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) (A mberlyst 15) and a perfluoroinated ion exchange polymer (Nafion-H, PFI EP) with the structurally related liquid acids methanesulfonic, sulfur ic, and trifluoromethanesulfonic acid (TFMSA), was conducted with mesi tyl 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% methanesulf onic acid or 60% sulfuric acid. Thus, the solids are much weaker acids than their liquid structural analogs. This seems to be a general prop erty, because the rigidity of the solids prevents the acid groups/site s from cooperating in the transfer of a hydron, an essential feature i n the manifestation of superacidity. The postulation of superacidity f or a number of solid acids appears to have no basis in fact. On the ot her hand, the acidity of the groups/sites on the surface can be increa sed by the interaction with a nonbasic solvent, capable of forming str ong hydrogen bonds with the anion of the site (anion-stabilizing solve nt). The anion-stabilizing solvent generates a new liquid phase around the acid site; for appropriate structures of the solid acid and solve nt this phase can be superacidic. The acidity-enhancing effect of the anion-stabilizing solvent was found to have an important effect in boo sting the catalytic activity of the solid for carbocationic reactions.