Adsorption Properties and Microscopic Mechanism of CO₂Capture in 1,1-Dimethyl-1,2-ethylenediamine-Grafted Metal-Organic Frameworks

The adsorption properties and microscopic mechanism of CO2 adsorption in 1,1-dimethyl-1,2-ethylenediamine (dmen) functionalized M2(dobpdc) (dobpdc4-=4,4′-dioxidobiphenyl-3,3′-dicarboxylate; M = Mg, Sc-Zn) have been completely unveiled for the first time via comprehensive investigations based on first-principles density functional theory (DFT) calculations. The results show that for the primary-primary amine, dmen prefers to interact with the open metal site of M2(dobpdc) via the end with smaller steric hindrance. The binding energies of dmen with MOFs are in the range of 104-174 kJ/mol. In presence of CO2, it fully inserts into the metal-N bond, forming ammonium carbamate. The CO2 binding energies vary from 53 to 89 kJ/mol, showing strong metal dependence. Among the 11 metals, dmen-Sc2(dobpdc) and dmen-Mg2(dobpdc) have the highest CO2 binding energies of 89 and 84 kJ/mol, respectively, and may have large CO2 adsorption capacity for practical applications. More importantly, the microscopic CO2 capture process of dmen-M2(dobpdc) is revealed at the atomic level. The whole reaction process includes two steps, that is, formation of zwitterion intermediate (step 1) and rearrangement of the zwitterion intermediate (step 2). The first step in which nucleophilic addition between CO2 and the metal-bound amine and proton transfer from the metal-bound amine to free amine simultaneously occur is a rate-determining step, with higher energy barriers (0.99-1.35 eV). The second step with much lower barriers (maximum of 0.16 eV) is extremely easy, which can promote the whole CO2 uptake process in dmen-M2(dobpdc). This study provides a fundamental understanding of the underlying mechanism of the rather complicated CO2 adsorption process and sheds important insights on design, synthesis, and optimization of highly efficient CO2 capture materials.