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

Abstract

Diamine-appended Mg₂(dobpdc) (dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) metal-organic frameworks are a promising class of CO₂ adsorbents, although their stability to SO₂─a trace component of industrially relevant exhaust streams─remains largely untested. Here, we investigate the impact of SO₂ on the stability and CO₂ capture performance of dmpn-Mg₂(dobpdc) (dmpn = 2,2-dimethyl-1,3-propanediamine), a candidate material for carbon capture from coal flue gas. Using SO₂ breakthrough experiments and CO₂ isobar measurements, we find that the material retains 91% of its CO₂ capacity after saturation with a wet simulated flue gas containing representative levels of CO₂ and SO₂, highlighting the robustness of this framework to SO₂ under realistic CO₂ capture conditions. Initial SO₂ cycling experiments suggest dmpn-Mg₂(dobpdc) may achieve a stable operating capacity in the presence of SO₂ after initial passivation. Evaluation of several other diamine-Mg₂(dobpdc) variants reveals that those with <i>primary</i>,<i>primary</i> (1°,1°) diamines, including dmpn-Mg₂(dobpdc), are more robust to humid SO₂ than those featuring <i>primary</i>,<i>secondary</i> (1°,2°) or <i>primary</i>,<i>tertiary</i> (1°,3°) diamines. Based on the solid-state ¹⁵N NMR spectra and density functional theory calculations, we find that under humid conditions, SO₂ reacts with the metal-bound primary amine in 1°,2° and 1°,3° diamine-appended Mg₂(dobpdc) to form a metal-bound bisulfite species that is charge balanced by a primary ammonium cation, thereby facilitating material degradation. In contrast, humid SO₂ 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 CO₂ capture applications.