Microenvironment regulation in hydroxyl-functionalized COF membranes for enhanced CO2 separation

Covalent organic frameworks (COFs) are promising membrane materials for gas separation due to their periodic pore structure and chemical tailorability. However, the COF membranes usually show oversized pores and a single separation mechanism, which limits their gas separation applications. In this work, we fabricate highly ordered covalent organic framework (COF) membranes through interfacial polymerization, guided by a nucleation–epitaxial growth mechanism that ensures dense and defect-free layers. By regulating monomer hydroxyl content and spatial distribution, we achieve tailored pore architectures and interlayer stacking. Furthermore, a microenvironment engineering strategy is implemented via post-modification with boric acid and piperazine, which fine-tunes the pore size and introduces CO 2 -philic sites, enabling a synergistic combination of diffusion selective and facilitated transport. The optimized membrane exhibits an evident pore contraction, with the average pore diameter decreasing from 2.04 nm to 1.06 nm, confirming the effective tuning of the internal channels. As a result, it delivers a high CO 2 permeance of 2149 GPU and a CO 2 /N 2 selectivity of 14.3. This microenvironment regulation approach reveals critical structure–property relationships and provides valuable guidance for the separation of small molecule mixtures by large-pore membrane materials.