Competing Kinetic Consequences of CO₂ on the Oxidative Degradation of Branched Poly(ethylenimine)

Amine-functionalized porous solid materials are effective sorbents for direct air capture (DAC) of CO₂. However, they are prone to oxidative degradation in service, increasing the materials cost for widespread implementation. While the identification of oxidation products has given insights into degradation pathways, the roles of some species, like CO₂ itself, remain unresolved, with conflicting information in the literature. Here, we investigate the impact of CO₂ on the oxidative degradation of poly(ethylenimine)-alumina (PEI/Al₂O₃) sorbents under conditions encompassing a wide range of CO₂-air mixture compositions and temperatures relevant to DAC conditions, thereby reconciling the conflicting data in the literature. Degradation profiles characterized by thermogravimetric analysis, <i>in situ</i> ATR-FTIR, and CO₂ capacity measurements reveal nonmonotonic effects of CO₂ concentrations and temperatures on oxidation kinetics. Specifically, 0.04% CO₂ accelerates PEI/Al₂O₃ oxidation more at low temperatures (<90 °C) compared to 1% and 5% CO₂, but this trend reverses at high temperatures (>90 °C). First-principles metadynamics, machine learning accelerated molecular dynamics simulations, and ¹H relaxometry experiments show that chemisorbed CO₂ acid-catalyzes critical oxidation reactions, while extensive CO₂ uptake reduces PEI branch mobility, slowing radical propagation. These contrasting kinetic effects of CO₂ explain the complex degradation profiles observed in this work and in prior literature. Collectively, this work highlights the importance of considering atmospheric components in the design of DAC sorbents and processes. Additionally, it identifies the unconstrained branch mobility and local acid environment as two of the major culprits in the oxidation of amine-based sorbents, suggesting potential strategies to mitigate sorbent degradation.