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