Lewis and Brønsted acidity at the surface of sulfate-doped ZrO2 catalysts

Both Brønsted (protonic) and Lewis (aprotic) acidic centres can exist at the surface of an homo-geneous family of sulfated zirconia catalysts, provided sulfate groups (either isolated or, better, polynuclear units) exist on the regular patches of few low-index crystal planes representing the top termination of the scale-like ZrO2 crystallites. The Brønsted/Lewis acidity ratio depends then on (at least) two variables: the surface hydration/dehydration degree, and the highest temperature reached by the system because Brønsted acidity is induced only by the thermally most labile fraction of surface sulfates.