Abstract: The catalytic activity of ionic liquids is closely related to their acidity, and the Hammett acidity function (H0) is the most effective way to represent acidity. This paper synthesized a series of pyrrolidinone based and imidazolium-based ionic liquids that can be used in the synthesis process of 1,3,5-trioxane, and conducted systematic experimental and theoretical studies on their H0 in aqueous solution. The influence of anionic and cationic structures and solvent selection on acidity were compared. The effect of salt effect on the H0 determination of 1-propylsulfonic-3-methylimidazolium methanesulfonate ([C3SMIM][MSA]), methanesulfonic acid, trifluoromethanesulfonic acid, and sulfuric acid was studied. The results indicate that anions have a more significant impact on acidity. When the anions are the same, the longer the carbon chain of the cation substituent, the stronger the acidity; When the cations are the same, the lower the charge density of the anions, the stronger the acidity; For the same type of ionic liquid, ionic liquids functionalized with sulfonic acid have stronger acidity than those that are not functionalized. Most salts have a salting out effect, which enhances acidity, while a few salts weaken acidity, such as sodium p-toluenesulfonate and 1-propylsulfonic-3-methylimidazolium salt (C3SMIM). The common characteristics of these salts are large ion size and low charge density.

Key words: Ionic liquids, Hammett acidity function, Salt effect; 1,3,5-Trioxane