First-principles investigation of dimethyl-functionalized MIL-53(Al) metal–organic framework for adsorption and separation of xylene isomers

Functionalization of organic linker in metal–organic frameworks (MOFs) has been a popular method to utilize the porous material for a wide range of applications. In xylene separation, dimethyl functionalization of MOFs had demonstrated improved adsorptions properties. However, the effect of different xylene isomers on the organic linker is still not fully understood. In this work, the MIL-53(Al) MOF was functionalized with two methyl groups in ortho-, meta- and para-positions. The energy and thermochemical properties of xylene isomers when complexed dimethyl functionalized MIL-53(Al) MOFs were investigated using semiempirical quantum chemical PM6 method. The adsorption model of xylene isomers over meta-dimethyl-MIL-53(Al) (MDM) produced the lowest energy and the most thermochemically stable complex. The electronic properties, natural charge, nuclear magnetic resonance, aromaticity and molecular electrostatic potential of MDM complexes were further characterized using density functional theory (DFT) calculations at B3LYP/6-31G(d) level. DFT calculations revealed that the complex of MDM with m-xylene had the lowest adsorption energy and the highest reactivity, caused by the oxygen atoms surrounding the Al metal ion. In contrast, the complex of MDM with o-xylene had the highest energy of adsorption and the lowest reactivity, mostly governed by weak π interactions. Based on the results, we propose the use of MDM MOF for adsorption and separation of xylene isomers.