Mechanism of desulfurization

For a review, see Bonner Grimm, in Kharasch Meyers The Chemistry of Organic Sulfur Compounds, vol. 2 Pcrgamon New York, 1966. pp. 35-71, 410-413. For a review of the mechanism of desulfurization on molybdenum surfaces, see Friend Roberts Acc. Chem. Res. 1988,21, 394-400. 

Talballa, M., P.K. Trojan, and L.O. Brockway. 1976. Mechanisms of desulfurization of liquid iron carbon alloy with solid CaC2 and CaO. American Foundrvmen s Society Transactions. 84 775-786. DesPlaines, Illinois American Foundrymen s Society. 

There is a variety of sulfur-containing molecules in a residuum or heavy crude oil that produce different products as a result of hydrodesulfurization reaction. Although the deficiencies of current analytical techniques dictate that the actual mechanism of desulfurization remain largely speculative, some attempt... 

Further developments of the work include a more accurate study of the mechanisms of desulfurization processes using instrumental improvements. This will enable an easy quantitation of gas yield and a thermochemical approach of elemental processes. We also have been using model polymers to better study the interactions of pyrite and sulfur with the organic matrix during coal pyrolysis, oxidation and combustion (34 and to examine more accurately the specific role of organic sulfur in thermal degradation processes. 

Reaction of various thiiranes 208, catalyzed by aminium salts (tris-(4-bromophenyl)aminium hexachloroantimonate 209 or tris-(2,4-dibromophenyl)aminium hexachloroantimonate 210) in freshly distilled dichloromethane, led to desulfurization of starting thiiranes to alkenes 211 in good yield (typically 80%) (Scheme 53) <1995T8935>. The mechanism of desulfurization of thiiranes catalyzed by aminium salts is presented in Scheme 54. 

Tin hydrides, generally BuaSnH (TBTH) or PhsSnH (TPTH) (which may be more efficient), usually used in hot toluene or benzene in the presence of AIBN, cleave C—S bonds by a radical mechanism. Because the C—S bond is not very reactive, reactions performed without AIBN are ineffective. Extensive reviews on tin hydride properties have been published. Information about their use in desulfurizations may also be obtained from other reviews. " Evaluation of TBTH in the desulfurization of sulfides has led to a number of important conclusions. Thus, the mechanism of desulfurization is that shown in Scheme 16. 

Finally, the new environmental regulations limiting the sulfur content of gas oil used for vehicle transportation involves a new and increasing effort to understand the mechanisms of desulfurization of the most refractive compounds. These molecules are mainly DBTs with alkyl groups near the sulfur atom (fourth and sixth positions) (160). The reasons for their low HDS reactivity is still intensively discussed and two hypotheses have been proposed. The first proposal suggests that the transformation of 4-alkyl- and 4,6-dialkyl-DBT is limited by the adsorption step by the sulfur atom, which would be sterically hindered by the presence of the alkyl groups (161). The second hypothesis supposes a fiat adsorption of the DBT compounds on the catalyst (162). In this case, sterical factors do not hinder the first step of adsorption on the catalyst but more an intermediate step of elimination. 

The flash roaster is flexible ia handling various flotation concentrates and reaching the degree of desulfurization desired, ie, 0.5—3.0% sulfate sulfur. Waste heat is easily recovered. However, grinding and rabbling must be done mechanically. 

In 1974, Gassman et al. reported a general method for the synthesis of indoles. For example, aniline 5 was reacted sequentially with r-BuOCl, methylthio-2-propanone 6 and triethylamine to yield methylthioindole 7 in 69% yield. The Raney-nickel mediated desulfurization of 7 then provided 2-methylindole 8 in 79% yield. The scope and mechanism of the process were discussed in the same report by Gassman and coworkers as well. 

In the desulfurization of 3-substituted thiophenes several stereoisomers may be formed in certain cases. Both meso and racemic compounds have been obtained from the desulfurization of 3,4-diaryl-substituted thiophenes. It is claimed, however, that only meso, -diphenyladipic acid is obtained upon desulfurization of 3,4-di-phenyl-2,5-thiophenedicarboxylic acid and only di-isoleucin from 3-thienylglycine. The formation of small amounts of dimeric products in the desulfurization has been discussed with reference to the mechanism of this reaction. ... 

IMP-S02 [49], and ECRD-1 [50], Many of these strains are similar in the mechanism of DBT desulfurization, but vary in terms of the desulfurization rate as well as physiological characteristics. 

Rhodococcus sp. Strain T09 A Rhodococcus strain T09 was isolated by enrichment on media-containing BT. The desulfurization mechanism of this organism was reported to be similar to Gordonia sp. 213E due to the observation of similar intermediates however, the substrate specificity was different. The strain T09 could use 2-methyl, 3-methyl and 5-methyl BT apart from BT as sole source of sulfur for growth, but not 7-methyl or ethyl derivatives. Additionally, it could also use methyl thiobenzothiazole, marcaptobenzothiazole, as well as benzene sulfide, benzene sulfonate, biphenyl sulfinate, dimethyl sulfate, dimethyl sulfone, dimethyl sulfide, methane sulfonic acid, thiophene, and taurine as sole sulfur sources. However, it could not grow on DBT or DBT sulfone. 

A thermophilic strain M. goodii X7B was reported to carry out desulfurization of BT and DBT [38], The growth of the strain on various sulfur substrates including DBT, BT, 4,6-dimethyl DBT, 5-methyl BT, 2-thiophene carboxylic acid and propylmercaptan was studied at 45°C and varying degree of growth was observed. A mechanism of sulfur removal was proposed for BT based on metabolites identified by GC-MS (Fig. 7 [125]). 

Desulfurization using cell-free extracts The first report of desulfurization by cell-free extract of R. erythropolis was by Ohshiro et al. [180], This report showed stoichiometric desulfurization of DBT by a cell-free system and identified NADH as a necessary co-factor for desulfurization. Subsequently, the enzyme activity of cell-free extracts of the strain R. erythropolis D-l was found to be inhibited by a 2-HBP, and its analog 2,2 -dihydroxybiphenyl (DBHP). Sulfate did not inhibit enzyme activity [90], further proving that its role is not in controlling enzyme activity directly but via a genetic repression mechanism as indicated above. 

The second important issue related to commercial use of desulfurization biocatalysts is their inhibition by sulfate. The sulfur repression mechanism in most Rhodococcus species limits their use or activity in presence of sulfate- and sulfur-containing amino-acids such as cysteine, methionine, etc. To alleviate this problem, expression of the dsz genes under the control of alternate promoters has been investigated. 

Another Pseudomonas strain P. delafieldii R-8 was reported to remove 90.5% sulfur from highly desulfurized diesel oil [259], The biocatalyst achieved desulfurization via a pathway similar to the 4S pathway. The rate of desulfurization was reported to be 11.25 mmol sulfur/kg dcw/h, with the sulfur being reduced from 591 to 56 mg/L. This was achieved via two biocatalyst treatments lasting 20 hours each, although the biocatalyst was active only for first 6h in each treatment. Up to C4-DBTs were reported to be removed. Almost 100% of Q and C2 DBTs were removed and about 94% C3 DBTs and 97% C4 DBTs were removed. This strain of Pseudomonas thus appears to have a mechanism to uptake up to C4 DBTs through its cell membrane. 

Hydrocarbon Microbiology biodegradation mechanisms of oil products (gasoline, kerosene, diesel, etc.), pyrolysis, polycyclic aromatic hydrocarbons, chlorinated solvents, and ether fuels refining processes (e.g., oil product microbial desulfurization) and oil production processes (e.g., bacterial corrosion). 

Fish rapidly absorb, metabolize, and excrete chlorpyrifos from the diet (Barron etal. 1991). The mechanism of action of chlorpyrifos occurs via phosphorylation of the active site of acetylcholinesterase after initial formation of chlorpyrifos oxon by oxidative desulfuration. In studies with channel catfish (Ictalurus punctatus), the oral bioavailability of chlorpyrifos was 41%, substantially higher than in mammals. Catfish muscle contained less than 5% of the oral dose with an... 

The formal mechanism of oxidative desulfuration has been discussed in detail in an earlier book (Chapt. 7 in [59]). As shown in schematic and simplified form in Fig. 9.12, monooxygenase-catalyzed 5-oxygenation of phosphorothioates (9.67, X = O) and phosphorodithioates (9.67, X = S) yields an... 

---