(519e) Novel Iptycene-Based Polybenzimidazole Membranes for H2/CO2 Separation

H₂/CO₂ separation relatable to pre-combustion CO₂ capture from syngas after water-gas shift reactions demands robust polymer membrane materials with high thermal stability and strong size-sieving capability at high temperatures. Polybenzimidazole (PBI), such as m-PBI (Celazole^®), has been the focus of increasing studies due to its good H₂/CO₂ separation properties and high thermal stability. However, m-PBI suffers from extremely low permeability. Although many previous studies were attempted to improve m-PBI’s permeability by incorporating bulky moieties or large substituent groups, these strategies typically resulted in compromised size-sieving capability, particularly at high temperatures. On the other hand, modified PBI-based membranes, such as crosslinked PBIs, that were able to achieve ultrahigh selectivity often exhibited a significant loss of H₂ permeability. To address this issue, we developed a novel macromolecular design to incorporate shape-persistent iptycene (i.e., triptycene and pentiptycene) units into PBI structure to instill configurational free volume elements. The non-collapsible and well-defined microcavities originating from intrinsic molecular configuration of iptycene units are able to significantly enhance gas permeability as well as size sieving property. Moreover, by systematically varying types of iptycene units and pendent groups, structure-property relationships of these novel iptycene-based PBIs were investigated. In this work, a triptycene-based PBI (TPBI) and a pentiptycene-based PBI (PPBI) have been produced, and gas transport properties have been characterized. We demonstrated that by incorporating triptycene units, H₂ permeability was effectively improved by 300% relative to m-PBI. The bulkier pentiptycene units could disrupt chain packing more efficiently, exhibiting 30 times higher H₂ permeability than that of m-PBI. Post treatments including thermal treatment and crosslink via acid doping were performed to further tailor microporosity, leading to versatile and significantly improved separation performance. We found that thermal treatment at 350 ^oC favored rearrangement of TPBI chains to unblock more microcavities, resulting in further enhanced H₂ permeability by 200% relative to the untreated sample, which indicates the good temperature tolerance of configurational free volume. After crosslinked by doping with phosphoric acid, the packing of polymer chains was tightened due to proton transfer and hydrogen bonding interactions. As a result, the acid-doped TPBI exhibited largely improved H₂/CO₂ selectivity by 100%, outperforming the 2008 upper bound for H₂/CO₂ separation. This talk will discuss the preparation and characterization of novel iptycene-based PBIs membranes as well as the influence of configurational free volume, thermal treatment, and acid doping on their gas transport properties.