(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 â 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 150oC, CMS pyrolyzed at 900 oC show H2 permeability of 39 Barrers and H2/CO2 selectivity of 53, which surpasses the Robesonâs 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.