On the strength of Lewis- and Brønsted-acid sites at the surface of sulfated zirconia catalysts

Abstract

The surface acidity (of both Lewis and Brønsted 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) system, and of another SZ t-ZrO₂ system prepared through a conventional method have been compared; the two SZ systems turned out to be essentially equivalent. By FTIR and microcalorimetry it was observed that the presence of sulfate modifies to a very limited extent both the concentration 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 typical acidic oxides (e.g., catalytic aluminas). IR spectroscopic data indicate that surface sulfates induce in SZ systems the presence of (protonic) Brønsted acidity, otherwise absent in (anion-free) t-ZrO₂. On SZ, Brønsted-acid centres are present over the whole vacuum-activation temperature range explored (300–823 K), though in decreasing amounts with increasing temperature. The acid strength of Brønsted sites is confirmed to be definitely lower than that of typical protonic oxide systems (such as, for instance, H-exchanged zeolites). As the Lewis-acid sites present on SZ systems are not particularly strong, and the Brønsted-acid sites present on SZ systems in catalytic conditions are not particularly strong (nor very abundant), it is concluded that the unique catalytic properties of SZ systems, sometimes termed superacid catalysts, must have another origin, possibly connected with the simultaneous presence at the surface of SZ of acid sites of both types.