Sulfur cathodic reduction

Electrolytic reductions generally caimot compete economically with chemical reductions of nitro compounds to amines, but they have been appHed in some specific reactions, such as the preparation of aminophenols (qv) from aromatic nitro compounds. For example, in the presence of sulfuric acid, cathodic reduction of aromatic nitro compounds with a free para-position leads to -aminophenol [123-30-8] hy rearrangement of the intermediate N-phenyl-hydroxylamine [100-65-2] (61). 

Special Methods.—Many special methods are employed to produce reducing salts, particularly cathode reduction. The following exercise illustrates one of these special methods, viz., the action of a metal (zinc) on sulfur dioxide. 

Aniline can be reused for the synthesis of the starting material diazoaminobenzene. 3.2.5 Cathodic Reduction of Sulfur Compounds... 

Industrially, H202 was first produced by the anodic oxidation of sulfuric acid or bisulfates to persulfate, followed by hydrolysis and distillation [93]. The original process dates back to 1853 and was dominant for decades in spite of its high power consumption [1], In 1882, Traube obtained H202 by a cathodic reduction of dissolved 02 [2,94]. This is noteworthy because a strong oxidant with ° = 1.78 V at pH 0 was not expected to be formed at the cathode. The electrogeneration process can be written as follows ... 

A CNDO/2 method extending the atomic orbitals basis to sulfur d-orbitals has been applied to the study of the cathodic reduction of 1,2-dithiolium cations. ... 

A procedure for the cathodic reduction of anthranilic acid to the corresponding alcohol is described in Organic Synthesis [36]. Because the carboxylic acid group is so difficult to reduce, the method cannot normally be used for acid substituted with easily reducible functions or groups. For instance, pentafluorobenzoic acid loses fluorine by cathodic cleavage at mercury, in 20% sulfuric acid, at —1.20 V (SCE) 2,3,5,6-tetrafluor-obenzoic acid is obtained in 75% yield [38]. However, at potentials more negative than... 

Imino ether hydrochlorides can be reduced to amines in good yields at lead cathodes in aqueous sulfuric acid [159]. Cathodic reduction of nitriles in ethanolic sulfuric acid should be a useful procedure, therefore, with the first step in situ formation of an imino ether salt. 

By Cathodic Reduction of Acrylonitriles at a Sulfur/Carbon Electrode... 

The carrier ampholytes adjacent to the electrodes must be protected from anodic oxidation or cathodic reduction. This is done by using special electrode solutions, which perform the function of a liquid lock. A dilute solution of an acid such as phosphoric or sulfuric acid is used at the anode. A dilute alkali such as sodium hydroxide is used at the cathode. Electrolysis attracts the acid and the base to the respective electrodes. The acid at the anode gives a positive charge to the carrier ampholytes there. Thus they are repelled from the anode. At the cathode, the base gives a negative charge to the carrier ampholytes nearby. They are then repelled from the cathode. 

With 1,2-dichlorocyclohexane the same products are obtained as in CH2CI2/CF3CO2H albeit in lower yields. The same is true for methylcyclohexane. Unpleasant in these electrolyses, that are conducted in an undivided cell, is the formation of sulfur by cathodic reduction of sulfur dioxide, which partially covers the anode and cathode as a sticky coat. 

Sulfur cathodes that undergo reversible stepwise reduction in the presence of Li ions from S to Li2S via a series of Li2S (Li sulfides) moieties The main substrates for sulfur cathodes are carbonaceous materials (usually on Al foil current collectors) [7]. 

The liquid (sulfur-based compounds) and sohd sulfur cathodes (items 6 and 7) do not develop surface chemistry that can be separated from their main electrochemical redox reactions. Hence, when the reduction of sulfur SO2 or SOCI2 produces insoluble species such as LiCl, LijS, and LijO, they precipitate on the current collector [9]. When formed, LijS can be reoxidized, up to elemental sulfur, via various LLS intermediate compounds [10]. Hence, the current collector, which may be aluminum (Al) plus carbon in the case of sulfur cathodes or carbon in the case of SOCI2 cathodes, does not develop intrinsic surface chemistry beyond the precipitation of the reduction products of the active mass. 

In this section, a summary of the chemical principles involved with membrane reactors for desulfurization are overviewed. The details will be covered in the following sections. Electrochemical desulfurization technologies assisted by membranes have been extensively explored for the removal of sulfur that exists in sulfur compounds in fossil fuels and in SO2 form in flue gas. In principle, SO2 can be absorbed by an aqueous electrolyte solution and then electrochemically converted into species such as sulfate, hydrogen sulfide, and sulfur, among others, by oxidation or reduction processes, whereas the sulfur compounds in fossil fuels can be similarly removed. The universal reaction mechanism of the electrochemical cathodic reduction of organic sulfur compounds in gasoline and diesel is shown in Eqn (14.1) (Lam et al., 2012) ... 

T] = E-Eq. a semi-logarithmic Tafel plot yields the lines of the current densities of anodic metal dissolution and cathodic reduction of the redox system, as presented for iron dissolution in 0.5 M H2SO4 in Fig. 1-30 (Kaesche, 1979). The intersection of both lines yields Er and the related corrosion current density 4 within the electrolyte. In the case of iron corrosion in sulfuric acid, the corrosion rates determined by the electrochemical evaluation of the Tafel plot and the chemical analysis of the dissolved species or the weight loss of the specimen for simple immersion tests agree sufficiently well (Kaesche, 1979). 

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