and purification ability is mainly ascribed to the extra-strong binding affinity between Cu⁺ and π-electron donor molecules via a spatially confined π-complexation process.