Novel reverse-selective poly(2,5-benzimidazole) derivatives for membrane-based gas separation

Three novel polymers were synthesized by introducing hexyl, dodecyl, and octadecyl groups to the side chains of poly (2,5-benzimidazole) (ABPBI). Fourier transform infrared, thermal stability, and mechanical strength tests were applied to characterize the three polymers. Elemental analysis was also carried out to determine the degree of substitution. Gas permeation behavior of the three derivative membranes was investigated with pure gas, including hydrogen (H₂), nitrogen (N₂), oxygen, methane, and carbon dioxide (CO₂). The results show that except the hexyl-substituted ABPBI the other two polymers are reverse selective because in both polymers CO₂ has a higher permeability than H₂. Among the three derivatives, octadecyl-modified ABPBI shows the best permselectivity of gas pairs, such as CO₂/N₂ (approximately 13) and CO₂/H₂ (approximately 2.8), with a quite high CO₂ permeability (approximately 100 barrer) at 35°C and 3.0 atm.