Dibenzothiophene organic sulfur removal

Thiophene [110-02-17, C H S, and dibenzothiophene [132-65-OJ C22HgS, are models for the organic sulfur compounds found in coal, as well as in petroleum and oil shale. Cobalt—molybdenum and nickel—molybdenum catalysts ate used to promote the removal of organic sulfur (see Coal CONVERSION... 

One method of sulfur removal from refinery streams is by hydrodesulfurization (HDS) in the refineries. This step also directly impacts the characteristics of low sulfur diesel fuels, such as density, aromatics content, cetane number, and cloud point. The magnitude of these changes will depend upon the type and setup of refinery HDS units. However, in the end some refractory compounds in fuel, e.g., 4,6-diraethyl dibenzothiophene, are very resistant to desulfurization, owing to the inaccessibility of the organically bound sulfur atom. Lower pressure HDS units which can work satisfactorily at 350 rag/kg sulfur levels, may have difficulty achieving reduction to 50 or 10 rag/kg sulfur level. 

For a number of years researchers have attempted with little success to develop a biological system to remove organic sulfur. However, in 1989, J.J. Kilbane at the Institute of Gas Technology succeeded in isolating a bacterium that oxidized dibenzothiophene to 2-hydroxybiphenyl and liberated sulfur. 

In hydrotreating units, reactions that convert organic sulfur and nitrogen into H2S and NH3 also produce light hydrocarbons. For example, as shown in Figure 5, the removal of sulfur from dibenzothiophene (boiling point =... 

Two strains were isolated and purified, Pseudomonas sp. CDT-4, and Nocardia aster-oides, CDT-4b (ATCC 202160 and 202161, respectively). The microbes were passed through a multiple screen, first to allow growth on dibenzothiophene (DBT) as a sole source of sulfur, and then on fossil fuels, to identify organisms capable of desulfurization without metabolizing the DBT phenyl ring structures. N. asteroides sp. CDT-4b was found to metabolize DBT. The Pseudomonas species was found to utilize trace levels of sulfate from media and was found to be incapable of growth on DBT as a sole source of sulfur. However, the co-culture could remove more than 20% sulfur, with supplementation of a second sulfur-free carbon source. 

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). 

Other Organosulfur Compounds. There have been reports of the microbial metabolism of other OSC. However, few of these studies have given the identities of intermediates or organic endproducts of the OSC. For example, aerobic cultures have been reported to remove sulfur from phenyl sulfide (62). Thioxanthene and thianthrene were transformed to water-soluble products by a dibenzothiophene-oxidizing bacterium (48). In addition, thianthrene and thioxanthene served as sole carbon sources for the aerobic thermophile S. acidocaldarius (69) which released sulfate from these compounds. 

The majorify of sulfur compounds (thioles and sulfides) have been successfully removed from liquid fuel using a hydrodesulfurization process where high temperature and high pressure are required [7-9, 159]. As mentioned in section 2.5 some sulfur species are very resistant to hydrodesulfurization and those include thiophenic compounds, especially dibenzothiophene and 4,6 dimethyldibenzothiophene [148]. Various methods based on extraction and adsorption have been proposed to remove these compounds [7, 8, 13,145, 147-149, 151-158]. In the extraction route, sulfiir species are first oxidized and then extracted using organic solvents as, for instance acetonitrille [13, 149]. On the other hand, an adsorption process is usually tailored to enhance either adsorption forces, selectivity, or to impose a chemical reaction. So fer the enhancement in the removal of thiophenic compounds was reported on materials where n-complexation can occur as on Cu-Y zeolites [151, 153], or on alumina with highly dispersed sodium [147]. In the latter case, mono- and disodium thiophene metallates are formed. Another desulfurization methods use formation and subsequent precipitation of S-alkylsulfonium salts [148]. 

Recent publications confirm that a hydrotreater removes sulfur from organic sulfides, disulfides, and mercaptans with relative ease. Thiophenes, benzothiophenes, unhindered dibenzothiophenes and hindered dibenzo-thiophenes are successively harder to desulfurize. 

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