Novel and versatile PEI modified ZIF-8 hollow nanotubes to construct CO2 facilitated transport pathway in MMMs

1D nanotube (HNT) based MMM presents high CO2 permeation due to its long and hollow structure. However, the improvement of selectivity is still challenging. In this study, small PEI molecules with abundant amino groups were introduced into the nanocages of ZIF-8 HNTs to construct facilitated CO2 transmission channel with low resistance. In the PEI@ZIF-8 HNTs, the -NH2 groups can facilitate CO2 transport in presence of H2O, resulting in an improvement in both CO2 permeability and selectivity. Second, the introduction of PEI into the nanocages of ZIF-8 can adjust the effective sieving size to enhance CO2 selectivity. Third, the hydrogen bond between PEI and Pebax can strengthen the interfacial compatibility, further improving CO2/N-2 selectivity of the MMMs. Finally, the 1D hollow structure of PEI@ZIF-8 HNTs provides CO2 molecules with low-resistance and fast transport channel, enhancing the CO2 permeability. Consequently, both the high CO2 permeability and the high CO2/N-2 selectivity are acquired by the versatile filler of PEI@ZIF-8 HNTs. PEI@ZIF-8 HNTs based MMM at filler loading of 7 wt% under dry mixed gas, presents CO2 permeability of 177 Barrer and CO2/N-2 selectivity of 72, which are 40% and 10.8% higher than those of ZIF-8 HNTs based MMM, respectively. Moreover, the permeability of CO2 and the selectivity of CO2/N-2 reach 306 Barrer and 79 respectively under the wet mixed gas, which is far beyond the Robeson upper bound in 2008 and exceeds the McKeown upper bound in 2019. The proposed PEI@ZIF-8 HNTs are believed to be promising filler for MMMs based CO2 capture.