Preparation And Performance Of Nickel Phosphide Catalyst For Hydrodenitrogenation

It is necessary to remove nitrogen from liquid fuels because NO_x, which are major source of air pollution, are formed in the exaust gas of a combustion engine. In addition, nitrogen-containing compounds significantly reduce the activity of catalysts for hydrodesulfurization (HDS), hydrocracking, reforming, and hydrogenation processes. Hence, hydrodenitrogenation (HDN) process has become increasingly important. The conventional catalysts for the HDN process are mainly alumina-supported Mo or W sulfides promoted by Ni or Co. Though the supported metal sulfides exhibit substantially high activities for HDS, their HDN activities are generally quite low. Transition metal phosphides have recently been the focus of research due to their higher activity compared to metal sulfides for HDS and HDN. Neverthelss, it is necessary to study on this class of new catalysts further in order to understand their structures and performances in HDN and HDS. Nickel phosphide was chosen as the HDN catalyst in the present dissertation because it is reported to have the highest activity among transition metal phosphides. For the supported nickel phosphide catalysts, mesoporous MCM-41 was used as the carrier. The effect of support modification and the introduction of a secondary metal species on the activity of supported nickel phosphide catalysts were studied. Moreover, the effect of TiO₂ addition to bulk nickel phosphide catalyst on the HDN was investigated. The catalysts were characterized by XRD, N2 adsorption, CO chemisorption, TPR, TEM, and XPS. The main contents are as follows:A series of MCM-41-supported nickel phosphide catalysts with initial Ni/P atomic ratio of 0.5 - 2 and loading level of 10 - 40 wt % in the oxidic precursors were prepared by an in-situ reduction method. Their catalytic performances were evaluated in the HDN of quinoline. The optimal initial Ni/P ratio is 1.25 and the optimal loading of NiO and P₂O₅ is 30 wt %. TEM showed that nickel phosphide particles on MCM-41, prepared under optimal conditions, are about 5 nm in size. The supported nickel phosphides with initial Ni/P ratio of 1.25 exhibited much higher HDN activity than the supported Ni-Mo (0.5) sulfide. It is also indicated that Ni-P(1.25) is twice more active in the HDN of quinoline than sulfided Ni-Mo (0.5) on a catalyst mass base, and the turnover frequency of Ni-P(1.25) is four times higher than that of sulfided Ni-Mo (0.5). Quinoline is easily hydrogenated to DHQ on nickel phosphides catalysts. The activity of hydrogenation reaction is higher than that of the C-N breaking and the C-N breaking is the rate-controlling step for quinoline HDN on Ni₂P/MCM-41. The aniline HDN was strongly stressed by the presence of quinoline and DHQ and the HDN of quinoline proceeds exclusively via a pathway which involves fully saturated intermediates on the MCM-41-supported nickel phosphides. The HDN activity of nickel phosphide catalyst was dramatically reduced by the presence of H2S and DBT, probably due to the strong adsorption of H₂S and DBT on active sites which inhibited the adsorption of quinoline.Nickel phosphide catalysts supported on different supports were prepared and their performances were investigated in HDN of quinoline. The nickel phosphide catalyst supported on MCM-41 exhibited the highest HDN activity probably due to the high surface area of MCM-41 and the weaker interaction between nickel phosphide and the surface of MCM-41. The effect of MCM-41 modification by K⁺ and the introduction of HY on the HDN performance of the supported nickel phosphides were investigated. The modification of K⁺ on MCM-41 hardly affected its activity in quinoline HDN. The introduction of HY contributed little to the performance of the catalysts. However, the HDN activity of nickel phosphide catalysts supported on MCM-41 decreased with the increase in the content of K+ or HY addition to MCM-41, probably due to the lower dispersion of nickel phosphide on MCM-41 caused by the addition of K₂O≥2.0 wt % and HY≥20.0 wt %. The performance of nickel phosphide catalyst supported on MCM-41 was improved by TiO₂. Howerve, except for TiO2, the performances of nickel phosphide catalyst supported on MCM-41 were not improved by the addition of other compound (V, Cr, Mn, Fe, Co, Ni, Cu, Zn, Mo) to nickel phosphide.A series of bulk nickel phosphide catalysts containing TiO₂ were prepared by two methods and investigated in HDN of quinoline. The activity of nickel phosphide for quinoline HDN increased significantly by the addition of TiO₂. The activity of bulk Ni2P comtaining TiO₂ is about 5 - 6 times higher than that of bulk Ni2P. TiO₂ enrichment on the surface of the catalyst was observed, which may play an important role in the enhancement of HDN activity. The product distribution analysis suggested that the ability of both hydrogenation and C-N breaking is enhanced by the addition of TiO₂. The synergetic effect of TiO₂ and nickel phosphide was observed. The increase in the quinoline HDN activity may be attributed to higher electron density caused by the transfer of the electron in Ti³⁺ species to nickel phosphide.