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 cm3 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.