Platinum tungstated zirconia isomerization catalysts Part I. Characterization of acid and metal properties

Abstract Tungstated zirconia catalysts containing 14–34 wt% W were prepared by bringing silica-modified hydrous zirconia into contact with appropriate amounts of metatungstate. The dry powders were extruded with an alumina binder and calcined at 1023 K. The extrudates were subsequently loaded with 0.1–1.5 wt% platinum and finally calcined at 753 K. The samples were characterized by means of XRD, nitrogen adsorption–desorption, and infrared spectroscopy. It is shown that silica stabilizes crystalline zirconia in the tetragonal phase and produces a high surface area and porous catalytic material. The presence of tungsten at coverages below the theoretical monolayer and the alumina binder hardly affects the structural and textural properties of such a silica-stabilized zirconia. Bulk tungsten oxide is detected for tungsten coverage exceeding the theoretical monolayer and induces the mechanical collapse of the alumina-bound extrudate. Low-temperature FT-IR spectra of adsorbed carbon monoxide gave the distributions of the Lewis and Bronsted acid sites. The tungstate species consume both Lewis and Bronsted sites of the zirconia up to the monolayer. A new Bronsted acidity of moderate strength is created at intermediate tungsten coverage. The concentration of these Bronsted sites, specific to the polytungstate species, increases with loading and reaches a maximum of one site for about three tungsten atoms at monolayer coverage. Beyond, this acidity decreases because of the coexistence of these polytungstates with bulk tungsten oxide. The high site density suggests the formation of a pseudo-heteropolyanion incorporating Zr 4+ and most probably Si 4+ ions, in line with a new infrared signal detected in the structure band region. The metallic function was characterized by the hydrogenation of toluene and CO adsorption. Both methods yield two linear domains with increasing platinum content, with a sharp increase around 0.2 wt% Pt. It is proposed that below this break point, the very low amount of accessible metal is due to the insertion of a large fraction of platinum into the zirconia subsurface. After saturation of the subsurface, additional platinum remains external and accessible. Correcting the platinum content for the imbedded species yields dispersion as high as 90% with a constant particle size of about 1 nm for accessible platinum in the whole range of concentration.