Removal of Confined Ionic Liquid from a Metal Organic Framework by Extraction with Molecular Solvents

Hybrid materials of ionic liquids (ILs) confined in metal organic frameworks (MOF) are promising materials for energy storage. The effects of exposing or treating such composite materials with molecular solvents, e.g., with the aim to extract and replace the IL, have not been studied to date. In this study, acetone, isopropanol, methanol, and water were used to remove the IL 1-ethyl-3-methylimidazolium ethyl sulfate confined in a Cu-based metal-organic framework (CuBTC). The consequences of the solvent extraction process were analyzed using vibrational spectroscopy (FTIR), powder X-ray diffraction (PXRD), N-2 adsorption, scanning electron microscopy (SEM), and transmission electron microscopy (TEM). Methanol was identified as the best solvent for IL removal as it shows high extraction efficiency without affecting the porous geometry and crystal structure of the MOF. On the other hand, acetone and isopropanol were not able to completely remove the IL from CuBTC under the conditions employed. Water effectively removed the IL, but it has a significant detrimental effect on the CuBTC structure. This impact manifests as changes in the infrared spectra and the PXRD patterns as well as in the electron micrographs. The degraded CuBTC exhibits a nonporous structure that presents itself as nonuniformly agglomerated microrods along with very few hexagonal/amorphous phases. The confinement of acetone, isopropanol, and methanol in the MOF was also investigated. The results show that CuBTC is stable in acetone, isopropanol, and methanol but unstable in water.