A comparison study on the properties of 1,3-dialkylimidazolium tetrafluoroborate salts prepared by halogen-free and traditional method

A series of 1,3-dialkylimidazolium tetrafluoroborate salts had been prepared via one-step alkylation of N-alkylimidazole with trialkyloxonium salts and traditional two-step method, respectively. The possible impurities in these ionic liquids (ILs) such as Cl−, Br−, Na+ and alkyl imidazoles were measured. Detailed characterizations of two approaches in the ILs properties and performances such as spectroscopic properties, phase behavior, viscosity, refractive index, and electrochemical properties were systemically conducted. The obtained results showed that the variety of synthesized methods resulted in the distinct changes in physicochemical properties for the same IL. Therein, the comparison of spectroscopic properties and phase behaviors for the employed ILs were emphasized. Furthermore, in most cases, the ILs prepared by traditional two-step possessed the higher viscosity, lower refractive index as well as lower ionic conductivity than that of ILs prepared by one-step. It was worth noting that the 1-decyl-3-ethylimidazolium tetrafluoroborate ([DEIm]BF4) prepared with the traditional and trialkyloxonium methods respectively exhibited distinctly different phase and fluorescence behaviors. The reason might be attributed to the significant improvement on the purities of ILs by the one-step strategy.