Methanol anodic oxidation

Ruthenium and osmium depositions on platinum are of special interest with reference to methanol anodic oxidation on fuel cells and were first reported in the early 1960s [116,117], Among the different methods of deposition of ruthenium or osmium on platinum [116-121], the spontaneous deposition method is attractive because of its simplicity and the fast achievement of a surface concentration plateau (reached in seconds) [26,122,123]. Contrary to the reversible behavior of the electrochemically deposited ruthenium on noble metal substrates, the films formed by spontaneous deposition normally do not dissolve easily from the metal surface. They are very stable and normally change to stable hydroxides and oxides when the electrode potential is increased. 

Okamoto H, Kawamura G, Ishikawa A, Kudo T. Activation of tungsten molybdenum carbide (W,Mo)C methanol anodic oxidation catalysts using alkaline solution. J Electrochem Soc 1987 134 1649-53. 

Experimental data fitting using the ECM was performed with ZPlot s complex nonlinear least square fitting program. It was confirmed that the experimental data were fitted very well at all four temperatures and potentials, and this demonstrates that the proposed model in Fig. 12.13 is feasible for description of the methanol oxidation process. The charge transfer resistance (R ) of the methanol anode oxidation was obtained from the ECM fitting at the four different temperatures and potentials, as plotted in Fig. 12.14. 

Anodic oxidation ui fluorosutfomc acid converts perfluoraalkyl methanols into perfluoroalkyl fluurondfates [4] (equation 42), (o-hydiododecafluorohexyl methanol can be converted to perfluorohexane-1,6-bisfluorosulfate when a higher ratio of Faraday charge per mole is applied 4] (equation 43)... 

Anodic oxidation of N-alkyl-N-fluoroalkylanilines m methanol mlroduces a methoxy group into fluonnated alkyls [ 9] (equation 81)... 

Carboxylic acids can be converted by anodic oxidation into radicals and/or carbo-cations. The procedure is simple, an undivided beaker-type cell to perform the reaction, current control, and usually methanol as solvent is sufficient. A scale up is fairly easy and the yields are generally good. The pathway towards either radicals or carbocations can be efficiently controlled by the reaction conditions (electrode material, solvent, additives) and the structure of the carboxylic acids. A broad variety of starting compounds is easily and inexpensively available from natural and petrochemical sources, or by highly developed procedures for the synthesis of carboxylic acids. 

The direct anodic oxidation of methanol became much more attractive after it was shown that platinum-ruthenium alloys are catalytically much more active in this reaction than pure platinum (pure ruthenium is totally inactive in this reaction). 

Very early during research into the anodic oxidation of methanol in the 1960s, it was repeatedly attempted to build experimental models of methanol-oxygen or methanol-air fuel cells. Most of these studies were conducted in snlfnric acid solntions... 

In the late 1960s it was discovered (Entina, 1968 Binder et al., 1972) that a strong synergy effect exists in the platinum-ruthenium system. Alloys of these two metals are two to three orders of magnitude more active catalytically for the anodic oxidation of methanol than pure platinum, whereas pure ruthenium is altogether inactive for this reaction. Prolonged exploitation of such anodes is attended by a gradual decrease in catalytic activity of the alloys because of slow anodic dissolution of ruthenium from the surface layer. A similar simation is seen for platinum-tin alloys, which... 

Ota K-I, Nakagawa Y, Takahashi M. 1984. Reaction products of anodic oxidation of methanol in sulfuric acid solution. J Electroanal Chem 179 179-186. 

The manner in which radicals attack furans has been studied deeply and earlier misconceptions have been corrected, especially the idea that anodic oxidation in methanol involves methoxy radicals.248 Furan cation radi-... 

In many furans the dicationic state is probably much less easily reached, and it is usual to regard anodic oxidations as ECEC sequences. Thus one electron is removed and cation radical 110 reacts with a nucleophile (methanol, for example). The radical center can lose another electron and then react with another methanol molecule giving the product (Scheme 56).287-289 Thus the net result is usually the same as in Scheme (55). 

Yoshida has studied anodic oxidations in methanol containing cyanide to elucidate the electrode processes themselves.288 He finds that, under controlled potential ( 1.2 V), 2,5-dimethylfuran gives a methoxynitrile as well as a dimethoxy compound (Scheme 57). Cyanide competes for the primary cation radical but not for the secondary cations so that the product always contains at least one methoxy group. On a platinum electrode the cis-trans ratio in the methoxynitrile fraction is affected by the substrate concentration and by the addition of aromatic substances suggesting that adsorption on the electrode helps determine the stereochemistry. On a vitreous carbon electrode, which does not strongly adsorb aromatic species, the ratio always approaches the equilibrium value. 

The anodic oxidation of 1,2-dienes in methanol takes place stepwise at each double bond yielding a tetramethoxylated compound as one of the products (equation if. This result is reasonable since a 1,2-diene is not a conjugated diene. 

It is well known that the anodic oxidation of 1,3-dienes in nucelophilic solvents such as methanol and acetic acid gives mainly 1,4-addition products together with a small amount of 1,2-addition products [31]. If the 1,3-dienes substituted... 

The diastereoselectivity of the anodic oxidation of 1-silyl-1,3-dienes is interesting [32]. The anodic oxidation of 5-phenyl-1-trimethylsilyl-1,3-hexa-diene in methanol gives a mixture of two diastereomers in 4 1 ratio. On the basis... 

The preparative electrochemical oxidation of silyl-substituted sulfides results in the cleavage of the C Si bond [36-38]. For example, the anodic oxidation of 1-phenylthio-l-trimethylsilylalkanes takes place smoothly in methanol in an undivided cell equipped with a carbon rod anode and a carbon rod cathode. Although 1-methoxy-l-phenylthioalkanes are formed as the initial products, they are converted into 1,1-dimethoxyalkanes during the course of the reaction (Scheme 8). The electrochemical reaction in the presence of diols such as ethylene glycol affords the corresponding cyclic acetals. 

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... 

Various nucleophiles other than methanol can be introduced onto the carbonyl carbon. Anodic oxidation of acylsilanes in the presence of allyl alcohol, 2-methyl-2-propanol, water, and methyl /V-methylcarbamate in dichlorometh-ane affords the corresponding esters, carboxylic acid, and amide derivatives (Scheme 24) [16]. Therefore, anodic oxidation provides a useful method for the synthesis of esters and amides under neutral conditions. 

The concept of electroauxiiiaiy is quite powerful to solve these problems. The pre-introduction of a silyl group as an electroauxiliary decreases the oxidation potential of dialkyl ethers by virtue of the orbital interaction. As a matter of fact, we demonstrated that the anodic oxidation of a-silyl ether took place smoothly in methanol.30 Selective dissociation of the C-Si bond occured and the methoxy group was introduced on the carbon to which the silyl group was attached. Therefore, a-silyl ethers seemed to serve as suitable precursors for alkoxycarbenium ions in the cation pool method. 

A wide variety of electrolyte compositions used for anodic oxidation of silicon can be found in the literature. The electrolytes can be categorized in inorganic or organic solutions. In the latter case electrolytes like ethylene glycol [Ja2, Me6, Ma5, Mel3], methanol [Ma2] or tetrahydrofuryl alcohol [Be3] are used, with salts such as KN03 added in order to improve the conductivity. Studies with pure water [Ga2, Mo3, Hu3] as an electrolyte were performed, as well as with additions... 

M. Morita, Y. Iwanaga, and Y. Matsuda, Anodic-oxidation of methanol at a gold-modified platinum electrocatalyst prepared by RE-sputtering on a glassy-carbon support, Electrichim. Acta 36, 947-951 (1991). 

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