Activation and isomerization of hydrocarbons over WO 3/ZrO 2 catalysts : II. Influence of tungsten loading on catalytic activity: Mechanistic studies and correlation with surface reducibility and tungsten surface species

Abstract

We studied the correlation among the catalytic behavior of WO 3/ZrO 2 samples toward unsaturated and saturated hydrocarbons transformation, tungsten surface species oxidation states, and the crystallographic structure of the zirconia support. Different tungsten-loaded catalysts were studied, from 9 wt% (near-monolayer coverage) to 30 wt%. The resulting WO 3/ZrO 2 materials were obtained by impregnation of a tungsten salt on either a commercially available monoclinic zirconia or an amorphous hydroxide, ZrO x (OH) 4−2 x , followed by a calcination step (according to the Hino and Arata procedure), leading to a tetragonal structure. In contrast to previous works, here we demonstrate that the crystallographic structure of zirconia has no influence on catalytic properties. Correlations with XPS analyses revealed two aspects of catalytic behavior that depend strongly on the catalyst reducibility and thus on the W surface species oxidation states. First, on hardly reducible (tungsten loadings <15 wt%) or slightly reduced (below 423 K) surfaces, a purely acidic monomolecular mechanism for both isomerization (largely predominant) and cracking reactions, associated with W 6+ and W 5+ surface species, was demonstrated. Second, on easily reducible (tungsten loadings >15 wt%) or deeply reduced (over 723 K) surfaces, a bifunctional mechanism associating dehydrogenating/hydrogenating properties occurring on metallic tungsten and acidic isomerization and cracking on W 5+ and W 6+ surface species was observed. However, in this last case, we could not exclude the participation of a purely metallic isomerization mechanism occurring through σ-alkyl adsorbed species on the β-W metallic phase. A more pronounced reduction then led to an increase in the extensive hydrogenolysis mechanism, causing catalyst deactivation.