Synthesis of high purity 1,3-bis-(3-sulfonic acid)propyl imidazolium ionic liquids

Ionic Liquids (ILs) of 1,3-bis-(3-sulfonic acid)propyl imidazolium bisulfate ([PS2Im]HSO4) can be used as acid catalyst for its specific acidity. However, it is difficult to obtain high-purity ILs by the reported synthetic methods. The preparation of [PS2Im]HSO4 with N-trimethylsilylimidazole (TSIm) as raw material was studied. The results show that 1-(3-sulfonic acid)propyl-3-H-imidazolium bisulfate ([PSImH]HSO4) and 1,3-bis-H-imidazolium bisulfate ([ImH]HSO4) are easily formed in the process of preparing [PS2Im]HSO4 from TSIm. Some TSIm is encapsuled in the formed solid intermediate 1-(propyl-3-sulfonate)-3-trimethylsilylimidazolium ([TSIm]PS), it leads to the formation of [ImH]HSO4. The solid intermediate [TSIm]PS can not be completely depleted in the following reaction for the formation of 1-(3-trimethylsilyl sulfonate)propyl-3-(propyl-3-sulfonate) limidazolium ([PSTSIm]PS). The residual solid intermediate [TSIm]PS leads to the formation of [PSImH]HSO4. A novel process using sodium imidazolate (ImNa) as raw material was proposed. [PS2Im]HSO4 was successfully prepared with a purity of 96.2 mol.% and a yield of 69 %. The synthesis mechanism of high purity [PS2Im]HSO4 was revealed by experimental research and DFT computation. Intermediate 1-(3-sodium sulfonate)propyl imidazole (PSNaIm) is formed by the reaction of ImNa and 1,3-PS with a molar ratio of 1:1. PSNaIm can easily be dissolved in DMSO and react with 1,3-PS to obtain 1-(3-sodium sulfonate)propyl-3-(propyl-3-sulfonate) imidazolium ([PSNaIm]PS). After cation exchange and reaction with sulfuric acid, high purity [PS2Im]HSO4 is easily obtained. The results provide a guidance for the synthesis of high-purity imidazole-based ILs containing dual-acidic functional groups.