Hydrodesulfurization of thiophene over TiO₂-supported nickel phosphide catalysts

TiO₂-supported nickel phosphide catalysts (Ni₂P/TiO₂) were prepared by temperature-programmed reduction of the corresponding phosphate in flowing gases mixture of 10% H₂/N₂. The obtained catalysts were characterized by the X-ray diffraction (XRD), and their specific surfaces were determined by N₂ BET method. The hydrodesulfuration (HDS) activity of the catalysts were examined by the HDS of thiophene in a fixed bed reactor, in which, the thiophene was used as a model compound for the HDS reaction. The results show that, using the precursors with molar ratios of Ni/P=1/2 and 1/3, the catalysts yielded contain phase-pure Ni₂P on their surfaces, whereas, using the precursors with molar ratio of Ni/P = 1/1, the catalysts yielded contain phase of Ni₂P and little amount of Ni₁₂P₅ phase as impurity on their surfaces. The specific surface areas of obtained catalysts decrease with increasing Ni and P contents in the precursors, and the catalytic activities of TiO₂-supported nickel phosphide catalysts for thiophene HDS depend strongly on the Ni/P molar ratio and the Ni loading of the precursors. The most active Ni₂P/TiO₂ catalyst for HDS was obtained from the precursor with Ni/P molar ratio close to 1/2 and the Ni loading around 15%(wt), and using it under a suitable reaction condition, such as reaction temperature = 370°C, pressure = 3.0 MPa, VHSV = 2 H^-1, S in oil = 0.1%(wt.) and H₂/oil=450(v/v), the HDS conversion rate can almost reach 100%. During reaction, the high HDS conversion rate of the Ni₂P/TiO₂ catalysts can maintain more than 200 h and it indicates that the catalytic activity of Ni₂P/TiO₂ is quite stable. The study shows that the reaction pressure, liquid VHSV and the H₂/oil ratio have a little effect on the HDS performance of the Ni₂P/TiO₂ catalyst. As to the temperature, when the reaction temperature is below 300°C, the temperature has a relatively big effect on the HDS activities of the Ni₂P/TiO₂ catalysts, and when the reaction temperature is over 300°C, the HDS activity will be high enough to cause the HDS conversion rate reaches 100% when other reaction conditions are suitable as mentioned above.