Effect of Zn Contents of W/Zn-Beta Catalysts on Their Catalytic Performance in the Selective Hydrocracking of Tetrahydronaphthalene into Benzene, Toluene, and Xylene

Selective hydrocracking (HDC) of tetrahydronaphthalene (THN) into light aromatic hydrocarbons, such as benzene (B), toluene (T), and xylene (X), was performed over bifunctional catalysts with different ZnO contents (W/Zn-Beta) at 400 °C and 6 MPa. In this work, the change of catalytic performance of the bifunctional catalysts with different ZnO contents in selective HDC of THN and the nine-lump reaction kinetic model was investigated. It was found for the first time that ZnO can not only effectively regulate the acid properties of Beta zeolite but also react with WO3 to form non-active ZnWO4 crystals, resulting in low HDC activity of the catalyst. At low ZnO contents of the catalyst (<1 wt %), as the interaction of ZnO–Beta is stronger than that of WO3–Beta, the interaction of ZnO–Beta would replace the interaction of WO3–Beta, leading to the agglomeration of WO3 and the dispersions of WO3 on Beta zeolite decrease. As a result, WO3 would be easily reduced to WS2, and the BTX selectivity would increase. At high ZnO contents of the catalyst (≥1 wt %), the strong acid sites of Beta zeolite would be covered by ZnO. In addition, the excess ZnO would react with WO3 to form non-active ZnWO4 crystals, and the BTX selectivity would decrease. The W(25)/Zn(1)-Beta-40 catalyst exhibited maximal BTX selectivity (45 wt %) at 94% THN conversion. The kinetic study further indicated that with the increase of ZnO contents, the ratio of formation reaction rate of BTX to the over-HDC rate of BTX increases first and then decreases, and the reaction path selectivity of THN isomerization increases.