Inhibitory Effects on the Degradation Behavior of Amine-Functionalized MOF-Based Adsorbents for CO2 Direct Air Capture

Solid amine-based adsorption using metal–organic frameworks (MOFs) has shown great potential in direct air capture (DAC) due to its excellent CO2 selectivity and adsorption capacity. Incorporation of amine functional groups into MOFs significantly enhances CO2 uptake, but the adsorption performance is highly dependent on the activity of surface amine groups, which are prone to irreversible degradation. This study systematically investigates the degradation behavior and structural evolution of MOF-based solid amine adsorbents under complex environmental conditions, elucidating the degradation products and underlying mechanisms. Thermal degradation leads to reduced crystallinity and pore structure collapse, while also inducing linear condensation reactions at amine sites that form urea-linkage fragments, thereby impairing adsorption performance. Oxidative degradation breaks C–N bonds in amine groups, generating carboxylic acids and releasing ammonia, accompanied by framework rearrangement and loss of active sites. A benefit from the topological protection provided by the three-dimensional MOF framework and strong coordination between metal centers and amine groups, which effectively inhibited the degradation behavior of N-(2-aminoethyl) ethanolamine (AEEA) (both thermal degradation and oxidative degradation) is enabling the MOF adsorbent to have better antidegradation performance compared to pure AEEA. After 5 weeks of high-pressure oxidative treatment, the amine utilization remains at 45.83%, representing a 30.9% improvement over AEEA and a 10.97% reduction in performance loss. A coupled reaction model was established to describe the synergistic effects of thermal and oxidative degradation, and a first-order kinetic equation was developed to predict long-term degradation behavior. This study provides both theoretical insight and practical guidance for the long-term stable operation of MOF-based solid amine adsorbents.