Preparation And Catalytic Properties Of Molybdenum Carbide And Nickel Phosphide Assembled In Mesoporous Molecular Seives

Today,many countries enact severe regulations to limit the sulfur contents in fuel.It has been recognized that the current commercial hydrodesulfurization(HDS) catalysts are not adequate to meet the regulation requirements.This urges the worldwide research efforts to find new HDS catalysts,in which molybdenum carbide and nickel phosphide are believed to be the next-generation HDS catalysts for their excellent performance in HDS. In previous studies,supported molybdenum carbide and nickel phosphide catalysts often used Al₂O₃ or SiO₂ as support.The Al₂O₃ or SiO₂ supported catalysts has poor dispersion of active components as their low specific surface area resulting in the low catalytic activity in the reaction.Mesoporous molecular sieves(MMSs) have been recognized as the good catalytic supports for their high specific area and pore volume as well as the uniform pore size distribution.Thus,in this paper,molybdenum carbide and nickel phosphide were introduced into the MMSs by different assembly methods.It expects that the new-type catalysts can be obtained by utilizing the advantages of MMSs. Then,the prepared catalysts were characterized by XRD,N₂ adsorption-desorption isotherms,TEM and FTIR.Finally,the catalytic performances of the catalysts were evaluated by HDS of thiophene and dibenzothiophene(DBT),respectively.Firstly,the MoO₃/SBA-15 samples with different Mo contents were prepared by impregnation method by using SBA-15 as support.Then,the corresponding Mo₂C/SBA-15 catalysts were obtained by temperature-programmed carburization(TPC) method.All obtained samples had well mesoporous structure.When Si/Mo≥15(molar ratio),nano-sized particles of molybdenum carbide were homogeneously dispersed into the mesoporous structure of SBA-15.Mo₂C/SBA-15 catalysts have excellent HDS activity of thiophene in atmospheric pressure.Secondly,the Mo-SBA-15 samples were hydrothermally synthesized in strong acidic condition by using sodium silicate as silicon source,H₂SO₄ as acid source,sodium molybdate as molybdenum source,P123 and CTAB as co-template agents.Then,the corresponding Mo_xC-SBA-15 catalysts were prepared by TPC method.All obtained samples had highly ordered mesoporous structure.Some Mo ions may be located in the pore walls of SBA-15 in synthesis.Compared with the samples prepared by impregnation, the samples prepared by hydrothermal synthesis had higher specific surface area and pore volume,as well as the better dispersion of active components.In the HDS of DBT,MO_xC-SBA-15(Si/Mo=7.5) had the best catalytic activity with the DBT conversion of 97.6%at 380℃.The HDS of DBT over the SBA-15-containing molybdenum carbide catalyts mainly followed the direct desulfurization(DDS) mechanism.Except for the catalyt with the Si/Mo molar ratio of 30,the catalytic activities of carbide catalysts prepared by hydrothermal synthesis were much higher than those of catalysts prepared by impregnation method. Based on the experimental experiences of SBA-15-containing molybdenum carbide catalysts,the MoO₃/SBA-16 and Mo-SBA-16 samples were prepared by impregnation method and hydrothermal synthesis, respectively.Then,the Mo₂C/SBA-16 and Mo_xC-SBA-16 catalysts were prepared by TPC method.All obtained samples have cubic mesoporous structure.Also,the samples prepared by hydrothermal synthesis had higher specific surface area and pore volume,as well as the better dispersion of active components,resulting in the higher catalytic activity in the HDS of thiophene and DBT,respectively.Then,a series of Ni_xP/SBA-15 catalysts with different P/Ni molar ratios (P/Ni=0.25,0.50,0.75,1.0,1.25,1.5 and 2.0) were prepared by temperature-programmed reduction(TPR) method,respectively.All obtained samples had mesoporous structure.The active phase is the mixture phase of Ni₂P and Ni₁₂P₅ for Ni_xP/SBA-15(P/Ni=0.5),whereas the active phase is Ni₂P for the catalysts with P/Ni≥0.75.The intrinsic activities of DBT HDS for the Ni_xP/SBA-15 catalysts with different P/Ni molar ratios were investigated by ruling out the existence of diffusional limitations under 3.0 MPa of total pressure,H₂/oil ratio of 600 and with weight hourly space velocities(WHSV) of 26.7 h^-1.It was found that the HDS activity can be affected by P/Ni,and Ni₂P had better acitivity than Ni₁₂P₅.Ni_xP/SBA-15(P/Ni=0.75) had the best intrinsic HDS activity of DBT with the DBT conversion of 95.8%at 340℃.The stability test was performed for Ni_xP/SBA-15(P/Ni=0.75).The DBT conversion was higher than 95%after 300 h on stream,and the selectivity of BP was stabilized to 70%.It indicated that the DBT HDS over Ni_xP/SBA-15 (P/Ni=0.75) mainly followed the DDS mechanism.After the stability test,the specific surface area,pore volume and pore diameter remained unchanged. The ordering of mesoporous structure was also not destroyed.It indicated that Ni_xP/SBA-15(P/Ni=0.75) has the excellent activity and stability in the HDS of DBT.Finally,the nickel phosphate precursors were introduced into the MMS by Evaporation-Induced Self-Assembly(EISA) method using ethanol as solvent,nickel chloride as nickel source,TEOS as silicon source,H₃PO4 as phosphor source and P123 as template agent.Then,the corresponding nickel phosphide catalysts were prepared by TPR method.The obtained samples had ordered SBA-16-like cubic mesoporous structure with high specific surface area.The active phase of catalysts was Ni₂P The DBT conversion can reach 98%at 340℃over Ni_xP/MMS(Si/Ni=2.5),Ni_xP/MMS(Si/Ni=5) and Ni_xP/MMS-0.5K.