Ultrathin Polycage Membranes Facilitate Molecular Shuttling for Efficient CO₂ Separation

Porous organic cages (POCs), owing to their high specific surface area and tunable chemical properties, hold significant potential in gas separation membranes. However, the construction of POC membranes featuring highly interconnected angstrom-scale channels remains a formidable challenge. In this work, an innovative amine functionalization strategy was employed to successfully synthesize a water-soluble, amine-rich functionalized POC. Crucially, pendant ethylamine side chains on the cage framework enabled the weaving of discrete POC units into ultrathin polycage membranes via an amidation crosslinking reaction with terephthaloyl chloride, thereby constructing highly interconnected angstrom-scale amine-rich channels. The abundant amine groups within these channels display strong preferential adsorption toward CO₂, functioning as highways for CO₂ molecular shuttling. The polycage membrane exhibited an exceptional CO₂ permeance of 920 GPU and CO₂/N₂ selectivity of 42.4, along with excellent long-term stability, significantly surpassing most previously reported POC membranes. Furthermore, applying this polycage membrane to a two-stage membrane process for separating CO₂/N₂ mixed gas in simulated flue gas yielded a CO₂ purity as high as 99.5%. This work not only innovatively proposes ultrathin polycage membranes featuring tailored pores and their molecular shuttling mechanism, but also establishes a novel pathway for the seamless assembly of discrete porous molecular units into functional membranes.