Preparation And Hydrotreating Performance Of Nickel Phosphide Catalysts

Catalytic hydrotreating is an important refining process used to produce clean fuel, andthe development of high active catalysts is one of key technologies. In recent years, with thehigher activity of hydrodesulfurization (HDS) and tolerance of sulfer, Ni₂P catalysts haveattracted more and more attention. However, these catalysts are generally prepared throughreducing nickel phosphates with complicated procedures under high temperature. In thisthesis, nickel hypophosphite was chosen as precusor instead of nickel phosphates to preparesupported Ni₂P catalysts, which might finally lead to lower preparation temperature andsimpler procedures.The high purity nickel hypophosphite was prepared by neutralization reaction betweenbasic nickel carbonate and nickel hypophosphite under temperature 40-60℃and ratio of P toNi 2.2-2.5, the yield is about 95wt%.The decomposition process of nickel hypophosphite in the nitrogen atmosphere wasstudied by X-ray diffraction (XRD), thermo-gravimetric/differential thermal analyzer(TG-DTA) and high resolution transmission electron microscopy (HRTEM) techniques. Theresults suggested that nickel hypophosphite decomposed severely at about 210℃andproduced PH₃. The generated PH₃ which is important reducing agent promoted the formationof crystal of Ni₂P and Ni₅P₄.The decomposition products were Ni₂P and a little Ni₅P₄ between150-250℃.The content of Ni₅P₄ increased gradually while that of Ni₂P decreased between300-400℃, accompanied by the appearance of a small amount of Ni(PO₃)₂. Increased thetemperature to 400-500℃, onlyNi(PO₃)₂ and Ni₅P₄ could be obtained.The alumina or silica supported nickel hypophosphite catalyst precursors were preparedby pore saturation impregnation method. The precursors was calcined at 250-300℃in N2flowing, then the supported Ni₂P could be obtained. The dispersible threshold value of the Ni₂P was below 15wt% on either alumina or silica carriers.The effect of presulfided treatment by H₂S on the hydrogenation performance of Ni₂Pcatalysts was studied usingα-methynaphthalene (α-MN) and dibenzothiophene (DBT) asmodel compounds in a micro-reactor with high pressure. The results indicated that thepresulfided process could not only increase the HYD rate ofα-MN but also improve the HDSrate of DBT greatly. FT-IR and HRTEM results implied that the sulfur might react with thesurface atoms of Ni₂P during the sulfidation and formed Ni-P-S species which might be thereal catalytic active sites.