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

High Resolution Image Download MS PowerPoint Slide 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.