Preparation, Characterization Of Supported Ni₂P Catalysts And Their Hydrodesulfurization Performance Study

Transition metal phosphides exhibit broad and amazing physical and chemical properties,have recently been reported as a novel catalyst group for deep hydrotreatment and havereceived much attention because of their high activity for the hydrodesulfurization of fuels.However, the unsupported Ni₂P catalysts have low surface area and low activity, thereforedispersion of the Ni₂P on a high-surface support is needed. TiO₂-Al₂O₃may form to be apromising support for the hydrodesulfurization of catalysts since it can overcome thedisadvantages of low surface of the TiO₂, with the advantages of the high activity of the TiO₂.Chelating agents are widely used in the field of modified zeolites and catalyst preparation,and citric acid （CA） as chelating agent can effectively improve the surface area ofhydrodesulfurization catalyst, change the existing state of the metal components and promotethe formation of Ni octahedral six-coordinate species. However, currently the essence ofchelating agent mechanism is not yet clear. Therefore, the exploration towards this essencehas great significance for improving its hydrodesulfurization performance.Composite TiO₂-Al₂O₃supports were prepared by sol-gel method, and supportednickel phosphide catalysts, Ni₂P/TiO₂-Al₂O₃with citric acid （CA） as chelating agent, wereprepared by the impregnation method. The catalysts were characterized by X-ray diffraction（XRD）, N₂-adsorption specific surface area measurements （BET）, transmission electronmicroscope（TEM）, standard electronic modules（SEM）, temperature programmed oxidation（TPO）, temperature programmed reduction （TPR） and X-ray photoelectron spectroscopy（XPS）. The catalysts were evaluated using a lab-scale continuous flow fixed-bed reactor forthe hydrodesulfurization （HDS） of dibenzothiophene（DBT）. The effects of differentchelating agents, CA/Ni molar ratio, different phosphorus sources, Ni/P molar ratio, Ni₂Ploading, calcination temperature of support, calcination temperature of catalyst, reductionpressure, reduction temperature, immersion methods were studied. The result showed thatthe addition of appropriate amount of CA into the catalyst can enrich the pores ofNi₂P/TiO₂-Al₂O₃catalyst, increase the surface area, which made the catalyst with a betterpore structure, a higher dispersion of metal active component and more uniform size of theactive component. The addition of CA resulted in an apparent decrease of reductiontemperature for nickel and phosphorus precursor, as well as promotion of the formation ofthe active phase, and it can restrain the formation of carbon deposition on the catalystsurface to some extent, improving its stability. CA/Ni of2/1was found to be optimal. The HDS reaction of DBT over the Ni₂P/TiO₂-Al₂O₃catalyst proceeded through both the dircethydrogenalysis of C-S bond and hydrogenation of dibenzothiophene routes, and the resultshowed that the addition of appropriate amount of CA into the catalyst could significantlyimprove the catalytic activity of the direct hydrogenolysis route. At reaction temperature of360oC, pressure of3.0MPa, hydrogen/oil ratio of500（V/V）, and liquid hourly spacevelocity of2.0h^-1, reaction time of4h, the conversion of dibenzothiophene was99.5%.