Molybdenum sulfide preparation

Of the many molybdenum sulfides which have been reported, only MoS, M0S2 and M02S3 are well established. A hydrated form of the trisulfide of somewhat variable composition is precipitated from aqueous molybdate solutions by H2S in classical analytical separations of molybdenum, but it is best prepared by thermal decomposition of the thiomolybdate, (NH4)2MoS4. MoS is formed by heating the calculated amounts of Mo and S in an evacuated tube. The black M0S2, however, is the most stable sulfide and, besides being the principal ore of Mo,... 

A cationic molybdenum sulfide cluster [Mo3S4(H20)9] " with incomplete cubane-type structure and a cationic nickel-molybdenum mixed sulfide cluster [Mo3NiS4Cl(H20)9p " with complete cubane-type structure were introduced into zeolites NaY, HUSY and KL by ion exchange. Stoichiometry of the ion exchange was well established by elemental analyses. The UV-visible spectra and EXAFS analysis data exhibited that the structure of the molybdenum cluster remained virtually intact after ion exchange. MoNi/NaY catalyst prepared using the molybdenum-nickel sulfide cluster was found to be active and selective for benzothiophene hydrodesulfurization. 

Another SIMS study on model systems concerns molybdenum sulfide catalysts. The removal of sulfur from heavy oil fractions is carried out over molybdenum catalysts promoted with cobalt or nickel, in processes called hydrodesulfurization (HDS) [17]. Catalysts are prepared in the oxidic state but have to be sulfided in a mixture of H2S and H2 in order to be active. SIMS sensitively reveals the conversion of Mo03 into MoSi, in model systems of MoCf supported on a thin layer of Si02 [21]. 

Chemical deoxygenation of sulfoxides to sulfides was carried out by refluxing in aqueous-alcoholic solutions with stannous chloride (yields 62-93%) [186 Procedure 36, p. 214), with titanium trichloride (yields 68-91%) [203], by treatment at room temperature with molybdenum trichloride (prepared by reduction of molybdenyl chloride M0OCI3 with zinc dust in tetrahydrofuran) (yields 78-91%) [216], by heating with vanadium dichloride in aqueous tetrahydrofuran at 100° (yields 74-88%) [216], and by refluxing in aqueous methanol with chromium dichloride (yield 24%) [190], A very impressive method is the conversion of dialkyl and diaryl sulfoxides to sulfides by treatment in acetone solutions for a few minutes with 2.4 equivalents of sodium iodide and 1.2-2.6 equivalents of trifluoroacetic anhydride (isolated yields 90-98%) [655]. 

Hariita et al. (14) prepared spherical particles of molybdenum sulfide and cobalt sulfide with a narrow size distribution by reaction of dilute ammonium orthomolybdate or cobalt(II) acetate with sulfide ions liberated from thioacetamide as a reservoir of S2- ions in weakly acidic media. The compositions of these metal sulfides were estimated to be Mo S 0 = 1.0 1.7 3.0 and Co S 0 = 1,0 4.5 6.4 by chemical analysis. Figure 3.1.4 shows an SEM of a thus prepared uniform molybdenum sulfide particles sample. These sulfide particles were of no distinct crystal structure as shown by x-ray diffractometry. The isoelectric points of the Mo sulfide and Co sulfide particles in terms of pH were 1.9 and 3.1, respectively. Both of them are useful as hydrodesulfurization catalysts. 

Work was carried out in this laboratory to reproduce these reactions7. Attempts to prepare Mo02S did not yield a simple molybdenum oxysulfide or a molybdenum sulfide, but a mixture of unidentifiable products. The reaction of MoS2 with water vapor yielded neither hydrogen nor hydrogen sulfide. The thermal decomposition of... 

In connection with the research on destructive hydrogenation at the Institute of High Pressures, Maslyanskii (224) passed benzene at 475° under 200 atm. hydrogen over molybdenum oxide (1 mole CeH6 16 moles Ha) to produce 58% methylcyclopentane, 14% cyclohexane, 8% 2-methyl-pentane, 5% n-hexane, and 8% unreacted. Over molybdenum sulfide the product distribution was similar. The preparation of these catalysts was described by him in 1940 (223). Isomerization and other conversions accompanying destructive hydrogenation were also pointed out by Prokopets and by others (257,311,314). 

Molybdenum sulfide nanoparticles were prepared by adding via a syringe 6.9 mL of a 0.005 M ammonium tetrathiomolybdate (ATTM) solution to a 0.1 M AOT/ n-heptane/ 0.5 M sulfuric acid microemulsion system with a water-to-surfactant molar ratio w of 20. It should be noted that the water-to-surfactant molar ratio accounts for both ATTM and sulfuric acid solution. The reaction can be schematically represented as follows ... 

Cobalt-promoted molybdenum sulfides supported on alumina are widely used in petroleum refining for sulfur removal [1]. However, there is now increasing necessity to further improve their performance (activity, selectivity, and stability), due to more and more stringent legislation on sulfur contents in transportation fuels [2]. It is thus primordial to understand the detailed structure and catalytic behavior of these catalysts. In this respect, controlled preparation of catalysts with desired composition and structure has great importance in permitting fundamental study of structure-performance correlation. 

Studies performed in previous years (ref.3-5) over molybdenum loaded zeolites in HDS reaction showed that reactivity of these catalysts depends on the type of zeolite used, concentration of transition metal and the way of preparation. However, preparation of molybdenum based catalysts applying ammonium heptamolybdate usually results in low dispersion of molybdenum sulfides and relatively low activity. Recent studies showed that saturation of Y-zeolites with molybdenum carbonyl can produce catalysts with high molybdenum dispersion (ref.6, 7). Subsequent sulfidation of these catalysts leads toward highly dispersed, supported sulfided molybdenum species (ref. 8,9) exhibiting high reactivity in HDS and water-gas shift reaction (ref. 9,10). 

Farlow and Signaigo196 describe the preparation of a molybdenum sulfide catalyst from a molybdenum-aluminum alloy ... 

Structure Control of Molybdenum Sulfide Clusters Encaged in Zeolite Prepared by a CVD Technique... 

Intrazeolite molybdenum sulfide catalysts (MoSx/NaY) were prepared by a CVD technique using Mo(CO)6. On the basis of the Mo-Mo atomic distance, it has been shown that a MoSs-like molybdenum sulfide dimer clusters are formed when the Si/Al ratio is between 3.5 and 2.4. When the Si/Al ratio was very high, finely dispersed M0S2 particles were formed. The specific activity of molybdenum sulfide clusters for the HDS of thiophene decreased as the Mo-Mo atomic distance increased. It has been substantiated that the morphology, structure, and catalytic activity of intrazeolite molybdenum sulfides are controlled by the host zeolite composition. 

Molybdenum sulfide-based catalysts have been used for hydrodesulfurization (HDS) [6]. The preparation of highly dispersed molybdenum sulfides is one of the ways to increase HDS activity. Zeolite-supported molybdenum sulfide catalysts have received extensive attention because of intrinsically high dispersion of Mo on the high surface area support [4,5]. Furthermore, a combination of highly dispersed metal sulfide and zeolite acidity... 

Hexacarbonyl molybdenum Mo(CO)6 was successfully used to prepare intrazeolite molybdenum sulfide clusters in the cavities of NaY (CVD technique) [4,5,7,8]. The decomposition and sulfidation of Mo(CO)e encaged in NaY were extensively studied by Okamoto et al. [7-11] by means of temperature programmed decomposition (TPDE), XPS, and XAFS techniques. It has been claimed that the structure of molybdenum sulfides is described as molybdenum dinuclear sulfide clusters M02S4. de Bont et al. [12] supported the formation of molybdenum sulfide dimer species. The extremely high dispersion of molybdenum sulfide clusters prepared fi"om Mo(CO)6 was also suggested by an NO adsorption capacity much hi er than those of other conventional catalyst systems such as M0S2/AI2O3 [9]. 

In this present study, we prepared molybdenum sulfide clusters encaged in a series of NaY zeolites with different Si/Al ratios. It is shown here that the local structure of molybdenum sulfide clusters and their intrinsic activity are finely controlled by the zeolite composition. 

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