Abstract:

Benzoates are important intermediates for preparing perfume essence and artificial essential oils. For conventional esterification of benzoic acid with methanol, protonic acids such as HCl, H₂SO₄, have been used, which is harmful to equipment and shows low product yield. Using ionic liquids as solvents or catalysts can make the reaction conditions stable, recyclable, and environment-friendly, and lead to high product yield. In this work, a series of sulfonic-acid-functionalized dual-core piperazine-based ionic liquids, such as 1, 4-bis [N-methyl-N-（3-sulfopropyl）]piperazine bis（p-toluenesulfonate） （[BMSP][PSA]）, 1, 4-bis [N-methyl-N-（3-sulfopropyl）] piperazine dimethanesulfonate （[BMSP][MSA]）, and 1, 4-bis [N-methyl-N-（3-sulfopropyl）] piperazine bis（hydrosulfate） （[BMSP][SA]）, were synthesized by a two-step process. The molecular structure of[BMSP][PSA] was characterized by FT-IR, ¹H NMR and TGA-DTG. It exhibited good thermal stability. The results showed that the three sulfonic-acid-functionalized dual-core piperazine-based ionic liquids had excellent catalytic activity, among which the synthesized [BMSP][PSA] exhibited the best catalytic activity. The conversion of benzoic acid reached 97.6% under the conditions of alcohol-acid ratio of 16∶1, reaction temperature of 70 ℃, reaction time of 3 h, and catalyst dosage of 3 mmol. The conversion for esterification still reached 93.9% after 4 cycles with the catalyst, which showed excellent catalytic activity. Therefore, it was a good candidate of green catalyst for industrial production.

Key words: ionic liquid, esterification, methyl benzoate, 1, 4-dimethylpiperazine