Studies On Siliceous MCM-41-Supported Ni-Mo Sulfides Or Co-Mo Sulfides As Deep Hydrodesulfurization Catalysts

High performance diesel hydrodesulfurization (HDS) catalysts were prepared by depositing Ni-Mo and Co-Mo sulfides on mesoporous MCM-41 molecular sieves with extremely high surface. The prepared catalysts were characterized by means of XRD, TPR and UV-Vis, and the performances of the prepared catalysts were evaluated by HDS of the most refractory polyaromatic sulfur-containing molecules and a straight-run diesel with high sulfur content. The kinetics of dibenzothiophene HDS catalyzed by either Ni-Mo/MCM-41 or Co-Mo/MCM-41 catalysts were studied. An isotope tracer method was used to investigate the dynamic behavior of sulfur atoms during HDS to elucidate the HDS mechanism, and a new catalytic circle was proposed accordingly. The main contents are as follows:(1) The synthesis of siliceous MCM-41 was studied using sodium silicate as a silica source and cetyltrimethylammonium bromide as a templating agent. And the effects of hydrothermal conditions and post-synthesis treatment on the mesostructure and thermal stability of the synthesized MCM-41 were also investigated. It is shown that siliceous MCM-41 were obtained when pH of the synthesis mixture fell in the range of 10.0-12.0 at temperatures of 100-130 癈 and for a time period of 12-24 h, and that the pore diameters of the synthesized MCM-41 could be finely tuned by adjusting the pH in the range. It was found that the mesoporous structure of the synthesized MCM-41 could be improved when the as-synthesized products were preheated in an atmosphere of N2 at 600 癈 prior to air calcinations in order to remove the organic templating agent. No change in XRD spectrum was observed after the synthesized siliceous MCM-41 had been heated at 700 癈 for 2 h in air, and the intensity of XRD peak of MCM-41 changed a little even heated at 800 癈 for 2 h in air, indicating that the synthesized MCM-41 was thermally stable.(2) Siliceous MCM-41-supported Ni-Mo and Co-Mo bimetallic HDS catalysts were prepared, and characterized by means of XRD, UV-Vis and TPR. The HDS activities of the prepared catalysts were investigated using a representative polyaromatic sulfur-containing model compound, such as dibenzothiophene (DBT), 4-methyldibenzothiophene (4-MDBT), or 4,6-dimethyldibenzothiophene. The HDS performance of the prepared catalysts were evaluated by the HDS of a high sulfur straight-run diesel (2.83 wt% S). The performace of the prepared catalysts were compared with a commercial deep hydrodesulfurization catalyst on Japanese market (DHDS). It is shown that these two series catalysts exhibited extremely high performance in HDS of DBT, 4-MDBT, 4,6-DMDBT or the straight-run diesel, higer than that of DHDS. The maximum HDS activity was observed at Ni(Co)/Mo atomic ratio of 0.75, which is higher than that of conventional alumina-supported Ni-Mo or Co-Mo sulfides.which is higher than that of conventional alumina-supported Ni-Mo or Co-Mo sulfides.(3) Charaterization by XRD and UV-Vis revealed that the introduction of Ni or Co greatly reduced the crystal size of MoO3 on the surface, and that some kind of new active phase might be generated. TPR patterns showed that high performance catalyst was characterized by a low main H2 consumption peak temperature.(4) From the product selectivities of DBT HDS, it is indicated that hydrogenolysis, to yield biphenyl(BP), was the predominate reaction path in HDS catalyzed by Co-Mo/MCM-41 whereas both hydrogenolysis path and hydrogenation path played important role in HDS catalyzed by Ni-Mo/MCM-41. It was found that cyclohexylbenzene (CHB), which is the product of hydrogenation path, was not stable at elevated temperatures, decomposing to form benzene and cyclohexane.(5) The effect of basic nitrogen-containding compound (such as piperidine) on HDS performance was also investigated. It is shown that the presence of a little amount of piperidine in the feed led to dramatic decrease of HDS activities on both Ni-Mo/MCM-41 and Co-Mo/MCM-41. Analyses of the product selectivities revealed that piperidine depressed HDS react...