Fourier transform Raman studies of methyl red adsorbed on γ-alumina and silica-alumina

Abstract

Fourier transform Raman spectra of methyl red adsorbed on untreated and pretreated γ-alumina and silica-alumina calcined at 900 °C under 1 atm steam flowing were recorded. Spectral analysis shows that the active species adsorbed on γ-alumina was to be deprotonated methyl red, and on silica-alumina to be di-protonated. This indicates that γ-alumina adapted in this work holds Brönsted basicity, and silica-alumina Brönsted acidity. Raman intensities of methyl red on pretreated γ-alumina are about three times stronger than on untreated γ-alumina, while spectral features are unchanged. For silica-alumina, spectral features show modified vibrational characteristics upon surface hydroxylations generated from pretreatment. Consequently, the acidity loss for silica-alumina and the basicity gain for γ-alumina were observed by increasing the surface hydroxyl groups on the catalysts through pretreatment of the steam calcination.