Organosulfur additives

In contrast to the foregoing treatment of the additive action of disulfides by an explicit physical and chemical model, the usual approach to interpreting results from lubricant bench testing is to look for empirical correlations. Table 11-7 shows some data obtained by Mould, Silver and Syrett [25] from experiments with organosulfur additives in the four-ball test. The additives are listed in order of in-... 

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. 

Other Rhodococcus strains similar to those described above in terms of the desulfurization ability have also been isolated [83], The purpose of identifying such Rhodococcus strains, in several cases, appears to be the development of in-house biocatalysts for BDS application. The specificity of the desulfurizing strains of organosulfur compounds in addition to DBT has also been studied (Table 3). 

Rhodococcus strain SY1 was reported to desulfurize dimethyl sulfide, dimethyl sulfoxide, and several alkyl sulfonates [41] in addition to DBT [78], Barium chloride has been used to precipitate sulfate and shown to alleviate sulfate repression partially. The authors proposed a tentative pathway for oxidative removal of sulfur from DBT and other organosulfur compounds. It should be noted that phenyl disulfide and thianaph-thene were not desulfurized by any of the Rhodococcus strains, but have been reported to be substrates of Gordonia CYKS2. 

A microbial biocatalyst in context of BDS is defined as a microorganism expressing enzymes capable of removing sulfur selectively from organosulfur compounds. The definition of a biocatalyst, in general, also includes use of one or more enzymes, by themselves or in a cellular extract (used either in suspended form or carrier-supported form) for removal of sulfur. Additionally, biocatalyst can be a microbial consortium as well. Aerobic as well as anaerobic pathways for sulfur removal have been reported. The anaerobic routes, however, have been plagued with lack of reproducibility preventing further development. 

The next patent awarded to IGT (in 1993) [93] was related to the organism, B. sphaericus ATCC 53969, and included extended protection for its capabilities to cleave C—S bonds from organosulfur molecules. That full protection also involved any known possible derivative, which might directly or indirectly carry or enhance the observed capabilities. Following these earlier patents, the latter patents introduced additional concepts and practices into the oil-refining biocatalysis arena, by broadening the traditional (whole cell or enzyme) concept of a biocatalyst. 

As Pd° and Ni° are capable of oxidative addition by C—S bonds, organosulfur compounds can take part in cross-coupling reactions as electrophilic reagents. Due to the formation of stable Pd—S... 

Unsaturated a-heteroatom organosili-con, organotin, and organosulfur compounds undergo selective cleavage of the C—Si, C—Sn, and C—S bond respectively, with the subsequent intramolecular addition of the generated carbocation to the double bond (Fig. 33) [154,155]. 

In addition, another organosulfur compound, S-allyl-mercaptocysteine (Fig. 2) has been shown to affect histone acetylation levels. This again is probably due to its ability to form allyl mercaptan upon metabolic reduction of its disulfide bond. ... 

In addition, several metal-coordinated thials have been described in studies pertaining to hydrodesulfurization (HDS) reactions. This catalytic process is used to remove sulfur from organosulfur compounds present in fossil fuel feedstocks by reaction with hydrogen and a transition metal (Rh, Ir) and possesses both commercial and environmental importance393,394. 

The Michael addition mechanism, whereby sulfur nucleophiles react with organic molecules containing activated unsaturated bonds, is probably a major pathway for organosulfur formation in marine sediments. In reducing sediments, where environmental factors can result in incomplete oxidation of sulfide (e.g. intertidal sediments), bisulfide (HS ) as well as polysulfide ions (S 2 ) are probably the major sulnir nucleophiles. Kinetic studies of reactions of these nucleophiles with simple molecules containing activated unsaturated bonds (acrylic acid, acrylonitrile) indicate that polysulfide ions are more reactive than bisulfide. These results are in agreement with some previous studies (30) as well as frontier molecular orbital considerations. Studies on pH variation indicate that the speciation of reactants influences reaction rates. In seawater medium, which resembles pore water constitution, acrylic acid reacts with HS at a lower rate relative to acrylonitrile because of the reduced electrophilicity of the acrylate ion at seawater pH. 

Salinity exerts a positive influence on the rate of the addition reaction depending on the polarizability of the organic molecule. This effect is pronounced for unsaturated molecules containing a terminal carboxyl group. These results suggest that hypersaline palaeoenvironmental conditions would have favored organosulfur formation by the Michael addition mechanism. 

Mikolajczyk, M., Grzeijszczak, S., and Zatorski, A., Organosulfur compounds IX NMR and structural assignments in a,)3-unsaturated sulphoxides using additive increments method, Tetrahedron, 32, 969, 1976. 

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