Simultaneously enhanced CO2 permeability and CO2/N2 selectivity at sub-ambient temperature from two novel functionalized intrinsic microporous polymers

Abstract

Most polymeric membranes are suffering from CO2 permeability and CO2/N2 selectivity trade-off effect. Here, a novel anti-trade-off effect by simultaneously increase CO2 permeability and CO2/N2 selectivity upon decreasing temperature was observed for two new functionalized fully carbon main chain PIMs (HSBI-4-CF3 and HSBI-3-CF3). They are synthesized by one-step Friedel-Crafts polycondensation reaction from cheap raw materials of 3,3,3’,3’-tetramethyl spirobisindane-6,6’-diol and 3(4)-trifluoromethylbenzaldehyde isomers. Both PIMs showed high molecular weight, good solubility, thermal and mechanical properties. HSBI-4-CF3 exhibited a higher BET surface area (318 vs 287 m2 g-1) and more open chain packing than HSBI-3-CF3. The two polymers showed modest permeability and selectivity as well as pressure and aging resistance. The HSBI-4-CF3 showed higher permeability but less gas pair selectivity than HSBI-3-CF3. Interestingly, upon decreasing temperature to −20 °C, both polymers showed improved CO2 permeability and CO2/N2 selectivity at the same time. The CO2 permeability of HSBI-3-CF3 increased from 292 to 453 Barrer and the CO2/N2 selectivity improved from 20.3 to 44.0. The higher permeability at low temperature was attributed to the increased solubility (SCO2 ∼ 3.2 fold) over diffusion coefficient drop (DCO2 ∼ 2.1 times) of CO2, which is opposite to that of N2, and thus enhanced the CO2/N2 selectivity. The origin of this effect comes from their minus activation energy of permeation (Ep) of CO2 (−0.78 and −1.1 kcal mol-1) while positive Ep for N2 (1.7 and 1.1 kcal mol-1), which is due to the diffusion activation energy (Ed) overweight the heat of sorption (-Hs) for N2 while inverse for CO2. This unique simultaneously increase CO2 permeability and CO2/N2 selectivity opened a new era in membrane-based sub-ambient temperature CO2 capture.