IR study of nitrotoluene adsorption on oxide surfaces

IR spectra of 4-nitrotoluene on silica, titania, zirconia, silica-alumina, mordenite and sulfated zirconia, are reported. Ligation to Lewis acidic surface sites was the dominant mode of adsorption for all oxides except silica for which hydrogen bonding between silanol groups and nitro groups occurred. Zirconia and sulfated zirconia catalysed the slow reductive dimerisation of adsorbates to the corresponding azoxybenzenes. There was no detectable IR evidence for adsorbate protonation, even for the highly acidic mordenite and sulfated zirconia surfaces. Colour changes accompanying adsorption may not necessarily imply that protonation has occurred.