Microwave-synthesized porous organic polymers with tunable CO₂ selectivity via amine functionalization

Porous organic polymers (POPs) are promising materials for CO₂ capture; however, the mechanistic origin of the trade-off between adsorption capacity and gas selectivity remains poorly understood. In this study, a tetraphenylethylene–bipyridine-based POP (MSN0) was synthesized through microwave-assisted Friedel–Crafts alkylation and systematically functionalized with amines of different structures and nitrogen contents (MSN1–MSN3) to explore how chemical functionality affects adsorption behavior. Thorough characterization confirmed successful framework formation and post-synthetic modification while maintaining thermal stability. Amine addition reduced BET surface area (from 1039 to 674 m² g⁻¹) due to partial pore blockage, resulting in decreased overall gas adsorption. Despite the reduction in CO₂ uptake caused by partial pore blockage, CO₂ selectivity over N₂ and CH₄ increased significantly. This improvement originated from stronger preferential CO₂ framework interactions together with a more pronounced suppression of N₂ and CH₄ adsorption, as supported by higher isosteric heats of adsorption and enhanced initial-slope and IAST selectivities (CO₂/N₂ up to ∼282 at 273 K). Notably, the diethylenetriamine-functionalized polymer (MSN3) showed the highest selectivity, indicating that higher amine density enhances CO₂ framework interactions and adsorption affinity. These findings demonstrate that the balance between pore accessibility and amine-induced affinity controls the adsorption–selectivity trade-off in POPs. This work reveals a clear correlation between amine incorporation, adsorption energetics, pore accessibility, and gas separation performance.