Vanadium sulfide, structure

A number of vanadium sulfide phases, ranging from VS4 to V3S, have been reported, although few have been well characterized and some may not exist at all. The structures, where known, are often complicated and difficult to describe. 

A compound identified as iron vanadium sulfide (JCPDS No. 31-657) was found ta every sample of the material submitted for analysis. Tbis compound has the stoichiometre formula (V, where x represents a small deviation from ideality resulting from structural defects. It is bcKcved that the Fc in this structure could be replaced by Ni without changing the difTraction pattern, providing a means for the incorporation of all three deposited metals together in one compound. 

With HREM, vanadium deposits were found in a layered structure both in the surrounding of the active phase, causing active site poisoning, and as isolated vanadium sulfide clusters, likely causing active site generation. In our vanadium deposition experiments (up to 9 wt.% vanadium) no significant catalyst deactivation occurred, indicating that both effects are compensated. 

Figure 6 shows a typical view of the spent Mo/SiOj catalyst with the layered structure of vanadium sulfide. According to EDX, vanadium sulfide is present on silica. The copper peaks in the EDX spectrum are due to the grid which supports the catalyst powder in the HREM. 

The class of layered transition metal dichalcogenides has been of great interest because of their varied electronic properties and chemical reactions. Most compounds of this class may be prepared by stoichiometric reactions of the elements above 500°. However, the highest vanadium sulfide that can be made in this manner is VjSg. An amorphous VS has been prepared by the metathetical reaction of LijS and VC. The method presented here allows preparation of polycrystalline VSj with the Cdiz structure. ... 

The most important undesired metallic impurities are nickel and vanadium, present in porphyrinic structures that originate from plants and are predominantly found in the heavy residues. In addition, iron may be present due to corrosion in storage tanks. These metals deposit on catalysts and give rise to enhanced carbon deposition (nickel in particular). Vanadium has a deleterious effect on the lattice structure of zeolites used in fluid catalytic cracking. A host of other elements may also be present. Hydrodemetallization is strictly speaking not a catalytic process, because the metallic elements remain in the form of sulfides on the catalyst. Decomposition of the porphyrinic structures is a relatively rapid reaction and as a result it occurs mainly in the front end of the catalyst bed, and at the outside of the catalyst particles. 

Vanadium trichloride or tribromide reacts with thioethers giving [VX3L2] (X = C1 or Br L = SMe2, tetrahydrothiophene or SEt2).288,289 The complex with di-n-propyl sulfide could not be isolated.288 These compounds are oxidized very easily. Solubility, molecular weights and conductance show that they are monomeric and non-ionic. Dipole moments, IR and electronic spectra are consistent with trans trigonal bipyramidal structures. 

TMS catalysts fell into a special category due to their exceptional resistance to poisons. In fact, the presence of sulfur compounds, the most common poison of metallic and oxide catalysts, does not decrease their catalytic activity, but is needed to maintain high activity. Sulfide catalysts are also very resistant to carbon deposition, which is illustrated by their use for converting residual oils. Arsenic, as well as nickel and vanadium contained in heavy petroleum fractions, are some of the few substances that cause significant deactivation, and this only occurs by physical blockage of pore structure in supported catalysts. 

Bolm and Bienewald discovered in 1995 that some chiral vanadium (IV)-Schiff base complexes were efficient catalysts (1 mol %) for sulfoxidation [71a]. The catalyst 20 was prepared in situ by reacting VO(acac)2 with the Schiff base of a fJ-aminoalcohol (Scheme 6C.8). Reactions were conveniently performed in air at room temperature by slow addition of 1.1 mol equiv. of aqueous hydrogen peroxide (30%). Under these experimental conditions the reaction of methyl phenyl sulfide gave the corresponding sulfoxide in 94% yield and 70% ee. The best enantioselectivity was obtained in the formation of sulfoxide 21 (85% ee). Many structural analogues of catalyst 20 were screened for their efficacy, but none of... 

This article is focused on HDN, the removal of nitrogen from compounds in oil fractions. Hydrodemetallization, the removal of nickel and vanadium, is not discussed, and HDS is discussed only as it is relevant to HDN. Section II is a discussion of HDN on sulfidic catalysts the emphasis is on the mechanisms of HDN and how nitrogen can be removed from specific molecules with the aid of sulfidic catalysts. Before the discussion of these mechanisms, Section II.A provides a brief description of the synthesis of the catalyst from the oxidic to the sulfidic form, followed by current ideas about the structure of the final, sulfidic catalyst and the catalytic sites. All this information is presented with the aim of improving our understanding of the catalytic mechanisms. Section II.B includes a discussion of HDN mechanisms on sulfidic catalysts to explain the reactions that take place in today s industrial HDN processes. Section II.C is a review of the role of phosphate and fluorine additives and current thinking about how they improve catalytic activity. Section II.D presents other possibilities for increasing the activity of the catalyst, such as by means of other transition-metal sulfides and the use of supports other than alumina. 

All heavy crude oil residues have heavy metals such as Ni, V or Fe in their structure. These metals are bonded as organometalic compounds. At high temperatures and for hydrogenation reactions, these compounds are cracked and heavy metals are deposited on the catalyst surface. These metals can also react with hydrogen sulfur from the gas phase to form metal sulfides. The deposition of sulfides of iron, vanadium or nickel leads to irreversible poisoning of the catalyst. This is the difference between catalyst deactivation by metals and deactivation by coke the former leads to an irreversible loss of the catalyst activity. 

Structurally, asphaltene contains flat sheets of condensed aromatic systems that may be interconnected by sulfide, ether, aliphatic chains or naphthenic ring linkages. Gaps and holes appear as defect centers in the aromatic systems with heterocyclic atoms coordinated to transition metals such as vanadium and nickel, most likely caused by free radicals. Due to the complexity and the large size of asphaltene molecules, asphaltene particles conveniently faU within the colloidal range. The stmcture of asphaltene has been determined previously by the x-ray diffraction method and is shown as Figure 2. 

We have taken catalytic hydrodemetallation (HDM) of crude oil for optimization of the pore structure of catalyst supports. Crude oil contains small quantities of nickel and vanadium in the form of porphyrins. During the demetallation process the metals are deposited on the catalyst in the form of sulfides. These sulfides cause irreversible fouling. In order to describe this deactivation, a realistic model of pore structure has to be employed. Therefore, we have used a three-dimensional network with a maximum connectivity of 18 (see Fig. 1). As in practice the connectivities are less than ten, we have used average connectivities 3, 6 and 9 in our calculations. 

The vanadium trioxide (V2O3) and vanadium tetraoxide (V2O4) compounds are typically associated with approximate unbumed carbon levels in the flyash 10 percent. This is consistent with the porphyrin structure of the vanadium in the crude oil, and in the petroleum coke. Nickel can occur as a sulfide, an oxide, or in a vanadate (e.g., nickel pyrovanadate) depending upon combustion condition. For example, r ucing conditions promote formation of sulfide compounds. Some of the conq)ounds shown (e.g., the sodium vanadate compounds) are nK>re commonly associated with oil firing than with petroleum coke firing, as a consequence of the low ash content in petroleum coke and the low... 

Carbasilatranes are another variation of the silatrane structure. The carba-silatranel44 has been used as a precursor to fix the optically active ligand system via the incorporated silicon atom on silica gel or mesoporous silicas (Scheme 39) and to prepare vanadium(V) complexes therefrom, which act as functiraial models for the sulfide-peroxidase [301]. Several 3,7,10-trimethylsilatranes and carbasilatranes have been synthesized and investigated by NMR spectroscopy [302]. 

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