Assessment of Long-Term Degradation of Adsorbents for Direct Air Capture by Ozonolysis

Porous adsorbents are a promising class of materials for the direct air capture of CO₂ (DAC). Practical implementation of adsorption-based DAC requires adsorbents that can be used for thousands of adsorption-desorption cycles without significant degradation. We examined the potential degradation of adsorbents by a mechanism that appears to have not been considered previously, namely, ozonolysis by trace levels of ozone from ambient air. We focused on amine-appended metal-organic frameworks, specifically amine-functionalized Mg₂(dobpdc), as a representative DAC adsorbent. Estimates based on the number of amine sites in these adsorbents and the ozone concentration in air suggest that degradation by ozone may be relevant over thousands of adsorption-desorption cycles if reactions with adsorbed ozone are fast. We used density functional theory calculations to estimate reaction rates for amine groups and carbon-carbon double bonds in amine-functionalized Mg₂(dobpdc).