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

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.