Influence of the Preparation and of the Activation Treatments on the Catalytic Activity of Mechanical Mixtures of Sulfated Zirconia and Pt/Al2O3

Zirconium hydroxides with specific surface areas in the range of 60 to 300 m2/g were obtained by varying between 6 and 12 the final pH of precipitation of the gels and the nature of the precursor salt. These compounds were sulfated by impregnation with 15 and 35 ml of aqueous 0.5MH2SO4. Their sulfur content was 3 to 5 wt%. Sulfation increases the thermal stabilities of the zirconias by nearly 200 K. The solids remain amorphous up to at least 673 K and then crystallize in the tetragonal and monoclinic phases. The properties of mechanical mixtures of the sulfated zirconias and a Pt/alumina catalyst were evaluated for the isomerization ofn-hexane. The study of the conditions of activation show that the highest performances of these catalysts result from a pretreatment under hydrogen at 573 and 623 K when the amounts of H2SO4added are, respectively, of 35 and 15 ml/g of zirconium hydroxide, whatever their specific surface areas. A close correlation is found between the activities of the catalysts and their specific surface areas. This suggests that the active sites are located at the low coordination sites of zirconia and that their efficiency increases with their dispersion. The acidity determined by IR spectroscopy of NH3adsorption is essentially of Lewis type after evacuation at 573 K. However, Brønsted acidity results from a reversible redox equilibrium between hydrogen and zirconia. The isomerization ofn-hexane is considered to follow a classical bifunctional mechanism.