Methane anodic oxidation

Anodic oxidation has been employed for water-soluble triphenyl-methane dyes. It has been shown that the formation of dye is an irreversible two-electron oxidation process.21-23 This method has been used for the oxidation of diamino triphenylmethane leuco compounds containing two to four sulfonic acid groups to obtain food-grade colored materials.24... 

Sulfides having two silyl groups are also oxidized electrochemically in methanol to give the corresponding methyl esters (Scheme 11 [36, 37]. The alkylation of (phenylthio)bis(trimethylsilyl)methane with alkyl halides followed by the anodic oxidation provides a convenient access to esters with one-carbon... 

The anodic oxidation of alkanes on a platinum anode in anhydrous hydrogen fluoride19 at low potentials is accompanied by exhaustive fluorination, with all of the hydrogen atoms being replaced by fluorine. Thus, the major fluorination products of methane and propane are carbon tetrafluoride and octafluoropropane, respectively. 

The anodic oxidation of ethers bearing two w-silyl groups is also useful. As shown in equation 33, the anion of methoxybis(trimethylsilyl)methane can be utilized as a synthon of the anion of the alkoxycarbonyl group. 

One of the main objective of SOFCs in the future is the use of gaseous mixtures of C0-H2-H20 produced by coal gasification plants or by steam reforming a hydrocarbon fuel, especially methane. Very little data is available about the direct oxidation of methane in SOFCs [96, 97], Steele et al. [97] have recently confirmed the poor electrocatalytic activity of Pt electrodes for the anodic oxidation of methane at 800 °C. Although nickel fulfills major requirements for anode materials when H2 and CO are employed as fuels, its use for the direct oxidation of methane encourages carbon deposition. To overcome this problem, alternative anode materials must be... 

A convenient method to produce porous surfaces is the anodic oxidation of aluminum plates. Such microstructured aluminum platelets have been coated by wet impregnation with Pt-, V- and Zr-precursors [35], and tested under catalytic methane combustion conditions. The conversion rate of oxygen followed directly the platinum content in the catalysts. These data were well reproducible even after five different runs. 

In the case of catalytic methane combustion, aluminum was chosen as an appropriate material for the catalyst wafers since anodic oxidation of aluminum can be used to obtain porous surfaces. Such micro structured aluminum platelets were coated by wet impregnation with Pt, V and Zr precursors [50],... 

Anthraquinone is being made at pilot plant scale from anthracene. The Ce3+/4+ couple is used with methane sulfonic acids. The steps involve anodic oxidation of Ce3+ and the use of Ce4+ outside the cell to convert naphthalene to napthaquinone, which is then converted to anthraquinone via a step involving butadiene. 

In a 1999 letter to Nature (Perry Murray etal., 1999), from North Western University, Illinois, the authors record the first laboratory achievement of useful oxidation rates for direct methane electrochemical oxidation, using an IT/SOFC. The cathode structures were porous lanthanum strontium manganite (LSM) on porous ( 203)0.15 (Ce02)o,85 or YDC. The anodes were cermets, porous YSZ with nickel in the pores. The laboratory operating temperatures were in the range 500-700 °C. The account of the North Western work, reporting on new anode types, continues on pp. 921-924 of Williams (2002). 

At p. 307 of Williams (2002), MIT enters the direct hydrocarbon field presenting alternative anode structures, asserted to be an improvement on copper-based anodes, as immediately above. See Figure A.5 on methane direct oxidation in Appendix A. 

Methoxy(trimethylsilyl)methane and methoxybis(trimethylsilyl)niethane have been proposed as new synthons for the formyl anion and the methoxycarbonyl anion, respectively after alkylation, C-Si cleavage is achieved by anodic oxidation. Similar electrochemical oxidative cleavage of acylsilanes reveals their potential as acyl cation synthons. Anodic oxidation of N-silylmethyl carbamates in methanol produces f -methoxymethyl carbamates in high yield. 

The oxidation of acetate by peroxodisulphate is much slower than that of formate. Glasstone and Hickling showed that the products, which include carbon dioxide, methane, ethane, and ethylene, are similar to those produced by the anodic oxidation of acetate ions (Kolbe electrolysis), and they inferred that the same organic radicals are formed as intermediates. Similar results are reported by Eberson et al. for the oxidations of ethyl terf.-butyl-malonate, tert.-butyl-cyanoacetate, and ferl.-butyl-malonamate ions. The oxidations of these ions and of acetate by peroxodisulphate are first order with respect to peroxodisulphate and zero order with respect to the substrate. Mechanisms involving hydroxyl radicals are excluded because the replacement of peroxodisulphate by Fenton s reagent leads to different products, so Eberson et al. infer that the initial attack on the substrate is by sulphate radical-ions. Sengar and Pandey report that the rate of the silver ion-catalysed oxidation of acetate is independent of the peroxodisulphate concentration. 

The anodic oxidation of 2,2-bis(ethylthio)propane producing acetone and an oxidation product of diethyl disulfide, claimed to be the corresponding disulfoxide, was reported many years ago [133]. A renewed interest in this reaction involving a C-S bond cleavage arose when bis(phenylthio)methane and some of its p-substituted ring derivatives were anodically oxidized in dry MeCN on Pt electrodes and afforded diaryl disulfides and formaldehyde, with the latter identified after aqueous workup [39, 134]. The same cleavage was observed when MeCN-H20 (9 1 v/v) was the solvent, but aryl benzenethiosulfonates were the main products [39] ... 

The anodic oxidation in methanol of sulifides and esters having two silyl groups leads to the formation of the corresponding methyl esters [Eq. (31)]. Since (phenylthio)-bis(trimethylsilyl)methane and methoxybis(trimethylsilyl)methane can be easily deproto-nated and alkylated, these compounds serve as effective synthons of the anion of the methoxycarbonyl group [118,119,122]. 

A Ritter-type reaction is also observed during the anodic oxidation of phenylthio-methane derivatives [186] of the type shown in Eq. (55), which summarizes the proposed reaction mechanism. 

Fig. 1.9 Comparison of electrolytes (3.0 M H2SO4 and 2.4 M CF3SO3H) upon the anodic oxidation of methane on Nafion -H coated R gas diffusion electrodes at 80 °C.

Lower chain alkanes that resist direct anodic oxidation in fluorosulfonic acid can be indirectly converted " . For that purpose peroxodisulfuryl difluoride is generated at the anode, being either platinum gauze or vitreous carbon, by constant current electrolysis in a divided cell charged with potassium fluorosulfonate in anhydrous fluorosulfonic acid then methane is introduced into the solution. Depending on the reaction conditions, at 45 °C selectively methyl fluorosulfonate (61% coulombic yield)... 

The anodic oxidation of alkanes in anhydrous hydrogen fluoride has been studied at various acidity levels from basic medium (KF) to acidic medium (SbFs) to establish optimum conditions for the formation of carbenium ions . The oxidation potential depends on the structure of the hydrocarbon methane is oxidized at 2.0 V, isopentane at 1.25 V vs Ag/Ag. Three cases of oxidation can be distinguished. In basic medium, direct oxidation of the alkane to its radical cation occurs. In a slightly acidic medium, the first-formed radical cation disproportionates to cation, proton and alkane. The oxidation is, however, complicated by simultaneous isomerization and condensation reactions of the alkane. In strongly acidic medium, protonation of the alkane and its dissociation into a carbenium ion and molecular hydrogen occurs. In acidic medium n-pentane behaves like a tertiary alkane, which is attributed to its isomerization to isopentane. The controlled potential electrolysis in basic medium yields polymeric species. 

Variation in the rate of anodic oxidation of normal saturated hydrocarbons with the number of carbon atoms parallels the variation in the rate of diffusion of the hydrocarbon through the electrolyte. This holds for different electrolytes (CsF/HF, H3PO4, HF, H2SO4) and electrodes (Pt-black, Raney-Pt). The rates are low for methane, highest for ethane and propane and then gradually decrease . ... 

The use of low molecular weight Refs. 33-37 hydrocarbons such as methane, ethane, and propane was considered to be an economic alternative to the anodic oxidation of hydrogen in the fuel cell. Extensive research and development was undertaken in this area in the 1960s. 

In 1859, Friedel electrolytically oxidized acetone and found a mixture of formic, acetic, and carbonic acids with evolution of carbon dioxide and oxygen at the anode in an acetone-sulfuric acid mixture. Further studies on ketones were not reported until 1931, when a similar study was carried out resulting in the formation of methane, ethane, and unsaturated hydrocarbons, in addition to carbon dioxide and oxygen at platinum anodes. The first anodic oxidation of benzene was reported in 1880, with the observation that the electrolytic oxidation of benzene in an ethanolic-sulfuric acid medium yielded unidentifiable substances. A few years later Gotterman and Friedrichs reported that hydrocarbons were obtained from the anodic oxidation of benzene in alcoholic-sulfuric acid solution at platinum anodes. 

Methane is the standard fuel used for SC-SOFCs with coplanar electrodes at intermediate and high SOFC operating temperatures [4]. At lower temperatures, the reduced catalytic activity of the anode for methane partial oxidation leads to... 

Murray, E.P., Tsai, T., and Barnett, S.A. (1999) Direct methane solid oxide fuel cell with ceria-based anode. Nature, 400, 649-651. 

Koizumi T, Fuchigami T, Nonaka T (1989) Anodic oxidation of (trimethylsilyl)methanes with tt-electron substituents in the presence of nucleophiles. Bull Chem Soc Jpn 62 219-225... 

Lin Y, Zhan Z, Barnett SA (2006) Improving the stability of direct-methane solid oxide fuel cells using anode barrier layers. J Power Sources 158 1313... 

This reaction scheme for methane is simplified but gives a flavor of the complexity of anodic oxidation reactions. The situation is more complicated for other hydrocarbons and shows that much more work is needed in this field in order to find even better electrocatalysts (combinations). More work is also needed before the vast knowledge of heterogeneous catalysis can be used for electrocatalysis. 

Fig. 2.16 (a) In situ FT-IR study of methane oxidation in [C4mPy][NTf2] (b) plots of methane anodic currents at 0.9 V apphed vs. methane concentration and (c) plots of normalized oxygen reduction currents at -1.2 V vs. oxygen concentration, (d) Scheme of methane oxidation and coupled oxygen reduction in [C4mPy][NTf2]... 

Binder and others [230] found that Raney platinum was the best single catalyst for electrochemical oxidation of hydrocarbons. Data obtained by Bianchi [231], Grubb [232], and Cairns and Mclnemey [203], who compared the catalytic activities of various platinum blacks in the anodic oxidation of propane, are consistent with this conclusion. As found by Petrii and Marvet [100], the oxidation rate of methane decreases sharply on changing from platinum to other platinum catalysts. Bockris and Dahms [192] showed that the oxidation rate of ethylene decreases in the series Pt > Rh > Ir and Pd > Au, where platinum is considerably more active than palladium. It was found in [233] that the oxidation rate of propane in alkali decreases in the order Pd > Pt > Ag, and those of ethane ethylene, propylene, and n-butane decrease in the order Pt > Pd... 

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