Unexpectedly Strong Size-Sieving Ability in Carbonized Polybenzimidazole for Membrane H2/CO2 Separation

Polymers with high permeability and strong size-sieving ability are needed for H-2/CO2 separation at temperatures ranging from 100 to 300 degrees C to enable an energy-efficient precombustion CO2 capture process. However, such polymers usually suffer from a permeability/selectivity tradeoff, that is, polymers with high permeability tend to exhibit a weak size-sieving ability and thus low selectivity. Herein, we demonstrate that carbonization of a suitable polymer precursor (i.e., polybenzimidazole or PBI) generates microcavities (leading to high H-2 permeability) and ultramicroporous channels (leading to strong size-sieving ability and thus high H-2/CO2 selectivity). Specifically, carbonization of PBI at 900 degrees C (CMS@900) doubles H-2 permeability and increases H-2/CO2 selectivity from 14 to 80 at 150 degrees C. When tested with simulated syngas-containing equimolar H-2 and CO2 in the presence of water vapor for 120 h, CMS@900 exhibits stable H-2 permeability of approximate to 36 barrer and H-2/CO2 selectivity of approximate to 53 at 150 degrees C, above Robeson's 2008 upper bound and demonstrating robustness against physical aging and CO2 plasticization.