| In order to satisfy the increasingly stringent regulations on the sulfur content of diesel oil, deep desulfurization of diesel oil becomes more and more important. Ultra low sulfur diesel having sulfur content lower than10μg.g-1can be produced under routine conditions using unsupported hydrogenation catalysts. Therefore, the preparation of this type of catalyst has garnered particular attention. In this paper, a series of unsupported bimetal and trimetal sulfide catalysts were prepared by the method of decomposition of tetraalkylammonium thiosalts. The pyrolysis process from precursors to catalysts was explored, as well as the effect of nickel content in unsupported sulfides on catalyst properties for desulfurization of diesel oil.The main contents are as follows:First, the aqueous solution method was used to successfully prepare a variety of different precursors at ambient temperature and pressure. These include ammonium thiomolybdate, ammonium thiotungstate, tetramethylammonium thiomolybdate, tetramethylammonium thiotungstate, tetraethylammonium thiomolybdate, tetraethylammonium thiotungstate, tetrabutylammonium thiomolybdate, tetrabutylammonium thiotungstate, cetyltrimethylammonium thiomolybdate and cetyltrimethylammonium thiotungstate, ammonium thiomolybdotungstate and tetramethylammonium thiomolybdotungstate.Second, the thermal decomposition of ammonium and tetramethylammonium thiosalts of molybdenum and tungsten in inert atmosphere were investigated as a function of temperature using thermogravimetric analysis (TGA), temperature-programmed decomposition with mass spectroscopy (TPD-MS), in-situ Fourier transform infrared (FTIR) and Raman spectroscopies, and X-ray photoelectron spectroscopy (XPS). The results allow for correlations to be made between the changes in the bulk and surface structures of the materials, and the evolution of gas-phase decomposition products. For both sets of precursors, XPS results show that the surface of the resulting materials at various treatment temperatures does not track directly with the state of the bulk material. While the ATM, TMATM, and TMATT-derived material surfaces are reduced to the+4oxidation state at the highest temperature, indicating disulfides, the ATT-derived materials still retained a significant amount of+6oxidation state consistent with the starting precursor.Third, a series of unsupported nickel-promoted MoS2catalysts was prepared by ex-situ decomposition of tetramethylammonium thiosalts. The XRD patterns of the samples containing different content of nickel show characteristic crystalline structure of MoS2. When nickel concentration R (R=Ni/(Ni+Mo)) increases from0.1to0.5, the intensity of the (002) crystal plane signal gradually decreases, which demonstrates a marked stacking diminution of the catalyst particle slabs. The specific surface area (SSA) of the samples decreased from60to21m2/g with increasing the amount of Ni promoter added to the catalyst. TEM images of MoS2and NixMoS2presented typical MoS2fringes. The stacking degree is estimated to be2-7slabs and the slab length of fringes reaches values up to8nm. Nickel sulfide was not found in the samples as determined by both TEM and XRD. Within the layers, the amount of nickel was close to the value determined by EDS and homogeneously distributed, suggesting the formation of a "Ni-Mo-S" phase. The catalytic activity and selectivity were tested by hydrodesulfurization (HDS) of dibenzothiophene (DBT).The results revealed that the addition of nickel had an evident positive impact on both the HDS of DBT activity and the selectivity towards the direct desulfurization (DDS) pathway. Moreover, with the increase of nickel content, the hydrogenation(HYD)/DDS ratios have a decreasing trend while the catalyst activity increases first before decreasing. Ni0.1-MoS2showed the best activity for HDS of DBT.Next, a family of unsupported NiMoW sulfides with different ratios were prepared by thermal decomposition of tetramethylammonium thiomolybdotungstate and Ni(NO3)2. The results show that this method is effective for preparing superior catalysts, and that the structure and properties of catalysts were determined by their nickel content and the decomposition conditions. Except for Ni1.5MoWSy, all the catalysts show type IV adsorption isotherms that are characteristic of a mesoporous structure with a sponge-like cave. The pore size distribution of the MoWSy was more narrow, with an average pore diameter of about16nm. The Ni0.5MoWSy pore size distribution was in the range of6-18nm, while the NiMoWSy pore size distribution was in the range of5-14nm. Compared with Ni0.5MoWSy and NiMoWSy, Ni1.5MoWSy has no clear pore size distribution curve and its structure shows a state of disorder. MoWSy has the largest SSA (130m2/g) compared with NixMoWSy (x=0.5,1,1.5). HRTEM images show the lattice fringes of MoS2and WS2, revealing2-8slab layers with the length of each layer up to10nm. The surface area normailized activity data shows that the low nickel content in MoWSy has a positive effect on both HDS of DBT and the selectivity of the HYD pathway. The Ni0.5MoWSy catalyst has the highest HDS of DBT activity compared with the other catalysts.Finally, a kinetics study of HDS of DBT on MoS2or Ni0.15MoS2was performed, with especially investigating the competitive effect of anthracene hydrogenation on HDS. The results showed that the HDS of DBT is in accordance consistent with a first order dependence on DBT. Anthracene hydrogenation impedes or inhibits the HDS of DBT. On the MoS2catalyst, HDS of DBT activation energies are91kJ/mol and105kJ/mol, respectively, before and after anthracene was added to the reaction solution. On the Ni0.15MoS2catalyst, HDS of DBT activation energies are59kJ/mol and83kJ/mol, respectively, before and after anthracene was added to the reaction solution.The selectivity of antheracene hydrogenationis shifted towards unsaturated products after adding dibenzothiophene into the reaction solution. For example, the selectivity of9,10-dihydroanthracene was increased. |
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