On the acidity of liquid and solid acid catalysts: Part 1. A thermodynamic point of view

Abstract

The protonation equilibria of weak bases (B) in solid acids (HClO₄/SiO₂, CF₃SO₃H/SiO₂, H₂SO₄/SiO₂) were studied by UV spectroscopy and the results were compared to those obtained for analogous compounds in concentrated aqueous solutions of strong acids (HClO₄, CF₃SO₃H, H₂SO₄). The behaviour of B in liquid (L) and solid (S) phase was analysed by titration curves, log[BH⁺]/[B] ratios and thermodynamic pK _BH+ values. It has been shown that the proton transfer process acid → base (i.e., from (H+A^-)_(L,S) to (BH+A^-)_(L,S)} can be described by the relationship observed between the activity coefficient terms that are to be taken into account for acid–base equilibria occurring in nonideal systems ( – log(f _B f _B+/f _BH+)_(L,S)= -n _BA log(f _A–f _H+/f _HA)_(L,S)) and can be estimated by the n _BA values. Two “activity coefficient functions” (i.e., Mc(B) = – log(f _B f _B+/f _BH+)and Mc(s) = – log(f _A–f _H+/f _HA)) were used to describe, respectively, the equilibria of B and the equilibria of the acids in concentrated aqueous solutions and the meaning of terms “activity coefficient function” and “protonating ability of an acid” were discussed. The difference between “acidity functions”, determined for solutes (Ac(i)) and solvents (Ac(s)) in aqueous acids, and the H_x acidity functions, the latter developed for solutes in analogous media by the Hammett procedure, was also shown.