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

Abstract

Direct air capture (DAC) processes for extraction of CO 2 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 Al 2 O 3 , is explored in detail. By combining CO 2 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.

Contributions of CO2, O2, and H2O to the Oxidative Stability of Solid Amine Direct Air Capture Sorbents at Intermediate Temperature

“…The enhancement of oxidative degradation in the presence of H 2 O vapor is consistent with the impact of H 2 O in accelerating oxidative degradation reported in our recent work. 15 The above results (Figure 2) show the implications of the copresence of CO 2 and O 2 , as they appear in ambient air, on the stability of the amine-based sorbents at an intermediate temperature. As briefly mentioned above, these results differ from what has been reported in the literature for different amine sorbents under slightly different conditions.…”

“…Previously, the presence of H 2 O was reported to have a positive effect during the oxidative degradation, retarding oxidation; 10 however, as seen in Figure 2 and as reported in our recent work, the outcome in our system is reversed. 15 When rationalizing these differing results, it is important to note the key differences between the two studies, which include the deactivation temperature (50−120 °C for the Heydari-Gorji and Sayari study and 70 °C for this study), CO 2 concentration (1−20% for the Heydari-Gorji and Sayari study and 0.04% for this study), CO 2 uptake temperature (75 °C for the Heydari-Gorji and Sayari study and 30 °C for this study), and support type (laboratorysynthesized ordered mesoporous silica vs disordered, commercial mesoporous alumina). Further discussion comparing and contrasting our results to the literature will be presented after additional analysis in the forthcoming sections.…”

“…29,39,41,47,48 A recent study from our group on the oxidative degradation of the PEI/γ-Al 2 O 3 sorbent at high temperatures illustrated the individual contributions of the imine and carbonyl species to the band in the region around 1666 cm −1 for the first time. 15 The study links the C−N bond cleavage events to the formation of carbonyl and imine species, new primary amine species, and other low-molecular-weight amine products. 15 To briefly discuss the proposed mechanism(s), in dry conditions, degradation begins with the formation of radical species (α-or β-amino alkyl radicals) due to thermal stress, which leads to chain relaxation and breakage, 15 or due to metal impurities within the sorbent that catalyze free radical formation.…”

“…Notably, a recent study from our group on the impact of atmospheric humidity on the stability of the PEI/Al 2 O 3 sorbent showed that H 2 O plays a significant role in accelerating degradation reactions on some sorbents, which is contrary to other reports in literature, as briefly mentioned above. 15 To this end, a broad understanding of the impact of ambient air components (H 2 O/CO 2 /O 2 /N 2 ) as they appear in the environment on sorbent degradation needs further indepth exploration.…”

Evaluating degradation of CO2 adsorbents in flue gas from bioenergy with carbon capture and storage

“…Accelerated aging testing can be employed to evaluate sorbent stability in reasonable time frames. 67–69 Ultimately, these results highlight the range of possible degradation mechanisms in BECCS due to structural differences in adsorbents and emphasize the need for stability research and mitigation strategies.…”