Effect of Different Acid Initiators on Branched Poly(propylenimine) Synthesis and CO₂ Sorption Performance

Branched poly(propylenimine) (PPI) provides an alternative to the prototypical amine polymer, commercially available branched poly(ethylenimine) (PEI), in composite adsorbents for CO₂ capture. Herein, we investigate the synthesis of PPI via cationic ring opening polymerization of azetidine using various acid initiators (HBr, HClO₄, HCl, CH₃SO₃H) and polymerization times, impacting the molecular weight and CO₂ sorption behavior. The polymerization kinetics and the amine distribution (i.e., primary:secondary:tertiary ratios) are monitored with ¹H NMR during polymerization, and a basic ion-exchange resin is used to neutralize charged amine centers and to remove unreacted acid. The polymers are impregnated into the model porous oxide support, mesoporous silica SBA-15, and the CO₂ capacities under both simulated ambient air and flue gas conditions are elucidated. In parallel, the oxidative stability of the PPI-based sorbents is assessed and compared with the prototypical PEI sorbents. Sorbents with 30 wt % polymers synthesized using HBr and HClO₄ exhibit higher CO₂ capacities than those made with HCl or CH₃SO₃H. Sorbents from HBr polymers only lost 24% of their CO₂ capacity after 12 h of oxidation in air at 383 K. Even trace amounts of residual ClO₄^- anions in HClO₄ initiated polymers, though, accelerated oxidation (decreased CO₂ capacity by 64%). Extended resin treatments were needed to leave undetectable Cl content in these polymers, which resulted in sorbents that are much more oxidatively stable.