Dibenzothiophene aromaticity

Omori, T. Saiki, Y. Kasuga, K., and Kodama, T., Desulfurization of Alkyl and Aromatic Sulfides and Sulfonates by Dibenzothiophene-Desulfurizing Rhodococcus Sp Strain Syl. Bioscience Biotechnology and Biochemistry, 1995. 59(7) pp. 1195-1198. 

Fig. 5a-c. A typical distribution of polycyclic aromatic hydrocarbons in a atmospheric fallout sample, Alexandria City - Egypt b bottom incineration ash leachate of municipal solid waste - USA c hydrothermal petroleum, Escanaba Trough, NE Pacific Ocean. PAH Compound identifications N = naphthalene, MN = methylnaphthalene, DMN = dimethylnaphthalenes, P = phenanthrene, MP = methylphenanthrene, Fl = fluoranthene, Py = pyrene, BaAN = benzol anthracene, DH-Py = dihydropyrene, 2,3-BF = 2,3-benzofluorene, BFL = benzo[fc,/c]fluoranthene, BeP = benzo[e]pyrene, BaP = benzo[a]pyrene, Per = perylene, Cx-228 = methyl-228 series, Indeno = indeno[ l,2,3-c,d]pyrene, DBAN = dibenz[a,/z]anthracene, BPer = benzo[g,/z,z] perylene, AAN = anthanthrene, DBTH = dibenzothiophene, Cor = coronene, DBP = dibenzo [a,e]pyrene, DBPer = dibenzo [g,h,i] perylene... 

Fig. 3 Electrochemical and homogeneous standard free energies of activation for self-exchange in the reduction of aromatic hydrocarbons in iV.A -dimethylformamide as a function of their equivalent hard sphere radius, a. 1, Benzonitrile 2, 4-cyanopyridine 3, o-toluonitrile 4, w-toluonitrile 5, p-toluonitrile 6, phthalonitrile 7, terephthalonitrile 8, nitrobenzene 9, w-dinitrobenzene 10, p-dinitrobenzene 11, w-nitrobenzonitrile 12, dibenzofuran 13, dibenzothiophene 14, p-naphthoquinone 15, anthracene 16, perylene 17, naphthalene 18, tra 5-stilbene. Solid lines denote theoretical predictions. (Adapted from Kojima and Bard, 1975.)...

Reduction of dibenzothiophene with sodium in liquid ammonia has been shown to be sensitive to the experimental methods employed however, the major product is usually 1,4-dihydrodibenzothiophene. 27 -28i The electrochemical reduction of dibenzothiophene in ethylene-diamine-lithium chloride solution has been shown to proceed via stepwise reduction of the aromatic nucleus followed by sulfur elimination. In contrast to the reduction of dibenzothiophene with sodium in liquid ammonia, lithium in ethylenediamine, or calcium hexamine in ether, electrolytic reduction produced no detectable thiophenol intermediates. Reduction of dibenzothiophene with calcium hexamine furnished o-cyclohexylthiophenol as the major product (77%). Polaro-graphic reduction of dibenzothiophene 5,5-dioxide has shown a four-electron transfer to occur corresponding to reduction of the sulfone group and a further site. ... 

An attempt to methylate the diketone (97) with methanolic hydrogen chloride gave the dimethoxy derivative (98), presumably by alkylation of the j8-dicarbonyl system by the activated aromatic nucleus. The possibility therefore exists of synthesizing polymethoxy derivatives of dibenzothiophene by the 2-chlorocyclohexanone route (Section IV, A), using this modification of the ring-closine step. 

Hydrogenation of other Doivcvciic aromatic compounds found in recycle solyents. These experiments were carried out at 400°C at 17 MPa for 2 h using a commercial 3%/15% CoMo cat yst. The first order rate constants are shown in Table IV it can be noticed that no values are reported for acenaphtiiylene and dibenzothiophene as hardly any of the starting compounds remained after the reaction and polymerisation was evident for both compounds. 

To characterize the relative gas-chromatographic retentions of condensed aromatics and heteroaromatics, inclu g thienothiophenes, benzo[b]thiophene, dibenzothiophene, naphthobenzothiophenes, and anthrabenzothiophenes, a system of indices. In, was proposed, In this system a series of similar linearly condensed hydrocarbons (such as benzene, naphthalene, anthracene, tetracene, pentacene,...) was used as a reference scale. The logarithm of the corrected retention volume (adjusted to 0°), log Ft, depends linearly upon the number of condensed benzene rings (z) in the molecule, both in the polar and nonpolar phases. In is expressed by Eq. (58) ... 

Kwok, E. S. C., R. Atkinson, and J. Arey, Kinetics of the Gas-Phase Reactions of Dibenzothiophene with OH Radicals, NO, Radicals, and O, Polycyclic Aromat. Compd., in press (1999). 

In describing catalytic activities and selectivities and the inhibition phenomenon, we will use a common format, where possible, which is based on a common reaction pathway scheme as outlined in Scheme 1. In contrast to the simple one- and two-ring sulfur species from which direct sulfur extrusion is rather facile, in the HDS of multiring aromatic sulfur compounds such as dibenzothiophene derivatives, the observed products are often produced via more than one reaction pathway. We will not discuss the pathways that are specific for thiophene and benzothiophene as this is well represented in the literature (7, 5, 8, 9) and, in any event, they are not pertinent to the reaction pathways involved in deep HDS processes whereby all of the highly reactive sulfur compounds have already been completely converted. 

For highly substituted dibenzothiophenes, ring hydrogenation prior to sulfur extrusion is the major route to hydrocarbon production as, relative to the parent molecules, aliphatic substituents on aromatic ring carbons... 

Recent computer modeling clearly shows that for molecules such as 4,6-dimethyldibenzothiophene (4,6-DMDBT), the methyl groups interfere with catalyst-molecule interactions as the sulfur atom adsorbs primarily through a one-point attachment and the dibenzothiophene ring system is nearly perpendicular to the catalyst surface. Hydrogenation of one ring of the 4,6-DMDBT causes the rigid planar aromatic structure to pucker and allow much better interaction between the sulfur atom and the catalyst surface (77). 

In the case of 4,6-DMDBT, it was possible to determine the rate constants for direct extraction of sulfur from the fully saturated sulfur-containing ring system (k0l) and for the secondary hydrogenation of the tetrahydro-dibenzothiophene intermediate (fcHs2)- As might be expected, the rate constants for direct sulfur extraction follow a clear trend in which A Dq < < d2 The reverse trend is observed in the aromatic ring hydrogenation rates, /cHs, > kHS, and kHPl > kw , which is consistent with the literature (see Fig. 10) (5, 35). 

It is clear from the data presented in Table XIV that H2S, though adsorbed competitively with dibenzothiophene, is not a major inhibitor for dibenzothiophene adsorption. Dibenzothiophene was shown to be preferentially adsorbed relative to biphenyl on both the cr and r sites. It is surprising that no adsorption of H2S was noted on the hydrogenation site (r) since it is known to be a strong inhibitor for many aromatic hydrogenations. A... 

These results show that, in equimolar concentrations, naphthalene would not be considered as a strong inhibitor toward direct sulfur extraction (A Do) for PASCs. However, as discussed earlier, the content of di- and trinuclear aromatics in diesel fuels and gas oils can be as high as 20-30%, whereas the level of sulfur compounds in today s diesel fuels is only 0.2% sulfur, or about 1 wt% PASCs. So the competition for the active site by aromatic hydrocarbons is very strong. Their effect on the direct desulfurization route will lower the rate to about one-third of the noninhibited rate in the case of dibenzothiophene and would lower that of 4,6-DMDBT even more. 

Inhibition by aromatic hydrocarbons is most severe for dialkyldiben-zothiophenes as these materials are preferentially desulfurized by hydrogenation of the aromatic ring prior to sulfur extraction and as aromatics are more strongly adsorbed on the hydrogenation site of Co(Ni)-Mo-S catalysts than are dibenzothiophenes. 

As described in Section IV.B, dibenzothiophenes, when substituted in positions adjacent to the sulfur atom, have reduced activity for direct sulfur extraction. As a result, catalysts that promote aromatic ring hydrogenation offer another route to desulfurization, as the partially hydrogenated ring presents much less steric restrictions to adsorption via r -S type bonding (17,21) or to oxidative addition to form a metallathiabenzene intermediate, as discussed in Section IV.E.3. In addition, the metal-S coordination bond strength is increased by increasing the electron density on sulfur, and the C-S bonds in hydrothiophenes are much weaker. 

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