Influence of the preparative route on the properties of WOx–ZrO2 catalysts: A detailed structural, spectroscopic, and catalytic study

Two series of tungstated zirconia (WZ) solid acids covering a wide range of tungsten surface densities (δ, W at/nm2) were prepared by nonconventional impregnation and coprecipitation routes, leading to samples with enhanced surface area (∼70–120 m2/g∼70–120 m2/g) on annealing at 973–1073 K. The materials were thoroughly characterized by N2 physisorption, XRD, Raman, XPS, H2-TPR, and DR UV–vis spectroscopy. The catalytic behavior of the Pt-promoted WZ catalysts (1 wt% Pt) was evaluated for the hydroconversion of n-hexadecane used as model feed representative of Fischer–Tropsch waxes. Both series of catalysts displayed a pronounced maximum in the reaction rate and a minimum in the selectivity to branched feed isomers (iso-C16) at an intermediate tungsten density (δmaxδmax). Interestingly, we found that δmaxδmax shifted toward higher values for coprecipitated catalysts (δmax,COP=6.8δmax,COP=6.8 W at/nm2) compared with the impregnated ones (δmax,IMP=5.2δmax,IMP=5.2 W at/nm2). This has been ascribed to a better inherent capacity of the coprecipitation route for dispersing tungsten species on the ZrO2 surface, as inferred from modeled XPS data. This determines that both the formation of highly interconnected amorphous WOx domains required for the generation of catalytically active Brønsted acid sites and the onset of growth of inactive three-dimensional WO3 crystallites (ascertained by XRD and Raman) occur at higher tungsten surface densities in WZ solids generated by coprecipitation than in those obtained by impregnation. Despite the observed shift in δmaxδmax, the two most active samples within each series displayed nearly the same intrinsic activity per total W atoms, suggesting that a similar nature and size for the supported active WOx domains should be attained by both impregnation and coprecipitation routes at δ=δmaxδ=δmax. Moreover, the method of preparation was found to affect the optical and electronic properties of the supported WOx species. Thus, coprecipitation provides WZ solids displaying a lower valence–conduction energy gap, as well as enhanced reducibility for the polytungstate domains due to an improved electronical linkage with the zirconia support, in opposition to a more isolated character of the WOx clusters generated by impregnation.