Hydrodesulfurization mechanisms

Characterization Using Spectroscopic Techniques 341 Theoretical Approach to the Comprehension of TMS 344 Hydrodesulfurization Mechanisms 344... 

Figure 9.8. Global reaction mechanism for the hydrodesulfurization of thiophene, in which the first step involves hydrogenation of the unsaturated ring, followed by cleavage ofthe C-S bond in two steps. Butadiene is assumed to be the first sulfur-free product,...

Studies of the Kinetics and Mechanisms of Ammonia Synthesis and Hydrodesulfurization on Metal Single-Crystal Surfaces... 

Thiophene is the typical model compound, which has been extensively studied for typifying gasoline HDS. Although, some results are not completely understood, a reaction network has been proposed by Van Parijs and Froment, to explain their own results, which were obtained in a comprehensive set of conditions. In this network, thiophene is hydrodesulfurized to give a mixture of -butenes, followed by further hydrogenation to butane. On the considered reaction conditions, tetrahydrothiophene and butadiene were not observed [43], The consistency between the functional forms of the rate equations for the HDS of benzothiophene and thiophene, based on the dissociative adsorption of hydrogen, were identical [43,44], suggesting equivalent mechanisms. 

The sulfidation mechanisms of cobalt- or nickel-promoted molybdenum catalysts are not yet known in the same detail as that of M0O3, but are not expected to be much different, as TPS patterns of Co-Mo/A1203 and Mo/Al203 are rather similar [56J. However, interactions of the promoter elements with the alumina support play an important role in the ease with which Ni and Co convert to the sulfidic state. We come back to this after we have discussed the active phase for the hydrodesulfurization reaction in more detail. 

Past reviews have described in detail the chemistry and mechanisms involved in the hydrodesulfurization of the most reactive organic species found in petroleum fuels (1-5). A vast amount of knowledge has been... 

Whitehurst, Isoda, and Mochida write about catalytic hydrodesulfurization of fossil fuels, one of the important applications of catalysis for environmental protection. They focus on the relatively unreactive substituted di-benzothiophenes, the most difficult to convert organosulfur compounds, which now must be removed if fuels are to meet the stringent emerging standards for sulfur content. On the basis of an in-depth examination of the reaction networks, kinetics, and mechanisms of hydrodesulfurization of these compounds, the authors draw conclusions that are important for catalyst and process design. 

The mechanism of hydrodesulfurization of benzo[6]thiophene and its derivatives involves, as a first step, chemisorption of the molecules (through the sulfur atom lone pair of electrons) in a more or less perpendicular fashion to the catalyst surface. Removal of the sulfur is then thought to produce a diradical which, on subsequent hydro-... 

Hydrodesulfurization (HDS) is a stepwise reaction of Paramount industrial and environmental relevance whose mechanism(s), however, is still poorly understood. In this process, sulfur, contained in various... 

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... 

In spite of all of the work, the kinetics and mechanism of alkyl-substituted dibenzothiophene, where the sulfur atom may be sterically hindered, are not well understood and these compounds are in general very refractory to hydrodesulfurization. Other factors that influence the desulfurization process such as catalyst inhibition or deactivation by hydrogen sulfide, the effect of nitrogen compounds, and the effect of various solvents need to be studied in order to obtain a comprehensive model that is independent of the type of model compound or feedstock used. 

There is a variety of sulfur-containing molecules in a residuum or heavy crude oil that produce different products as a result of hydrodesulfurization reaction. Although the deficiencies of current analytical techniques dictate that the actual mechanism of desulfurization remain largely speculative, some attempt... 

In the same manner as in hydrocracking (Dolbear, 1997), hydrogen is added at intermediate points in hydrodesulfurization reactors. This is important for control of reactor temperatures. The mechanical devices in the reactor, called reactor internals, which accomplish this step are very important to successful processes. If redistribution is not efficient, some areas of the catalyst bed will have more contact with the feedstock. This can lead to three levels of problems ... 

In addition, it has been found that isomerization sites are completely poisoned by the carbon deposit and that the hydrogenation reaction is more sensitive to poisoning than the hydrodesulfurization reaction indicating either the existence of different sites or of different reaction mechanisms. 

Removal of thiophene impurities from petroleum feedstocks is accomplished by a process called hydrodesulfurization (HDS) which involves the insertion of metals into the thiophene ring between the C-S bond. In order to better understand the mechanism of this reaction, different groups have utilized selenophene model systems due to the enhanced NMR characteristics of Se. Metal complexes of selenophenes that have been studied include rhodium <19970M2751>, molybdenum <2006POL499>, manganese <20010M3617, 19950M332>, chromium... 

For most reaction systems, the intrinsic kinetic rate can be expressed either by a power-law expression or by the Langmuir-Hinshelwood model. The intrinsic kinetics should include both the detailed mechanism of the reaction and the kinetic expression and heat of reaction associated with each step of the mechanism. For catalytic reactions, a knowledge of catalyst deactivation is essential. Film and penetration models for describing the mechanism of gas-liquid and gas-liquid-solid reactions are discussed in Chap. 2. A few models for catalyst deactivation during the hydrodesulfurization process are briefly discussed in Chap. 4. 

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