(681a) Highly Selective Carbon Molecular Sieve Membranes Derived from Polybenzimidazole Doped with Pyrophosphoric Acid for H2/CO2 Separation

Membrane technology with superior H₂/CO₂ separation properties at high temperatures has attracted significant interest for pre-combustion CO₂ capture. Polybenzimidazole (PBI) is a leading material for this separation with H₂ permeability of 27 and H₂/CO₂ selectivity of 16 at 150 ^oC because of the strong size-sieving ability. We demonstrate that the PBI can be first cross-linked with pyrophosphoric acid (PPA) and then carbonized to form carbon molecular sieves (CMS) membranes achieving more superior H₂/CO₂ separation properties. The effects of the doping level (defined as the ratio of the PPA to the PBI repeating units) and carbonization temperature on the separation properties are thoroughly characterized. For example, at a doping level of 0.34, the blend (PBI-PPA_0.34) exhibits H₂/CO₂ selectivity of 72 with H₂ permeability of 32 Barrer at 150 ^oC. However, a further increase in the doping level to 0.42 decreases H₂ permeability to 18 Barrer and H₂/CO₂ selectivity to 42. When carbonized at 600 ^oC, the obtained CMS of PBI-PPA_0.42 exhibits H₂ permeability of 460 Barrer and H₂/CO₂ selectivity of 37 at 150 ^oC, surpassing the Robeson’s upper bounds. By comparison, the CMS derived from PBI at 600 ^oC show a similar H₂ permeability (~450 Barrer) but much lower H₂/CO₂ selectivity of 5.3 at 150 ^oC. Moreover, we have optimized the carbonization to be 600 ^oC or below so that the PBI can still retain its mechanical properties, enabling excellent processability for the CMS membranes. We will also present the long-term testing results with simulated syngas in the presence of water vapor. The structure/property relationship in these CMS materials will also be elucidated.