Ionic liquid entrapped UiO-66: Efficient adsorbent for Gd3+ capture from water

An ionic liquid (IL), 1-butyl-3-methylimidazolium bromide ([C4mim]+[Br]−) was entrapped within the cavities of the robust metal-organic framework (MOF) UiO-66 via a ship-in-a-bottle technique. To this end, 1-methylimidazole and 1-bromobutane were introduced sequentially to diffuse through the pores of UiO-66 to form the IL species [C4mim]+[Br]− inside the MOF structure (C4[email protected]). It was characterized by XRD, N2 adsorption-desorption isotherms, elemental analysis, SEM, and FTIR spectroscopy. [C4mim]+[Br]− remained entrapped within the cavities of UiO-66 by its bulky size and interaction with UiO-66 framework as confirmed by FTIR, TGA, and density functional theory calculations. The material was subsequently applied for Gd3+ adsorption in water. A pseudo-second-order kinetic behavior was observed and a practical adsorption equilibrium was attained after 3 h of contact time at 25 °C. The maximum adsorption capacity of pristine UiO-66 for Gd3+ in water (17 mg/g) was greatly increased in C4[email protected] (85 mg/g) because of the strong interaction between Gd3+ ions and the N-sites of the imidazole rings of the entrapped [C4mim]+[Br]− molecules. The thermodynamic analysis indicated endothermic adsorption for Gd3+ ions on C4[email protected] with ΔHo of 24.4 kJ/mol. C4[email protected] showed a high selectivity of 82% for Gd3+ against other metal ions (Na+, Ca2+, Mg2+, Al3+, and Fe3+) present in the solution. Further, C4[email protected] could be used for five repeated runs without any significant drop in the adsorption capacity.