Hydroconversion of n-hexadecane over Pt/WOx–ZrO2 catalysts prepared by a PVA-template coprecipitation route: The effect of tungsten surface coverage on activity and selectivity

A series of high surface area tungstated zirconia catalysts (WZ) with tungsten densities in the range of 4.8–9.9 W/nm2 were prepared by a coprecipitation route in the presence of polyvinyl alcohol (PVA) at a constant pH of 10 and final oxidation at 800 °C. Bifunctional catalysts were obtained by impregnating Pt (1 wt.%) on the calcined WZ samples. The materials were characterized by XRD, N2 adsorption, Raman spectroscopy, DRIFTS of adsorbed CD3CN, and H2 chemisorption, and then evaluated for the hydroconversion of n-hexadecane taken as representative molecule of the long-chain n-alkanes produced in Fischer–Tropsch processes. Raman spectroscopy clearly revealed that saturation of the zirconia surface was attained at a tungsten density of 6.9 W/nm2, which practically matched the theoretical monolayer coverage. IR of adsorbed CD3CN on pre-reduced Pt/WZ samples showed the presence of Brönsted acid sites of lower strength at W surface coverages below the monolayer. The n-C16 conversion rate sharply increased with tungsten density and reached its maximum at the monolayer coverage. Interestingly, we found that the product selectivity significantly varied with the tungsten surface coverage. Thus, the highest hydroisomerization selectivity was obtained for Pt/WZ with a tungsten density below the monolayer capacity. The trends in activity and selectivity have been rationalized on the basis of differences in acid properties of the Pt promoted tungstated zirconias. The activity and product selectivity of Pt/WZ materials were compared with that of bifunctional catalysts based on USY zeolite and amorphous silica–alumina.