Regeneration of [Bmim]BF₄ ionic liquid by ozonation: hydrogen bond roles, synergistic effect, and DFT calculation

Ionic liquids (ILs) are a kind of promising extractants for removing undesirable organics from industrial wastewater and crude oil, etc. Herein, IL regeneration by ozonation was proposed for its recycling. 1-Butyl-3-methylimidazolium tetrafluoroborate ([Bmim]BF₄) and phenol were used as the representative IL and organic matter, respectively. The regeneration performance, stability of [Bmim]BF₄, and roles of hydrogen bonds between O₃ and [Bmim]BF₄ in the phenol removal were studied. The experimental results showed that 90.2% of phenol was degraded in [Bmim]BF₄ after ozonation treatment for 30 min with the O₃ concentration of 1.2 mg/L. The nonvolatility of [Bmim]BF₄-made phenol was adequately degraded, while the concentration of phenol increased during ozone treatment in the conventional extractants (ethanol, ethyl acetate, and dichloromethane) due to their high volatility. The reuse experiments, spectra of Fourier transform infrared spectroscopy and ¹H nuclear magnetic resonance spectroscopy indicated that [Bmim]BF₄ was steady under the ozonation process. Density functional theory simulation confirmed that hydrogen bonds between [Bmim]⁺ and O₃ could be formed, and calculational results of Hirshfeld charges indicated that the nucleophilicity of O₃ was enhanced by the hydrogen bonds. The enhanced nucleophilicity resulted in the increase of phenol degradation efficiency by ozonation, and the synergistic effect between O₃ and ILs was proportional to the hydrogen bond donor ability of ILs.