Electrochemical processes, indirect oxidation

HCIO4, one of the strongest of the mineral acids. The perchlorates are more stable than the other chlorine oxyanions, ie, chlorates, CIO chlorites, CIO or hypochlorites, OCf (3) (see Chlorine oxygen acids and salts). Essentially, all of the commercial perchlorate compounds are prepared either direcdy or indirectly by electrochemical oxidation of chlorine compounds (4—8) (see Alkali and chlorine products Electrochemical processing). 

An alternative to the direct anodic oxidation of organic contaminants are the methods of indirect oxidation with the aid of oxidizers formed electrochemically in situ. These oxidizers (or mediators) can be obtained in both anodic and cathodic processes. Anodic agents are the salts of hypochloric acid (hypochlorites), the permanganates, the persulfates, and even ozone. 

Flash rusting exhibited in neutral to alkaline water borne formulations appears to occur through a localised corrosion process probably Involving grit "activity" present from blasting, either directly or indirectly, in an electrochemical process. At such pH the rapid oxidation of ferrous to ferric ion produces... 

Anodic addition to an electron-rich heteroaromatic compound is used to transform furan to 2,5-dimethoxy-2,5-dihydrofuran, a valuable synthetic intermediate. Again, an indirect electrochemical process occurs. The bromide ion as redox catalyst is electrochemically oxidized to give bromine, which then acts as chemical oxidant for furan [7] ... 

In the case of A-acylated amino acid derivatives, the oxidation has to take place via an indirect electrochemical process using chloride ions as redox catalysts. In methanol as solvent, methylhypochlorite is formed as oxidant, which yields, via -chlorination, HC1 elimination, and methanol addition, the a-methoxylated amino acids as amidoalkylation reagents [27] ... 

Figure 22.1 Schematic representation of an indirect electrochemical process (given for an oxidation Med0Jt, Medred oxidized and reduced forms of the redox catalyst = mediator Sox, Sred oxidized and reduced forms of the substrate).

Wellmann J, Steckhan E. Indirect electrochemical processes 2. Electro-catalytic direct oxidation of aromatic compounds by hydrogen peroxide. Chem Ber 1977 110 3561-3571. 

Anodic processes may occur by direct electrochemical reaction of the waste at the anode, or by indirect oxidation when an oxidizing agent is generated in the waste stream. For example, oxygen and ozone may be generated from water in the waste stream, or the addition of sodium chloride to the waste stream can be used to generate free chlorine or hypochlorite at the anode. In addition, metal ions of higher valence states can be produced at the anode to oxidize wastes. 

Indirect electrochemical oxidation encompasses all those processes in which the pollutant is indirectly oxidized, either by the generation of oxidants (e.g., CI2, C10 , O3) or by the presence of a redox couple (Ag /Ag, Fe /Fe, Co /Co " ) that is used as an electron carrier for oxidation. 

Principles and applications of electrochemical remediation of industrial discharges are presented by Pallav Tatapudi and James M. Fenton. Essentials of direct and indirect oxidation and reduction, membrane processes, electrodialysis, and treatment of gas streams, and of soils, are complemented by discussions of electrode materials, catalysts, and elements of reactor design. 

In contrast to the direct process, an indirect process is one in which a foreign molecule or ion serves to shuttle electrons between the electrode and the substrate molecules. An indirect oxidation process may also involve the transfer of a hydrogen atom from a suitable substrate to a radical generated electrochemically. Indirect processes are typically observed for saturated aliphatic hydrocarbons and substrates that are more difficult to oxidize than the solvent-supporting electrolyte system. 

Similarly, carbanions from 1,3-dicarbonyl compounds can act as nucleophiles [153]. In a somewhat related reaction, enamines of cyclohexanone or cycloheptanone are oxidized in an iodide-mediated indirect electrochemical process leading to a Favorskii-type rearrangement. Thus, bicyclo[(n — 1).1.0] systems containing a nucleophile like a cyano or a methoxy function are formed [Eq. (27)] [154]. 

The indirect electrochemical process for the regeneration of NAD" from NADH is very fast, which is indicated by a strong catalytic current in the presence of glycerol dehydrogenase, the oxidized cofactor, and 7e.so-l,2-cyclohexanediol as substrate [109]. 

According to the reaction scheme given in Fig. 26, the anaerobic oxidation of l-glycerol 3-phosphate was performed under the conditions of an indirect electrochemical process using a water-soluble ferrocene derivative as mediator in the presence of o-fruc-tose-1,6-diphosphate aldolase from rabbit muscle for the in situ generation of the carbohydrate product. With a mediator concentration fo 1.5 mM using about 80 U of immobilized enzyme, after 25 h a 75% turnover of the substrate L-glycerol phosphate... 

Pyruvate oxidase (Pyox) is a FAD- and thiamine diphosphate (ThDP)-dependent enzyme that catalyzes the reaction of pyruvate to give acetyl phosphate or vice versa (see Fig. 15). If used in the oxidative way, it can be activated and reactivated under nonaerobic conditions using ferrocene mediators. Kinetic parameters of the indirect electrochemical process using the enzyme incorporated into a biomimetic supported bilayer at a gold electrode have been reported [142]. Similarly, FAD-dependent amino oxidases may also be applied. 

Within a potential range where radicals (like CF ) are not readily reduced or/and using indirect formation in solution by means of electron carrier, a chemistry—rather similar to that already developed by the Kolbe method by oxidation of CF3COO at a platninum anode—can be developed. Thus, Medebielle [193] showed that radical additional onto electron-rich olefins was possible and applied this for convenient synthesis of fluorinated heterocyclic compounds. Let us stress that the electrochemical process is in this case stoichiometric in electrons. 

The indirect, ex-cell chromic acid oxidation is the oldest industrial organic electrochemical process. 

H Marzouk. EDF, France. The Modified Grignard Reactor for Electrosynthesis Scale-Up of the Indirect Oxidation of Galacturonic Acid Electrochemical Processing, The Versatile Solution, Conference, ICI and EA-Technology. Barcelona, Spain, April 14-18, 1997. 

C Comninellis. The Industrial Application of the Indirect ex-cell Oxidation Process. 5th European Conf on Electrochem Processing, Chester. ICI, EA-Technology, eds. April 12-16, 1998. 

The oxidation of sulfite and thiosulfate becomes facile in the presence of iodide and novel disposable microband sensor electrodes have been developed by Williams and coworkers [187] to allow fast amperometric determination. A similar approach was proposed for the determination of sulfite in wine [188]. In this method, a coulometric titration is carried out in which S(IV) is indirectly oxidized to S(VI). Speciation of SO and sulfite was achieved down to micromolar levels. Sulfide and hydrogen sulfide can be determined electrochemically in the presence of an iodide mediator [189]. This process may be further enhanced at elevated temperatures. 

Electrochemical Oxidation Process. An electrochemical oxidation/ destruction treatment of various pollutants can be subdivided into two important categories direct oxidation at the anode, and indirect oxidation using appropriate anodically formed oxidants (Jiittner, Galla, and Schmieder, 2000). A critical review on electrochemical oxidation of organic pollutants for the wastewater treatment can be found elsewhere (Martmez-Huitle and Ferro, 2006). 

In fact, it is suggested that the indirect mechanism itself comprises two submechanisms the contact mechanism and the noncontact mechanism. While the contact indirect mechanism still takes into consideration the sessile bacteria and biofilm formation and their role in launching the electrochemical processes that will result in the dissolution of the sulfide mineral, the contact indirect mechanism describes the ferrous-ferric cycle by the oxidizing effect of the planktonic bacteria. A brief review of these submechanisms has been given elsewhere. ... 

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