Role of Polymer Architecture in CO₂ Capture from Air Using Supported Poly(alkylenimine)s: Linear vs Branched Polymers

Direct air capture (DAC) of CO₂ coupled with geologic storage is a promising climate change mitigation strategy, with some applications employing amines supported on porous solids as CO₂ sorbents. While branched poly-(ethylenimine) (PEI) is the standard benchmark amine material, it suffers from limited oxidative stability. Poly-(propylenimine) (PPI), as an alternative, has previously demonstrated improved resistance to degradation under harsh oxidative conditions. Linear and branched PEI are commercially available, though at different molecular weights, while PPI is not commercially available. For this reason, a comparative study of all four polymers (linear PEI, branched PEI, linear PPI, branched PPI) has not been reported for DAC. In this study, we synthesize and compare low-molecular-weight (∼800 g/mol) linear (L) and branched (B) PEI and PPI supported on a model support, SBA-15 silica. These materials are evaluated for CO₂ adsorption under dry, DAC-relevant conditions (400 ppm of CO₂, 30 °C). LPPI exhibited the highest amine efficiency at all loadings, reaching a maximum of 0.14 mmol CO₂/mmol N, outperforming BPEI, while LPEI consistently showed the lowest uptake capacity. Temperature-programmed desorption reveals that the structure of the amine polymer impacts the CO₂ binding strength, with branched polymers displaying higher desorption energies of 102-111 kJ/mol. In situ infrared spectroscopy experiments show that all sorbents preferentially capture CO₂ as ammonium carbamate. Isobaric CO₂ uptake studies further underscore the influence of polymer mobility and support pore crowding on performance, while demonstrating the sorbents' performance at elevated temperatures and CO₂ concentrations. All materials demonstrated good stability over 25 adsorption-desorption cycles using thermal regeneration in an inert gas purge, with only BPPI displaying a 10-11% decrease in capacity/amine efficiency during cycling, possibly due to the loss of low molecular weight, oligomeric amines. This is the first side-by-side comparison of the CO₂ sorption properties of linear and branched PEI and PPI with similar molecular weights. These findings highlight the significant role of polymer architecture in CO₂ capture efficiency and inform future designs of durable, high-performance DAC sorbents.