Boosting CO2 transport of poly (ethylene oxide) membranes by hollow Rubik-like 'expressway' channels with anion pillared hybrid ultramicroporous materials

In order to overcome “trade-off” Robeson upper bound 2008 and simultaneously increase the CO2 permeability and selectivity of traditional poly (ethylene oxide) polymer membrane, three anion pillared hybrid ultramicroporous materials (SIFSIX-2-Cu-i, TIFSIX-2-Cu-i and GEFSIX-2-Cu-i) were incorporated into Pebax/PEGDME polymer to fabricate mixed matrix membranes (MMMs). DFT-D2 calculation revealed that the C···F distance (2.62 A) between CO2 molecule and pillared anion (GeF62-) in GEFSIX-2-Cu-i was lower than SIFSIX-2-Cu-i and TIFSIX-2-Cu-i. The synergistic effects of C···F van der Waals (vdW) interactions and O···H hydrogen bonding interactions in GEFSIX-2-Cu-i gave a higher CO2 binding energy (34.5 kJ/mol) than SIFSIX-2-Cu-i and TIFSIX-2-Cu-i. It was verified that GEFSIX-2-Cu-i with higher CO2 binding energy and specific surface area exhibited higher CO2 adsorption capacity of 5.02 mmol/g than SIFSIX-2-Cu-i and TIFSIX-2-Cu-i. These anion-pillared hybrid ultramicroporous materials with suitable porosity and surface chemistry, enabling multiple host–guest interactions, created unique Rubik’s tube-like “expressway” channels for CO2 molecule passage through the membrane. Results showed that the MMMs with 1 wt% GEFSIX-2-Cu-i nanoparticles exhibited optimal CO2 separation performance in terms of CO2 permeability (460 Barrers) and CO2/H2 (17, an increase of 12.2%), CO2/CH4 (18, an increase of 24.1%), and CO2/N2 (57, an increase of 9.6%) selectivities, as compared with the pure Pebax/PEGDME membrane. As GEFSIX-2-Cu-i loading increase from 1 to 10 wt%, the overall CO2 separation performance decrease due to the aggregation of these nanoparticles. Positron annihilation lifetime spectroscopy (PALS) experiments revealed that the fractional free volume of MMMs with 1 wt% GEFSIX-2-Cu-i nanofillers (FFV = 3.63%) was higher than those of MMMs incorporated with a higher loading (2.5–10 wt%) of GEFSIX-2-Cu-i.