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

摘要

Catalyst acidity is among the crucial parameters affecting the direction and degree of acid-catalyzed reactions. The Hammett acidity functions (H-0) of binary mixtures of triflic acid (TfOH) and ionic liquids (ILs), namely, [BMim][HSO4], [BMim] [TfO], and [BMim][TFA], were measured by the C-13 NMR method using mesityl oxide as a probe. The results show that the H-0 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 -H-0 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 (-H-0 > 13.00) when the IL mole fraction is less than 0.05, a sharp decline in acidity (-H-0 = 13.00 -> 8.00) at an IL mole fraction of 0.05 to 0.20, and a relatively stable acidity (-H-0 = 8.00 -> 5.40) at an IL mole fraction of 0.20 to 0.50. These mixtures share nearly the same H-0 value when the IL mole fraction is less than 0.20. The addition of [BMim] [TFA] can alter the Hammett acidities of the binary systems more easily than [BMim][HSO4] or [BMim] [TfO] at the same concentration. Models to predict the H-0 values of the three IL/TfOH binary systems as a function of acid concentration are also proposed.