Triple Framework Isomerism and Efficient Ethane/Ethylene Separation of Single‐Crystal Covalent Organic Frameworks With 2D hcb Sheets

Constructing triple framework isomerism with atom‐resolution structures is of great significance but remains highly challenging for covalent organic frameworks (COFs). Herein, [3+3] imine condensation of 1,3,5‐trimethyl‐2,4,6‐tris(4‐aminophenyl)benzene with 1,3,5‐triethyl‐2,4,6‐tris(4‐formylphenyl)benzene affords three single‐crystal COF isomers composed of hcb nets. Their AB hcb stacking, double [2D+2D], and triple [2D+2D+2D] crossing entangled structures have been resolved by 3D electron diffraction. Among the three desolvated COFs, GZU‐4a exhibits the highest ethane uptake (100.7 cm 3 g −1 ) and ethane/ethylene selectivity (1.6), enabling one‐step production of high‐purity ethylene (>99.99%) in dynamic breakthrough experiments. The superior and inverse separation performance originates from its rigid non‐interpenetrated framework with optimized pore confinement for strengthened ethane binding. This work not only presents an unprecedented example of triple isomeric COFs with atom‐resolution structures from non‐interpenetration to inclined interpenetration of 2D nets, but also shows an inverse adsorption selectivity for ethane over ethylene, greatly enriching the diversity of COFs.