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

Abstract

Aminopolymer-based sorbents are preferred materials for extraction of CO₂ from ambient air [direct air capture (DAC) of CO₂] owing to their high CO₂ 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 CO₂, O₂, and H₂O 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)/Al₂O₃ sorbent is assessed over continuous and cyclic dry and humid conditions to determine the effect of the copresence of CO₂ and O₂ 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 CO₂ with O₂ accelerates sorbent deactivation using these sorbents in dry and humid conditions compared to that using CO₂-free air in similar conditions. The in situ HATR-IR spectroscopy results of PEI/Al₂O₃ sorbent deactivation under a CO₂-air environment show the formation of primary amine species in higher quantity (compared to that in conditions without O₂ or CO₂), which arises due to the C-N bond cleavage at secondary amines due to oxidative degradation. We hypothesize that the formation of bound CO₂ 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 CO₂-containing air (0.04% CO₂/21% O₂ balance N₂). However, the loss in capacity during cyclic studies is significantly less than that during continuous deactivation, as expected.