Evaluating equilibrium and kinetics of CO2 and N2 adsorption into amine-functionalized metal-substituted MIL-101 frameworks using molecular simulation

Abstract

The primary objective of the present research is to evaluate the equilibrium and kinetics of CO2 and N2 adsorption into different varieties of MIL-101 including Cr-, Al-, and Fe-based amine-functionalized MIL-101 metal organic frameworks (MOF) using grand canonical Monte-Carlo (GCMC) and molecular dynamics (MD) simulation methods. In the present study, an attempt is also made to clarify the adsorption mechanisms. To achieve this goal, the adsorption isotherms and self-diffusion coefficients of CO2 and N2 in these MIL-101 MOF adsorbents are computed at different temperatures using the universal force field (UFF). Isosteric heat of adsorption and the selectivity values of the CO2/N2 mixture are also calculated. Among the three computed atomic point charge models, including the QM-based Mulliken population analysis, electrostatic potential (ESP) derived, and semi-empirical charge equilibration (QEq) methods, the latter produces more accurate results compared to the others. The results show that molecular simulation using GCMC and MD methods with UFF and QEq can be an appropriate alternative to experiment for predicting equilibrium and kinetic data of the CO2 and N2 adsorption into different varieties of MIL-101. The radial distribution functions (RDF) indicate that the metal centers are dominant active sites compared to the amine groups in the amine-functionalized Cr-based MIL-101. However, it is not the case for the amine-functionalized Al- and Fe- based MIL-101 frameworks, for which the amine groups are more capable than metal sites to capture CO2 molecules.