Influence of the preparative route on the properties of WO x–ZrO 2 catalysts: A detailed structural, spectroscopic, and catalytic study

Abstract

Two series of tungstated zirconia (WZ) solid acids covering a wide range of tungsten surface densities ( δ, W at/nm 2) were prepared by nonconventional impregnation and coprecipitation routes, leading to samples with enhanced surface area ( ∼ 70 – 120 m 2 / g ) on annealing at 973–1073 K. The materials were thoroughly characterized by N 2 physisorption, XRD, Raman, XPS, H 2-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-C 16) at an intermediate tungsten density ( δ max ). Interestingly, we found that δ max shifted toward higher values for coprecipitated catalysts ( δ max , COP = 6.8 W at/nm 2) compared with the impregnated ones ( δ max , IMP = 5.2 W at/nm 2). This has been ascribed to a better inherent capacity of the coprecipitation route for dispersing tungsten species on the ZrO 2 surface, as inferred from modeled XPS data. This determines that both the formation of highly interconnected amorphous WO x domains required for the generation of catalytically active Brønsted acid sites and the onset of growth of inactive three-dimensional WO 3 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 δ max , 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 WO x domains should be attained by both impregnation and coprecipitation routes at δ = δ max . Moreover, the method of preparation was found to affect the optical and electronic properties of the supported WO x 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 WO x clusters generated by impregnation.