Effect of Phosphorus Content in Nickel Phosphide Catalysts Studied by XAFS and Other Techniques

A series of novel, high-activity supported nickel phosphide hydroprocessing catalysts (Ni2P/SiO2) was synthesized by means of temperature-programmed reduction (TPR), and the effect of phosphorus content on hydroprocessing performance and catalyst structure was studied. The catalysts were characterized by BET surface area determinations, CO uptake titrations, X-ray diffraction (XRD) analysis, elemental analysis, and extended X-ray absorption fine structure (EXAFS) measurements. The activity of the catalysts was studied in a three-phase trickle-bed reactor operated at 3.1 MPa and 643 K in the hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) of a model liquid feed containing 2000 ppm nitrogen (quinoline), 3681 ppm sulfur (dibenzothiophene), 500 ppm oxygen (benzofuran), 20 wt% aromatics (tetralin), and balance aliphatics (tetradecane). The samples were prepared with initial Ni/P ratios of 2/1, 1/1, 1/1.8, 1/2, 1/2.2, and 1/3, but the samples with excess P lost some of their P content during reduction and the main phase obtained was Ni2P. Activity and stability of the catalysts were affected profoundly by the phosphorus content, both reaching a maximum with an initial Ni/P ratio of about 1/2 (actual Ni/P=1/0.57 after reaction). At this optimal P content, the activity was excellent, with steady state HDS conversion of 100% and HDN conversion of 81%, which were much higher than that of a commercial Ni–Mo–S/Al2O3 catalyst with corresponding HDN conversion of 76% and HDN conversion of 38%. The stability of the optimal composition was also high, with no deactivation observed over 90 h in HDS and only a slight deactivation in HDN. EXAFS analysis of the catalysts indicated the formation of a Ni2P phase for the sample with an initial Ni/P ratio of 1/2, which was retained after reaction. At lower P content some Ni metal and Ni12P5 was obtained, and at higher P content, the Ni2P active phase was blocked by excess P. The activity results indicate that on these novel catalysts, the HDN reactions are structure sensitive while the HDS reactions are structure insensitive.