A computational investigation on the roles of binding affinity and pore size on CO2/N2 overall adsorption process performance of MOFs through modifying MIL-101 structure

To develop an appropriate metal-organic framework to capture CO2 from dry flue gas, understanding the roles of structural modifications, location of primary adsorption sites and underlying adsorption mechanisms is crucial. Herein, effects of type, position and concentration of nitrogen-containing functional groups and C/N-substituted ligands are investigated on overall CO2/N2 adsorption process performance and mechanism of pristine and metal-substituted MIL-101 MOFs. In this investigation, density functional theory computations, grand-canonical Monte-Carlo, and molecular dynamics simulations are applied. Taguchi-based overall evaluation criteria objective function is also used to assess the overall adsorption performances of these MIL-101 MOFs in terms of CO2 uptake capacity, CO2/N2 selectivity, isosteric heat of adsorption and mass transfer resistance. The PBE/DZVP computations resulted in the CO2-framework binding energies ranging from −3.85 to −21.01 kJ/mol implying an acid-base type of physisorption. A correlation is introduced to estimate the CO2 uptake capacity from binding affinity and pore diameter of these MIL-101 MOFs. Mechanistic investigations show that to enhance CO2 uptake capacity and CO2/N2 selectivity, functionalization of the linker ligands is more successful than functionalization of the open metal sites having strong interactions with CO2 molecules. Results also showed that the best possible scenario to improve the overall adsorption process performance of MIL-101 is to substitute the C atom(s) of the BDC ligands with the N atom(s), regardless of the central metal atom type. The comprehensive molecular design approach introduced in this work can be used as a methodology by scientists for the development of new MOFs for practical CO2 capture applications.