Cobalt oxidation catalysts thiol

In summary, the oxidation of thiols to disulfides is quantitative in aqueous alkaline solution and may best be effected at high oxygen pressures in the presence of a catalyst. The catalyst should dissolve in the alkaline solutions, and of the simple metal salts, the addition of copper, cobalt, and nickel results in the most effective catalysis. 

A poly(propylenamine) dendrimer (11, Fig. 6.37) functionalised with poly-(N-isopropylacrylamide) (PIPAAm) (see Section 4.1.2) was used as dendritic host for anionic cobalt(II)-phthalocyanine complexes (a, b) as guests, which are held together by supramolecular (electrostatic and hydrophobic) interactions [57]. These dendritic complexes were investigated as catalysts in the above-mentioned oxidation of thiols, where they show a remarkable temperature dependence the reaction rate suddenly increases above 34°C. One attempted explanation assumes that the dendritic arms undergo phase separation and contraction above the Lower Critical Solubility Temperature (LCST). At this temperature the phthalocyanine complex site is more readily accessible for substrates and the reaction rate is therefore higher. 

A latex-supported catalyst has been used to isolate sites. Styrene has been polymerized in the presence of an ionene diblock copolymer (a water-soluble cationic copolymer) to form a graft copolymer latex.31 The cobalt phthalocyanine sulfonate catalyst [CoPc(S03 Na+])4 was added and became attached to the cationic polymer. When this catalyst was used for the oxidation of thiols to disulfides by oxygen, the activity was 15 times that in a polymer-free system. 

Attention has also been focused on the oxidation of thiols in the presence of solid catalysts. One of the more comprehensive investigations into systems of this type has been made by Wallace et al. [133,145, 146] with a view to the possible use of phthalocyanine type complexes as commercial sweetening catalysts. Comparisons were drawn with metal pyrophosphates, phosphomolybdates, phosphotungstates, and phosphates. Pyrophosphates were found to be effective catalysts, possible due to the existence of six-membered rings involving the cobalt cation [147], which enhances the ability of the cation to donate an electron to oxygen and stabilises each oxidation state of the cation. For a series of pyrophosphates, the order of activity was Co > Cu > Ni > Fe, an activity pattern which was explained in terms of the stability of the 3d electron shells. 

Isolation of a cobalt phthalocyanine catalyst known to be active in autooxidation and to be deactivated by dimerization has been reported by Schutten (36). In this case, a polyvinylamine poly-dentate ligand was added to a dilute aqueous solution of the cobalt(II) phthalocyanine tetra(sodium sulfonate) in order to prepare a thiol oxidation catalyst. By employing dilute solutions, the polydentate polyamine polymer in effect isolated the cobalt(II) catalyst within an individual polyamine coil minimizing dimerization and significantly increasing catalyst activity. 

Phthalocyanlnes. Gebler (18) has reported the attachment of a variety of metal phthalocyanines to both 8% and 20% dlvlnylbenzene polystyrene copolymer beads. The attachment of the phthalocyanine unit was either ly a sulfonamide or a sulfone linkage. Nickel, vanadyl, cobalt, iron and manganese complexes were formed in this way. Since solution aggregation accounts for a diminution of the catalytic activity, it was anticipated that polymer immobilization would Increase reactivity. Such an effect was not observed and little advantage over the homogeneous catalysts could be observed in the oxidation of cyclohexene. Oxidations of thiols by immobilized phthalocyanines have been reported (19-20) by both Schutten and Brouwer. 

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. 

The continuous oxidation of thiols involved in the sweetening of light naphtha, with air to the disulphides using cobalt phthalocyanine complexes as catalysts, involves the formation of a stable complex between the thiolate ions and the metallocyanine catalyst . 

The oxidation of a series of alkyl and aryl thiols in aqueous alkaline solution has been studied in the presence of various metal ions. Quantitative amounts of disulfide were produced in all cases. The oxidation rate of thiols has been found to be affected by the geometric size and electron-directing properties of substituent groups in the organic chains of the thiols. The best three catalysts, when added as simple salts, have been found to be copper, cobalt, and nickel. The dependence of the rates of oxidation on the concentrations of reactants have been investigated in some detail. 

Using the ethanethiol system as a model, we investigated the dependence of the oxidation rate on the concentration of thiol, of oxygen, and of hydroxide ion. The results for the copper- (10"5M), cobalt-(10"3M), and nickel- (10 3M) catalyzed oxidations, together with the comparable system in the absence of added catalysts are recorded in Table V. 

Further evidence has been obtained to support the contention that the active catalysts are metal complexes dissolved in solution. With experiments reported in Table II, the kinetics of oxidation under standard conditions in the presence of various metal salts are compared with the rates of reaction when solid residues have been filtered from solution. The agreement between the rates in Cases 1 and 3 of Table II (where the amount of metal available is dictated by the solubility of metal complexes) shows that solid precipitates play little or no part in catalysis in all the systems studied. The amount of metal in solution has been measured in Cases 2 and 3 metal hydroxide complexes (Case 2) are not as soluble as metal-thiol complexes, and neither is as soluble as metal phthalocyanines (19). The results of experiments involving metal pyrophosphates are particularly interesting, in that it has previously been suggested that cobalt pyrophosphates act as heterogeneous catalysts. The result s in Table II show that this is not true in the present system. 

The catalytic effectiveness of the well-crystallized catalysts (DS3 to DSe) was evaluated by a simple laboratory test (screenings). Mercaptan removals of 70 to 83 % are obtained. This important catalytic activity is attributed to a) the meaningful contamination by C03 anions in the interlayer domain, which induces the pursued basic character as described by Constantino et al. [27,28] for the decomposition of MBOH to acetone and acetylene by MgAlC03 hydrotalcite at 353 K, and b) to the dispersion of the cobalt phtalocyanine complex in the interlayer space, since an aggregation of the cobalt phtalocyanine complexes decreases the thiol oxidation effectiveness [22]. 

Detailed investigation of the oxidation of ethane thiol in the presence of copper-, cobalt-, and nickel-containing catalysts was also carried out [138]. The reaction was stoichiometric to disulphide, and the dependence of the rates of oxidation on the concentration of individual reactants is summarised in Table 4. It can be seen that the concentrations of added metal bear little resemblance to the concentrations of catalytically active metal. The change from initial to final rates usually occurred at about 10— 30% of total conversion and was attributed to the formation of disulphides which can compete for coordination sites on the metal ion. 

The reaction was successfully carried out with various aryl(hetaryl) iodides and bromides involving different aryl thiols and alkyl thiols. A plausible catalytic cyde includes reduction of Co(II) complexes to Co(I), substitution of iodide ligand by SR leading to the formation of ArSCo(I), oxidative addition of ArX to this cobalt complex with formation of Co(III) derivative, followed by reductive elimination (Scheme 3.57) resulted in the product formation and regeneration of Co(I) catalyst. 

Active oxygen transfer in the reactions of aldehydes and alkenes with hydrogen peroxide in carboxylic acid medium is possible both via catalyst and via carboxylic acid.220 Micellar effects on the cobalt(II)-catalysed oxidation of jn-nitroso-lV,lV-dialky-lanilines by H2O2 have been studied with a variety of surfactants of different charge types. Acid-catalysed oxidations of aryl sulfides and phosphorus esters of thiols with... 

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