Design of [EtOHMIM][PF6]-incorporated UiO-66 for adsorptive removal of methylene blue: deep eutectic solvent-assisted MOF synthesis and computational ionic liquid screening

Abstract

The rapid development of the dye industry has led to a significant amount of wastewater being discharged. As a widely used dye, toxic and non-degradable methylene blue is becoming a serious threat to our ecosystem. Thus, wastewater containing methylene blue must be treated properly. Considering the strengths of ionic liquids (ILs) and metal–organic frameworks (MOFs) in molecular separation, a convenient solution strategy is proposed in this work for designing and preparing an IL@MOF composite adsorbent in order to efficiently remove methylene blue from water. As a kind of water-stable MOF, UiO-66 was synthesized using a one-pot method under very mild conditions using deep eutectic solvents (DESs) as reaction media instead of conventional organic solvents. The easily obtained UiO-66-DES has superior MB adsorption kinetics but limited adsorption capacity. To mitigate this weakness, a computational and systematic IL screening framework is developed. [EtOHMIM][PF₆] is eventually selected from 3569 candidates and incorporated into the UiO-66-DES. The resulting IL@MOF composite maintains the fast MB adsorption while importantly increasing the adsorption capacity from 45.85 mg g⁻¹ to 227.57 mg g⁻¹. Given a 10 mg L⁻¹ MB solution, the removal rate is improved from 34% to 91% in 5 minutes. Meanwhile, the adsorption capacity is almost unchanged after 5 regeneration cycles, which offers great potential for practical applications. Finally, the detailed mechanism analysis reveals that the loaded IL introduces new adsorption sites with stronger interactions with MB. These results show that the [EtOHMIM][PF₆]@UiO-66-DES composite, resulting from DES-assisted MOF synthesis and IL-based post-synthesis, is a promising material for efficient MB wastewater treatment.