Infrared study of adsorption of acetophenones on silica-alumina

Infrared spectra are reported of acetophenone and 4-methoxyacetophenone adsorbed on silica-alumina catalyst. The dominant modes of adsorption involved ligation via carbonyl groups to Lewis acidic Al3+ sites and H-bonding of surface silanol groups to carbonyl groups or aromatic rings. Isotopic H/D exchange between SiOH groups and CD3 groups or SiOD and CH3 occurred for acetophenone probably via an enolisation mechanism, although no enolic species could be detected. There was also little evidence for condensation reactions. Band positions for adsorbed acetophenones compared with the predictions of Hartree-Fock calculations suggested that acetophenone was not protonated, but 4-methoxyacetophenone was protonated by Bronsted acidic hydroxy groups for which -6.15 less than or equal to H-0 less than or equal to -4.81. (C) 1998 Elsevier Science B.V. All rights reserved.