Programmable triazacyclononane nanomembranes: A platform for precision ion sieving

Abstract Efficient ion separation is essential for sustainable water purification and energy recovery under the water–energy nexus. Polyamide (PA) nanofiltration membranes are a key platform, with performance governed by monomer‐defined network structures. Herein, a macrocyclic triamine 1,4,7‐triazacyclononane (3N) was reacted with different acyl chlorides, trimesoyl chloride (TMC), isophthaloyl chloride (IPC) and terephthaloyl chloride (TPC) to construct programmable NF membranes. This pairing enables tunable pore size and cross‐linking density for targeted ion separations. The 3N‐IPC membrane, featuring a balanced pore structure achieved high heavy metal rejection (>95%) with a water permeability of 25.9 L m −2 h −1 bar −1 , while the 3N‐TPC membrane formed a compact network exhibiting an outstanding selectivity of 90.4 and water permeance of 20.9 L m −2 h −1 bar −1 in Li/Mg separation. These findings elucidate the molecular design principles governing selective ion transport and provide a scalable strategy for developing precision‐engineered PA membranes for advanced water purification and resource recovery.