A DFT study on the selective adsorption and separation of CO2/CH4 on the Mg-decorated 2-D covalent organic framework (COF-5)

Covalent organic frameworks (COFs), an emerging class of crystalline polymeric materials, have garnered growing interest due to their ideal chemical and thermal stability and ordered microporous architectures, which make them effective agents for selective CH4/CO2 separation. In this work, adsorption and separation of methane and carbon dioxide molecules on the two-dimensional pristine and Mg-decorated COF-5 (MgCOF-5) was investigated using density functional theory, employing B3LYP. Both CH4 and CO2 molecules were found to weakly adsorbed through van der Waals interactions to the bare sheet via physisorption, releasing energies ranging from -3.8 to -5.6 and -8.7 to 12.8, respectively and the sheet's electrical characteristics don't alter all that much. To overcome this weak selectivity/sensitivity, multiple Mg atoms were decorated atop aromatic rings of COF-5. Our results show that up to four CO2 molecules can be adsorbed on each Mg atom exothermically, whereas Ead of CH4 is near zero so the theoretical CO2 capacity of a full Mg-covered sheet is 0.51 gCO2/g MgCOF-5. Also, the decorating of Mg atoms on the surface of COF-5 induces certain changes in the sheet's electrical characteristics and that the sheet's Eg changes up to 80% following the adsorption of several CO2 molecules, making it a potential candidate for CO2 detection.