Insights into the capture mechanism of CO2 by diamine-appended Mg2(dobpdc): a combined DFT and microkinetic modeling study

The development of energy-efficient materials is highly desirable for CO2 capture to diminish CO2 emissions from fossil fuel combustion. Herein, we explore the mechanism of CO2 capture in mmen-Mg-2(dobpdc), combining the use of density functional theory (DFT) calculations with microkinetic simulations. Our results demonstrate that CO2 capture in mmen-Mg-2(dobpdc) via outer-amine binding and inner-amine binding is feasible, and CO2 adsorption on the outer amine of the mmen molecule is kinetically more favorable than that on the inner amine. In addition, we have considered that CO2 molecules continuously adsorb on the outer and inner amine sites simultaneously. We found that the activation barrier of CO2 carboxylation varied from 1.29 to 1.52 eV via dual-amine binding modes, providing a reasonable explanation for the experimental observation of Mg-coordinated ammonium carbonate and outer carbamic acid species in mmen-Mg-2(dobpdc). Moreover, we found that CO2 desorption from the outer and inner ammonium carbonate complexes occurred at 128 and 142 ?, respectively, which is consistent with previous experimental results. Furthermore, our results demonstrate that the CO2 adsorption barriers can be effectively reduced by considering the deprotonated mmen molecules in an alkaline environment. This study provides detailed insights and the actual mechanism of CO2 adsorption in mmen-Mg-2(dobpdc). We hope that this work could shed new light on the understanding and development of diamine-functionalized metal-organic frameworks for efficient CO2 capture.