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; dobpdc4– = 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.碳捕获与封存作为气候变化减缓策略的大规模部署，可通过开发更节能的吸附剂得以加速推进。通式为diamine–M₂(dobpdc)的二胺接枝金属有机框架（其中M=Mg、Mn、Fe、Co、Ni、Zn；dobpdc⁴⁻=4,4'-二氧基联苯-3,3'-二羧酸根）在碳捕获应用中展现出良好潜力，但此类材料中CO₂吸附的分子机制仍存在诸多待厘清的问题。本研究依托该类框架的结晶性与可调控性，对CO₂化学吸附开展系统性研究：针对13种diamine–M₂(dobpdc)变体，结合多核核磁共振（nuclear magnetic resonance, NMR）光谱实验与范德华校正密度泛函理论（density functional theory, DFT）计算，本研究证实经典的CO₂化学吸附产物——氨基甲酸铵链与氨基甲酸对可被清晰区分，且在diamine–Mg₂(dobpdc)材料中，氨基甲酸铵链的形成占据主导地位。此外，本研究阐明了dmpn–Mg₂(dobpdc)材料（dmpn=2,2-二甲基-1,3-丙二胺）中全新的化学吸附机制：该机制通过生成氨基甲酸铵与氨基甲酸1:1的混合物，解释了该材料反常的吸附特性。最后，本研究证实水的存在对diamine–M₂(dobpdc)材料中CO₂吸附机制的调控具有重要作用。综上，本研究结合NMR与DFT的联用方法，可全面刻画并阐明diamine–M₂(dobpdc)化合物内的CO₂吸附过程，未来可为其他胺功能化吸附剂的相关研究提供参考。