Atomistic level mechanism of CO2 adsorption in n-ethylethylenediamine-functionalized M2(dobpdc) metal-organic frameworks

We, for the first time, unveil the underlying mechanism of CO2 adsorption in Nethylethylenediamine (e-2) functionalized M2(dobpdc) (M = Mg, Sc-Zn) metal-organic frameworks using van der Waals (vdW) corrected density functional theory (DFT-D3) calculations. Our results show that the e-2 molecule strongly interacts with M2(dobpdc) through its primary amine. The binding energies between e-2 molecule and M2(dobpdc) series range from 127 to 175 kJ/mol for different metals. Besides the experimentally synthesized structure, we unexpectedly discovered a novel configuration of CO2-e-2-M2(dobpdc) with 0.34-0.48 eV energy lower than the experimental one. For the experimental configurations, the CO2 binding energies are in the range of 41-76 kJ/mol. Systematic investigations indicate that the adsorption mechanism includes two important steps in the reaction pathway. In the first step, CO2 is added nucleophilically into the metal-bound amine forming a zwitterion intermediate with proton transfer, which is the rate-determining step with energy barriers ranging from 0.99 to 1.48 eV for different metals. The second step is the rearrangement of the zwitterion intermediates to form ammonium carbamate, which is relatively easy with low barriers (<0.50 eV). The large heat released by this exothermic reaction, and the very low barrier of the second step causes the reaction to proceed rapidly at process temperatures. This results in large CO2 adsorption capacities of e-2-M2(dobpdc) with unusual step-shaped isotherms. This study for the first time provides detailed analysis of the pathways for this complicated CO2 capture process. This solid evidence for the chemical evolution will provide fundamental understanding on the atomic scale reaction mechanism of CO2 adsorption and shed insights on design and synthesis of novel and efficient adsorbent materials for CO2 capture, and promote the experimental efforts in this field.