Supramolecular Polymer Networks of Ion-Coordinated Polybenzimidazole with Simultaneously Improved H2 Permeability and H2/CO2 Selectivity

Cross-linking polymers using covalent bonds often improves polymer chain packing efficiency and thus molecular size-sieving ability but decreases gas permeability. Herein, we demonstrate that supramolecular polymer networks (SPNs) comprising polybenzimidazole (PBI) and dissociable metal salts (including palladium trifluoroacetate and nickel trifluoroacetate) display unusual behavior of breaking the permeability/selectivity tradeoff for H2/CO2 separation. With metal ions coordinating the imidazole rings on PBI chains and trifluoroacetate anions plasticizing the PBI chains, the SPNs achieve higher fractional free volumes yet lower d-spacing than PBI, simultaneously increasing H2 permeability and H2/CO2 selectivity. For instance, doping PBI with 13.7 mass% nickel trifluoroacetate increases H2 permeability by 82%, from 34 to 62 Barrer, and H2/CO2 selectivity from 10 to 15 at 150 °C. The SPNs also exhibit mechanical properties superior to PBI and stable mixed-gas H2/CO2 separation performance in the presence of water vapor. This study unveils a facile approach to design materials exhibiting small but dense free volume elements and thereby achieving high gas permeability and selectivity.