Evaluating Autoxidation Radical Scavengers and Additives to Enhance Aminopolymer Sorbent Stability

Solid amine sorbents have shown promise in the removal of ultradilute CO₂ from the atmosphere. Despite being a promising candidate material type for this application, these sorbents are prone to degradation during long-term exposure to environmental components such as CO₂, O₂, and H₂O, with amine oxidation being a particularly challenging problem. In this study, we investigate the potency of different radical scavengers and additives in mitigating the degradation of a model poly-(ethylenimine) (PEI)/Al₂O₃ sorbent under direct air capture (DAC)-relevant conditions. The results reveal that a 4,4'-bis-(α,α-dimethylbenzyl)-diphenylamine (BDDPA)-incorporated PEI/Al₂O₃ sorbent showed the most resistance toward oxidative degradation at varying exposure times and BDDPA loadings under CO₂-free air (21% O₂/balance N₂) at 120 °C. Interestingly, under humid (∼43% relative humidity (RH) at 26 °C) and dry 0.04% CO₂-air, the BDDPA/PEI/Al₂O₃ sorbent showed enhanced sorbent stability both at 70 and 120 °C after 4.5 h of exposure. Under humid CO₂-free air, at 120 °C, the antioxidant performance slightly declined (in comparison to the dry CO₂-free air condition) but displayed a much higher stability than the pristine sorbent. Overall, the ability of BDDPA to inhibit sorbent degradation under dry and humid, CO₂-free and CO₂-containing (0.04%) air at intermediate (70 °C) and elevated (120 °C) temperatures is promising in prolonging sorbent stability and underscores the importance of performing accelerated oxidation studies in the presence of all species that are expected to be present in DAC processes to identify suitable stabilization treatments for sorbent materials.