CO₂Adsorption Properties of a N, N-Diethylethylenediamine-Appended M₂(dobpdc) Series of Materials and Their Detailed Microprocess

Herein, we report the adsorption energy and reaction path of CO2 capture by N,N-diethylethylenediamine (ee-2)-functionalized M2(dobpdc) (M = Mg, Sc-Zn; dobpdc4-= 4,4′-dioxidobiphenyl-3,3′-dicarboxylate) calculated using density functional theory (DFT). Our calculations reveal that both the amine (ee-2) binding energies and CO2 adsorption energies have strong metal center dependence. The ranges are 131.3 (Cr) to 184.1 kJ/mol (V) and 37.7 (Cu) to 79.2 kJ/mol (Sc) for the ee-2 binding energies and CO2 adsorption energies, respectively. In addition, we determined the reaction intermediates and barriers of the CO2 adsorption process. The entire process consists of two steps: first, the combination of CO2 with ee-2-M2(dobpdc) changes from a vdW complex to a chemically bonded intermediate, accompanied by the transfer of a proton in the primary amine. Then, a molecular rearrangement reaction occurs, forming a stable ammonium carbamate structure. The first step of the process has a higher barrier (1.04-1.49 eV) in comparison to that (0.01-0.27 eV) of the second step. This first step is the decisive step of the overall reaction of CO2 with ee-2-M2(dobpdc). This work provides a fundamental understanding of the microprocess of CO2 capture by amine-functionalized MOFs and sheds some insight into the design and optimization of highly efficient CO2 capture materials.