Unveiling the Molecular Mechanism of CO₂Capture in N-Methylethylenediamine-Grafted M₂(dobpdc)

For the first time, the adsorption properties and detailed microscopic mechanism of CO2 capture in N-methylethylenediamine (m-2) functionalized M2(dobpdc) (dobpdc4- = 4,4′-dioxidobiphenyl-3,3′-dicarboxylate; M = Mg, Sc-Zn) have been systematically investigated using van der Waals-corrected (vdW) density functional theory (DFT) calculations. Our results show that the m-2 binding energies in two binding modes via primary amine binding and secondary amine binding are similar. These binding energies range from 129/133 to 177/181 kJ/mol for 1°/2°-m-2-M2(dobpdc) (M = Mg, Sc-Zn), providing a reasonable explanation for the experimental observations that 1°-amine-bound and 2°-amine-bound products coexist. For CO2 capture by 1°/2°-m-2-M2(dobpdc), we have considered both the chain structures of the experimental configurations and the pair structures proposed by Planas et al. For each metal, the ammonium carbamate chain model is preferable to the carbamic acid pair model, and CO2-1°-m-2-M2(dobpdc) is more stable than CO2-2°-m-2-M2(dobpdc). The CO2 binding energies of chain-CO2-1°-m-2-M2(dobpdc) range from 45-102 kJ/mol. Moreover, the reaction path of CO2 capture in chain-1°-m-2-M2(dobpdc) is described in detail in two parts. The first part is a proton transfer and nucleophilic addition between the CO2 and the metal-bound amine. This forms a zwitterion intermediate. The first step is found to be rate limiting with relatively high barriers of 0.62-1.44 eV. The second step is the rearrangement of the zwitterion intermediates with lower barriers (<0.50 eV). The present study provides detailed insights into the absorption of CO2 onto these materials. We hope that these results will provide inspiration for improved design of future CO2 capture materials.