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, dobpdc4- = 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.