Oxidation of Supported Amines for CO2 Direct Air Capture: Assessing Impact on Physical Properties and Mobility via NMR Relaxometry

Abstract

Aminopolymer-based adsorbents are commonly investigated for CO₂ direct air capture (DAC). In the presence of high temperature and oxygen, which can occur during adsorption-regeneration cycles, oxidative degradation can significantly reduce adsorbent lifetime. Degradation is typically measured using adsorption performance metrics or chemical characterization. This work demonstrates that the polymer's physical properties, as measured via nuclear magnetic resonance (NMR) relaxometry and differential scanning calorimetry, can be used to quantitatively track degradation. The extent of oxidation of an alumina-poly(ethylenimine) sample, measured by reductions in amine efficiency (A.E.), is correlated with proton T₂ relaxation times. This work hypothesizes that T₂ relaxation accurately tracks oxidative degradation in aminopolymers due to the reduction in polymer mobility occurring during the oxidation process. The ability to use NMR relaxometry as a noninvasive technique to probe degradation is demonstrated on a 1-inch square-channel monolith adsorbent exposed to actual DAC service conditions. This highlights the potential for relaxometry to evaluate the state of the adsorbent accurately and rapidly compared to typical analytical methods.