Effect Of TiO₂ On Hydrodesulfurization Over Ni(Co)-Mo Catalysts Supported On MCM-41

With the increasing consumption of transport fuel, especially gas oil and diesel oil, the environmental pollution progressively increases by automobile exhaust gas. More stringent fuel regulation has been established, especially for sulfur content. Moreover, the sulfur content of petroleum also increased gradually. Therefore, the deep hydrodesulfurization (HDS) of petroleum fractions has become an issue of high economic value and realization values problem. TiO₂ modified Ni(Co)-Mo/MCM-41 hydrodesulfurization (HDS) catalysts were prepared by a co-impregnation method, MCM-41 and catalysts were characterized by XRD, N₂ adsorption, SEM, TEM, TPR, UV-Vis et al. the effect of TiO₂ as well as its introduction order on the HDS performance of Ni(Co)-Mo/MCM-41 was studied using a model fuel containing 0.8 wt% dibenzothiophene (DBT) in decalin.The experiment result indicated that both the coordination states and the reducibility as well as the surface acidity of the oxidic precursor of Ni-Mo/MCM-41 were not remarkably affected by the introduction of TiO₂, but the formation of NiMoO₄ was promorted. Both the distribution and the coordination states of the oxidic precursor of Co-Mo/MCM-41 were not affected by the introduction of TiO₂, but the reducibility of the catalyst was promoted. The addition of TiO₂ to the surface of oxidic precursor suppressed the formation of low activeβ-CoMoO₄ minicrystal and facilitated the formation of high active Co-Mo-S, whereas promoted the formation of low activeβ-CoMoO₄ minicrystal and not to facilitated the formation of high active Co-Mo-S.The hydrodesulfurization of DBT over Ni-Mo/MCM-41 mainly via two parallel pathway, hydrogenation(HYD) and direct desulfurization(DDS). But the hydrodesulfurization of DBT mainly via the rout of direct desulfurization over Co-Mo/MCM-41. The activities of DBT HDS over these catalysts follow the order TiO₂-Ni-Mo/MCM-41＞Ni-Mo/TiO₂-MCM-41＞Ni-Mo/MCM-41, TiO₂-Co-Mo/MCM-41＞Co-Mo/MCM-41＞Co-Mo/TiO₂-MCM-41.The HDS results showed that the addition of TiO₂ suppressed the direct desulfurization pathway activity of Ni-Mo/MCM-41, but significantly enhanced its hydrogenation pathway (HYD) activity, leading to enhanced global HDS activity. On the base of the product compositions, it is suggested that the improved HYD and HDS activities of TiO₂-containg Ni-Mo/MCM-41 catalysts may be attributed to the enhanced acidity of the sulfides. The introduction order of TiO₂ into Co-Mo/MCM-41 was not remarkable affected the HYD activity, but affected the DDS activity. The DDS activity was significantly enhanced when TiO₂ was introduced after the impregnation of Co-Mo over MCM-41, leading to enhanced total HDS activity. Whereas, the DDS activity was suppressed, leading to decreased global HDS activity. The introduction of TiO₂ onto the surface of Co-Mo/MCM-41 maybe palyed an role of electron assistant, which improved the electron cloud density of Co-Mo, leading to its DDS activity.