(659b) Pentiptycene-Based Polybenzimidazole Membranes for High-Temperature H2/CO2 Separation

H₂/CO₂ separation using polymer materials to capture precombustion CO₂ following the water-shift reaction in syngas requires membranes with robust thermal stability and exceptional separation performance at elevated temperatures. Polybenzimidazoles (PBI), specifically m-PBI (Celazole ®), offer superior size-sieving capabilities. However, their performance suffers from extremely low H₂ permeability due to highly dense chain packing. Previous studies have enhanced permeability via disruption of chain packing by introducing large bulky groups into PBI’s structure, yet avoiding a significant reduction in H₂/CO₂ selectivity has remained challenging, especially at high temperatures. Further processing, such as acid doping, has been shown to reduce excess free volume for drastic improvements in selectivity but consequently reduces permeability and fails to improve overall performance. To overcome the permeability-selectivity tradeoff effect, we incorporate shape-persistent pentiptycene units into the PBI structure. The configurational free volume delineated by the shape of pentiptycene unit is permanent and implements well-defined microcavities that significantly enhance permeability while maintaining size-sieving properties at high temperatures. Pentiptycene-PBI (PPBI) has shown a significantly higher H₂ permeability and overall separation performance that surpasses upper bounds as temperature increases, indicating that configurational free volume stability is thermally resilient. To fully explore the potential of PPBI membrane for H₂/CO₂separation, a series of physical blends containing systematically varied molar ratios between PPBI and m-PBI were prepared, and the gas transport properties of each blend membrane were comprehensively evaluated under both pure and mixed-gas feed conditions at elevated temperatures up to 180°C. We have demonstrated that in equimolar H₂:CO₂ mixed gas feed conditions, all blend compositions surpass the 35°C H₂/CO₂ upper bound and approach the 180°C upper bound. The best performance was displayed by a composition of 50% PPBI having 345% higher permeability for H₂ than pure m-PBI and an H₂/CO₂ selectivity 135% greater than pure PPBI. These attractive performances clearly indicate that the non-collapsing configurational free-volume of pentiptycene units effectively disrupts chain packing, and fine-tuning PPBI concentration can minimize reductions in selectivity promoting the overall separation performance. This talk will discuss the preparation and characterization of pentiptycene-based PBI membranes and the role of configurational free volume on their gas transport properties.