The chemical stability of amine-based solid sorbents under operational conditions is a major challenge for the long-term viability of Direct Air Capture (DAC) technologies. Repeated cycles of CO₂ adsorption from ambient air, followed by desorption at elevated temperatures, lead to temperature-dependent oxidative degradation of the sorbent. This degradation significantly reduces CO₂ capture capacity over time, limiting sorbent lifespan and increasing operational costs. Despite its critical impact, the underlying mechanisms remain poorly understood. Elucidating elementary reaction pathways associated with oxidative sorbent degradation, and understanding the influence of CO₂, H₂O, and temperature, are essential for properly characterizing sorbent durability and optimizing broader system design and cost.
To address this challenge, we construct a microkinetic model of the reaction cascade initiated by a polymeric benzyl amine reacting with O2. Our sorbent model is representative of a polystyrene-derived aminated resin, one of the common classes of solid sorbents for DAC. Using a representative atomistic model of the sorbent structure, we compute reaction activation energies and rate constants using first-principles quantum chemistry calculations combined with statistical thermodynamics. Our study shows that commonly accepted reactions pathways fail to capture both the timescales for degradation and the nature of degradation products. We present novel elementary reaction pathways, whose microkinetic simulations reproduce key experimental trends in amine loss and degradation rates without empirical fitting. We then discuss how CO2 and humidity may influence the rate-limiting steps in this newly identified degradation pathway and recommend best practices for increased lifespan of DAC sorbents for practical operation.
