(365d) Multi-Technique Study of Oxidative Degradation of Supported Poly(ethylenimine) sorbents for CO2 capture

The increase in CO₂ concentration in the air has been a growing subject of attention for scientists in the past few decades. By holding on to the current policies and regulations, it is anticipated that the CO₂ level in air will be more than 450 ppm by 2035, leading to a minimum increase of 2 ^oC in average atmospheric temperature. ¹ Therefore, technologies that lead to a reduction of the CO₂ concentration in the air, such as direct adsorptive CO₂ capture from air (Direct Air Capture, DAC), are expected to play an important role in the coming years.

DAC sorbents can be prepared by loading amines into different mesoporous inorganic supports such as alumina, silica, and carbon. Typically, primary and secondary amine groups react with CO₂ to form carbamates and bicarbonates. However, one of the key challenges involved with employing amine sorbents is their sensitivity to deactivation by NOx and SOx, amine leaching, and simultaneous exposure to heat and oxygen. Previous studies have shown that primary and secondary amines can readily oxidize to imines and carbonyl/imide species at the elevated temperatures used for sorbent regeneration if significant oxygen is also present.² Considering the significant concentration of oxygen in air, understanding the chemistry and kinetics of this oxidative degradation process plays a crucial role in designing long-lifetime sorbents for direct-air-capture.

In this research, the oxidative degradation of a prototypical class 1 sorbent [poly(ethylenimine) impregnated into commercial mesoporous γ-Al₂O₃] at different operational conditions (temperature, water content, oxygen content, and oxygen exposure time has been analyzed. A suite of analytical techniques including calorimetry, thermogravimetric, elemental, and IR analysis, has been used to measure the rate of oxidation in situ. The results are then used to understand the kinetics and thermodynamics of the oxidative degradation reactions. The trend observed in the obtained heat of reaction from calorimetry studies, carbonyl peak intensity from IR, and CO₂ loss from thermogravimetric analysis are in agreement with each other. Using the calorimetry and elemental analysis data, a rate expression has been developed, and an overall activation energy is calculated for the concerted suite of oxidation reactions.

References:

1. Anwar, M. N.; Fayyaz, A.; Sohail, N. F.; Khokhar, M. F.; Baqar, M.; Yasar, A.; Rasool, K.; Nazir, A.; Raja, M. U. F.; Rehan, M.; Aghbashlo, M.; Tabatabaei, M.; Nizami, A. S., CO₂ utilization: Turning greenhouse gas into fuels and valuable products. Journal of Environmental Management 2020, 260, 110059, DOI: 10.1016/j.jenvman.2019.110059.
2. Ahmadalinezhad, A.; Sayari, A., Oxidative degradation of silica-supported polyethylenimine for CO₂ adsorption: insights into the nature of deactivated species. Physical Chemistry Chemical Physics 2014, 16, 1529-1535, DOI: 10.1039/C3CP53928H.