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.