Hydrodesulfurization And Hydrodenitrogenation Mechanisms On Supported NiMoS Catalyst

The oxidized metal components in the conventional supported Co（Ni）-Mo（W） catalysts can not be fully sulfided because of the strong metal-support interaction generated in the calcination process at high temperature. Prefabricated sulfide NiMoS/γ-Al₂O₃ catalyst has been prepared successfully using tetrathiomolybdate as precursor in our previous research. The hydrodesulfurization （HDS） and hydrodenitrogenation （HDN） activities of NiMoS/γ-Al₂O₃ were higher than that of commercial catalysts. The preparation methods and pretreatment conditions of NiMoS/γ-Al₂O₃ have been thoroughly investigated, but the formation mechanism of catalytic active phases and the role of nickel and phosphorus in this kind of catalyst have not been studied systemically up to now.In this dissertation, MoS₂/γ-Al₂O₃ （MA） and MoO3/γ-Al₂O₃ （MO） catalysts were prepared using ATTM and ammonium paramolybdate as precursors, respectively. The differences of activity and selectivity between two kinds of catalysts were interpreted on the basis of the structure characterization of catalysts and their hydrotreating performances for model compounds. The effects of nickel on MoS₂/γ-Al₂O₃ and phosphorus on NiMoS/γ-Al₂O₃ were studied and their promotion mechanisms were put forward, respectively. The mutual influence of hydrodenitrogenation and hydrodesulfurization were also studied, and the results can present a theoretical basis for the development of higher efficiency catalysts. The results demonstrate that the morphology of MoS₂ on the catalyst surface is related to both metal loading and the property of precursor. There exists obvious difference in the bonding type of MoS₂ clusters on the support for MA catalysts and MO catalysts. The basal-bonding MoS₂ clusters with the larger size are dominated on MA catalysts, while the edge-bonding MoS₂ clusters with higher dispersion are dominated on MO catalysts. It is found that the hydrogenation selectivity for the HDS of dibenzothiophene （DBT） is closely related to the microstructure of MoS₂ clusters on the catalysts. MA catalyst shows inferior activity to MO catalyst at lower temperatures, and superior activity at higher temperatures for the HDS of DBT due to the larger activation energy on the former than that on the later.It is also found that nickel and phosphorus inhibit the growth of MoS₂ crystalline along the basal plane （002） in calcination and H₂ activation processes. Ni-Mo-S active phase is formed when Ni atoms decorate the edge sites of MoS₂ clusters. The nickel promotes the reduction in the strength of the metal–sulfur bonding energy for the Ni–Mo–S structure as compared to bulk MoS₂ crystallines. Moreover, another role of the nickel promoter is to facilitate the formation of sulfur vacancies at lower temperatures. Phosphorus has no influence on the metal-sulfur bonding energies in the Ni-Mo-S phases and bulk MoS₂ crystallines, but it increases the proportion of Ni-Mo-S phases in the metal sulfides. Nickel promotes both the hydrogenation （HYD） and desulfurization （DDS） activities for HDS of DBT, but the promotion on the latter is more notable. This effect can be attributed to an enhancement of the basicity of certain sulfur anions promoted by nickel, which is beneficial for the cleavage of C-S bonds in the dihydrointermediate through aβ-elimination process. Nickel mainly improves the hydrogenation of phenyl ring in the HDN of aniline, but it is detrimental to the cleavage of C（sp²ï¼‰-N bonds. Phosphorus shows a promotive effct on the hydrogenation of phenyl ring only in the presence of H₂S. The rupture of C（sp²ï¼‰-N bonds is accelerated by the addition of phosphorus.The results indicated that DBT and H₂S increase the HDN percent of quinoline and conversion of indole. In the absence of sulfides, the poor denitrification capability of quinoline is caused by the low cleavage rate of C（sp³ï¼‰-N bonds in decahydroquinoline and 1,2,3,4-tetrahydroquinoline molecules, while the low HDN percent of indole results from the poor hydrogenation reactivity of pyrrole ring. In the presence of sulfides, the reactions of 1,2,3,4-tetrahydroquinoline→o-propylaniline and indoline→o-ethylanline are improved, the reactivities of alkyl-anilines are still poor. Increase in the rupture rate of C（sp³ï¼‰-N bonds in the presence of H₂S is offset by the inhibition on the hydrogenation of benzene ring in quinoline and indole molecules. The results suggest that both the higher activity for rupture of C（sp³ï¼‰-N bonds and that for hydrogenation of benzene ring is required for the HDN reaction.It is identified from experimental and density functional theory （DFT） calculation results that both the hydrogenation （HYD） and hydrogenolysis （DDS） path ways are inhibited by N-containing compounds, but the suppression on the former is stronger. Moreover, both the parent molecules and their N-containing intermediates play an important role in this aspect. The stronger inhibition on HDS of DBT by quinoline than indole is mainly from the influence on DDS route. This is relevant to the hydrogenation abilities of parent molecules and the relative concentration of N-containing intermediates.