Benzothiophenes and Dibenzothiophenes

Benzothiophene and dibenzothiophene with Cp Co(C2H4)2 give products where the carbon—sulfur bond has been cleaved, such as 308 [94JOM(472)311, 97P3115]. 

Oldfield, C. Wood, N. T. Gilbert, S. C., et al., Desulphurisation of Benzothiophene and Dibenzothiophene by Actinomycete Organisms Belonging to the Genus Rhodococcus, and... 

Tanaka, Y. Matsui, T. Konishi, J., et al., Biodesulfurization of benzothiophene and dibenzothiophene by a newly isolated Rhodococcus strain. Applied Microbiology and Biotechnology, 2002. 59(2-3) pp. 325-328. 

Figure 28 presents the calculated electron densities and bond orders of a variety of thiophenes, benzothiophenes, and dibenzothiophenes. It can be seen in the figure that the electron density on sulfur actually increases as the thiophene ring system becomes more condensed, e.g., with the parent molecules thiophene < benzothiophene < dibenzothiophene. This has also been noted by Rauchfuss (92). Thus, if electron density were the only controlling factor, one might expect dibenzothiophenes to be more reactive than thiophenes. However, as was discussed previously, the reverse is true. Thus other factors must also be important. 

As mentioned earlier, the MOPAC-PM3 calculations also helped to determine the importance of bond order in the hydrogenative route to desulfurization. Figure 28 shows the calculated bond orders of all bonds in a wide variety of thiophenes, benzothiophenes, and dibenzothiophenes (38). These values were correlated with the rates of desulfurization of sterically hindered alkyl-substituted benzothiophenes and alkyl-substituted 1,1 -diox-... 

This statement could well be expanded to include studies describing the kinetics of model compounds. In reviewing the literature, one finds that there are almost as many kinetic representations as there are researchers and/or model compounds. Even the same authors have found it necessary to use different equations to describe the different responses to inhibitors for closely related sulfur species such as thiophene, benzothiophene, and dibenzothiophene (104, 122, 123). The inhibiting effect of H2S for the hydrogenation of a simple molecule, such as toluene, has been found to require extremely complex equations to adequately describe mathemati-... 

Perhaps the largest discrepancies in reported results are the relative values for the adsorption constants of H2S and thiophene molecules (THs, including thiophene, benzothiophene, and dibenzothiophene). The reported preference for adsorption on the direct desulfurization site ranges from H2S THs (122,123,125) to about the same (104) to H2S < c THs (125). 

As complex as the desulfurization of thiophene might appear, projection of the kinetic picture to benzothiophene and dibenzothiophene, and to their derivatives, is even more complex. As has already been noted for bond energy data, kinetic data derived from model compounds cannot be expected to include contributions from the various steric effects that are a consequence of complex molecules containing three-dimensional structures. Indeed, such steric effects can lead to the requirement of additional catalyst and process parameters for sulfur removal (Isoda et al., 1996a, 1996b). 

Kiezel, L., Liszka, M., and Rutkowski, M., Carbon-13 magnetic resonance spectra of benzothiophene and dibenzothiophene, Spec. Lett., 12, 45, 1979. 

The results in Figure 4 clearly show that a number of alkyl benzothiophenes and dibenzothiophenes can be metabolized (or co-metabolized) by microorganisms and the biodesulfurization studies suggest that other OSC can undergo biodegradation. Yet, the metabolism of only a few OSC has been studied in detail. 

Thiophenes are common pyrolysis products of the Murchison macromolecular material and thiophene, methylthiophene, dimethylthiophene, and benzothiophene have been detected in this way (Biemann, 1974 Holtzer and Oro, 1977 Sephton et al, 1998). Sodium dichromate oxidation has revealed the presence of substituted benzothiophene and dibenzothiophene moieties... 

The hds of thiophen, benzothiophen, and dibenzothiophen and their methyl-substituted derivatives were compared in pulse experiments (623-723 K) over a sulphided CoO(5.6)-Mo03(11.2)/Al203 catalyst. Reactivities at 1 atm pressure were roughly the same but at hi er pressures, reactivity decreased with the number of rings. For benzothiophen, methyl substituents did not affect the reactivities, but for dibenzothiophen methyl substituents in the 4- or 4 - and 6- positions caused a decrease of desulphurization rate. Aromatic ring hydrogenation was not a prerequisite of C-S scission, which was the slow step. 

PASH (polycyclic aromatic sulfur hydrocarbon) benzothiophene and dibenzothiophene... 

Of particular current interest is the fact that the most abundant residual sulfur compounds in diesel fuels are found in the form of alkyl-substituted benzothiophenes and dibenzothiophenes, 4-methyldibcnzothiophene and 4,6-dimethyldibcnz.othiophcnc being the prototypes of the most difficult compounds to degrade. The removal of these very stable molecules through what has come to be known as deep desulfurization in order to meet the severe new standards for sulfur contents in fuels is certainly a most difficult task and a satisfactory solution for this problem has not yet been found [14-16, 19, 20]. 

Sulfur is present in petroleum as sulfides, thiophenes, benzothiophenes, and dibenzothiophenes. In most cases, the presence of sulfur is detrimental to the processing because sulfur can act as catalytic poisons during processing. 

Figure 1. Organosulfur compounds present in petroleum feedstocks are thiols, thioethers, disulfides, thiophenes, benzothiophenes and dibenzothiophenes.

Thiophenic compounds are naturally present in crude oil. Although hydrodesulfurization targets them and converts toward hydrocarbon and hydrogen sulfide [7] still significant quantities are present after this treatment. The reactivities of the 1- to 3-ring compounds decreases in the order thiophenes > benzothiophenes > dibenzothiophenes. In gasoline benzothiophenes are present, in jet fuel - benzothiophenes and dibenzothiophenes whereas in diesel fuel dibenzothiophenes and 4, 6-dimethyldibenzothiophene (4,6-DMDBT) are found. The latter is considered as the most refractory suffur compound towards HDS. The most common refractory sulfur compounds in liquid fuels are presented in Fig. 4. 

Aromatic organosulfur compounds such as thiophenes, benzothiophenes and dibenzothiophenes are frequently contained in fossil oil and their sulfur atoms are generally difficult to remove in HDS process [106], In the industrial HDS process, Mo/Co/S or Ni/Mo/S heterogeneous catalysts supported on alumina are widely employed. In order to obtain ideas to develop more efficient catalysts as well as to shed some light on their mechanisms at a molecular level, transition metal complex-mediated cleavages of C-S bond are extensively studied. On the other hand, thiiranes and thietanes are frequently employed for preparation of transition metal sulfides, in which their C-S bonds are smoothly cleaved. In this section, the C-S bond cleavages of thiophene derivatives, thiiranes, thietanes, vinylic sulfides, allylic sulfides, thiols and dithioacetals are overviewed. 

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