Electrooxidation indirect

Anthraquinone has been produced commercially from anthracene by indirect electrooxidation based on the Cr3+/Cr6+ couple, cf. Fig. 9. 

Pulping additives such as quinoid compounds increase the yield of the pulp mass up to 4% [128]. For commercial application the most promising additives are anthraquinone (AQ) or the more convenient soluble salt of tetrahydroan-thraquinone (THAQ). If AQ or THAQ could be obtained at a price below 2/kg it would find a substantial market as a pulping additive [129], Commercial production of THAQ is now based on the partial thermochemical oxidation of naphthalene. In recent years, however, the lure of the pulp market has promoted several attempts to develop a process for the electrosynthesis of THAQ based on the indirect electrooxidation of naphthalene to naphthaquinone (NQ) with Ce4+, according to the stoichiometry of the reactions 1, 2 and 3. 

Fig. 12. Indirect electrooxidation of toluene and toluene derivatives using Mn(III) as mediator [140] Industrial plant...

Wang et al240 reported the electrooxidation of MeOH in H2S04 solution using Pd well-dispersed on Ti nanotubes. A similar reaction was studied by Schmuki et al.232 (see above), but using Pt/Ru supported on titania nanotube which appear a preferable catalyst. Only indirect tests (cyclic voltammetry) have been reported and therefore it is difficult to understand the real applicability to direct methanol fuel cell, because several other aspects (three phase boundary to methanol diffusivity, etc.) determines the performance. 

Halide ions, in general, are typical nonmetal type electron carriers. The following halogen redox systems for indirect electrooxidation are well established Cr/[C1]+, Br /[Br]+, Br-/BrO , I /[I]+, I03 /I04, and so on [29, 30]. The oxidation potentials (E°) of halogen redox... 

Besides metal and nonmetal redox systems, organic redox systems are also important electron carriers for indirect electrooxidation. 

The oxidation of secondary alcohols (66) to (67) is possible by indirect electrooxidation utilizing thioanisole as an organic redox catalyst in a PhCN-2,6-lutidine-Et4NOTs-(C/Pt) system at 1.5 V vs. SCE (Scheme 25) [81] and is also performed in the presence of 2,2,2-trifluoroethanol [82]. It is suggested that the initially formed cation radical sulfide species derived from the direct discharge of the sulfide provides phenylmethyl-alkoxysulfonium ions, which are transformed to (67) and thioanisole. 

Scheme 31) [109]. 4-Chlorobutanal, cyclopropylaldehyde, and m-phenoxyl-benzaldehyde are also prepared in the two-phase system [110]. Indirect electrooxidation of 6y3-methyl-3y3, 5a-dihydroxy-16a, 17a-cyclohexanopregnan-20-one are indirectly electrooxidized to the corresponding 5a-hydroxy-3,20-dione using sodium bromide and substituted TEMPO as the mediating couple [111]. 

Redox systems other than halide salts are also used for catalytic indirect electrooxidation. For instance, p-methoxytoluene (90) can be oxidized in an aqueous K2S2O8-(C02K)2 system to give anisaldehyde (91)... 

Success in indirect electrooxidation with a metal redox carrier depends on the choice of a metal ion (1) that is best suited for the desired functionalization (2) that is soluble in the electrolysis media in both the high and the low oxidation states (3) that is expected to undergo electrooxidative regeneration with high current efficiency as well as to react with the substrate in a high yield (4) that can be readily separated from the products ... 

Chromic acid is a potential electron carrier for indirect electrooxidation. For example, p-xylene may be converted to terephthalic acid on treatment with Cr(VI). Similarly, methyl groups on aromatic rings... 

A method for preparing a Ti/Cr203 electrode has been developed for the indirect electrooxidation of 2-propanol to acetone [149]. Epoxidation of alkenes has been performed using oxo[5,10,15,20-tetrakis(2,6-dimethyl-3-sulfonatophenyl)... 

The electrogenerated Mn(lll)-assisted coupling of various olefins with active methylene compounds proceeds by indirect electrooxidation with a small amount of Mn(OAc)2 in the presence or absence of Cu(OAc)2 [196]. The Mn(Ill)-assisted carboxymethylation of styrenes (142) affords y-aryl-y-lactones (143) in good current yields (Scheme 57) [194, 195, 206, 207], (Table 9). 

The oxidation of toluene to benzaldehyde (max. yield 98.8%) can be performed in a Ce(Cl04)3-HCl04-(Pt/Ti-Cu) system by using the in-cell method in an undivided cell [28]. Indirect electrooxidations of organic compounds with Ce(IV) are listed in Table 12 [221-230]. For the electrogeneration of Ce(IV), platinized titanium or platinum oxide-on-titanium electrodes are known to be suitable for continuous oxidation of Ce(III) in perchloric acid. 

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