Interaction effects of nickel polyoxotungstate with the Al2O3–MgO support for application in dibenzothiophene hydrodesulfurization

In order to increase the activity of WS2/Al2O3 hydrotreating catalysts promoted by Ni, an Al2O3–MgO support with 5 mol% of MgO was prepared by sol–gel method and two catalyst series were obtained on this support by co-impregnation using pH 4 or 9 solutions. Support characteristics and W concentration effects were studied during impregnation and calcination steps by ultraviolet–visible (UV–Vis) diffuse reflectance and Raman spectroscopies while, after sulfidation, slab morphology and proportion of promoted NiWS phase were determined on the as-obtained sulfide catalysts by X-ray photoelectron spectroscopy (XPS), energy dispersive X-ray spectroscopy (EDX), and high-resolution transmission electron microscopy (HRTEM). The catalysts prepared at pH 4 showed image nickel heteropolyoxotungstates on dried catalyst and distorted image on the calcined catalyst. After sulfidation, the catalysts prepared at pH 4 have a higher proportion of promoted NiWS phase with an homogeneous Ni/W atomic ratio distribution and short (2.2–3.3 nm) WS2 monolayers. The catalysts prepared at pH 9 series present image nickel heteropolyoxotungstates and after calcination, image with strong Ni–W interaction. After sulfidation, heterogeneous W/Al and Ni/W atomic ratio distributions and longer WS2 stacks were obtained. This latter series was more difficult to sulfide, particularly for Ni. This heterogeneity and the stronger Ni–W interaction lead to a less dispersed active phase leading to a dibenzothiophene hydrodesulfurization activity lower on the pH 9 series than on the pH 4 series. The catalysts prepared at pH 4 were more active than a commercial NiW/Al2O3 catalyst. MgO addition has therefore a beneficial effect on the activity of NiW/Al2O3 catalyst.