Oxidatively-Stable Linear Poly(propylenimine)-Containing Adsorbents for CO₂ Capture from Ultradilute Streams

Aminopolymer-based solid sorbents have been widely investigated for the capture of CO₂ from dilute streams such as flue gas or ambient air. However, the oxidative stability of the widely studied aminopolymer, poly(ethylenimine) (PEI), is limited, causing it to lose its CO₂ capture capacity after exposure to oxygen at elevated temperatures. Here, we demonstrate the use of linear poly(propylenimine) (PPI), synthesized through a simple cationic ring-opening polymerization, as a more oxidatively stable alternative to PEI with high CO₂ capacity and amine efficiency. The performance of linear PPI/SBA-15 composites was investigated over a range of CO₂ capture conditions (CO₂ partial pressure, adsorption temperature) to examine the tradeoff between adsorption capacity and sorption-site accessibility, which was expected to be more limited in linear polymers relative to the prototypical hyperbranched PEI. Linear PPI/SBA-15 composites were more efficient at CO₂ capture and retained 65-83 % of their CO₂ capacity after exposure to a harsh oxidative treatment, compared to 20-40 % retention for linear PEI. Additionally, we demonstrated long-term stability of linear PPI sorbents over 50 adsorption/desorption cycles with no loss in performance. Combined with other strategies for improving the oxidative stability and adsorption kinetics, linear PPI may play a role as a component of stable solid adsorbents in commercial applications for CO₂ capture.