Strength of solid acids and acids in solution. Enhancement of acidity of centers on solid surfaces by anion stabilizing solvents and its consequence for catalysis

A comparison of acidity of two solids, a poly(styrenesulfonic acid) (Amberlyst 15) and a perfluoroinated ion exchange polymer (Nafion-H, PFIEP) with the structurally related liquid acids methanesulfonic, sulfuric, and trifluoromethanesulfonic acid (TFMSA), was conducted with mesityl oxide as probe base (determination of the Delta delta(1) parameter) and for the fluorinated materials also with hexamethylbenzene as the probe base. It was found that Nafion-H is similar in strength to 85% sulfuric acid, whereas Ambelyst 15 is much weaker than 80% methanesulfonic acid or 60% sulfuric acid. Thus, the solids are much weaker acids than their liquid structural analogs. This seems to be a general property, because the rigidity of the solids prevents the acid groups/sites from cooperating in the transfer of a hydron, an essential feature in the manifestation of superacidity. The postulation of superacidity for a number of solid acids appears to have no basis in fact. On the other hand, the acidity of the groups/sites on the surface can be increased by the interaction with a nonbasic solvent, capable of forming strong hydrogen bonds with the anion of the site (anion-stabilizing solvent). The anion-stabilizing solvent generates a new liquid phase around the acid site; for appropriate structures of the solid acid and solvent this phase can be superacidic. The acidity-enhancing effect of the anion-stabilizing solvent was found to have an important effect in boosting the catalytic activity of the solid for carbocationic reactions.