Acidic properties and stability of sulfate-promoted metal oxides

Methanol dehydration, used as a test reaction, and CO and pyridine adsorption studied by infrared spectroscopy, allow comparison of the acidity of pure and sulfated metal oxides such as ZrO2, TiO2-anatase, -rutile and Al2O3. Sulfation enhances the strength of the weakest Lewis acid sites but poisons the strongest. It also creates Brönsted acidity in the case of highly loaded samples. This latter effect explains the high activity of persulfated TiO2 and ZrO2 samples due to the formation of sulfate species different from those previously characterized on low loaded samples. They have been tentatively described as polymeric forms or SO3-like species. In contrast, sulfation tends to decrease the very high activity of Al2O3. The active species are stable at 300 °C on alumina and zirconia, but are quickly decomposed on the TiO2 samples. This low stability, compared to that observed under vacuum, is due to interaction with the methanol and/or water formed, increasing the ionic character of the sulfate species, and making them less stable.