Formation mechanism of ammonium carbamate for CO₂ uptake in N,N′-dimethylethylenediamine grafted M₂(dobpdc)

The adsorption properties and formation mechanism of ammonium carbamate for CO₂ capture in N,N′dimethylethylenediamine (mmen) grafted M₂(dobpdc) (dobpdc⁴- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate; M = Mg, Sc-Zn, except Ni) have been studied via density functional theory (DFT) calculations. We see that the mmen molecule is joined to the metal site via a M-N bond and has hydrogen bonding with neighboring mmen molecules. The binding energies of mmen range from 135.4 to 184.0 kJ/mol. CO₂ is captured via insertion into the M-N bond of mmen-M₂(dobpdc), forming ammonium carbamate. The CO₂ binding energies (35.2 to 92.2 kJ/mol) vary with different metal centers. Furthermore, the Bader charge analysis shows that the CO₂ molecules acquire 0.42 to 0.47 |e|. This charge is mainly contributed by the mmen, and a small additional amount is from the metal atom bonded with the CO₂. The preferred reaction pathway is a two-step reaction. In the first step, the hydrogen bonded complex B changes into an N-coordinated intermediate D with high barriers (0.69 to 1.58 eV). The next step involves the translation and rotation of the chain in the intermediate D, resulting in the formation of the final O-coordinated product I with barriers of 0.22 to 0.61 eV. The higher barriers of CO₂ reaction with mmen-M₂(dobpdc) relative to attack the primary amine might be due to the larger steric hindrance of mmen. We hope this work will contribute to an improved understanding and development of future amine-grafted materials for efficient CO₂ capture.