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 sal-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 Ni-(oh)(2+)/W(12)O(42)(10-)or Ni-(oh)(2+)/W(9)O(34)(9-)and Ni-(oh)(2+)/WO42- nickel heteropolyoxotungstates on dried catalyst and distorted Ni (2+)((d-oh))/W(12)O(42)(10-)and Ni-(d-oh)(2+)/W9O349- 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 Ni-oh(2+)-W12O4210-, Ni-oh(2+) - W(9)O(34)(9-)and Ni-oh(2+) -WO42- nickel heteropolyoxotungstates and after calcination, Nig(oh)(2+)-O3W-O-WO3 and Ni-oh(2-) -WO4(dist)2- with strong Ni-W interaction. After sulfidation, heterogeneous W/A1 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. (C) 2014 Elsevier Inc. All rights reserved.