Oxidation of mercaptans

Due to the presence of hydrocarbons in the gas feed to the burner section, some undesirable reactions occur, such as the formation of carbon disulfide (CS2) and carbonyl sulfide (COS). A good catalyst has a high activity toward H2S conversion to sulfur and a reconversion of COS and CS2 to sulfur and carbon oxides. Mercaptans in the acid gas feed results in an increase in the air demand. For example, approximately 5-13% increase in the air required is anticipated if about 2 mol% mercaptans are present. The increase in the air requirement is essentially a function of the type of mercaptans present. The oxidation of mercaptans could be represented as ... 

When free radicals or other reactive organic species are generated in the electron transfer step, the pattern of surface coverage may have a dramatic impact on the yields of various possible organic oxidation products. Consider the oxidation of mercaptans (RSH) by a metal oxide surface. One-equivalent oxidation forms mercapto radicals (RS1) which quickly couple with neighboring radicals to form a disulfide product (36). 

Sulfoxide Radical. Denoted by the formula —SO—, Sulfoxides are organic compds of the general formula R.SO.R which are obtd by oxidation of mercaptans (analogous to ketones). They form both metal perchlorate complexes and nitrated organic derivatives. Examples of these compds of Ordn interest are presented below... 

Copper sweetening processes involving the oxidation of mercaptans to disulfides by oxygen in the presence of cupric chloride. 

Hypochlorite sweetening the oxidation of mercaptans in a sour stock by agitation with aqueous, alkaline hypochlorite solution used where avoidance of free-sulfur addition is desired, because of a stringent copper strip requirements and minimum expense is not the primary object. 

Oxidation of Mercaptans by Ov. Mercaptans are autoxidized in the presence of 02 in alkaline medium. In general, the oxidation is slow in the absence of catalyst because of unfavorable spin state symmetries that result from differences in the electronic configuration of the reactants (54). However, the reaction proceeds rapidly in the presence of traces of metal ions or transition metal phthalocyanines (55—58). The catalyst tends to alter the electronic structure of either the reductant and/or 02 so as to surmount the activation energy barrier imposed on the reaction by spin-state symmetry restriction. The coupled oxidation system in the presence of catalyst can be represented by ... 

Oxidation of Mercaptans by H>Q-> Mercaptans undergo rapid oxidative coupling reaction in alkaline medium in the presence of H202 without the necessity of metal catalysis. The overall reaction in the presence of a base can be written as ... 

Oxidation of Mercaptans by O Oxidation of methyl mercaptan in aqueous solution by 03 produces methane sulfonic acid as the major product. The reaction mechanisms are complex with the formation of dimethyl disulfide (CH3SSCH3), methyl methane thiolsulfonate (CH3S02SCH3), and methyl methanethiolsulfinate (CH3SOSCH3) as minor products. Continued ozonation could result in slow formation of sulfuric acid ... 

Minalk A variation of the Merox process in which a small quantity of a weak aqueous alkali is continuously added to the catalyst bed in order to promote the oxidation of mercaptans to disulfides. 

The end product of the oxidation of mercaptans, sulfides, disulfides, sulfoxides, sulfones, etc., is a sulfonic acid. From a preparative stand-... 

The amount of methyl mercaptan (MM) adsorbed (and converted to dimethyl disulfides (DMDS) depends on the sur e pH [ 1, 2], and the presence of various impregnants, such as potassium iodide, potassium iodite, potassium carbonate or ammonia [3,4], It has also been pointed out in the literature that different functional groups on the carbon sur ce or/and metal ions such as iron can catalyze oxidation of mercaptans to disulfides [3-6]. As we have found recently, there is an indication of a competition for high-energy adsorption sites between dimethyl disulfide and water molecules when adsorption occurs in the presence of moisture [1,2]. This happens as a result of big differences between water and DMDS in the strength of adsorption forces and their incompatibility (DMDS has very low solubility in water) [7]. 

Gentle oxidation of mercaptans and thiols leads to coupled products— disulfides. The oxidants used for this purpose are air [9], hydrogen peroxide [186], dichromates [663], chlorochromates [619], manganese dioxide [816], copper permanganate [896], ferric chloride [907], and diethyl azodicar-boxylate [977] (equations 544-546). 

The oxidation of mercaptans with chlorine in water gives sulfonyl chlorides. Ethyl mercaptan and pentyl mercaptan afford ethanesulfonyl chloride and pentanesulfonyl chloride in yields of 73 and 78% at 5 and 1-5 C, respectively [6S3]. 

Some of these reaction flavors occur in nature. The oxidation of mercaptans to form disulfides and other products is in part responsible for the loss of flavor from fresh roasted coffee only a short time after brewing. Furfiiryl Mercaptan is one of the most potent flavor components in coffee and is responsible for a fresh brewed coffee flavor. While it is present at only 1.55 ppm versus Furfuryl Alcohol (515 ppm) and Furfural (157 ppm), its flavor contribution is 310,014 times its flavor threshold (0.005 ppb). If there were a 100% conversion of Furfuryl Mercaptan to Diflirfuryl Disulfide, the coffee would appear to be 10 times weaker due to Difurfliryl Disulfide s flavor threshold being 0.05 ppb, or ten times weaker than Furfuryl Mercaptan (equation 10). The overall effect of this oxidation is that the coffee has lost its flavor. 

The oxidation of mercaptans can be useful to prepare other flavor molecules. Furfuryl Mercaptan and Methyl Mercaptan can be oxidized together to give Methyl Furfuryl Disulfide, a potent material useful for bread, pork and other meat products. It is also the third most active flavor material in fresh brewed coffee, being present at 0.38 ppm or 9,623 times its flavor threshold (0.04 ppb) (equation 11). 

Thermal and photochemical reactions of other, stronger reductants with Fe(III) oxides, including oxalate (Baumgartner et al., 1983 Blesa et al., 1987), ascorbate (Zinder et al., 1986), and citrate (Waite and Morel, 1984b) have been studied. The oxidation of mercaptans by Fe(in) oxides has been examined (Baumgartner et al., 1982 Waite et al., 1986 Waite and Torikov, 1987) in order to understand chemical transformations of mercaptans in the environment, and to improve formulations for rust and scale removal. 

Oxidation of mercaptanes is possible as well [25], while applications in the sweetening of the petroleum fractions in the MEROX process [26] seem possible. 

A.A. Oswald, F. Noel, and A.J. Stephenson, Organic sulfur compounds. V. Alkylammonium thiolate and peroxide salts possible intermediates in amine-catalyzed oxidation of mercaptans by hydroperoxides, J. Org. Chem. 1961, 26, 3969-3974. 

On the other hand, mild oxidation of mercaptans may result in the formation of disulfides. Thus ... 

Substantial quantities of soip- gasoline are being sweetened by oxidation of mercaptan Sulfur by means of oxygen and copper chloride solution. 

Buck, T., Bohlen, H., Wohrle, D., and Schulz-Ekloff, G. (1993) Influence of substituents and ligands of various cobalt(ll) porphyrin derivatives coordinatively bonded to silica on the oxidation of mercaptan. J. Mol. Catal., 80, 253-267. 

Scheme 4.5 Proposed pathway of the photocatalytic oxidation of mercaptane over hydroxyapatite [50],...

Further thorough studies would be necessary to define in detail the mechanism of these reactions. The limited evidence available is, however, consistent with the reasonable assumption that the reaction proceeds by a radical chain mechanism, probably initiated by heavy metal ions and involving thiyl radicals following a scheme similar to that proposed for the oxidation of mercaptans by molecular oxygen (see section IV). 

Thiols undergo an easy photolytic reaction (sec chapter 10 on photochemistry) which is in fact an oxidation of mercaptans to disulphides (equation 109). 

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