Abstract: Silica MCM-41 without and with sulfuration treatment was measured by in situ pyridine adsorption infrared spectroscopy. Cluster models of pure SiO2 and SO2-4/SiO2 were built up, and density functional theory calculations on pyridine adsorption were carried out to analyze the surface acidity of SO2-4/SiO2. The combined results of experimental and theoretical studies showed that there were no Lewis acid sites on the surface of SO2-4/SiO2. The IR frequencies assignable to Lewis acidity corresponded to the enhanced hydrogen-bonded adsorption of pyridine by sulfuration treatment. The chelating structure of HSO-4 acts as Brnsted sites on the surface of SO2-4/SiO2, with an acid strength higher than that of sulfonic group-functionalized silica SO3H-MSU but weaker than that of HZSM-5. The acid catalytic activity of SO2-4/SiO2 results from the Bronsted acidity.

Key words: silica, sulfate anion, surface acidity, pyridine adsorption, infrared spectroscopy, density functional theory