Hydrodesulfurization reactivity

Figure 4. Hydrodesulfurization reactivity of dibenzothiophene using Co/Mo MSC catalysts.

Ma, X.L., Sakanishi, K.Y., and Mochida, I. Hydrodesulfurization reactivities of various sulfur-compounds in diesel fuel. Industrial Engineering Chemistry Research, 1994, 33, 218. 

Ma, X., Sakanishi, K., and Mochida, 1. Hydrodesulfurization Reactivities of Various Sulfur Compounds in Diesel Fuel, Ind. Eng. Chem., 1994, 33, 218-222. 

Saturation of olefins other than reactive olefins usually is not desired. The added hydrogen is often expensive or useful elsewhere, and it does not provide any real improvement in product quality. Acmally, product quality may be reduced in the case of gasolines. Research octane number losses may be correlated with increasing olefin saturation. So in many cases, hydrodesulfurization conditions are selected with an eye toward minimizing olefin saturation over and above that needed for product quality improvement. There is one exception saturation of certain olefins shows substantial improvements in Motor octane number. This is true for iso- and n-pentenes and to a lesser extent for higher boiling isoolefins. The higher n-olefins show octane losses upon saturation. 

Although desulfurization is a process, which has been in use in the oil industry for many years, renewed research has recently been started, aimed at improving the efficiency of the process. Envii onmental pressure and legislation to further reduce Sulfur levels in the various fuels has forced process development to place an increased emphasis on hydrodesulfurization (HDS). For a clear comprehension of the process kinetics involved in HDS, a detailed analyses of all the organosulfur compounds clarifying the desulfurization chemistry is a prerequisite. The reactivities of the Sulfur-containing structures present in middle distillates decrease sharply in the sequence thiols sulfides thiophenes benzothiophenes dibenzothio-phenes (32). However, in addition, within the various families the reactivities of the Substituted species are different. 

The general reaction occurring in hydrodesulfurization has been described in Section 2.1.1. The most studied model compound is DBT. The reactivity towards hydrogenation of the phenyl substituents already mentioned (Section 2.1.1) is also observed in the hydroprocessing of sulfur compounds. The reactivity towards hydrogenolysis of the C-S bond masks the effects associated to aromatics hydrogenation. The DBT reaction network is sketched in Fig. 8 the pseudo-first-order reaction constants measured by Houalla [68] have been included. 

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... 

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). 

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... 

The use of hydrodesulfurization in the determination of the structures of unknown benzo[6]thiophenes is becoming increasingly recognized about 100 examples have now been recorded. Two sets of experimental conditions are commonly used the benzo[6]thiophene may be treated with Raney nickel alloy directly in alkaline medium (the method of Papa et al.7M), or it may be boiled in ethanol with Raney nickel of varying reactivity (commonly W-7). Even under mild conditions, nonaromatic double bonds are usually saturated, halogens are removed, and nitro groups are reduced. Raney cobalt has about one-tenth the activity of Raney nickel in the hydrodesulfurization of benzo [6]thiophenes.765... 

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... 

Under the usual commercial hydrodesulfurization conditions (elevated temperatures and pressures, high hydrogen-to-feedstock ratios, and the presence of a catalyst), the various reactions that result in the removal of sulfur from the organic feedstock (Table 4-3) occur. Thus, thiols as well as open chain and cyclic sulfides are converted to saturated and/or aromatic compounds depending, of course, on the nature of the particular sulfur compound involved. Benzothio-phenes are converted to alkyl aromatics, while dibenzothiophenes are usually converted to biphenyl derivatives. In fact, the major reactions that occur as part of the hydrodesulfurization process involve carbon-sulfur bond rupture and saturation of the reactive fragments (as well as saturation of olefins). 

Table 5-6 Reaction and Relative Reactivity of the Common Hydrodesulfurization Catalysts...

Considerable hydrodesulfurization of the thiophene type compounds is consistent with the known reactivity of dibenzothio-phene (18,22). The furans exhibited a net nonreactivity. 

In a number of petrochemical processes, a gas (hydrogen) is present as reactant. In hydrodesulfurization (HDS), hydrocracking (HC), and hydrodenitrogenation (HDN), the reaction products H2S and ammonia, respectively, are known to decrease the catalyst activity, but are partly transferred to the gas phase. Therefore, also these processes profit from reactive stripping. 

An example of a product-inhibited conversion is the hydrodesulfurization of medium-to-heavy petroleum fractions. The removal of sulfur from such oils can generally be described as a second-order reaction in total sulfur [1]. This high apparent order is a reflection of the presence of a variety of sulfur-containing compounds that have widely differing reactivities for hydrodesulfurization and implies that a relatively large proponion of sulfur is removed from the oil by conversion of the bulk of more reactive compounds in an early stage of the reaction. The conversion of the more refractive sulfur compounds occurs far more slowly in a later stage. 

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