Resonance Raman spectroscopic studies of probe molecules interacting with acidic sites on silica-alumina

Resonance Raman spectra have been recorded from Methyl Red, Ethyl Red, alpha-Naphthyl Red, o-aminoazotoluene and 4-(phenyl-azo)diphenylamine adsorbed from toluene solutions on to untreated and calcined silica-alumina. The spectra show clear evidence for the protonation of all five molecules, resulting from interaction with Bronsted acidic sites on the silica-alumina surface. Adsorption data over a wide range of equilibrium concentrations give satisfactory fits to Langmuir isotherms. The densities of acidic sites increase after calcination at 300 degrees C.