Aminal-Linked Covalent Organic Frameworks with hxl-a and Quasi-hcb Topologies for Efficient C₂H₆/C₂H₄ Separation

In reticular chemistry, topology is a powerful concept for defining the structures of covalent organic frameworks (COFs). However, due to the lack of diversity in the symmetry and reaction stoichiometry of the monomers, only 5% of the two-dimensional topologies have been reported to be COFs. To overcome the limitations of COF connectivity and pursue novel topologies in COF structures, two aminal-linked COFs, KUF-2 and KUF-3, are prepared, with dumbbell-shaped secondary building units. Linear dialdehydes and piperazine are condensed at a ratio of 1:2 to construct an aminal linkage, leading to unreported hxl-a (KUF-2) and quasi-hcb (KUF-3) structures. Notably, KUF-3 displays top-tier C2H6/C2H4 selectivity and C2H6 uptake at 298 K, outperforming most porous organic materials. The intrinsic aromatic ring-rich and Lewis basic pore environments, and appropriate pore widths enable the selective adsorption of C2H6, as confirmed by Grand Canonical Monte Carlo simulations. Dynamic breakthrough curves revealed that C2H6 can be selectively separated from a gas mixture of C2H6 and C2H4. This study suggests that topology-based design of aminal-COFs is an effective strategy for expanding the field of reticular chemistry and provides the facile integration of strong Lewis basic sites for selective C2H6/C2H4 separation.