Elucidating CO₂ Chemisorption in Diamine-Appended Metal-Organic Frameworks

The widespread deployment of carbon capture and sequestration as a climate change mitigation strategy could be facilitated by the development of more energy-efficient adsorbents. Diamine-appended metal-organic frameworks of the type diamine-M₂(dobpdc) (M = Mg, Mn, Fe, Co, Ni, Zn; dobpdc^4- = 4,4'-dioxidobiphenyl-3,3'-dicarboxylate) have shown promise for carbon-capture applications, although questions remain regarding the molecular mechanisms of CO₂ uptake in these materials. Here we leverage the crystallinity and tunability of this class of frameworks to perform a comprehensive study of CO₂ chemisorption. Using multinuclear nuclear magnetic resonance (NMR) spectroscopy experiments and van-der-Waals-corrected density functional theory (DFT) calculations for 13 diamine-M₂(dobpdc) variants, we demonstrate that the canonical CO₂ chemisorption products, ammonium carbamate chains and carbamic acid pairs, can be readily distinguished and that ammonium carbamate chain formation dominates for diamine-Mg₂(dobpdc) materials. In addition, we elucidate a new chemisorption mechanism in the material dmpn-Mg₂(dobpdc) (dmpn = 2,2-dimethyl-1,3-diaminopropane), which involves the formation of a 1:1 mixture of ammonium carbamate and carbamic acid and accounts for the unusual adsorption properties of this material. Finally, we show that the presence of water plays an important role in directing the mechanisms for CO₂ uptake in diamine-M₂(dobpdc) materials. Overall, our combined NMR and DFT approach enables a thorough depiction and understanding of CO₂ adsorption within diamine-M₂(dobpdc) compounds, which may aid similar studies in other amine-functionalized adsorbents in the future.