Acidic Properties of Sulfated Zirconia: An Infrared Spectroscopic Study

Sulfated zirconia with S content of 2 wt.% equivalent to complete coverage of its surface was studied by infrared spectroscopy. At least four sulfated species were identified and exhibited an important and reversible sensitivity to water as schematized below: (SO 3) ads + H 2O ⇆ (H 2SO 4) ads (H 2SO 4) ads + 2(H 2O) ads ⇆ (H 3O +) ads + (HSO − 4 ads + (H 2O) ads ⇆2(H 3O +) ads + (SO 2− 4) ads. These equilibria were demonstrated to exist by the study of adsorption of incremental amounts of water. D 2O and (H 2 18O isotopically enriched water molecules were used to assist interpretation of IR spectra. To characterize acidity features, the probe molecules butane, CO, and H 2O (as weak bases) or pyridine (as a strong base) were adsorbed. Two Lewis acid sites (L 1 and L 2) were observed and one Brønsted site (B) related to the zirconia support (L 1) and the sulfated species (L 2, B). They were evidenced by pyridine adsorption which was shown to partly displace adsorbed sulfate species. With the help of previous theoretical calculations using an ab initio method and representing the zirconia surface by a mononuclear zirconium complex, it is emphasized that the sulfated zirconia can be visualized as a H 2SO 4 compound grafted onto the surface of zirconia in a way which makes it very sensitive to water but in a reversible way. Its acidity is similar to that of sulfuric acid but it is not really superacidic. Comparison with other oxides leads us to suggest that the cationic charge borne by the metallic cation is of prime importance for the acidity strength. The role of water on the acidic and catalytic properties for n-butane isomerization reaction is emphasized.