Effect of phosphorus content in nickel phosphide catalysts studied by XAFS and other techniques

A series of novel, high-activity supported nickel phosphide hydro-processing 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), 3000 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-MoS/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 NO 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. (C) 2002 Elsevier Science (USA).