MICROCALORIMETRIC CHARACTERIZATION OF STRUCTURAL AND CHEMICAL HETEROGENEITY OF SUPERACID SO4 ZRO2 SYSTEMS/

The population of coordinatively unsaturated (cus) Zr4+ ions acting as strong Lewis acid sites on superacid sulfated zirconia catalysts, as well as the relevant heats of adsorption, were measured by means of CO adsorption at 303 K. The presence of charge-withdrawing SO4 groups sl ightly enhances the electron-accepting ability of the residual free su rface cations, not involved in sulfate/metal complexes. H2O adsorbed a t 303 K by a stepwise procedure progressively eliminates the Lewis aci dic sites, through a mechanism that suggests a competition in strongly binding H2O between cus Zr4+ cations and SO4 groups. Quantitative and energetic data are reported and correlated to IR spectroscopic and ca talytic data previously reported on the same catalysts. A model for th e catalytic active site is proposed, based on the assumption that a st rict cooperation between Lewis and Bronsted acidic sites is likely to exist.