Effect of citric acid on hydrodesulfurization performance of the supported nickel phosphide catalyst

The TiO₂-Al₂O₃ complex support was prepared by the sol-gel method. A nickel phosphide catalyst, Ni₂P/TiO₂-Al₂O₃ with citric acid (CA) as chelating agent, was prepared by the impregnation method. The catalysts were characterized by X-ray diffraction (XRD), N₂-adsorption specific surface area measurements (BET), thermogravimetry-differential thermal analysis (TG-DTA), temperature programmed reduction (TPR) and transmission electron microscope (TEM). The effects of different chelating agents and CA addition for dibenzothiophene (DBT) hydrodesulfurization (HDS) were studied. The result showed that the addition of appropriate amount of CA into the catalyst can enrich the pores of Ni₂P/TiO₂-Al₂O₃ catalyst, increase the surface area, which made better pore structure, higher dispersion of metal active component and more uniform size of the active component. CA can weaken the interaction between the active phase and the support, resulting in an apparent decrease in reduction temperature for nickel and phosphorus precursor as well as promotion of the formation of the Ni-P-O active phase. At reaction temperature of 360℃, pressure of 3.0 MPa, hydrogen/oil ratio of 500(volume ratio), liquid hourly space velocity of 2.0 h^-1 and reaction time of 4 h, the initial dibenzothiophene conversion was 99.5% and stabilized at about 95.0% during 48 h reaction.