Thermoreversible, Hydrophobic Ionic Liquids of Keggin-type Polyanions and Their Application for the Removal of Metal Ions from Water

A series of Keggin-based ionic liquids are synthesized by reacting in situ generated first-row transition-metal ion (Mn2+, Fe3+, Co2+, Ni2+, Cu2+, and Zn2+) substituted monolacunary Keggin with tetraoctylammonium (TOA) cations. These ionic liquids contain highly charged bulky anions and are found to be hydrophobic, thermoreversible, and self-healing with melting temperatures less than 100 °C. The ionic liquids are characterized by Fourier transform infrared, Raman, X-ray diffraction, scanning electron microscopy, thermogravimetric analysis, differential scanning calorimetry, NMR, electron paramagnetic resonance, X-ray photoelectron spectroscopy, and UV–vis spectroscopic techniques. These ionic liquids show flakelike microscopic morphology and possess lamellar structure packed with Keggin and TOA cation layers alternately at room temperature. They contain highly charged anionic oxide clusters of size ∼1 nm and are excellent hydrophobic solvents for the removal of Cd2+ and Pb2+ metal ions.