(519b) Polyformamidine As Fixed-Site Carrier for CO2/N2 Separation Membranes

Amidine species was perceived as better alternatives for amine species in the facilitated transport membranes for having faster CO₂ reaction kinetics. The synthesis of poly(ethylene formamidine) (PFA) was carried out via the polycondensation of ethylenediamine and triethyl orthoformate catalyzed by acetic acid. The synthesized PFA possesses the highest amidine functional group density owing to the case using the shortest-chain diamine, ethylenediamine. Moreover, in comparison with mobile carriers, it can have a much higher boiling point, which practically eliminates the possibility of carrier volatility and leaching out. In order to obtain the preliminary gas permeation results, PFA (10 kDa MW) was blended with polyvinylalcohol (PVA, 1.1 MDa MW) and then coated onto nanoporous polyethersulfone (PES) substrates to form 170-nm thin membranes. Increasing the content of PFA from 36 wt.% to 66 wt.% in these membranes led to a substantial increase in membrane performance, especially for CO₂ permeance. Further increasing the PFA content to 83 wt.% still enhanced the CO₂ permeance but showed a moderate drop in CO₂/N₂ selectivity. Under an extremely high PFA content of 90 wt.%, the membrane showed a low CO₂ permeance of 1019 GPU but a high CO₂/N₂ selectivity of 236, indicating severe pore penetration. The membrane containing 83 wt.% PFA exhibited the best results with 2807 GPU CO₂ permeance and 72 CO₂/N₂ selectivity at 77°C. In view of the superior transport performance, it is necessary to increase the MW of PFA to make it capable of forming the membrane matrix alone to enable the incorporation of CO₂-reactive mobile carriers, thereby improving the overall CO₂ separation performance. By adopting the design of experiment technique, the synthesis conditions of PFA were improved in terms of the temperature profile, catalyst amount, and monomer concentrations, through which the MW was successfully increased to 3.3 MDa.