Diaminopropane-Functionalized Metal-Organic Frameworks with Controllable Diamine Loss and One-Channel Flipped CO₂ Adsorption Mode

Diamine-functionalized metal–organic frameworks (MOFs) based on Mg₂(dobpdc) (dobpdc⁴⁻=4,4’-dioxidobihyenyl-3,3’-dicarboxylate) are considered promising CO₂ adsorbents owing to their characteristic stepwise adsorption behavior. However, the high temperatures required for CO₂ desorption from diamine-Mg₂(dobpdc)-based adsorbents induce gradual diamine loss. Additionally, the existence of an exotic CO₂ adsorption mode remains experimentally unanswered. Herein, we present CO₂ adsorbents obtained by functionalizing Mn₂(dobpdc) with a series of diaminopropane derivatives. The low regeneration energies of these adsorbents allow for CO₂ 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 Mn₂(dobpdc) is greater than that between the diamine and Mg ions in Mg₂(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 CO₂ adsorbents.