Oxidation, mercaptans

Merox [Mercaptan oxidation] A process for removing mercaptans from petroleum fractions by extracting them into aqueous sodium hydroxide and then catalytically oxidizing them to disulfides using air. The catalyst is an organometallic compound, either a vanadium phthalocyanine supported on charcoal, or a sulfonated cobalt phthalocyanine. Developed by UOP in 1958 and widely licensed by 1994, more than 1,500 units had been built, worldwide. Unzelman, G. H. and Wolf, C. J., in Petroleum Processing Handbook, Bland, W. F. and Davidson, R. L., Eds., McGraw-Hill, New York, 1967, 3-128. 

Disulfides are manufactured by the oxidation of unhindered mercaptans. This reaction is one of the most facile reactions in organic chemistry. On prolonged exposure to air, mercaptans become contaminated with several percent of the corresponding disulfides. Primary mercaptans oxidize more readily than secondary while tertiary mercaptans oxidize the least readily. 

Novel one step synthesis of cobalt (II) phtalocyanine-hydrotalcite catalysts for mercaptan oxidation in light oil sweetening... 

Mercaptan oxidation can proceed through coordination of the mercaptan to manganese dioxide, with the manganite ion providing a base for deprotonation of the mercaptan. 

Cobalt complexes of polymeric phthalocyanines have been employed in aqueous alkaline solution as heterogenous catalysts in the oxidation of thiols to disulfides (MEROX, mercaptan oxidation process in the petroleum industry, Eq. 6-12, see Sections 5.2 and 5.4, Experiments 5-11 and 5-12). The catalytic activities of the polymer 31 (M = Co(II)) are higher than those of dissolved low molecular weight phthalocyanines, and both complexes exhibit better activities on charcoal than on Si02 as carrier. This is the result of better electrical contact between different reaction centers, which facilitates a multi-electron process in the oxidation of R-S to R-S-S-R and reduction of O2 to H2O [95]. Another advantage of the heterogeneous catalysts in comparison to the dissolved low molecular weight phthalocyanines is their easy re-use. 

In our initial attempts to study the effect of (DODA) Chloride, (DODA)C, sonicated vesicles on the thiolysis of p-nitrophenylacetate (NPA) by n-heptyl mercaptan (HM) we found that metal from the probe catalyzed the oxidation of the mercaptan [Cuccovia, I.M., unpublished]. In order to avoid mercaptan oxidation we developed an alcohol injection method for the preparation of (DODA)C vesicles, that were characterized by gel filtration and by the incorporation of lipid soluble probes [15]. It is not clear, today, if the aggregates prepared by ethanol injection are highly permeable closed vesicles or other types of bilayer assemblies [16]. 

Since mercaptan oxidation is not complete, it also limits the COD reduction. This is independent of any possible transfer of a fraction of volatile mercaptans into the air gaseous phase. 

Merox process A catalytic process used in petroleum refineries to remove mercaptans from LPG, propane, butane, naphtha, kerosene, and jet fuel by oxidation to form hydrocarbon disulphides. An abbreviation of mercaptan oxidation, the process uses an aqueous solution of sodium hydroxide and a water-soluble catalyst to remove the foul-smelling mercaptans. 

---