Generation of Strong Acid Sites on Yttrium-Doped Tetragonal ZrO2-Supported Tungsten Oxides: Effects of Dopant Amounts on Acidity, Crystalline Phase, Kinds of Tungsten Species, and Their Dispersion

Abstract

A tungsten oxide catalysts supported on yttrium- or ytterbium-doped crystalline zirconium oxide (WOx/Y(Yb)-ZrO2) with different dopant concentrations but the same tungsten loadings and calcination temperatures were prepared to investigate the mechanism responsible for the strong solid acidity of so-called tungstated zirconia. The surface acid–base properties of WOx/Y(Yb)-ZrO2 and the support were characterized by catalyses of n-butane skeletal isomerization, alkylation of anisole with benzyl alcohol, and 2-butanol decomposition. The structural characterization and quantitative analysis were conducted by XRD, XAFS, and UV–Vis techniques. Crystalline zirconium oxides were effective as the starting material for tungsten–zirconium-based strong solid acid catalysts instead of amorphous Zr(OH)x if the crystalline form was tetragonal. Tetragonal-phase Y(Yb)-ZrO2 substitutional solid solutions with dopant concentrations of 3–5 mol % was a suitable support for stabilization of active small WO3 clusters. Tungsten species aggregated to form inactive large WO3 particles on the monoclinic ZrO2 polymorph with Y-dopant amount less than 2 mol % even though the surface density of tungsten species and calcination temperature were the same as those for the active catalysts. Inactive yttrium–tungsten binary oxides formed when the yttrium doping level exceeded 10 mol %. Intrinsic role of crystalline phase of tetragonal zirconia on generating the strong solid acidity is confirmed to stabilize WO3 small clusters without aggregation.