Effect of ZnO additives and acid treatment on catalytic performance of Pt/WO₃/ZrO₂ for n-C₇ hydroisomerization

The effect of WO₃ (5-50 wt%) and ZnO (0.7-22 wt%) on the catalytic properties of Pt/WO₃/(ZnO)-ZrO₂ for n-heptane (n-C₇) hydroisomerization was investigated. The optimized WO₃ and ZnO contents are 20 wt% and 3.4 wt%, respectively. The catalytic performance is achieved at 81% n-C₇ conversion and 89% C₇ isomer selectivity at 250 °C, which is reproducible and can be kept constant over 82 h under reaction conditions. Both WO₃ and ZnO can stabilize the tetragonal phase of ZrO₂. The Brønsted acid-to-Lewis acid ratio should be optimized to achieve high catalytic performance. The activity for Pt/WO₃/ZrO₂ using Zr(OH)₄ as the catalyst support (n-C₇ conversion, 88% at 250 °C) is much higher than that for Pt/WO₃/ZrO₂ with ZrO₂ as the support (n-C₇ conversion, 9% at 250 °C) with the same Pt and WO₃ loadings. BET, SEM-EDX, and pyridine-FTIR analyses show that acid treatment can successfully enhance the surface area (from 73 to 91 m²/g), increase the number of Brønsted acid sites, and lower the surface Zn:Zr ratio (from 0.43 to 0.15) for ZnO-ZrO₂ with 22 wt% ZnO. The yield of C₇ isomers is increased from nil to 47% at 300 °C on Pt/WO₃/ZnO-ZrO₂ catalyst after acid treatment. It is suggested that n-heptane hydroisomerization activity is related to acidity, surface area, and crystalline phase of ZrO₂.