Application Of Titanium Modified Support In Nickel Phosphide Catalyst For Hydrodenitrogenation

It is necessary to remove nitrogen from engine fuels which generate large quantities of NOx,the major source of air pollution.In addition,nitrogen-containing compounds significantly reduce the activity of catalysts for reforming,hydrodesulfurization and hydrocracking processes.Hence,the study of hydrodenitrogenation process and developing high-activity catalyst has its theoretical and practical significance for producing cleaner engine fuels and protecting living environment.Nickel phosphide catalyst has rencently attracted more attention due to its higher hydrodesufurization,hydrodenitrogenation?HDN?and stability.So far,it is worth studying how to modify support to improve HDN activity of Ni₂ P catalyst,and how to synthetize Ni₂ P with an enviromental and mild method.In this thesis,Ti modified supports including TiO₂-SiO₂,TiO₂-Al₂O₃ composite support and Ti-SBA-15 support were prepared with different methods,and the effect of as-prepared supports on the HDN activity of Ni₂ P catalyst was investigated in depth.Besides,hydrothermal synthesis of Ni₂ P was preliminarily discussed.The main contents are as follows:A series of TiO₂-SiO₂ composite supports were prepared by sol-gel method,and the supported nickel phosphide catalysts,Ni₂P/TiO₂-SiO₂,were prepared by incipient wetness impregnation and in situ H2 reduction method.The effect of?NiO+P2O5?loading,initial Ni/P molar ratio and Ti/Si molar ratio was investigated on the catalyst HDN activity.The results showed that TiO₂-SiO₂ composite support was amorphous state with Ti-O-Si bond formed between TiO₂ and SiO₂.The specific surface area of composite support with Ti/Si molar ratio of 1/2 was the largest and the amount of acidity was moderate among series of supports.The initial Ni/P molar ratio of precursor had great effect on the final phase after reduction.A mixture of Ni3 P and Ni12P5 was found in the supported catalyst prepared with initial moalr ratio of Ni/P=2.0,while pure Ni₂ P was obtained in the supported catalyst prepared with initial moalr ratio of Ni/P=1.0 or Ni/P=0.5.The optimal HDN activity was found on the Ni₂P/TiO₂-SiO₂ catalyst with initial molar ratio of Ti/Si =1/2,molar ratio of Ni/P= 1.0,the loading of?NiO+P2O5?= 25%.The order of HDN activity for different supports was as follows:Ni₂P/ TiO₂-SiO₂?1/2?> Ni₂P/ SiO₂> Ni₂P/ TiO₂?A series of TiO₂-Al₂O₃ composite supports were prepared by in situ sol-gel and co-precipitation method,and the supported nickel phosphide catalyst,were prepared by incipient wetness impregnation and in situ H2 reduction method.The hydrodenitrogenation?HDN?activity of the supported nickel phosphide catalysts were evaluated on a continuous-flow fixed-bed reactor by using quinoline as the model molecules.The results showed that the composite support prepared by in situ sol-gel method basically retained the original pore properties of ?-Al₂O₃ with larger surface area and decentralized pore size distribution,and TiO₂ was enriched on tubular ?-Al₂O₃ surface.The composite support prepared by co-precipitation method had smaller surface area and centralized pore size distribution,and TiO₂ was evenly dispersed on massive ?-Al₂O₃ surface.The main active phases after reduction were Ni₂ P and Ni12P5 phase for the sol-gel prepared TiO₂-Al₂O₃ supported catalyst,but only Ni₂ P appeared for the co-precipitation TiO₂-Al₂O₃ supported catalyst.The introduction of TiO₂ obviously decreases the interaction between P species and Al₂O₃,which is beneficial not only to the formation of Ni₂ P active phase but also to the C-N bond cracking ability of catalyst.Different TiO₂-Al₂O₃ preparation techniques and different Ti/Al molar ratio had great effect on HDN activity of catalysts.The co-precipitation TiO₂-Al₂O₃ supported catalyst exhibited better reducibility and HDN activity than the in situ sol-gel TiO₂-Al₂O₃ supported catalyst.Two kinds of catalyst exhibited the same order with varying Ti/Al molar ratio,that is,Ni₂P/ TiAl?1/8?> Ni₂P/ TiAl?1/16?> Ni₂P/ TiAl?1/4??Ti modified SBA-15 support was synthesized by one-step hydrothermal synthesis method,and the supported nickel phosphide catalysts were prepared by incipient wetness impregnation and in situ H2 reduction method.The effect of Ti doping on SBA support and HDN activity of their supported nickel phosphide catalysts was investigated.The results showed that the Ti doping had not changed highly ordered mesoporous structure of SBA-15.The active phase was found in the form of Ni₂ P while Ti in the form of TiO₂ and Ti9O17 after reduction of catalyst precursor.HDN activity of Ni₂P/Ti-SBA-15 catalysts were much higher than Ni₂P/SBA-15 catalyst,and the catalyst with the molar ratio of Si/Ti=25 exhibited the best HDN performance.The introduction of moderate TiO₂ enhance the hdrogenation ability and C-N bond cracking ability of Ni₂ P catalyst.Low valent Tin+ species on the surface of support may be as assistant agent and contribute to the higher HDN activity of catalyt.Nickel phosphide catalysts were successfully synthetized by using non-toxic red phosphorus instead of white phosphorus,nickel nitrate as raw materials,water as solvent at a low temperature by hydrothermal?solvothermal?method.The influence of several factors in the process of synthesis was studied,including molar ratio of P/Ni,hydrothermal reaction temperature,reaction time,different solvent system and surfactant.The results showed that the pure hexagonal nickel phosphide?Ni₂P?could be successfully prepared under the initial P/Ni mole ratio of 6 and the reaction temperature of 120 ? for 12 h.With initial P/Ni mole increment,pure tetragonal nickel phosphide?Ni12P5?could be transformed into pure hexagonal nickel phosphide?Ni₂P?,while with temperature incrrement,Ni₂ P could be gradually transformed into Ni12P5.The crystallinity of products increased with reaction time prolonging.When the solvent water was changed into water-glycol system,Ni₂ P could be obtained as well,but the micro-mophology changed from small sphere to pine lamina shape,and to perpendicular slice shape.