Determination of the Hammett Acidity Functions of Triflic Acid/Ionic Liquid Binary Mixtures by the 13C NMR-Probe Method

Abstract

Catalyst acidity is among the crucial parameters affecting the direction and degree of acid-catalyzed reactions. The Hammett acidity functions (H0) of binary mixtures of triflic acid (TfOH) and ionic liquids (ILs), namely, [BMim][HSO4], [BMim][TfO], and [BMim][TFA], were measured by the 13C NMR method using mesityl oxide as a probe. The results show that the H0 values of the mixtures can be effectively controlled by tailoring the structure of the IL or tuning the amount of IL added to the system. The −H0 values of the binary mixtures decrease with increasing amount of IL. Mixtures of [BMim][HSO4]/TfOH and [BMim][TfO]/TfOH show minimal changes in acidity (−H0 > 13.00) when the IL mole fraction is less than 0.05, a sharp decline in acidity (−H0 = 13.00 → 8.00) at an IL mole fraction of 0.05 to 0.20, and a relatively stable acidity (−H0 = 8.00 → 5.40) at an IL mole fraction of 0.20 to 0.50. These mixtures share nearly the same H0 value when the IL mole fraction is less than 0.20. The addition of [BMim][TFA] ...