Comparison of Mo/MgO and Mo/γ-Al2O3 catalysts: impact of support on the structure and dibenzothiophene hydrodesulfurization reaction pathways

The MgO and P2O5-promoted gamma-Al2O3 supports with alkaline and acidic natures, respectively, were prepared, impregnated with Mo atoms, and compared for dibenzothiophene (DBT) hydrodesulfurization (HDS) reaction. Ultraviolet spectroscopy and the principal component analysis were used to identify the impact of the supports on the reaction pathways. The catalysts were characterized by BET surface analysis, X-ray diffraction, temperature-programmed reduction, Fourier transform infrared, and X-ray photoelectron spectroscopy. The gamma-Al2O3-supported catalyst favors the hydrogenation pathway relative to the MgO-supported catalyst, which facilitates the direct desulfurization route. The different performance was attributed to the dissimilar Mo phases that emerged during the activation procedure. The activation under sulfo-reductive condition changed the Mo atoms on gamma-Al2O3 support into the sulfide phase while extra oxidation took place for the MgO-supported catalyst. The migration and consumption of loosely bonded bulk oxygen atoms with under-coordinated Mo atoms on the MgO support were introduced as a possible reason for such extra oxidation. DFT calculations predicted an interaction between the Mo/MgO catalyst and DBT via the electron donation from the catalyst oxygen atoms to the aromatic rings, resulting in weakening and breaking of the C-S bonds. In spite of the higher resistance of the MgO-supported catalyst toward coking and its superior activity, its lower hydrogenation capability suggested using a dualfunction catalyst. Accordingly, two catalysts were mixed and the synergism was observed in the HDS reaction of thiophene.