Influence of polybenzimidazole main chain structure on H2/CO2 separation at elevated temperatures

• Four high molecular weight PBIs with modified main chain structures were synthesized.
• Film processing conditions were carefully studied to obtain high quality PBI films.
• Optimized PBIs reached H2 permeability of up to 13-fold higher than base m-PBI.
• All PBIs exhibited H2/CO2 separation performance beyond the Robeson upper bound.

Four polybenzimidazole (PBI) derivatives were prepared to study the effects of main chain chemistry and structure on H2/CO2 perm-selectivity of cast films. These structural variations were designed to exhibit high localized mobility at elevated temperatures, contain rigid and bent configurations that frustrated close chain packing, or possess bulky side groups. The modified PBIs exhibited high molecular weights, slightly lower thermal stabilities, and higher organo-solubilities compared with commercial m-PBI. Dilute polymer solutions (<3.0 wt%) were used to fabricate high quality thin films under carefully optimized film processing conditions. Gas permeation properties of these PBI films were evaluated at elevated temperatures (up to 250 °C) and pressures (up to 50 psia). It was found that the main chain structural variations effectively disrupted the PBI chain packing resulting in much improved film H2 permeability (up to 997.2 barrer) compared with m-PBI (76.81 barrer) at 250 °C and 50 psia. However, lower H2/CO2 selectivities (5–7 (modified PBIs) versus 23 (m-PBI)) were also measured and reflected the general trade-off between gas permeability and selectivity. When tested at 250 °C, PBI-based materials exhibited gas separation performance higher than the Robeson upper bound prediction and are promising materials for high temperature H2 separation from syngas.