Mechanistic Studies of Oxidative Degradation in Diamine-Appended Metal-Organic Frameworks Exhibiting Cooperative CO2 Capture.

Understanding the impact of O₂ during a carbon capture process is vital for designing robust, cost-effective materials for carrying it out. However, mechanistic studies of the O₂-induced degradation of materials are not easily undertaken owing to the complex sequential reaction pathways that arise. Here, we report comprehensive mechanistic investigations of the O₂-induced degradation of diamine-appended metal-organic frameworks (MOFs) exhibiting cooperative CO₂ adsorption. Oxygen exposure experiments were performed on seven different diamine-appended MOFs, including e-2-Mg₂(dobpdc) (e-2 = N-ethylethylenediamine, dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate), under various temperatures and O₂ pressures. These experiments show that diamine degradation inhibits CO₂ chemisorption and that the degradation rate is significantly influenced by the diamine structure. In contrast, the parent frameworks remain essentially intact upon O₂ exposure. Detailed characterization of O₂-exposed e-2-Mg₂(dobpdc) revealed the formation of various degradation products, including acetaldehyde, carbon dioxide, water, ethylamine, and other aldehyde- and imine-containing species. Together, these observations suggest that diamine degradation occurs via C-N bond cleavage through pathways involving C-centered radicals. Furthermore, computational evaluation of the initiation and propagation pathways for amine degradation in diamine-appended MOFs indicates that (i) degradation is likely initiated by OH^•, (ii) carbon-centered radicals generated via radical transfer reactions react with O₂, leading to amine degradation, and (iii) the rate-limiting step of the degradation reactions likely involves O-O bond cleavage. Overall, these mechanistic insights could inform strategies for mitigating O₂-induced amine degradation in next-generation carbon capture technologies.