Solvation-Induced Synthesis of COF Nanosheets for Enhanced H₂/CO₂ Separation

Two-dimensional covalent organic framework nanosheets (2D-CONs) have garnered significant attention in gas separation owing to their ultrathin nature and vertically aligned ordered nanopores. However, the efficient synthesis of CONs with high crystallinity, minimal defects, and large aspect ratios remains a major challenge. Herein, a scalable and rapid fabrication based on a "solvation-induced reassembly" strategy is developed to construct CONs with exceptional aspect ratios from colloidal particles, wherein water, acting as a poor solvent, plays a pivotal role in directing the assembly of nanospheres. Mechanism insights reveal that the pronounced difference in Hansen solubility parameters between water and acetonitrile enables water not only to disrupt the acetonitrile solvation shell and promote covalent organic framework (COF) colloid aggregation, but also to activate the reversible dissociation and reformation of imine bonds, thereby driving the transformation from nanospheres to nanosheets. The versatility of this method is demonstrated by the successful preparation of high aspect ratio CONs from a range of commonly used aldehydes. Membranes were fabricated by coassembling the as-produced CONs with graphene oxide (GO), and they achieved highly efficient H₂/CO₂ separation performance that exceeds most reported COF-based separation membranes, significantly surpassing the 2008 Robeson upper bound. This study provides a versatile strategy for the controlled preparation of CONs and opens new avenues for the processing and integration of COF-based materials in advanced gas separation technologies.