Ab Initio Study of Gas Adsorption in Metal–Organic Frameworks Modified by Lithium: The Significant Role of Li-Containing Functional Groups

Metal–organic frameworks (MOFs) are promising materials for gas adsorption. Introducing metal cations, for example, lithium cations (Li+), in the framework is an effective way to alter the gas adsorption features of MOFs. In this work, Li+ carried by different functional groups was incorporated onto a benzene linker, which is one type of the most common liker used in MOF synthesis. The interactions between the Li-modified linkers and various gas molecules were studied using MP2 method. Compared to the original benzene ring, the structures and orbitals of Li-modified linkers were significantly changed toward the direction of enhancing gas adsorption. For nonpolar gas species (CH4, H2, N2, and CO2), the induced polarizations greatly enhance the interactions between gas molecules and MOF linkers. Particularly, the expanded binding energy differences of H2/N2, CH4/CO2, and N2/CO2 will make them easier to get separated. For polar gas species (H2O, H2S, SO2, and CO), the electrostatic interactions between gas molecules and Li+ play a significant role in enhancing gas adsorption. The strong affinities between polar gases and Li-modified linkers denote that the binding sites around Li+ can be first occupied by polar molecules such as H2O and SO2 during the practical adsorption process. This can result in the reduced adsorption capacities of other gases, such as CO2.