The data on the use of mediator oxidation systems activated by electric current (anodic or parallel anodic and cathodic) in organic electrosynthesis are considered and generalised. Electrochemical activation of these systems permits successful application of catalytic versions and easy scaling of mediator-promoted processes. Chemical and environmental advantages of electrochemical processes catalysed by mediator oxidation systems are demonstrated. Examples of the application of organic and inorganic mediators for the oxidation of various classes of organic compounds under conditions of electrolysis are given.

https://iopscience.iop.org/article/10.1070/RC2009v078n02ABEH003886/pdf

Mediator oxidation systems in organic electrosynthesis

The investigation of electrocatalytic processes involved in the electrooxidation of organic compounds became a subject of growing interest at the beginning of the 1960s. A tremendous effort was undertaken to develop fuel cells, which theoretically have the ability to directly convert the chemical energy of hydrocarbons into electrical energy without the limitations due to Carnot’s theorem (i.e., with a high energy efficiency), an advantage which is particularly attractive for autonomous power sources.1–3 In such applications the rate of conversion of the organic compound by oxidation into carbon dioxide has to be as high as possible, in order to obtain the maximum energy available from the fuel and thus the maximum efficiency. However, the incomplete oxidation of organic compounds is of great interest as well, if one considers the potential applications for organic electrosynthesis, so that many attempts have been made in this field as well. Rigorous control of the experimental conditions may allow such reactions to become highly selective, leading to the development of industrial processes based on electrochemical reactions for the production of chemicals.4,5

Electrocatalytic Oxidation of Oxygenated Aliphatic Organic Compounds at Noble Metal Electrodes

The synthesis of benzaldehyde derivatives by oxygenation of methylarenes is of significant conceptual and practical interest because these compounds are important chemical intermediates whose synthesis is still carried out by nonsustainable methods with very low atom economy and formation of copious amounts of waste. Now an oxygenation reaction with a 100% theoretical atom economy using a polyoxometalate oxygen donor has been found. The product yield is typically above 95% with no “overoxidation” to benzoic acids; H2 is released by electrolysis, enabling additional reaction cycles. An electrocatalytic cycle is also feasible. This reaction is possible through the use of an aqueous sulfuric acid solvent, in an aqueous biphasic reaction mode that also allows simple catalyst recycling and recovery. The solvent plays a key role in the reaction mechanism by protonating the polyoxometalate thereby enabling the activation of the methylarenes by an electron transfer process. After additional proton transfer and ox...

Oxygenation of methylarenes to benzaldehyde derivatives by a polyoxometalate mediated electron transfer-oxygen transfer reaction in aqueous sulfuric Acid.

Fenton's reagent, regenerated by the electrochemical reduction Fe3++e-→Fe2+ has been used for the hydroxylation of benzene into phenol in both continuous and batch processes. In a batch process, the current yield for this reaction is relatively low because of the oxidation, in the aqueous phase, of phenol and its oxidation products by HO0 radicals. When the phenol is continuously extracted in the form of phenate ions, the current yield is found to increase from 20 to 70% for a nine-fold rise in the rate of benzene circulation. Varying the sulphuric acid concentration in the range 0.05 to 0.8m has little effect on current yield, while varying the Fe3+ concentration gives a maximum yield of about 70% for a concentration of 0.1m. At higher Fe3+ concentrations the rate of production of HO0 radicals becomes too high in comparison with the rate of benzene hydroxylation. The presence of a co-solvent like CCl4, although unreactive with HO0, reduces the yield. A numerical model taking into account the 13 reactions for which rate constants could be found or estimated, was developed for the continuous process. This model can only predict the general trends in yield and is not quantitatively predictive. Attempts at hydroxylating fluorobenzene using Fenton's reagent with electrochemical regeneration did not give appreciable yields of fluorophenols.

The electrochemical regeneration of Fenton's reagent in the hydroxylation of aromatic substrates: batch and continuous processes

Photobleaching of methyl orange in titanium dioxide suspended in aqueous solution

Oxidation of toluene to benzaldehyde by Mn3+ and OH free radicals, generated in the anodic and cathodic reactions respectively, was carried out simultaneously in the cathodic and anodic compartments of a cell. The selectivity of benzaldehyde was very high in both the anodic and cathodic reactions. The maximum total current efficiency for benzaldehyde production in the paired electrooxidation was 171%.

Paired electro-oxidation. I. Production of benzaldehyde

The catalytic effects of the silver ion on toluene oxidation by Ce4+ as well as the anodic generation of Ce4+ were investigated. The rates of toluene oxidation were first order with respect to both Ag+ and Ce4+. Argentic oxides formed on the surface of the platinum anode when Ag+ was present in the anolyte, which increased the activity of the electrode. About 3×10−3M of Ag+ was needed to obtain maximum current efficiency.

Catalytic effects of the silver ion on the indirect electrochemical oxidation of toluene to benzaldehyde using Ce3+/Ce4+ as mediator

Adsorption and electrokinetics of glucose reduction

Product distributions and kinetics of cathodic reduction of vanillin in aqueous solution

The interfacial mass transfer of mediators of anodic oxidation of glucose using bromide ion and hypobromite ion as the redox mediator was studied. Both the experimental results and the theoretical analysis indicate that the rate of oxidation of glucose is independent of the concentration of glucose but significantly affected by the interfacial mass transfer and the initial concentration of the mediator, the ratio of the surface area of electrode to the volume of solution, and the rate of generation of the active mediator. The interfacial mass transfer coefficient and the rate constants of the generating mediators were obtained.

Jiin-Jiang Jow

Papers

Paired electro-oxidation. I. Production of benzaldehyde

Catalytic effects of the silver ion on the indirect electrochemical oxidation of toluene to benzaldehyde using Ce3+/Ce4+ as mediator

Adsorption and electrokinetics of glucose reduction

Product distributions and kinetics of cathodic reduction of vanillin in aqueous solution

The roles of redox mediators in the anodic oxidation of glucose