Hydrodesulfurization, thiophenes

While metal complexes capable of desulfurizing or hydrodesulfurizing thiophenes are relatively numerous, homogeneous catalysts are very rare, being limited to rhodium and iridium TRIPHOS precursors that are unique in tolerating the harsh experimental conditions required for the second C-S bond cleavage of thiophenes [2 b]. 

Reduction and Hydrodesulfurization. Reduction of thiophene to 2,3- and 2,5-dihydrothiophene and ultimately tetrahydrothiophene can be achieved by treatment with sodium metal—alcohol or ammonia. Hydrogen with Pd, Co, Mo, and Rh catalysts also reduces thiophene to tetrahydrothiophene [110-01-0] a malodorous material used as a gas odorant. 

Rigorous hydrogenating conditions, particularly with Raney Nickel, remove the sulfur atom of thiophenes. With vapor-phase catalysis, hydrodesulfurization is the technique used to remove sulfur materials from cmde oil. Chemically hydrodesulfurization can be a valuable route to alkanes otherwise difficult to access. 

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. 

Thiophenes continue to play a major role in commercial applications as well as basic research. In addition to its aromatic properties that make it a useful replacement for benzene in small molecule syntheses, thiophene is a key element in superconductors, photochemical switches and polymers. The presence of sulfur-containing components (especially thiophene and benzothiophene) in crude petroleum requires development of new catalysts to promote their removal (hydrodesulfurization, HDS) at refineries. Interspersed with these commercial applications, basic research on thiophene has continued to study its role in electrocyclic reactions, newer routes for its formation and substitution and new derivatives of therapeutic potential. New reports of selenophenes and tellurophenes continue to be modest in number. 

Here we illustrate how to use kinetic data to establish a power rate law, and how to derive rate constants, equilibrium constants of adsorption and even heats of adsorption when a kinetic model is available. We use the catalytic hydrodesulfurization of thiophene over a sulfidic nickel-promoted M0S2 catalyst as an example ... 

Figure 7.16. Dependence of the rate of thiophene hydrodesulfurization on the partial pressures of the reactants thiophene and hydrogen and of the product hydrogen sulfide,...

Figure 7.18. Dependence of the rate of thiophene hydrodesulfurization on the partial pressures of thiophene at different temperatures, along with fits according to the Langmuir-Hinshelwood model, Eq. (32). [Fron A. Borgna and J.W. Niemantsverdriet, to be published (2003).]...

Mechanistically, hydrotreating of ring-type molecules such as thiophene and related molecules involves the hydrogenation of the unsaturated ring followed by cleavage of the bonds between the heteroatom and its carbon neighbors. Figure 9.8 illustrates this for the hydrodesulfurization of thiophene. 

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,...

Can you think of reasons why substituted dibenzothiophenes are more difficult to desulfurize than thiophene or simple thiols (see Fig. 9.2) Depending on the choice of catalyst, hydrodesulfurization can be accompanied by hydrogenation to various extents. In which of the product streams in the refinery would you choose hydrogenative HDS and in which would you not ... 

In this paper we will discuss the results that we have obtained studying two important processes, the synthesis of ammonia over Fe and Re catalysts and the hydrodesulfurization of thiophene on Mo. These examples illustrate two problems in catalysis to which our methods are particularly sensitive. The first is the effect of the structure of the catalyst on reaction rates. The sensitivity of the... 

Kinetics over the Mo(lOO) Crystal Surface. We have studied the hydrodesulfurization of thiophene over the initially clean Mo(lOO) single crystal surface in the temperature range 520K - 690K and at reactant pressures of 100 Torr < P(H ) 800 Torr and 0.1 Torr P(Th) < 10 Torr. Under these conditions the reaction is catalyzed at a constant rate for a period of approximately one hour after which the rate begins to decrease with time. The rates reported here are all initial rates of reaction calculated from data collected in the period over which they remain constant. 

The studies of ammonia synthesis over Fe and Re and the hydrodesulfurization of thiophene over Mo, described above, illustrate the importance and success of our approach of studying catalysis over single crystal samples at high pressures. The use of surfaces having a variety of orientations allows the study of reactions that are surface structure sensitive 6Uid provides insight into the nature of the catalytic site. Here we have shown that the ammonia synthesis... 

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. 

One of the major challenges in the petroleum industry today is the removal of sulfur compounds, especially refractive ones such as 4,6-dimethyldibenzo-thiophene (DMDBT), from petroleum fractions such as diesel to concentrations <5-10 ppm from the current values of 50-500 ppm. The current technology is hydrodesulfurization catalyzed by cobalt-nickel-molybdenum sulfides at high pressures. Reducing sulfur concentratios in diesel fuels below 5-10 ppm... 

From the preceding discussion, it can be seen that the mathematical description of the chemical transformations involved in product formation can be extremely difficult. However, knowledge of the response of HDS reaction rates to different kinds of feed components and byproducts is extremely important for designing new processes that will allow refineries to meet the stringent standards of the future. The following text attempts to summarize the observations reported in the literature on the effects of inhibitors on the hydrodesulfurization rates of alkyl-substituted dibenzo-thiophenes. It is quite possible that many reports have been overlooked, and the present authors apologize for any oversights that may have occurred in this review. 

Smectite-type materials containing transition metal divalent cations (Ni2+, Co2, and Zn2+) in octahedral sheets were synthesized. The synthetic smectites were thermally stable and had large surface areas and high pore volumes after evacuation at 873 K. Catalytic activities of synthetic smectites were investigated. The Ni2 -containing smectites were active for the isomerization of 1-butene and the oligomerization of ethylene. The Co2+-containing smectites were active for the hydrodesulfurization of thiophene. 

Figure 5. The conversion and selectivity (0 -butane, A 1-butene, A wa/iv-2-butene, m-2-butene) of thiophene hydrodesulfurization (a,b) Co-380 and (c,d) 5wt% Co/Si02.

The positive identification of the sulfur compounds in crude oils is a difficult problem often complicated by the lack of reference compounds. This difficulty has been overcome by hydrodesulfurization (see Section VIII), which converts the sulfur compounds into known hydrocarbons. Treatment of a petroleum oil fraction with calcium hexammine converts the benzo[6]thiophenes present into aryl mercaptans, which are readily separable from accompanying aromatic hydrocarbons (e.g., naphthalene) and then identified by hydro-... 

Unwanted sulfur-containing components may be removed from petroleum oils by hydrodesulfurization over a Co-Mo catalyst at high temperatures under pressure ( unifining or hydrofining ).38 However, benzo[6]thiophene is hydrodesulfurized with difficulty over a molybdenum catalyst89 40 and it is difficult to remove completely from petroleum oils by hydrofining.41... 

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