M. Del Valle

Preparation of MoS2 and WS2 catalysts by in situ decomposition of ammonium thiosalts

The in situ decomposition of ammonium thiometallates during the hydrodesulfurization (HDS) of dibenzothiophene (DBT), to obtain molybdenum disulfide and tungsten disulfide catalysts, was investigated. It was found that very efficient catalysts for the HDS of DBT were obtained by in situ decomposition. Mechanical uniaxial pressing of the precursors (ammonium thiometallates) affected both textural and catalytic properties of the catalysts. Surface areas of molybdenum and tungsten disulfides increased as a function of uniaxial pressing, while catalytic activities went through a maximum. For MoS2, the hydrogenation selectivity was much higher for in situ catalysts than for ex situ ones. For WS2 catalysts, the hydrogenation selectivity was less sensitive to the condition of decomposition (ex situ/in situ). The surface S/M (M = Mo, W) atomic ratio from the Auger signal decreased as a function of uniaxial pressing, while the C/M ratio remained almost constant at 1.6. The best catalyst showed an experimental S/Mo ratio that is slightly higher than the stoichiometric value. The effect of in situ decomposition and mechanical deformation of thiometallate precursors is discussed.

Preparation of WS2 catalysts by in situ decomposition of tetraalkylammonium thiotungstates

Tungsten tetraalkylammonium thiosalts are used as precursors for the in situ formation of WS2 catalysts in dibenzothiophene (DBT) hydrodesulfurization. The thermal decomposition of alkyl-ammonium thiosalts proceeds directly to WS2 without WS3 formation, as in the case of ammonium thiotungstate (ATT), allowing good control of the catalyst’s stoichiometry. The alkyl-ammonium thiosalts give WS2 particles with different characteristic morphologies. The hydrodesulfurization (HDS) activities of WS2 catalysts derived from alkylthiosalts are higher than those of catalysts derived from the ammonium thiosalt. The reaction rate increases with the size of the cation in the precursor. No correlation of catalytic activities with surface areas is found. The S/W and C/W surface ratios determined by Auger electron spectroscopy decrease with increasing cation size. Surface composition is WS2:25C1:7 ,W S 1:7 C0:9 and WS1:3 C0:7 for the in situ catalysts derived from ammonium, methylammonium, and butylammonium precursors, respectively. The improved catalytic properties of WS2 catalysts derived from alkylammonium thiosalts in the HDS of DBT are attributed to the formation of carbon-containing tungsten sulfide phases on the catalyst’s surface.

Hydrodesulfurization activity of MoS2 catalysts modified by chemical exfoliation

The surface area of unsupported MoS2 catalysts prepared by thiosalt decomposition is found to increase after undergoing a treatment known as chemical exfoliation. Rate measurements of dibenzothiophene hydrodesulfurization in a batch reactor show that activity decreases for the chemically modified MoS2 catalysts, along with the hydrogenation/hydrodesulfurization ratios (HYD/HDS). These results indicate that both basal and edge planes of the layered sulfides are rearranged by the exfoliation treatment, but that other processes must also be involved. Reference crystalline MoS2 is also discussed in the work.

Catalytic properties of WS2 catalysts prepared by in situ decomposition of tetraalkyl-ammonium thiotungstates

Abstract Tungsten disulfide unsupported catalysts obtained by in situ decomposition of tetramethyl- and tetrabutylammonium thiosalts (TMATT and TBATT) presented better hydrodesulfurization performance than catalysts derived from the ammonium thiosalt (ATT). The reaction rate increased with the size of alkyl group in the precursor, however, no correlation of activity with surface area was observed. Auger analysis revealed that the surface concentration of sulfur and carbon varied with the precursor. The improved performance of WS2 catalysts derived from alkylammonium thiosalts in the HDS of DBT is attributed to the formation of tungsten carbide-sulfide species on the surface.

A New Route for the Synthesis of Ammonium Thiotungstate, a Catalyst Precursor

Ammonium thiotungstate (ATT), (NH4)2WS4, is prepared by a new method using (NH4)2S, and compared with ATT obtained by conventional sulfidation with H2S(g). The XRD spectra of both samples are very similar to the simulated spectrum of ATT. UV–Vis and FT-IR spectra of both samples have characteristic bands for ATT. SEM micrographs show a similar morphology between the samples. TGA-DTA curves of both samples are the same, corresponding to the decomposition of ATT. Analytical results thus show that the ATT obtained by this new, safer, method is the same as the ATT obtained by sulfidation with poisonous H2S(g). The HDS activity of the WS2 catalyst derived from ATT prepared by the new method described here is equivalent to that of ATT prepared by the conventional method.Graphical Abstract