Effect of Storage Conditions on Efficacy of Poly(ethylenimine)-Alumina CO₂ Sorbents

Solid amine sorbents are one of the primary components of DAC technologies that allow for the removal of ultradilute CO₂ from the atmosphere. A main drawback in the implementation of solid amine sorbents in industrial-scale DAC applications is their instability under certain operational or storage conditions over an extended period. In this work, the effect of storage temperature and gas composition in the storage headspace on the long-term stability of a poly-(ethylenimine)-alumina (PEI/γ-Al₂O₃) sorbent is explored. PEI/γ-Al₂O₃ sorbents with 70 and 100% pore filling are aged under varying gases (N₂, O₂, Ar, 0.04% CO₂-N₂, CO₂, and ambient air) in an oven (40 °C), at common ambient indoor temperature conditions (23 °C), or in a freezer (-4 °C). The CO₂ sorption capacity, as measured by thermogravimetric analysis (TGA), along with FTIR spectra of the fresh and aged sorbents, reveal that at 23 and -4 °C, storage under ambient air or inert gas (Ar) provides reasonable long-term stability, with <13% degradation over 12 and 5 months of storage. Interestingly, with storage at 40 °C, similar levels of deactivation were observed under pure O₂ and N₂ after 4 months of storage, which suggests that nonoxidative thermal reactions can occur under prolonged storage conditions under N₂. In contrast, with storage under CO₂, sorbent degradation is substantially suppressed compared to storage under N₂, ambient air, O₂, or Ar, yielding sorbents with no observable loss in capacity after 2 months, compared to a 66, 63, and 62% loss under N₂, ambient air, and N₂ in the same period at 40 °C, respectively. Overall, these findings provide guidance for practical amine sorbent storage in academic or industrial settings where amine sorbents are used for carbon capture.