ON THE STRENGTH OF LEWIS-ACID AND BRONSTED-ACID SITES AT THE SURFACE OF SULFATED ZIRCONIA CATALYSTS

The surface acidity (of both Lewis and Bronsted type) of an anion-free t-ZrO2, stabilized in the tetragonal form by forming a solid solution with ca. 3 mol% Y2O3, of the corresponding sulfated t-ZrO2 (SZ) syste m, and of another SZ t-ZrO2 system prepared through a conventional met hod have been compared; the two SZ systems turned out to be essentiall y equivalent. By FTIR and microcalorimetry it was observed that the pr esence of sulfate modifies to a very limited extent both the concentra tion an strength of the types of strong (aprotic) Lewis sites that can be revealed by CO adsorption at 300 K. In absolute terms, the acidity of these Lewis sites turns out to be definitely lower than that of ty pical acidic oxides (e.g., catalytic aluminas). IR spectroscopic data indicate that surface sulfates induce in SZ systems the presence of (p rotonic) Bronsted acidity, otherwise absent in (anion-free) t-ZrO,. On SZ, Bronsted-acid centres are present over the whole vacuum-activatio n temperature range explored (300-823 K), though in decreasing amounts with increasing temperature. The acid strength of Bronsted sites is c onfirmed to be definitely lower than that of typical protonic oxide sy stems (such as, for instance, II-exchanged zeolites). As the Lewis-aci d sites present on SZ systems are not particularly strong, and the Bro nsted-acid sites present on SZ systems in catalytic conditions are not particularly strong (nor very abundant), it is concluded that the uni que catalytic properties of SZ systems, sometimes termed superacid cat alysts, must have another origin, possibly connected with the simultan eous presence at the surface of SZ of acid sites of both types.