(491c) Polybenzimidazole-Derived Carbon Molecular Sieves with Microcavities and Ultra-Microporous Channels Achieving Superior Membrane H2/CO2 Separation Properties

Polymeric membranes are of great interest for H₂/CO₂ separation in H₂ purification and CO₂ capture, because of inherently high energy-efficiency, low cost, and easy scale-up. The key to the success of this technology is membrane materials with high H₂ permeability and H₂/CO₂ selectivity at 100 – 300 ^oC. However, polymers with higher permeability often exhibit lower selectivity, as indicated by Robeson’s upper bound plot. Herein we report novel carbon molecular sieve (CMS) membranes derived from polybenzimidazole (PBI), a leading polymer for H₂/CO₂ separation. These membranes are prepared by pyrolysis of PBI in inert gas, forming microcavity with high gas permeability, and ultra-microporous channels with strong size-sieving ability. We systematically study the effect of the pyrolysis temperature on the physical properties and gas separation properties in the CMS. The pyrolysis leads to the formation of graphite-like structure, as confirmed by WAXD. As the pyrolysis temperature increases, the CMS density increases before decreasing, while the porosity increases consistently up to 30% for CMS with a pyrolysis temperature of 900 ^oC. In general, the carbonization of PBI dramatically increases H₂ permeability while retaining or slightly increasing H₂/CO₂ selectivity. For example, pure PBI exhibit H₂ permeability of 12 Barrers and H₂/CO₂ selectivity of 14 at 100 ^oC, while the CMS with a pyrolysis temperature of 800 ^oC shows an H₂ permeability of 672 Barrers and H₂/CO₂ selectivity of 18 at 100 ^oC. We have also demonstrated that the doping of the PBI with H₃PO₄ before the pyrolysis can dramatically increases the H₂/CO₂ selectivity. For example, the PBI doped with 0.05 wt% H₃PO₄ shows an H₂ permeability of 8.5 Barrers and H₂/CO₂ selectivity of 49, while the pyrolysis at 600 ºC increases the H₂ permeability to 100 Barrers and retaining the H₂/CO₂ selectivity at 40 at 100 ºC. The effect of temperature on gas permeability and selectivity are studied, and the fundamental gas solubility and diffusivity in these CMS are determined. This talk will also describe the mixed-gas separation properties, and the effect of water vapor on the H₂/CO₂ separation properties. These CMS with intrinsic micropores exhibit H₂/CO₂ separation properties far above the Robeson’s upper bound, demonstrating their potential for practical H₂ purification and CO₂ capture.