Diaminopropane-Functionalized Metal-Organic Frameworks with Controllable Diamine Loss and One-Channel Flipped CO 2 Adsorption Mode.

Diamine-functionalized metal-organic frameworks (MOFs) based on Mg ₂ (dobpdc) (dobpdc ^4- =4,4'-dioxidobihyenyl-3,3'-dicarboxylate) are considered promising CO ₂ adsorbents owing to their characteristic stepwise adsorption behavior. However, the high temperatures required for CO ₂ desorption from diamine-Mg ₂ (dobpdc)-based adsorbents induce gradual diamine loss. Additionally, the existence of an exotic CO ₂ adsorption mode remains experimentally unanswered. Herein, we present CO ₂ adsorbents obtained by functionalizing Mn ₂ (dobpdc) with a series of diaminopropane derivatives. The low regeneration energies of these adsorbents allow for CO ₂ desorption at temperatures lower than those reported for Mg-based analogs. Our first-principles density functional theory calculations indicated that the bond strength between the diamine and Mn ions in Mn ₂ (dobpdc) is greater than that between the diamine and Mg ions in Mg ₂ (dobpdc). This stronger bonding prevents diamine loss even at high temperatures and enables efficient regeneration. Additionally, the computational and experimental results showed that MOFs functionalized with primary-tertiary diamine exhibit unique one-channel flipped adsorption structures that have not been previously observed. Our findings provide valuable insights into the role of metal ions in diamine loss for the future development of efficient amine-based CO ₂ adsorbents.
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