(381ax) Polybenzimidazole-Derived Carbon Molecular Sieves with Microcavities and Ultra-Microporous Channels Achieving Superior Membrane H2/CO2 Separation Properties

Membrane technology is highly attractive for H₂/CO₂ separation for pre-combustion CO₂ capture, which requires membrane materials with both high H₂ permeability and H₂/CO₂ selectivity at 100 – 300 ^oC. Herein, we present carbon molecular sieve (CMS) membranes with superior H₂/CO₂ separation properties via the pyrolysis of polybenzimidazole (PBI, a leading polymer for H₂/CO₂ separation). We thoroughly investigated the effect of the pyrolysis temperature on physical properties (pore size, d-spacing, density, solubility, diffusivity) and H₂/CO₂ separation properties of the CMS films. PBI exhibits H₂ permeability of 27 Barrers and H₂/CO₂ selectivity of 14 at 150 ^oC, while the CMS films pyrolyzed at 600 ^oC, 850 ^oC, and 900 ^oC exhibit H₂ permeability of 370 Barrers, 190 Barrers, and 54 Barrers at 150 ^oC, respectively, and their corresponding H₂/CO₂ selectivity of 8.9, 16, and 80, respectively. The pyrolysis forms micro-cavities leading to high gas permeability and ultra-microporous channels leading to strong size-sieving ability, which is also confirmed by the Positron Annihilation Lifetime Spectroscopy (PALS). As the pyrolysis temperature increases from 600 ^oC to 850 ^oC and then 900 ^oC, the free volume element size increases from 5.08 Ã… to 5.46 Ã… first and then decreases to 4.90 Ã…. When tested with a mixture containing 50% H₂ and 50% CO₂ at 150^oC, CMS pyrolyzed at 900 ^oC show H₂ permeability of 39 Barrers and H₂/CO₂ selectivity of 53, which surpasses the Robeson’s upper bound for H₂/CO₂ separation at 150 ^oC. The film shows stable mixed-gas separation performance for 40 h. When water vapor of 0.31 mol% is introduced, the H₂ permeability slightly decreases to 37 Barrers while the H₂/CO₂ selectivity remains the same. The mixed-gas H₂ permeability increases to 39 after the water vapor is removed. The stability and robust H₂/CO₂ separation properties demonstrate the potential of PBI-derived CMS for practical H₂ purification and CO₂ capture.