Behavior of NiMo/SBA-15 catalysts prepared with citric acid in simultaneous hydrodesulfurization of dibenzothiophene and 4,6-dimethyldibenzothiophene

In the present work, NiMo catalysts supported on SBA-15 were prepared using citric acid (CA) during the synthesis. The objective of this work was to realize a comparative study of NiMoCA/SBA-15 catalysts prepared under different conditions in order to get a deeper insight into the effect of the thermal treatment and pH of the impregnation solution used on the catalytic behavior in deep hydrodesulfurization (HDS). Catalysts were prepared by simultaneous impregnation of Ni and Mo species and CA, using impregnation solutions of acidic or basic pH values (pH = 1 or 9, respectively). The speciation diagrams of Ni(II) and Mo(VI) species in aqueous solution as a function of pH were established. Nicit24- complex was formed in aqueous solution at pH = 9. After the impregnation, NiMoCA/SBA-15 catalysts were dried at 100 °C and some of them were calcined at 500 °C in air atmosphere. Prepared catalysts were characterized by thermogravimetric analysis (TGA/DTG), nitrogen physisorption, powder X-ray diffraction (XRD), UV–Vis diffuse reflectance spectroscopy (UV–Vis DRS), temperature-programmed reduction (TPR), and high-resolution transmission electron microscopy (HRTEM) and tested in simultaneous HDS of dibenzothiophene (DBT) and 4,6-dimethyldibenzothiophene (4,6-DMDBT) in a batch reactor at 300 °C for 8 h. XRD characterization showed that Ni and Mo oxide species were well dispersed in all catalysts prepared with CA. In contrast, reference NiMo/SBA-15 catalysts prepared by co-impregnation of Ni and Mo species, without the addition of CA, showed signals of crystalline phases: (NH4)4[Ni(OH)6Mo6O18] 4H2O after drying at 100 °C and NiMoO4 after calcination at 500 °C. HDS of DBT showed differences in activity and selectivity of the catalysts depending on the pH of the impregnation solutions and the temperature at which the catalysts were treated: NiMoCA/SBA-15 catalysts prepared from acidic impregnation solutions were more active for HDS of DBT than those prepared using basic ones. Both dried and calcined catalysts prepared at pH = 1 were selective toward the hydrogenation (HYD) route of hydrodesulfurization. However, the selectivity of the catalysts prepared from basic solutions (pH = 9) was strongly affected by the thermal treatment: dried catalyst was highly selective for the direct desulfurization (DDS) of DBT, whereas the calcined one for the HYD route. NiMoCA/SBA-15 catalysts with high hydrogenation ability showed high activity in hydrodesulfurization of 4,6-DMDBT.