Oxidation of Aliphatic Hydrocarbons

It is well known today that perhaps the most dramatic application of the fuel cell—an electrochemical device that may be based in the future upon the oxidation of aliphatic hydrocarbons— was in the Gemini Space Mission. In this application, the cell was based upon the use of a solid polymer electrolyte —a cation-exchange membrane in its acid form—but with hydrogen and oxygen as the fuels rather than an aliphatic hydrocarbon. Considerable research and development preceded and supported these successful missions and the units demonstrated that indeed the H2/O2 fuel cell was capable of extended performance at relatively high current densities—2l capability of fundamental importance in commercial applications. 

Other applications for fuel cells have been identified and development continues today, an important one being to provide an alternative and economically competitive source of electrical energy to meet the projected demand through the next decade. United Technologies has expended considerable effort toward this goal, believing that success can only be achieved if commercially available hydrocarbons and air are the reactants in the fuel cell. 

The fundamental studies of the electro-oxidation of low molecular weight, saturated hydrocarbons in aqueous electrolytes such as sulfuric and phosphoric acid, showed that adsorption of the organic molecule upon the surface of the catalytic electrode is a critical Partial oxidation occurs rapidly, 

The electrocatalytic oxidation of alkenes has similarly been the subject of extensive study, as part of the development of fuel cell technology. The overall reaction is thought to be similar to gas phase oxidation processes, involving both adsorption of the olefin and oxidative adsorption of water molecules, the latter forming intermediate species which react with the 

The reaction pathways involved in the anodic oxidation of a hydrocarbon molecule are shown below  

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Taking all these considerations into account, it is possible to postulate a general mechanism for the oxidation of aliphatic hydrocarbons namely,... 

Ortiz De Montellano PR, Stearns RA. Radical intermediates in the cytochrome P-450 catalysed oxidation of aliphatic hydrocarbons. Drug Metab Rev 1989 20 183-191. 

Markovetz, A. J. (1971). Subterminal oxidation of aliphatic hydrocarbons by microorganisms. CRC Critical Reviews in Microbiology, 1, 225—38. 

An interesting extension of the cpe technique is pulse electrolysis. The electrode is maintained not at one single potential, but at a series of potentials of controlled duration according to a predetermined program. Tills is done by means of a pulse generator (also commercially available). Pulse techniques have hitherto been used mainly for mechanistic studies 91,92-1 but hold great promise for synthetic applications too 90,2 65 As an example, in the anodic oxidation of aliphatic hydrocarbons in non-aqueous medium at a platinum anode, the electrode activity falls rapidly with time if the potential is kept constant, probably because of the formation of an adsorbed film of intermediates or products. However, regular, short cathodic pulses reactivate the anode and the reaction proceeds without difficulties 30 ... 

Tomat, R. and Rigo, A. (1985) Electrochemical oxidation of aliphatic hydrocarbons promoted by inorganic radicals. I. Hydroxyl radicals. J. Appl. Electrochem. 15, 167-173. 

At relatively low temperature (for example, 100 to 200 C), the gas-phase oxidation of aliphatic hydrocarbons in the absence of catalysts is immeas-... 

Indirect oxidation mechanisms have been proposed in some cases, such as for the oxidation of aliphatic hydrocarbons [25], for which voltammetric studies do not give unambiguous information about the nature of the electroactive species. Often, they are assumed to involve the oxidation of the anion of the supporting electrolyte to form an inorganic radical [Eq. (16)], which subsequently either attacks a C-H bond in the organic substrate [Eq. (17)] or oxidizes the substrate in an electron transfer process [Eq. (18)] ... 

Besides acting as an electron scavenger, the precise role of O2 has been elusive for some time and continues to be a subject of some debate. In several studies on the photocatalysed oxidation of aliphatic hydrocarbons, alcohols, ketones, carboxylic acids and aldehydes, Heller (1995) concluded that molecular O2, and not the photogenerated hole or OH radical, is the primary oxidising agent. As evidence, data were given that photooxidation of n-octane films on water produced only traces of octanols. 

Y. A. Kalvachev, T. Hayashi, S. Tsubota, M. Haruta, Vapor-phase selective oxidation of aliphatic hydrocarbons over gold deposited on mesoporous titanium silicates in the co-presence of oxygen and hydrogen, J. Catal. 186 (1999) 228. 

ORTIZ DE MONTELLANO, P.R. and STEARNS, R.A. (1989) Radical intermediates in the cytochrome P-450 catalysed oxidation of aliphatic hydrocarbons. DrugMetab. Rev., 20, 183. PARKE, D.V. (1968) The Biochemistry of Foreign Compounds (Oxford Pergamon). 

The products formed vary somewhat with the catalyst and the temperature employed, but in general represent all the stages of the oxidation of aliphatic hydrocarbons from alcohols to oxygenated acids, together with hydrocarbons and oxidized bodies resulting from secondary reactions. Alcohols, ketones, aldehydes, naphthenic acids and other substances formed make up a mixture of such complexity that quantitative analysis, even, is a hopelessly complicated task. 0... 

In the one-screen semi-industrial apparatus the excess heat developed was carried off by a system of cooling pipes, the closed ends of which were embedded in the catalyst. Either air or water could be used as the cooling medium. In this oxidation of aliphatic hydrocarbons the author usually prefers to keep the temperature of the catalyst below 400° C. (usually 280° to 380° C.) hence, in an apparatus larger than that used in the laboratory, the temperature tends to rise, because of the greater distance to any radiating surface and the non-conducting character of the catalyst and its carrier, which is usually asbestos. 

The hydroxylation theory of Bone2 and his co-workers has had wide acceptance as far as the oxidation of aliphatic hydrocarbons is concerned. The mechanism postulated involves the successive formation of hydroxyl compounds, which may add oxygen to form additional hydroxyl groups or which may lose water and decompose. In this way methane would first form methanol, then methylene glycol which would be decomposed to formaldehyde and water formaldehyde would be oxidized to formic acid or decomposed to carbon monoxide and hydrogen. The theory, however, is open to a number of criticisms. 

The treatment of such problems is more complicated than those involving only dissolved species, because one must choose an adsorption isotherm, which involves the introduction of additional parameters and, in general, nonlinear equations. In addition, the treatment must include assumptions about (a) the degree to which adsorption equilibrium is attained before the start of the electrochemical experiment (i.e., how long after the formation of a fresh electrode surface the experiment is initiated) and (b) the relative rate of electron transfer to the adsorbed species compared to that for the dissolved species. These effects complicate the evaluation of the voltammetric data and make the extraction of desired mechanistic and other information more difficult. Thus adsorption is often considered a nuisance to be avoided, when possible, by changing the solvent or changing concentrations. However, adsorption of a species is sometimes a prerequisite for rapid electron transfer (as in forms of electrocatalysis), and can be of major importance in processes of practical interest (e.g., the reduction of O2, the oxidation of aliphatic hydrocarbons, or the reduction of proteins). Our discussion here will deal with the basic principles and several important cases. 

Fig. 9-6. Schematic layout for pressure oxidation of aliphatic hydrocarbons.

In the catalytic oxidation of aliphatic hydrocarbons at atmospheric pressure, the oxidation reactions are probably stepwise, each successive step occurring with greater ease. The type of reaction for the succesave steps is the same, and when a carbon compound containing oxygen is oxidized further, it is the hydrogen atoms joined to the carbon atoms already in combination with oxjgen that ar attacked. Thus, the point of first attack continues to be the point for successive attacks. This has been clearly shown for the case of the oxidation of the isomeric octanes, where... 

Aliphatic nitro compounds cannot as a rule be prepared in the same way as the aromatic nitro compounds. The more rapid oxidation of aliphatic hydrocarbons by nitric acid is the main interfering factor, so that conditions must be chosen which minimize oxidation and promote nitration. The oxidation reactions are of such complexity in these cases that no attempt will be made to formulate them. Only a summary of the conditions favoring nitration will be given. The use of a solvent such as ether for carrying out the reaction is often successful. Also dilute nitric acid has been used, and alkyl (generally ethyl) nitrate. In the Friedel-Crafts reaction with ethyl nitrate, aluminium chloride is used as catalyst. In aliphatic nitrations with ethyl nitrate, alkalis such as metal alkoxides (NaOC Hs) are found to be best. The use of alkalis brings out the similarity of this reaction to aldol condensations which are also favored by alkalis. An example of aliphatic nitration, in comparison with an aromatic one may be given ... 

As with the electro-oxidation of aliphatic hydrocarbons, several pathways may be involved in anodic reactions of aromatic molecules (a) direct electron transfer from the aromatic substrate to form cationic species, (b) reaction of... 

Fleischmann, M. and Pletcher, D. (1968) The electrochemical oxidation of aliphatic hydrocarbons in acetonilrile. Tetrahedron Letters, 6255. 

Figure 1 Oxidation of aliphatic hydrocarbons by microorganisms to form organic acids. (Information from Wyatt, 1984 Smith, 1991)...

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