Synthesis, characterization, and hydrotreating activity of several iron group transition metal phosphides

A series of iron, cobalt, and nickel metal phosphides of chemical formula Fe₂P, CoP, and Ni₂P with specific surface areas of around 3 m² g^-1 were synthesized by means of temperature-programmed reduction (TPR) of the corresponding phosphates. These phosphides were also successfully prepared in dispersed form on a silica support (90 m² g^-1) for use as catalysts. The phase purity of these materials was established by X-ray diffraction (XRD), and surface properties were determined by N₂ BET specific surface area (S_g) measurements and CO uptake determinations. The activity of the silica-supported catalysts in hydrodenitrogenation (HDN) and hydrodesulfurization (HDS) was evaluated in a three-phase trickle-bed reactor using a model liquid feed containing 2000 ppm nitrogen as quinoline, 3000 ppm sulfur as dibenzothiophene, 500 ppm oxygen as benzofuran, 20 wt% aromatics as tetralin, and balance aliphatics as tetradecane. The reactivity study showed that the HDS activity sequence for the three samples was Ni₂P/SiO₂>CoP/SiO₂>Fe₂P/ SiO₂, while the HDN activity followed the sequence CoP/SiO₂>Ni₂P/SiO₂>Fe₂P/ SiO₂. Compared with a commercial Ni-Mo-S/γ-Al₂O₃ catalyst, Ni₂P/SiO₂ had a higher HDS activity (90 vs 76%), but a lower HDN activity (14 vs 38%), based on equal sites loaded in the reactor. The sites were determined by CO chemisorption for the phosphide and low-temperature O₂ chemisorption for the sulfide. XRD and X-ray photoelectron spectroscopy characterizations of the spent catalysts indicated that the Ni₂P/SiO₂ catalyst was tolerant of sulfur.