Confinement-gradient crosslinking strategy for dynamic-gated iCOF@PEI ultrathin composite membranes toward precision H2/CO2 separation

Covalent organic frameworks (COFs) are regarded as disruptive membrane materials owing to their versatile functionalities, high stability and ordered nanochannels. However, a major obstacle to the application of COF-based membranes in gas separation is the inherent size difference between their pore sizes and target gas molecules. Herein, we employed a confined cross-linking strategy to graft CO 2 -philic poly-ethyleneimine (PEI) onto carboxylic acid-type iCOF membranes, constructing a gradient wedge-shaped pore wall structure to enhance H 2 separation. High-density carboxyl group-containing iCOF nanosheets were prepared via an improved single-phase solution method and assembled into membranes. 1-ethyl-3-(3-dimethylami-nopropyl) carbodiimide/N-Hydroxysuccinimide (EDC/NHS) coupling chemistry was used to functionalize iCOF membranes with PEI. The introduction of a large number of amino groups via PEI enhances CO 2 adsorption by 81.4%. Meanwhile, the grafting of PEI reduces defects or pinholes on the membrane surface and improves the mechanical properties and stability. As a result, the PEI-functionalized iCOF@PEI-4 membrane exhibits a H 2 permeance of 1103 GPU and an H 2 /CO 2 selectivity of 21.3, positioning its performance among advanced membranes and surpassing the 2008 Robeson upper bound. Customized structural design of iCOF membranes through tailored functionalization may provide insights for designing molecularly selective COF membranes.