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 CO 2 permeability and CO 2 /N 2 selectivity trade-off effect. Here, a novel anti-trade-off effect by simultaneously increase CO 2 permeability and CO 2 /N 2 selectivity upon decreasing temperature was observed for two new functionalized fully carbon main chain PIMs (HSBI-4-CF 3 and HSBI-3-CF 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-CF 3 exhibited a higher BET surface area (318 vs 287 m 2 g -1 ) and more open chain packing than HSBI-3-CF 3 . The two polymers showed modest permeability and selectivity as well as pressure and aging resistance. The HSBI-4-CF 3 showed higher permeability but less gas pair selectivity than HSBI-3-CF 3 . Interestingly, upon decreasing temperature to −20 °C, both polymers showed improved CO 2 permeability and CO 2 /N 2 selectivity at the same time. The CO 2 permeability of HSBI-3-CF 3 increased from 292 to 453 Barrer and the CO 2 /N 2 selectivity improved from 20.3 to 44.0. The higher permeability at low temperature was attributed to the increased solubility (S CO2 ∼ 3.2 fold) over diffusion coefficient drop (D CO2 ∼ 2.1 times) of CO 2 , which is opposite to that of N 2 , and thus enhanced the CO 2 /N 2 selectivity. The origin of this effect comes from their minus activation energy of permeation (E p ) of CO 2 (−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 (E d ) overweight the heat of sorption (-H s ) for N 2 while inverse for CO 2 . This unique simultaneously increase CO 2 permeability and CO 2 /N 2 selectivity opened a new era in membrane-based sub-ambient temperature CO 2 capture. • Two novel –OH and –CF 3 functionalized PIMs with fully carbon backbone were synthesized from two isomer monomers. • The polymerization is a one-step reaction from cheap materials and easy scale-up. • The two polymers showed simultaneously improved P CO2 and a CO2/N2 upon decreasing temperature. • The increased P CO2 is originated from the enhanced S CO2 at low temperature. • The increased P CO2 and decreased P N2 at low temperature caused the improved a CO2/N2 .