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

Most polymeric membranes are suffering from CO2 permeability and CO2/N-2 selectivity trade-off effect. Here, a novel anti-trade-off effect by simultaneously increase CO2 permeability and CO2/N-2 selectivity upon decreasing temperature was observed for two new functionalized fully carbon main chain PIMs (HSBI-4-CF3 and HSBI-3CF(3)). 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 m(2) 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/N-2 selectivity at the same time. The CO2 permeability of HSBI-3-CF3 increased from 292 to 453 Barrer and the CO2/N-2 selectivity improved from 20.3 to 44.0. The higher permeability at low temperature was attributed to the increased solubility (S-CO2 similar to 3.2 fold) over diffusion coefficient drop (DCCO2 similar to 2.1 times) of CO2, which is opposite to that of N2, and thus enhanced the CO2/ N-2 selectivity. The origin of this effect comes from their minus activation energy of permeation (E-p) of CO2 (-0.78 and-1.1 kcal mol(-1)) while positive E-p for N-2 (1.7 and 1.1 kcal mol(-1)), which is due to the diffusion activation energy (Ed) overweight the heat of sorption (-H-s) for N-2 while inverse for CO2. This unique simultaneously increase CO2 permeability and CO2/N-2 selectivity opened a new era in membrane-based sub-ambient temperature CO2 capture.

Automated summary

This study observed a novel effect where two functionalized fully carbon main chain polymers simultaneously increased CO2 permeability and CO2/N-2 selectivity upon decreasing temperature. This effect opens up new possibilities for CO2 capture at sub-ambient temperatures.