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.