In this study, two series of WOx/ZrO2 catalysts were synthesized by incipient wetness impregnation of (1) crystalline ZrO2 and (2) amorphous metastable support Zr(OH) 4 using ammonium metatungstate precursor. The nC5 conversion catalytic activity and product selectivity of these materials were studied with respect to surface density between 2 and 11 tungsten atoms per nm2 (W/nm2), calculated from WO3 weight loading and BET surface area values). Maximum activity in nC5 conversion is observed at ~5.2 W/nm2, indicating that a strong acid-demanding reaction like nC5 isomerization follows a volcano-shape dependence similar to less acid demanding reactions. Results further show that, at a given tungsten surface density, Zr(OH) 4 serves as a better ZrO2 source than crystalline ZrO2 for this reaction, in agreement with observations by other research groups. Pyridine FTIR experiments gave a better insight in the acidic properties of this system. With respect to Brønsted acidity, tungstated zirconia has similar concentrations of Brønsted sites with commercial FCC equilibrium catalysts. The deactivation of Brønsted sites by 44% due to coke deposition indicated the direct correlation between catalytic activity and Brønsted acidity.
By establishing the surface density model and understanding its impact on to the acidic properties of tungstated zirconia, we can improve upon WOx/ZrO2 catalysis through rational nanostructure control.