Brønsted-acidic ionic liquids as catalysts for synthesizing trioxane

The batch reaction experiments have been made for the first time to investigate the effect of ionic structure on the reactivity and selectivity of the trioxane-forming reaction catalyzed by a Brønsted-acidic ionic liquid (IL). The ILs considered include 1-cyclohexyl-2-pyrrolidinonium trifluoromethanesulfonate (NCyP]TfO]), 1-cyclohexyl-2-pyrrolidinonium benzenesulfonate (NCyP]BSA]), 1-cyclohexyl-2-pyrrolidinonium p-toluenesulfonate (NCyP]p-TSA]), 1-octyl-2-pyrrolidinonium 2,4-dinitrobenzenesulfonate (NOP]DNBSA]), 1-octyl-2-pyrrolidinonium benzenesulfonate (NOP]BSA]), 1-octyl-2-pyrrolidinonium methanesulfonate (NOP]MSA]), and 1-octyl-2-pyrrolidinonium trifluoromethanesulfonate (NOP]TfO]). It is found that the yield of trioxane in the reaction solution correlates inversely with the Hammett acidity function H₀ of the aqueous solution of the corresponding ILs. Variation of the cation structure fromNCyP]⁺ toNOP]⁺ exerts little influence on the yield and the selectivity of trioxane in the reaction solution. UsingTfO]^- orDNBSA]^- in place ofMSA]^- orBSA]^- apparently increases the yield of trioxane, but only slightly increases the concentration of formic acid in the reaction solution. The continuous production experiments have been made to investigate the performance ofNOP]MSA],NOP]DNBSA],NCyP]TfO], and H₂SO₄ as an extraction distillation agent. Such effect ofNCyP]TfO] considerably overrides that of H₂SO₄. The yield and the selectivity of trioxane are both increased whenNCyP]TfO] is used instead of H₂SO₄ at a reaction time 5 h.