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

Authors: 
Omidvar, M., University at Buffalo, The State University of New York
Lin, H., University at Buffalo, The State University of New York
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.