Article pubs.acs.org/IECR
Decomposition of Perfluorinated Ionic Liquid Anions to Fluoride Ions in Subcritical and Supercritical Water with Iron-Based Reducing Agents Hisao Hori,* Yoshinari Noda, Akihiro Takahashi, and Takehiko Sakamoto Department of Chemistry, Faculty of Science, Kanagawa University, 2946 Tsuchiya, Hiratsuka, Kanagawa 259-1293, Japan ABSTRACT: Decomposition of perfluorinated ionic liquid anions [(CF3SO2)2N]− and [(C4F9SO2)2N]− in subcritical and supercritical water was investigated with the aim of developing a technique to recover the fluorine component from ionic liquid wastes. Addition of zerovalent iron to the reaction system dramatically increased the yield of F−: when the reaction of [(CF3SO2)2N]− was carried out at 344 °C for 6 h, the F− yield was 69.0%, which is 186 times the yield without iron. Increasing the temperature and reaction time in the presence of zerovalent iron further increased the yield: when [(CF3SO2)2N]− and zerovalent iron were heated in supercritical water at 375 °C for 18 h, 76.8% of the fluorine content in the initial [(CF3SO2)2N]− was transformed into F−. [(CF3SO2)2N]− also decomposed in the presence of FeO, which underwent in situ disproportionation to form zerovalent iron, which acted as the reducing agent. Although the FeO-induced decomposition of [(CF3SO2)2N]− was initially slower than the zerovalent iron-induced decomposition, after prolonged reaction (18 h) at 378 °C, the F− yield of the former reaction reached 85.7%, which was the highest yield obtained. This result suggests that the zerovalent iron that formed in situ reacted preferentially with the substrate, as opposed to water.
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INTRODUCTION Ionic liquids (ILs) have been widely investigated as reaction and extraction media for green chemistry applications.1,2 Furthermore, there has been increasing interest in the electrochemical applications of ILs, mainly as electrolytes safer than organic solvents.3 In particular, perfluorinated IL anions are being introduced in many electrochemical devices, including lithium-ion batteries, polymer electrolyte membrane fuel cells, and dye-sensitized solar cells, owing to their nonflammability, high thermal stability, wide electrochemical windows, high ion conductivity, and low viscosity.3−6 Wider use of ILs will require that waste treatment be established. Incineration is one method for decomposing these chemicals. However, incineration requires high temperatures to break the strong C−F bonds, and hydrogen fluoride gas is formed, which can seriously damage the firebrick of an incinerator. In addition, these anions do not biodegrade under either aerobic or anaerobic conditions,7 which indicates that conventional microbial degradation processes are not applicable for treatment of waste ILs. If perfluorinated IL anions could be decomposed to F− by means of environmentally benign techniques, the well-established protocol for treatment of F− could be used, whereby Ca2+ is added to the system to form CaF2, which is a raw material for hydrofluoric acid. Thus, the development of such a method would allow the recycling of fluorine, the global demand for which is increasing. Reactions in subcritical and supercritical water have been recognized as an innovative and environmentally benign wastetreatment technique, owing to the high diffusivity and low viscosity of these media, as well as their ability to hydrolyze many organic compounds.8 Subcritical water is defined as hot water with sufficient pressure to maintain the liquid state, and supercritical water is defined as water at temperatures and pressures higher than the critical point, 374 °C and 22.1 MPa. © 2013 American Chemical Society
Recently, such water was used for pilot- and practical-plantscale decomposition of hazardous compounds such as trinitrotoluene9 and polychlorinated biphenyls.10 We previously demonstrated that a perfluorosulfonic acid membrane polymer for fuel cells is efficiently decomposed in subcritical water in the presence of metals.11 In this study, we investigated the decomposition of two typical perfluorinated IL anions, [(CF 3 SO 2 ) 2 N] − and [(C4F9SO2)2N]−, in subcritical and supercritical water in the presence of an oxidizing agent (O2) or an iron-based reducing agent (zerovalent iron, FeO, or Fe3O4). The latter conditions (except Fe3O4) effectively decomposed the anions to F−. This is the first report not only on the decomposition of perfluorinated IL anions in subcritical and supercritical water but also on a technique aimed at waste treatment that successfully achieves the efficient formation of F− ions from the anions.
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EXPERIMENTAL SECTION Materials. Bis(trifluoromethanesulfonyl)imide ([(CF3SO2)2N]−, >99.5%) and bis(nonafluorobutanesulfonyl)imide ([(C4F9SO2)2N]−, >99%) were obtained from SynQuest Laboratories (Alachua, FL) and Mitsubishi Material Electronic Chemicals (Akita, Japan) as lithium and potassium salts, respectively. Zerovalent iron (>99.9%, 99.9%, 99.9%,
