Hydrodesulfurization complexes

Interestingly, the Mo2S " 4 (Fig- 3f) core stmcture can be viewed as occupying six of the eight vertices of a distorted cube. Reaction of the dinuclear complexes having the Mo2S " 4 core with appropriate metal ions leads to the plaimed assembly of M2M02S4 thiocubane stmctures (19,20). When M = Co (Fig. 3h) the compounds are potential precursors for hydrodesulfurization catalysts (15). 

No. 6 fuel oil contains from 10 to 500 ppm vanadium and nickel in complex organic molecules, principally porphyrins. These cannot be removed economically, except incidentally during severe hydrodesulfurization (Amero, Silver, and Yanik, Hydrode.suljurized Residual Oils as Gas Turbine Fuels, ASME Pap. 75-WA/GT-8). Salt, sand, rust, and dirt may also be present, giving No. 6 a typical ash content of 0.01 to 0.5 percent by weight. 

It should be noted, however, that this reaction sequence may be different from what may actually be occurring in the reactor. The reactions proceed at different rates depending on the process variables. Hydrodesulfurization of complex sulfur compounds such as dibenzothiophene also occurs under these conditions. The desulfurized product may crack to give two benzene molecules ... 

These metals form chalcogenolate complexes in several oxidation states, and from the application-oriented point of view manganese compounds have been synthesized as models for hydrodesulfurization processes and rhenium and technetium derivatives as models for radiopharmaceuticals. 

Metal sulfides and polysulfides have been extensively studied because of their key role in important catalytic processes such as the hydrodesulfurization of crude oil or the biosynthesis of metalloproteins. The coordination chemistry of polysulfides85 86 has been comprehensively reviewed similar to that of the heavier polychalcogenides.10,12 15 Polysullido complexes are themselves reactive and their exothermic desulfurization can be exploited as a means of... 

Catalysts based on molybdenum disulfide, M0S2, and cobalt or nickel as promoters are used for the hydrodesulfurization (HDS) and hydrodenitrogenadon (HDN) of heavy oil fractions [48,49]. The catalyst, containing at least five elements (Mo, S, Co or Ni, as well as O and A1 or Si of the support), is rather complex and represents a real challenge for the spectroscopist. Nevertheless, owing largely to research in the last twenty years, the sulfided C0-M0/AI2O3 system is one of the few industrial catalysts for which we know the structure in almost atomic detail [49, 50],... 

It should also be noted that polysulfido complexes are not only interesting because of their structures and reactivity, but also because of their possible applications. They can, for example, be used to prepare sulfur rings of predetermined size and they are also suspected to play a role in catalysis (particularly in hydrodesulfurization). 

Electron-transfer and intramolecular redox reactions (related to 82 complexes). The redox behavior of 82 complexes is of particular interest because it can probably provide a foundation for understanding the course of reactions involved in relevant enzymes and catalysts (especially hydrodesulfurization catalysts). Intramolecular redox reactions related to type la 82 ligands can be summarized as follows ... 

Few studies have attempted to relate catalytic activity to catalyst parameters. Ueda and Todo (47, 105) have developed a complex correlation between hydrodesulfurization of thio-/3-naphthol and paramagnetic species present on the catalyst. Their correlation involves Mo3+, Co2+, and a surface complex containing an organic species. 

Catalytic hydrodesulfurization (HDS) is a very important industrial process that involves removal of sulfur from crude oils by high-temperature ( 400°C) treatment with hydrogen over Co- or Ni-promoted Mo or W catalysts supported on alumina. In an attempt to determine the mechanism of this process, many transition metal complexes of thiophene, a sulfur-containing heterocycle that is particularly difficult to desulfurize, have been prepared and their reactivities studied in order to compare their behavior with those of the free thiophenes that give H2S and C4 hydrocarbons under HDS conditions (88ACR387). Thiophene can conceivably bind to the catalyst surface by either cr-donation via a sulfur electron pair or through a variety of -coordination modes involving the aromatic system... 

J-Bonded metal complexes, hydrodesulfurization models with benzothiophene, 1, 769 with dibenzothiophene, 1, 769 Bonding studies energetics, 1, 285 overview, 1, 573—603 ring size effects, 1, 396 strength, 1, 609... 

Perylene, with tetrapalladium sandwich complex, 8, 345 PES, see Photoelectron spectroscopy PE spectroscopy, see Photoelectron spectroscopy Petroleum, and hydrodesulfurization and hydrodenitrogenation, 1, 760 (—)-PF1163B, via ring-closing metathesis, 11, 243 PFSs, see Polyferrocenylsilanes pH, role in aqueous media, 1, 828 Pharmaceuticals... 

Molybdenum oxide - alumina systems have been studied in detail (4-8). Several authors have pointed out that a molybdate surface layer is formed, due to an interaction between molybdenum oxide and the alumina support (9-11). Richardson (12) studied the structural form of cobalt in several oxidic cobalt-molybdenum-alumina catalysts. The presence of an active cobalt-molybdate complex was concluded from magnetic susceptibility measurements. Moreover cobalt aluminate and cobalt oxide were found. Only the active cobalt molybdate complex would contribute to the activity and be characterized by octahedrally coordinated cobalt. Lipsch and Schuit (10) studied a commercial oxidic hydrodesulfurization catalyst, containing 12 wt% M0O3 and 4 wt% CoO. They concluded that a cobalt aluminate phase was present and could not find indications for an active cobalt molybdate complex. Recent magnetic susceptibility studies of the same type of catalyst (13) confirmed the conclusion of Lipsch and Schuit. 

The data indicate the types of reactions that can occur during the hydrode-sulfurization reaction and include those reactions that will occur at the upper end of the temperature range of the hydrodesulfurization process whether it is a true hydrodesulfurization reaction or a cracking reaction. Even though some of the reactions given here may only be incidental, they must nevertheless be taken into account because of the complex nature of the feedstock. The several process variations (Chapter 9) which (in addition to the fact that the overall hydrodesulfurization process is exothermic (Table 4-2) also contribute to the complexity of the product mix. 

The structural differences between the various sulfur-containing molecules make it impractical to have a single rate expression applicable to all reactions in hydrodesulfurization. Each sulfur-containing molecule has its own hydrogenolysis kinetics that is usually complex because several successive equilibrium stages are involved and these are often controlled by internal diffusion limitations during refining. 

It is also generally accepted that the simpler sulfur compounds (e.g., thiols, R-SH, and sulfides, R-S-R1) are (unless steric influences offer resistance to the hydrodesulfurization) easier to remove from petroleum feedstocks than the more complex cyclic sulfur compounds such as the benzothiophenes). It should be noted here that, because of the nature of the reaction, steric influences would be anticipated to play a lesser role in the hydrocracking process. 

Residua hydrodesulfurization is considerably more complex than the hy-... 

Thus, the hydrodesulfurization process is a very complex sequence of reactions due, no doubt, to the complexity of the feedstock. Furthermore, the fact that feedstocks usually contain nitrogen and oxygen compounds (in addition to metal compounds) increases the complexity of the reactions that occur as part of the hydrodesulfurization process. The nitrogen compounds that may be present are typified by pyridine derivatives, quinoline derivatives, carbazole derivatives, indole derivatives, and pyrrole derivatives. Oxygen may be present as phenols (Ar-OH, where Ar is an aromatic moiety) and carboxylic acids (-C02H). The most common metals to occur in petroleum are nickel (Ni) and vanadium (V) (Reynolds, 1997). 

Because of their high molecular weight and complexity, the asphaltenes remain an unknown entity in the hydrodesulfurization process. There are indications that, with respect to some residua and heavy oils, removal of the asphaltenes prior to the hydrodesulfurization step brings out a several fold increase in the rate of hydrodesulfurization and that, with these particular residua (or heavy oils), the asphaltenes must actually inhibit hydrodesulfurization. As a result of the behavior of the asphaltenes, there have been several attempts to focus attention on the asphaltenes during hydrodesulfurization studies. The other fractions of a... 

Hydrodesulfurization, through the application of hydroprocesses is linked to the relevant environmental regulations as well as to the desired distribution of products. Achieving these goals is compounded when heavy oil or residua are the feedstock because of the molecular complexity of the constituents. 

The composition of the various feedstocks may, at first sight, seem to be of minor importance when the problem of the hydrodesulfurization of heavy oils and residua comes under consideration. However, consideration of the variation in process conditions that were outlined in the previous section (Table 6-6) for different feedstocks (where the feedstocks are relatively well-defined boiling fractions of petroleum) presents some indication of the problems that may be encountered where the feedstocks are less well defined. Molecular composition is as important as molecular weight (or boiling range). Such is the nature of the problem when dealing with various residua and heavy oils which are (to say the least) unknown in terms of their chemical composition. In fact, the complexity of these particular materials (Chapter 3) has allowed little more than speculation as to the molecular structure of the constituents. 

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