Effect of water vapor on CO2 and N2 permeability in PIM based membranes

The present study investigates the impact of relative humidity on the permeability and selectivity of three polymers of intrinsic microporosity (PIMs), namely PIM-EA-TB, PIM-SBF, and AO-PIM-1. To that aim, single gas permeation tests were conducted at 25 degrees C and 1 bar with nitrogen and carbon dioxide with different humidity levels, from 0 to 80 %. Water sorption was measured independently to correlate relative humidity to the water content in the membrane. The effect of water vapor on polymer ageing was also considered, by testing the same sample multiple times. Results indicate that water vapor strongly affects the permeability of the three materials; a drastic decrease in CO2 permeability, higher than 90 %, was observed for all the polymers when increasing the relative humidity of the gas. In addition to that, in the case of PIM-EA-TB and PIM-SBF, permeability was not completely recovered after the 30 days of testing due to irreversible changes likely caused by ageing and water exposure. After three permeation runs in humid conditions the permeability of dry CO2 decreased, from 2285 to 249 Barrer and from 4070 to 1150 Barrer for PIM-EA-TB and PIM-SBF respectively. In AO-PIM-1, on the other hand, dry gas permeability remained substantially unchanged even after exposure to water vapor. This difference is also observed in sorption experiments which showed a strong decrease in water uptake in subsequent tests for PIM-EA-TB and PIM-SBF while smaller reductions were observed for AO-PIM-1. CO2/N2 selectivity was in the order of 10 and 25 for the PIM-EA-TB and PIM-SBF and remained substantially unchanged during the tests, while for AO-PIM-1 a more than two-fold increase was observed, from about 40 to more than 100, when relative humidity was increased from 0 to 80 %. This increase, coupled with a CO2 permeability still higher than 200 Barrer, allowed this polymer to surpass the 2019 Robeson's plot for the CO2/N2 gas pair.