Tailoring of alumina surfaces as supports for NiMo sulfide catalysts in the ultra deep hydrodesulfurization of gas oil: case study of TiO2-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: TiO₂ (from Degussa, 50 m²/g), Al₂O₃ (Nikki, 186 m²/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-Al₂O₃ without forming precipitates up to ca. 11 wt.% 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 (573 K, 3 MPa) show that sulfide catalysts supported on the composite support (ca. 11 wt.%) are more active than those supported on to TiO₂ or Al₂O₃. 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.