Effects of aprotic solvents on the physicochemical properties and ferric ion oxidation activity of iron-based ionic liquids

Abstract

In recent years, iron-based ionic liquids (e.g. BmimFeCl₄, Fe-IL) have been widely used in the catalytic oxidation removal of hydrogen sulfide owing to their excellent redox reversibility and stability. Nevertheless, the high viscosity and poor Fe³⁺ activity of BmimFeCl₄ limit its large-scale industrial application. The addition of aprotic organic solvents to BmimFeCl₄ is an effective strategy to enhance its mass transfer efficiency and catalytic oxidation desulfurization performance. In this work, the effects of two kinds of aprotic organic solvents, weak polar polyether alcohols (NHD, PEG200) and strong polar amides (DMAC, DMF, and NMP), on the density, viscosity, conductivity and ferric activity of Fe-IL were investigated. The Eyring equation fitted well for the relationship between the viscosity and the temperature of the composites. When the mass ratio of BmimFeCl₄ to solvent was 7 : 3 at 298.2 K, the viscosity of BmimFeCl₄/DMAC and BmimFeCl₄/NHD was 8.67 mPa s and 27.19 mPa s, respectively. The excess molar volume (V^E) and viscosity deviation (Δη) of the two composite systems were calculated and fitted using the Redlich–Kister equation. The study of V^E implies that DMAC has stronger solvation to the BmimFeCl₄ ion pairs, and NHD could cause a more obvious volume shrinkage. For the composites investigated, Δη of BmimFeCl₄/DMAC is negative while that of BmimFeCl₄/NHD is positive, showing that DMAC could significantly weaken the combination ability of [Bmim]⁺ and [FeCl₄]⁻, and NHD may form a stronger interaction with [Bmim]⁺. The FT-IR spectra and DFT calculations demonstrated that both polyether alcohol and amide could interact with C2–H on [Bmim]⁺. The CV curves and the MK charges show that the addition of aprotic polar solvents could effectively improve the activity of Fe³⁺ under the action of a hydrogen bond, and the effect of amide solvents on the activation of Fe³⁺ is stronger than that of polyether alcohol solvents. In conclusion, it is found that the composites with stronger ferric activity have much better catalytic oxidation ability for the conversion performance of hydrogen sulfide, and the the interactions induced by the molecular weight and the polarity of the solvent have a significant effect on the configuration of the Fe-IL ion pairs.