(401g) Impacts of Environmental Conditions on the Stability of Aminopolymer Sorbents for Direct Air Capture

Amine-functionalized sorbents are primary candidates for large-scale CO₂ removal due to their unique chemistry that allows for the removal of ultra-dilute/trace CO₂ in high quantities. However, these sorbents lose their high CO₂ adsorption capacity when operating for extended adsorption/desorption cycling. Along with process upsets that induce undesired states (e.g. exposure to high temperature and high oxygen concentrations simultaneously), extended exposure to environmental components such as CO₂, O_2, and H₂O can contribute significantly to long-term sorbent instability.

This work employs advanced simulations and experimental studies to identify the role CO₂ and temperature play in accelerating oxidative degradation of a model aminopolymer sorbent (poly(ethylenimine)-alumina (PEI/Al₂O₃)). The results reveal that adsorbed CO₂ species (carbamic acid and carbamates) reduce the free energy barriers of C–N bond cleavage promoting chain branching reactions that enhance free radical concentration and further destabilize the sorbent. However, sorbed CO₂’s tendency to accelerate oxidative degradation depends on several factors, including the presence of radicals on the chain, CO₂ loading, and temperature. For each CO₂-air mixture studied (0.04%, 1%, and 5% CO₂-air), there is a region of temperature (55 – 80 °C for 0.04% CO₂-air, 75 – 85 °C for 1% CO₂-air, and 80 – 100 °C for 5% CO₂-air) in which sorbent deactivation accelerates (more than 50% of the initial CO₂ adsorption capacity is lost) due to the combined effects of enhanced carbamic acid/carbamate-catalyzed C-N bond cleavage and increased mobility of the aminopolymers. Deep potential molecular dynamics simulations and experimental results show that increasing the CO₂ loading minimizes PEI branch mobility and increases the energy barrier for radical propagation, minimizing sorbent degradation. This work demonstrates the role CO_2­ plays in sorbent stability and the importance of considering various atmospheric components in DAC sorbent design and development.