Molecularly Engineering Polymeric Membranes for Pre-Combustion CO2 Capture at 150 ? and Above

Membrane materials with high H₂ permeability and H₂/CO₂ selectivity at 150–300 ^oC hold promise for pre-combustion CO₂ capture. However, polymers with high permeability tend to exhibit weak size-sieving abilities and thus low selectivity. Polybenzimidazole (PBI) is one of the leading membrane materials for H₂/CO₂ separation for H₂ purification and pre-combustion CO₂ capture. For example, PBI exhibits H₂ permeability of 27 Barrers and H₂/CO₂ selectivity of 14 at 150 ^oC, which is on the Robeson’s upper bound. Herein we demonstrate that PBI can be engineered to further improve the H₂/CO₂ separation performance. For example, PBI can be doped by polyprotic acid to increase the size-sieving ability and thus H₂/CO₂ selectivity to as high as 130; PBI can be carbonized to obtain “hourglass” free volumes and thus improve the H₂ permeability to 54 Barrer and H₂/CO₂ selectivity to 80; and PBI can be doped with Palladium nanoparticles to increase H₂ solubility and thus achieve H₂ permeability of 48 Barrer and H₂/CO₂ selectivity of 43. We determined the mixed-gas separation properties of these materials, demonstrating their stability for long-term operation in the presence of water vapor. The structure/property relationship in these materials will also be elucidated.