Tailoring of alumina surfaces as supports for NiMo sulfide catalysts in the ultra deep hydrodesulfurization of gas oil: Case study of TiO ₂-coated alumina prepared by chemical vapor deposition technique

The present paper gives a detailed review of the different studies under investigation in our laboratory concerning the use of TiO₂ and TiO₂-Al₂O₃ composites prepared by chemical vapor deposition (CVD) as support for sulfide catalysts in the HDS of dibenzothiophene (DBT) derivatives. The supports investigated here are: TiO ₂ (from Degussa, 50m²/g), Al₂O₃ (Nikki, 186m²/g) and TiO₂-Al₂O₃ supports prepared by CVD of TiCl₄ on alumina. Using several characterization techniques, we have demonstrated that the support composite presents a high dispersion of TiO₂ over γ-Al₂O ₃ without forming precipitates up to ca. 11wt.% loading. Moreover, the textural properties of the support composite are comparable to those of alumina. XPS investigations of Mo and NiMo catalysts supported on the different carriers show that Mo-oxide species exhibit a higher degree of sulfidation on the surface of TiO₂ and TiO₂-Al₂O₃ than on alumina. The HDS tests of 4,6-DMDBT under mild operating conditions (573K, 3MPa) show that sulfide catalysts supported on the composite support (ca. 11wt.%) are more active than those supported on to TiO₂ or Al ₂O₃. This higher HDS catalytic activity is attributed to the promotion of the hydrodesulfurization pathway, whereby the pre-hydrogenation of one of the aromatic rings adjacent to the thiophenic one may reduce the steric hindrance caused by the two methyl groups adjacent to the sulfur atom during the C-S bond cleavage.