Synthesis And Catalytic Performance Of Hierarchical Zeolites Supported Nickel-tungsten Hydrodesulfurization Catalysts

Desulfurization of fuel oil is an important research topic. More and more restrict regulations were drawn up in many countries to limit the concentration of sulfur below 10 ppm or 15 ppm in transportation fuel oil. This means that the dibenzothiophene molecules with alkyl- groups like 4,6-dimethyl-dibenzothiophene(4,6-DMDBT) must be desulfurized. However, it is difficult to meet the demand by industrial hydrodesulfur ization(HDS) method because conventional HDS catalysts were not active in the HDS of 4,6-DMDBT due to the steric hindrance from alkyl groups. To overcome this problem, hierarchical zeolite Beta and Mordenite supported nickel- tungsten catalysts were prepared and their catalyt ic performance in HDS reactions was evaluated in this dissertation.Firstly, hierarchical zeolite Beta was prepared by post-treatment and characterized by X-ray diffraction(XRD), N2-sorption, scanning electron microscopy(SEM),transmission electron microscopy(TEM) and nuclear magnetic resonance(NMR) techniques. It was found that the number of acid sites in Beta was reduced after base-treatment due to the decrease of tetrahedral coordinated aluminum content. Subsequent acid treatment could increase the amount of tetrahedral coordinated aluminum and partially recover the acidity of zeolite Beta. Moreover, the hierarchization of zeolite Beta could greatly increase the number of accessible acid sites in zeolite Beta. As a result, hierarchical zeolite Beta prepared by base-acid treatment exhibited excellent catalyt ic activity in acid-catalyt ic reactions, especially towards large molecules.Secondly,N i W catalysts supported on Al2O3, commercial zeolite Beta and Mordenite(HB and HM), and hierarchical Beta and Mordenite prepared by post treatment(HB-M and HM-M) were characterized by XRD, N2 sorption, Infrared spectroscopy(IR), TEM and X-ray photoelectron spectroscopy(XPS). Their catalyt ic performance was evaluated in the hydrodesulfur ization(HDS) of dibenzothiophene(DBT) and 4,6- DMDBT. In the HDS of DBT, N i W/Al2O3 exhibited better catalyt ic activity than zeolite supported catalysts. O n the contrary, zeolite supported catalysts exhibited higher catalytic performance in the HDS of 4,6-DMDBT, which could be attributed to the strong acidity of the zeolitic support that supplied an additional isomerization reaction route. The use of hierarchical zeolite Beta and Mordenite could further improve the isomerization ability of catalyst. As a result, the HDS activit y of 4,6- DMDBT over N i W/HM-M and Ni W/HB-M was more than twice as that of Ni W/Al2O3 in the HDS of 4,6-DMDBT.Finally,the deactivation of hierarchical Beta zeolite supported catalysts was studied in the HDS of thiophene. The results indicated that N i W/HB as well as N i W/HB-M endured rapid and serious deactivation during the reaction. The introduction of additional mesopores/macropores in support could not prevent the deactivation tendency. Interestingly, N i W/HB or N i W/HB-M mixed with N i W/Al2O3 and the utilization of N a+-exchanged Beta as support improved remarkably the anti- deactivation ability of the catalysts. As a result, the catalysts N i W/Na B-M(N i W catalyst supported on N a+-exchanged hierarchical Beta) and N i W/20HB-M(20% N i W/HB-M mixed with 80% N i W/Al2O3) gave superior catalyt ic activity than N i W/HB-M and N i W/Al2O3. Moreover, this two methods may be utilized to enhance the anti- deactivation ability of zeolites with different famework types supported Ni W catalysts in the HDS of thiophene.