Hydrogen bond strengths and acidities of hydroxyl groups on silica–alumina surfaces and in molecules in solution

Abstract

The properties of hydroxyl groups on mixed silica–alumina gels (0–50 % Al₂O₃) have been studied by adsorbing weak H bond accepting molecules onto the gels and recording the OH stretching infrared band.

Additional measurements on silica gel and compilation of numerous literature data have shown that frequency shift measurements, or series of Δv_OH measurements condensed into BHW (Bellamy, Hallam and Williams) slopes, provide a reliable estimate of the OH acidity. The resulting scale is independent of the atom to which the hydroxyl is attached; it may be used for surfaces of solids as well as for solutions.

The results obtained for silica–alumina show that two distinct populations of hydroxyl groups are present. Hydroxyl groups of type 1 are identical with those present on the surface of silica. The spectra show the presence of hydroxyl groups of a second type, the proportion of which increases as the Al content of the gel rises; their acidity corresponds to a pK_a between –4 and –8 and they are presumably the sites responsible for the protonation observed upon adsorption of pyridine or ammonia.