Li-modified metal–organic frameworks for CO2/CH4 separation: a route to achieving high adsorption selectivity

Abstract

In this work three Li-modified metal–organic frameworks (MOFs) were constructed from MOF-5, by substituting the H atoms with O–Li groups in the organic linkers. A multiscale approach combining grand canonical Monte Carlo (GCMC) simulation and density functional theory (DFT) calculation was adopted to investigate the separation of CO₂/CH₄ mixtures in these new Li-modified MOFs, as well as in a previously proposed Li-doped MOF-5 for hydrogen storage and the original MOF-5. The results show that the selectivity of CO₂ from CH₄/CO₂ mixtures in Li-modified MOFs is greatly improved, due to the enhancement of electrostatic potential in the materials by the presence of the metals. One of the new Li-modified MOFs, chem-4Li, shows a higher CO₂ selectivity than any other known MOFs. Therefore, this work provides a route to improve the separation performance of MOFs for gas mixtures with components that have large differences in dipole and/or quadrupole moments. In addition, the mechanisms for selectivity enhancement in the Li-modified MOFs were elucidated at the molecular level, and we found that the location of doped metals can change the adsorption sites for CO₂, and in turn may change the active sites in MOFs when used as catalysts.