Assessment of Long-Term\nDegradation of Adsorbents\nfor Direct Air Capture by Ozonolysis

Porous adsorbents are a promising class of materials\nfor the direct\nair capture of CO<sub>2</sub> (DAC). Practical implementation of adsorption-based\nDAC requires adsorbents that can be used for thousands of adsorption–desorption\ncycles without significant degradation. We examined the potential\ndegradation of adsorbents by a mechanism that appears to have not\nbeen considered previously, namely, ozonolysis by trace levels of\nozone from ambient air. We focused on amine-appended metal–organic\nframeworks, specifically amine-functionalized Mg<sub>2</sub>(dobpdc),\nas a representative DAC adsorbent. Estimates based on the number of\namine sites in these adsorbents and the ozone concentration in air\nsuggest that degradation by ozone may be relevant over thousands of\nadsorption–desorption cycles if reactions with adsorbed ozone\nare fast. We used density functional theory calculations to estimate\nreaction rates for amine groups and carbon–carbon double bonds\nin amine-functionalized Mg<sub>2</sub>(dobpdc).