Unexpectedly Strong Size-Sieving Ability in Carbonized Polybenzimidazole for Membrane H2/CO2 Separation
Maryam Omidvar, Hien Nguyen, Liang Huang, Cara Doherty, Anita Hill, Christopher M. Stafford, Xianshe Feng, Mark Swihart, Haiqing Lin
Polymers with high permeability and strong size-sieving ability are needed for H2/CO2 separation at temperatures ranging from 100 °C to 300 °C to enable an energy-efficient pre- combustion CO2 capture process. However, such polymers usually suffer from a permeability/selectivity tradeoff, i.e., polymers with high selectivity tend to exhibit low gas permeability. Herein we demonstrate that carbonization of polybenzimidazole (PBI) generates desirable hourglass structures with microcavities (leading to high H2 permeability) and ultra-microporous channels (leading to strong size sieving ability and thus high H2/CO2 selectivity). Specifically, PBI is a leading polymer for H2/CO2 separation with H2 permeability of 27 Barrers (1 Barrer = 3.35 × 10-16 mol m m-2 s-1 Pa-1) and H2/CO2 selectivity of 14 at 150 °C, while carbonization at 900 °C (CMS@900) increases the H2 permeability to 54 Barrers and H2/CO2 selectivity to 80. When tested with simulated syngas containing equimolar H2 and CO2 in the presence of water vapor for 120 h, the CMS@900 exhibits stable H2 permeability of ≈36 Barrers and H2/CO2 selectivity of ≈53 at 150 °C, which is above Robesons upper bound and demonstrates their potential for practical H2 purification and CO2 capture.