Evaluation of the Stability of Diamine-Appended Mg₂(dobpdc) Frameworks to Sulfur Dioxide

Diamine-appended Mg 2 (dobpdc) (dobpdc 4– = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) metal–organic frameworks are a promising class of CO 2 adsorbents, although their stability to SO 2 ─a trace component of industrially relevant exhaust streams─remains largely untested. Here, we investigate the impact of SO 2 on the stability and CO 2 capture performance of dmpn–Mg 2 (dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine), a candidate material for carbon capture from coal flue gas. Using SO 2 breakthrough experiments and CO 2 isobar measurements, we find that the material retains 91% of its CO 2 capacity after saturation with a wet simulated flue gas containing representative levels of CO 2 and SO 2, highlighting the robustness of this framework to SO 2 under realistic CO 2 capture conditions. Initial SO 2 cycling experiments suggest dmpn–Mg 2 (dobpdc) may achieve a stable operating capacity in the presence of SO 2 after initial passivation. Evaluation of several other diamine–Mg 2 (dobpdc) variants reveals that those with primary, primary (1°,1°) diamines, including dmpn–Mg 2 (dobpdc), are more robust to humid SO 2 than those featuring primary, secondary (1°,2°) or primary, tertiary (1°,3°) diamines. Based on the solid-state 15 N NMR spectra and density functional theory calculations, we find that under humid conditions, SO 2 reacts with the metal-bound primary amine in 1°,2° and 1°,3° diamine-appended Mg 2 (dobpdc) to form a metal-bound bisulfite species that is charge balanced by a primary ammonium cation, thereby facilitating material degradation. In contrast, humid SO 2 reacts with the free end of 1°,1° diamines to form ammonium bisulfite, leaving the metal–diamine bond intact. This structure–property relationship can be used to guide further optimization of these materials for CO 2 capture applications.