Orientation of Isomeric Phthalic Acids on Alumina and Magnesia Surface Observed by Inelastic Electron Tunneling Spectroscopy

Abstract

Abstract The vibrational spectra of phthalic, isophthalic, and terephthalic acid adsorbed onto alumina and magnesia surfaces from the gas phase have been measured by inelastic electron tunneling spectroscopy. The analysis of the tunneling spectra shows that phthalic and isophthalic acid react with the surface OH groups on Al2O3 and MgO to give the adsorbed dicarboxylate ions on both surfaces. Phthalic acid is adsorbed with its benzene ring perpendicular to the surfaces, while isophthalic acid is adsorbed parallel to the surfaces. However, terephthalic acid is adsorbed predominantly as the monocarboxylate ion almost perpendicular to the alumina surface, while it is adsorbed as the dicarboxylate ion parallel to the magnesia surface. These results suggest the different properties of the surface OH groups on Al2O3 and MgO. There are more than one type of OH group on the alumina surface and they interact with each other through hydrogen bonding. The magnesia surface has an isolated and a hydrogen-bonding OH group. The OH groups on MgO are more basic than those on Al2O3, and thus have higher reactivity. The different properties and basicity of the OH groups cause the different orientation and reactivity of the adsorbed terephthalic acid on the alumina and magnesia surfaces.