Hydrodesulfurization of sulfur refractory compounds: Effect of gallium as an additive in NiWS/γ-Al2O3 catalysts

The effect of gallium over the NiW/gamma-Al2O3 catalyst was investigated in the hydrodesulfurization (HDS) of 4,6-dimethyl-dibenzothiophene (4,6-DMDBT). The alumina carrier was modified with gallium by the addition of an aqueous solution of Ga(NO3)(3)center dot H2O in order to obtain supports with a nominal composition between 0.0 and 3.0 wt.% of Ga. The Ni-W catalysts were prepared with respectively 2.8 W atoms and 1.9 Ni atoms per nm(2) of initial surface area for the alumina support. The catalysts were characterized by different techniques such as temperature-programmed reduction, X-ray diffraction, UV-vis, Raman spectroscopy, X-ray photoelectron spectroscopy and transmission electron microscopy and they were evaluated in the HDS of 4,6-DMDBT. The highest HDS activity for NiW catalysts was observed when the amount of gallium was 2.4 wt.%, improving the value by ca. 90% as compared with that for the NiW gallium-free catalyst. This result was correlated to the effect of Ga over the dispersion of W and Ni entities. Indeed, the amount of WO particles calculated by the Raman (O=W=O + W=O)/W-O-W band intensities ratio was the highest for NiW/Ga(2.4)-gamma-catalyst and HRTEM examination on the sulfided catalysts showed a decrease for the average stacking number and for the average length of WS2 particles with the gallium content. Moreover, change of the promoter (Ni2+) interaction with the support was induced by the affinity of gallium (at low loadings) to the tetrahedral sites of alumina, inducing an increase of the octahedral species of Ni in the oxidic state of the catalysts as evidenced from UV-vis and to a higher amount of the NiWS mixed phase in the sulfided samples as deduced from XPS analysis. (C) 2012 Elsevier B.V. All rights reserved.