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

Abstract

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-Mg₂(dobpdc), combining the use of density functional theory (DFT) calculations with microkinetic simulations. Our results demonstrate that CO₂ capture in mmen-Mg₂(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-Mg₂(dobpdc). Moreover, we found that CO₂ desorption from the outer and inner ammonium carbonate complexes occurred at 128 and 142 °C, 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-Mg₂(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.