A series of Al-containing mesoporous molecular sieves SBA-15 with different Si/Al molar ratios (50, 30, 20 and 10) was prepared by chemical grafting method using aluminum(III) chloride as alumina source. Mo and NiMo catalysts prepared using Al-SBA-15 supports were tested in the 4,6-dimethyldibenzothiophene (4,6-DMDBT) hydrodesulfurization (HDS) reaction. The synthesized solids were characterized by N-2 physisorption, small-and wide-angle XRD, Py FT-IR, Al-27 MAS-NMR, temperature-programmed reduction (TPR), UV-vis diffuse reflectance spectroscopy (DRS), chemical analysis and HRTEM. It was found that post-synthetic alumination of SBA-15 by reacting with AlCl3 has a little influence on the ordered mesoporous structure of the parent SBA-15 and leads to amorphous silicoaluminate materials with moderate Bronsted acidity. For all Si/Al molar ratios studied, tetrahedrally coordinated Al3+ cations predominated over octahedrally coordinated ones. However, as Al loading increases, the proportion of the latter ones was also increased. The dispersion of oxidic and sulfided Mo species increases with the incorporation of aluminum in the SBA-15 support due to the strong interaction of Mo and Ni species with aluminum atoms which serve as anchoring sites on the support surface. Catalytic activity of Mo and NiMo/SBA-15 catalysts increased with Al incorporation in the support reaching a maximum at Si/Al molar ratio of 20. High activity of NiMo/Al-SBA-15 catalysts in 4,6-DMDBT HDS can be attributed to a good dispersion of Ni and Mo active phases and to the bifunctional character of these catalysts. It was shown that both types of sites, coordinatively unsaturated sites of NiMoS active phase and Bronsted acid sites of the support participate in the catalytic transformations of 4,6-DMDBT. (c) 2007 Elsevier B.V. All rights reserved.