Contributions of CO(2), O(2), and H(2)O to the Oxidative Stability of Solid Amine Direct Air Capture Sorbents at Intermediate Temperature

[Image: see text] Aminopolymer-based sorbents are preferred materials for extraction of CO(2) from ambient air [direct air capture (DAC) of CO(2)] owing to their high CO(2) adsorption capacity and selectivity at ultra-dilute conditions. While those adsorptive properties are important, the stability of a sorbent is a key element in developing high-performing, cost-effective, and long-lasting sorbents that can be deployed at scale. Along with process upsets, environmental components such as CO(2), O(2), and H(2)O may contribute to long-term sorbent instability. As such, unraveling the complex effects of such atmospheric components on the sorbent lifetime as they appear in the environment is a critical step to understanding sorbent deactivation mechanisms and designing more effective sorbents and processes. Here, a poly(ethylenimine) (PEI)/Al(2)O(3) sorbent is assessed over continuous and cyclic dry and humid conditions to determine the effect of the copresence of CO(2) and O(2) on stability at an intermediate temperature of 70 °C. Thermogravimetric and elemental analyses in combination with in situ horizontal attenuated total reflection infrared (HATR-IR) spectroscopy are performed to measure the extent of deactivation, elemental content, and molecular level changes in the sorbent due to deactivation. The thermal/thermogravimetric analysis results reveal that incorporating CO(2) with O(2) accelerates sorbent deactivation using these sorbents in dry and humid conditions compared to that using CO(2)-free air in similar conditions. The in situ HATR-IR spectroscopy results of PEI/Al(2)O(3) sorbent deactivation under a CO(2)-air environment show the formation of primary amine species in higher quantity (compared to that in conditions without O(2) or CO(2)), which arises due to the C–N bond cleavage at secondary amines due to oxidative degradation. We hypothesize that the formation of bound CO(2) species such as carbamic acids catalyzes C–N cleavage reactions in the oxidative degradation pathway by shuttling protons, resulting in a low activation energy barrier for degradation, as probed by metadynamics simulations. In the cyclic experiment after 30 cycles, results show a gradual loss in stability (dry: 29%, humid: 52%) under CO(2)-containing air (0.04% CO(2)/21% O(2) balance N(2)). However, the loss in capacity during cyclic studies is significantly less than that during continuous deactivation, as expected.