(377b) Facilely Cross-Linking Polybenzimidazole with Polycarboxylic Acids to Enhance H2/CO2 Selectivity

Membrane technology with superior H₂/CO₂ separation properties at high temperatures has gained significant attention for pre-combustion CO₂ capture. Polybenzimidazole (PBI) is a leading material for this separation for its strong size-sieving ability and excellent thermal stability. Herein we demonstrate that PBI can be facilely cross-linked to achieve high H₂/CO₂ selectivity with five polycarboxylic acids with various pKa values and number of protons, including oxalic acid (OA), trans-aconitic acid (TaA), phthalic acid (PhA), trimesic acid (TrA), and pyromellitic acid (PyA). These acids react with the imidazole rings on PBI chains via proton transfer and hydrogen bondings, thereby decreasing free volume and enhancing size-sieving ability. The physical properties of the doped PBI were thoroughly characterized, including density, thermal degradation behavior, d-spacing, and free volume. Increasing the concentration of the proton groups decreases the d-spacing and free volume, and the pKa values of the acids have a negligible effect on free volume. For example, the introduction of PyA in PBI increases the H₂/CO₂ selectivity of 12 to 52 at 35 ^oC. The PBI-PyA exhibits the best combination of H₂ permeability (19 Barrer) and H₂/CO₂ selectivity (26) at 150 ^oC among all of the samples studied, which are higher than those of state-of-the-art polymers and surpass Robeson’s upper bound. Similarly, PBI-TrA and PBI-TaA also exhibit stable H₂/CO₂ selectivity of ~ 30 when challenged with simulated syngas containing water vapor at 150 ^oC, indicating their potential for practical applications. This work shows a facile way to post-modify polymers to achieve high selectivity, and acids with smaller molecules and more protons are preferred for cross-linking.