| Hydrogen sulfide (H2S), which is mainly produced in various industrial production and solid waste treatment processes, is a kind of virulent and noxious gas and should be removed. Recently, iron-based ionic liquids have shown outstanding performances for catalyzing and oxidizing H2S to sulfur. However, in most cases, oxygen has been used for the regeneration of desulfurizers. The regeneration rate and efficiency are very low because of the limitation of the reaction kinetics. Besides, H in H2S is underused but to form water. So if the electrolytic regeneration is used, the regeneration rate and efficiency will be increased by stealing electrons from Fe(â…¡) in the systems directly, and H in H2S will be used. That will be more economic and green in chemical production.In order to recycle and reuse the iron-based ionic liquids, a composite system of iron-based ionic liquid (Fe(â…¢/â…¡)-IL) has been established by introducing Fe(â…¡) to simulate the iron-based ionic liquid (Fe(â…¢)-IL) after desulfurization. Then its electrochemical property, desulfurization performance, electrolytic regeneration and circulation have been studied.A platinum microelectrode was applied to reduce the effect of the uncompensated resistance. The reaction mechanism of high concentration Fe(â…¢)/Fe(â…¡) in Fe(â…¢)-IL and Fe(â…¢/â…¡)-IL on the electrode has been studied by cyclic voltammetry without any other electrolytes. The values of the viscosity (η), conductivity (σ) and apparent diffusion coefficients of the electroactive particles (D’) of Fe(â…¢)-IL and Fe(â…¢/â…¡)-IL have been measured and the influence of solvents on the two ionic liquids has been discussed. Results show that the electrode reaction of Fe(â…¢)/Fe(â…¡) at platinum electrode is still a quasi-reversible process as the addition of Fe(â…¡). The electrode reaction is simplified as:FeCl4-+e-= FeCl42-; 3FeCl42-= Fe3Cl7-+5Cl-.Both migration and diffusion of electroactive ions affect the electrochemical behavior of Fe(â…¢)/Fe(â…¡) on the microelectrode. D’increases with increasing temperature, and D’Fe3Cl7-(Fe(â…¢/â…¡)-IL)>D’FeCl4-(Fe(â…¢/â…¡)-IL>D’FeCl4-(Fe(â…¢/â…¡)-IL),ηFe(â…¢/â…¡)-IL>η7Fe(â…¢)-IL, and σFe(â…¢/â…¡)-IL<σFe(â…¢)-IL at the same temperature. VFT equation is more appropriate for these iron-based ionic liquids on the relationship of viscosity and temperature than Arrhenius equation. Adding solvents especially strong polar solvents can decrease the viscosity and increase the molar conductivity and the apparent diffusion coefficient of Fe(â…¢)/Fe(â…¡) of the ionic liquids.Certain Fe(â…¡) is beneficial for the matching of desulfurization and electrolytic regeneration process. So in this paper, Fe(â…¢/â…¡)-IL has been used to remove H2S and showes higher desulfurization efficiency than Fe(â…¢)-IL. In order to further improve the desulfurization performance of iron-based ionic liquids, some neutral or weakly basic polar solvents have been chosen to mix with Fe(â…¢/â…¡)-IL as desulfurizer and the stability and the desulfurization efficiency of the systems have been discussed. Then the desulfurization mechanism of Fe(â…¢)-IL and Fe(â…¢/â…¡)-IL/N,N-dimethylformamide (DMF) has been analyzed. The addition of neutral or weakly basic solvents can reduce the acidity of iron-based ionic liquids, increase the absorption of H2S and promote the gas-liquid mass transfer, so the desulfurization efficiency increases obviously. After desulfurization, orthorhombic sulfur has been produced. The desulfurization mechanism is simplified as:6FeCl4-+14[Bmim]++3H2S â†'2Fe3Cl7-+3HC1+3S↓+7[Bmim]2Cl+3H+. The framework of Fe(â…¢/â…¡)-IL hasn’t been changed after the addition of DMF. But there are some interaction between them, which can affect the thermo and electrochemical stability of the system, promote the ionization and oxidation of H2S, and prevent the produced HCl from escaping.Based on the electrochemical activity of H+, Cl- and the other ions in ionic liquids, a diaphragm bath with Fe(â…¡)-rich ionic liquid as anolyte and HCl solution as catholyte has been constructed, and the electrolytic regeneration at constant potential and current has been used respectively. The optimum anode potential is 0.8 V (vs.Ag/AgCl) in the constent potential electrolysis. The electrolysis rate of Fe(â…¡) increases with increasing the initial concentration of Fe(â…¡). The addition of solvents can increase the electrolysis efficienty and average oxidation rate obviously. A little bit of water is benefit to electrolytic regeneration but shows little effect on the stability of the reaction. The conversion efficiency of Fe(â…¡) by using YAB membrane is higher than by using Nafion membrane in Fe(â…¢/â…¡)-IL/DMF at the same time, but the stability of reaction is worse. The operation of the constant current electrolysis is more simiple. The conversion efficiency of Fe(â…¡) increases with increasing electrolytic current. However, side reaction is more easily to occur at higher current.Finally, an indirect circulation of desulfurization and electrolytic regeneration using Fe(â…¢/â…¡)-IL/DMF has been implemented. The desulfurization efficiencies are higher than 98% and the electrolytic rates stay the same. Mass balance in the cycle indicates a steady process for desulfurization. The skeletal structure of the system and the redox property of iron have not changed after recycling. So this system can be used for desulfurization recurrently. |
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