Disclosing the microscopic mechanism and adsorption properties of CO2 capture in N-isopropylethylenediamine appended M2(dobpdc) series

Abstract

The detailed picture of the microscopic mechanism for CO₂ capture in N-isopropylethylenediamine (i-2) functionalized M₂(dobpdc) (dobpdc⁴⁻ = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate; M = Mg, Sc–Zn) has been determined for the first time via systematic computations with van der Waals (vdW) corrected density functional theory (DFT) methods. The results show that acting as a Lewis base, the i-2 molecule can strongly interact with the acidic open metal sites of M₂(dobpdc) via its primary amine with binding energies of 132 to 178 kJ mol⁻¹ for different metals. After exposure to gaseous CO₂, CO₂ is captured by inserting into the metal–N bond. The corresponding CO₂ binding energies (43–69 kJ mol⁻¹) vary depending on the metal centers. i-2-Sc₂(dobpdc) and i-2-Mg₂(dobpdc) with high CO₂ binding energies have promising potential for CO₂ capture. Moreover, the results demonstrate that the CO₂ capture process involves two steps, consisting of simultaneous nucleophilic attack of the CO₂ onto the metal-bound N atom with proton transfer. This results in the formation of a zwitterion intermediate (step1), and then rearrangement of the zwitterion intermediate into the final product ammonium carbamate (step2). The first step with relatively high barriers (0.99–1.49 eV) is rate-determining. The second step with low barriers (less than 0.50 eV) can easily occur and will promote the reaction. This work uncovers the complicated microscopic mechanism of CO₂ capture with i-2 functionalized MOFs at the molecular level. This study provides fundamental understanding of the adsorption process and insights into the design and synthesis of highly efficient CO₂ capture materials.