Volatile Products of the Autoxidation of Poly(ethylenimine) in CO2 Sorbents

Poly(ethyleneimine) (PEI)-based CO2 sorbents are a promising material class for use in CO2 capture applications, particularly direct air capture (DAC), due to their high amine content and CO2 capacities. The sorbent lifetime is a key uncertainty in the deployment of these materials in such applications, as they oxidize under conditions relevant to candidate DAC process cycles. Here, we utilize thermogravimetric analysis/differential scanning calorimetry/FTIR spectroscopy to characterize the nature, rate, and quantity of volatile species formed from the oxidation of PEI when supported on mesoporous Al2O3. We show that NH3, CO2, and H2O are primary volatile species formed from PEI oxidation and that their production rate matches the overall oxidation rate as measured via heat flow from a differential scanning calorimeter. We show that the total quantity of NH3 evolved is consistent with terminal primary amines being cleaved from the polymer chain and that approximately one hydrogen atom per PEI repeat unit reacts to form H2O during the reaction. Finally, metadynamics and quantum chemical simulations performed on a simplified system are used to highlight the likely importance of radicals in the elementary reactions that comprise the oxidation reaction set. The findings here represent the first characterization of the volatile products from the oxidation of PEI in CO2 sorbents and lead to a greater understanding of the mechanisms behind the oxidative degradation of these materials.