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Study on sulphidation degree and morphology of MoS2catalyst derived from various molybdate precursors

The MoS2 catalysts were prepared from various molybdate precursors including inorganic and organic molybdate compounds.

Zhang Le, Li Mingfeng and Nie Hong
Research Institute of Petroleum Processing, SINOPEC
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Article Summary
The sulphidisation degree and morphology of active phases of MoS2 activated by various molybdate precursors in H2S/H2 stream at different temperatures were studied by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The organic molybdate precursors lead to MoS2 catalysts with higher sulphidisation degree and smaller active phases to demonstrate higher catalytic activity during hydrodesulphurisation (HDS) of 4,6-DMDBT.

Environmental concerns lead to increasingly tightening regulations on sulphur, nitrogen and aromatics content in fuels1. Conversion of these compounds is therefore of paramount importance, and such an objective needs the development of more active catalysts. Molybdenum sulphide materials have emerged as a class of promising catalysts for hydrotreating reactions.

Many researches have dealt with the activation treatment by means of gas phase (H2/H2S) and liquid phase (DMDS) activation of catalysts, with the effects on morphological and catalytic properties of treated catalysts properly reported2-3. Especially, the decomposition of thiosalts has been widely used in preparing molybdenum or tungsten disulphide with high surface area4-5. L. Alvarez6 reported that the method of activation (in-situ or ex-situ) of tetraalkylammonium thiomolybdates and the nature of the thiosalt precursor (with or without C) can influence strongly the textural and catalytic properties of the final MoS2 and Co/MoS2 catalysts. The use of a tetraalkylammonium thiomolybdate precursor (with C) reduces significantly the formation of a MoS2-like intermediate and can lead to a final meso-structure of MoS2. H. Nava and co-workers7 prepared unsupported nickel-molybdenum-tungsten sulphide catalysts from tri-metallic NiMoW alkyl precursors with tetraalkylammonium thiomolibdotungstates salts, (R4N)4MoWS8 (where R=H, methyl, propyl, butyl, or cetyl-trimethyl). The nature of the alkyl group can strongly affect both the specific area and the HDS activity. The catalytic activity is strongly enhanced when the carbon-containing precursors are used. So the effect of molybdate precursors (with or without C) was warmly discussed with respect to their influence on the structure, morphology and activity of MoS2 catalysts. However, this investigation is mainly aimed at thiosalt precursor and sulphidisation degree, and the morphology of active phases of MoS2 activated by various molybdate precursors at different temperatures is less studied. In the present work, the activation law, the difference in morphology, and performance of MoS2 activated by various molybdate precursors have been studied and the effect of carbon contained in the molybdate precursors has been discussed.

Catalyst preparation

MoS2 catalysts were prepared from five different molybdates precursors listed in Table 1. The molybdates precursors Mo-1, Mo-2 and Mo-4 are chemical reagents. The molybdate precursor Mo-3 was prepared by heating a solution of citric acid and molybdenum trioxide (at a molar ratio of 1:1). The molybdate precursor Mo-5 was obtained by the following experiment. Under heating and stirring the ammonium heptamolybdate tetrahydrate solution was added to a solution of hexadecyl trimethyl ammonium bromide (CTAB) prior to being refluxed at 373 K for 4 h. The white precipitate formed thereby was isolated by filtration, washed with water and dried at 393 K for 3 h to obtain Mo-5. The carbon content in the molybdates precursors Mo-3 and Mo-5 was analysed by a carbon-sulphur analyser. Then the molybdate precursors are all sulphided in a flow of 15% H2S/H2 mixture at 473 K and 573 K, respectively, for 4 h to produce the MoS2 catalysts (Table 2).

The MoS2 catalysts were characterised by X-ray photoelectron spectroscopy (XPS) and high-resolution transmission electron microscopy (HRTEM). The XPS experiments were performed in a VG Scientific ESCALab 220i- XL spectrometer, with source of X-rays, Al Ka (1486.6 eV) anode and 300 W of power. HRTEM was carried out on a TECNAI F20 G2 apparatus, made by the FEI Company with a resolution of 0.24 nm.

The hydrodesulphurisation (HDS) of 4,6-dimethyldibenzothiophene (4,6-DMDBT) was carried out in a fixed-bed micro-reactor made by the American Autoclave Engineers Company. The molybdate precursors were in-situ sulphided with a solution of 5% CS2 in cyclohexane at a flow rate of 0.4 mL/min at 360°C, 4.14 MPa of H2 pressure and a H2 flow rate of 400 mL/min for 3 h. Then the reactor was switched to treatment of the reactant feed (0.45% of 4,6-DMDBT in decane) at a flow rate of 0.2 mL/min in the same hydrogen atmosphere. The reaction products were analysed by the on-line GC-FID directly. The conversion of 4,6-DMDBT was calculated using the internal standard method. The HDS activities were calculated using the following equation: Total HDS activity: AHDS=F0×conversion/m where F0 is the molar flow rate of the reactant (mol/s) and m is the mass of the catalyst (g).

Results and Discussion
The sulphidation laws of the catalysts

The XPS spectra of the MoS2 catalysts were collected. Table 6 gives the binding energies of S2p and Mo3d derived from decomposition of the XPS spectra of MoS2 catalysts. The XPS spectra of S2p on the MoS2 catalysts exhibit only one peak at about 162.2 eV, which corresponds to S2-8-9. The absence of any signal at 169.0 eV after sulphidisation indicates that no oxidation of the catalysts occurs during the transfer of the solid from the sulphidising reactor to the XPS spectrometer. In the Mo3d spectra, the peaks are attributed to Mo4+ species (229 eV and 232 eV for the 3d5/2 and 3d3/2, respectively) and Mo6+ species (233 eV and 236 eV)10.

The sulphidisation degree of surface species has been calculated based on the area of the XPS peaks of the various species as shown in Table 3. The peaks at around229 eV and 232 eV are attributed to Mo4+ species and can be used to calculate the sulphidisation degree of surface Mo species.

There are great difference in the sulphidisation degree for the molybdate precursors at a sulphidisation temperature of 473 K. The sulphidisation degree of inorganic molybdate precursors (Mo-1 and Mo-2) dips to as low as 50%—60% upon activation at 473 K. The organic molybdate precursors (Mo-3, Mo-4 and Mo-5) have much higher sulphidisation degree than inorganic molybdate precursors, and especially Mo-4 and Mo-5 are almost totally sulphide upon activation at 473 K. Interestingly, the sulphidisation degree increases with the increase in carbon content of the molybdate precursors (Figure 1).

Upon sulphidisation at 573 K, the sulphidisation degree of catalysts activated by different molybdate precursors shows no large difference. The sulphidisation degree of catalysts activated by organic molybdate precursors is still higher than that of catalysts activated by inorganic molybdate precursors. The sulphidisation degree of catalysts activated by inorganic molybdate precursors reach up to 82% under this condition, and the catalysts are almost totally sulphided by organic molybdate precursors.

Morphology of active phases of MoS2 catalysts
Figure 2 presents the HRTEM photographs of the sulphided MoS2 catalysts prepared from various molybdate precursors upon activation at different temperatures.

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