Insights into the Oxidative Degradation Mechanism of Solid Amine Sorbents for CO2 Capture from Air: Roles of Atmospheric Water.

Direct air capture (DAC) processes for extraction of CO2 from ambient air are unique among chemical processes in that they operate outdoors with minimal feed pretreatments. Here, the impact of humidity on the oxidative degradation of a prototypical solid supported amine sorbent, poly(ethylenimine) (PEI) supported on Al2 O3 , is explored in detail. By combining CO2 adsorption measurements, oxidative degradation rates, elemental analyses, solid-state NMR and in situ IR spectroscopic analysis in conjunction with 18 O labeling of water, a comprehensive picture of sorbent oxidation is achieved under accelerated conditions. We demonstrated that the presence of water vapor can play an important role in accelerating the degradation reactions. From the study we inferred the identity and kinetics of formation of the major oxidative products, and the role(s) of humidity. Our data are consistent with a radical mediated autooxidative degradation mechanism.

Effect of Extended Aging and Oxidation on Linear Poly(propylenimine)-Mesoporous Silica Composites for CO2 Capture from Simulated Air and Flue Gas Streams.

Physical aging or degradation of amine-containing polymers and supported amine adsorbents is a critical issue that could limit the practical application of such materials for CO2 capture. However, to date, there is a scarcity of studies that evaluate the long-term stability of amine-based sorbents without the exclusive use of accelerated aging tests. Here, we demonstrate that extended aging (∼2 years) of linear poly(propylenimine) (LPPI) confined in mesoporous silica (SBA-15) supports does not drastically impact the CO2 adsorption performance under simulated flue gas (10% CO2) and direct air capture (DAC, 400 ppm CO2) conditions, although the behavior of the aged sorbents and polymers in the two CO2 concentration regimes differs. The sorbents made with aged LPPI have modestly decreased CO2 uptake performance (≲20% lower) compared to the fresh polymers, with overall good CO2 cycling performance. The data indicate that only slow degradation occurs under the deployed ambient storage conditions. Even after extended aging, the LPPI-based sorbents preserved their ability to display stable temperature-swing cycling performance. In parallel, the impact of blending LPPI polymers of different number-average molecular weights, Mn, is evaluated, seeking to understand its impact on adsorbent performance. The results demonstrate that the blends of two Mn aged LPPI give similar CO2 adsorption performance to adsorbents made from a single-Mn LPPI, suggesting that molecular weight will not negatively impact adsorbent performance in the studied Mn range. After an accelerated oxidation experiment, the aged LPPI sorbents retained a larger portion of the samples' original performance when cycling under simulated flue gas conditions than under DAC conditions. However, in each case, the oxidized sorbents could be cycled repeatedly with consistent uptake performance. Overall, these first of their kind extended aging tests suggest that LPPI-based amine adsorbents offer promise for long-term, stable use in carbon capture applications.