Hydrodesulfurization Reaction Mechanisms

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,...

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. 

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 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. 

The financial and environmental importance of hydrodesulfurization reactions assures continued interest in HDS research. The knowledge of the mechanisms in commercial HDS reactions is still considerably limited determination of the intimate mechanisms of these reactions constitutes a formidable challenge. The use of metal clusters as models for HDS catalysts is continuously developing and we might therefore expect that several new model systems will be investigated in the future. 

Beuther H, Schmid BK. Reaction mechanisms and rates in residue hydrodesulfurization. Sixth World Petroleum Congress June 1963 Frankfurt. Section 3 297-310. 

Take hydrodesulfurization catalysis, for example. We used an active Ti02 as the catalytic support to load the active species for catalysis. Till now, there are many works devoted to research the hydrodesulfurization catalysis mechanism by DFT and experiment, and the reaction pathway of hydrodesulfurization has been figured out. DFT was almost focused on the simulation of the active site. For example, Raybaud et al investigated the interaction of the reactants and intermediate with the M0S2 or NiMoS catalyst by DFT (Dupont et al, 2011). Also, they studied the interaction between active species and support (Costa et al, 2007). However, there was litde work on the interaction of reactants or resultants with catalytic support. The ReaxFF method can be apphed to investigate the reaction on Ti02. It can combine the effect of surface and the effect of transport into reaction process so that the reaction time and the transport time can be separately studied to find out the key influence factor. 

Bataille, F. Lemberton, J.L. Michaud, P. Perot, G. Vrinat, M. Lemaire, M. Schulz, E. Breysse, M. Kasztelan, S. Alkydibenzothiophenes Hydrodesulfurization-Promoter Effect, Reactivity, and Reaction Mechanism. J. Catal., 2000,191,409-422. 

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. 

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. 

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... 

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. 

Those deactivation models accounting for both coke and metal sulfides are rather simple. Coke and metals foul residue hydrodesulfurization catalysts simultaneously via different processes, and decrease both intrinsic reaction rate and effective diffusivity. They never uniformly distribute in the commercial reactors. We have examined the activity and diffusivity of the aged and regenerated catalysts which were used at the different conditions as well as during the different periods. This paper describes the effects of vacuum residue conversion, reactor position, and time on-stream on the catalyst deactivation. Two mechanisms of the catalyst deactivation, depending on residue conversion level and reactor position, are also proposed. 

Many operations in chemical engineering require the contact of two liquid phases between which mass and heat transfer with reaction occurs. Examples are hydrometallurgical solvent extraction, nitrations and halogenations of hydrocarbons, hydrodesulfurization of crude stocks, emulsion polymerizations, hydrocarbon fermentations for single-cell proteins, glycerolysis of fats, and phase-transfer catalytic reactions. A most common method of bringing about the contact of the two phases is to disperse droplets of one within the other by mechanical agitation. 

If the PMcj and dppe (diphenylphosphino ethane) PtLj analogues are used, complete hydrodesulfurization of DBT and 4-MeDBT to biphenyl and 3-Me-biphenyl, respectively, takes place under 20 atm H, at 100 °C interestingly, the presence of acid-washed alumina promoted the HDS reaction (see e.g. Eq. 4.9). Neither the fate of the sulfur nor the HDS mechanism is clear in this case [42, 43]. 

In this chemistry, it is natural to focus on models for the catalytic reactions that are most important economically or which are most poorly understood because of difficulties of direct study. One which best fits these criteria is the catalytic hydrotreatment of petroleum feedstocks, which is used to remove sulfur and other heteroatoms, which interfere with subsequent catalytic reactions such as petroleum reforming, from the hydrocarbons. Molybdemun sulfide is the most common metal sulfide used in this catalysis, and hydrogen activation and C-S bond hydrogenolysis are known to be key reactions occurring at the catalyst surface but details are difficult to obtain. Study of model binuclear and cluster complexes has elucidated mechanisms of several of the key reactions and Section 2.6 describes important recent advances in this field, with the focus being on models for hydrodesulfurization catalysts. 

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