Hydrocarbons oxidative dimerization

Keller and Bhasin were first to report in 1982 [1] on the catalytic one-step oxidative dimerization or coupling of methane (OCM) to C2 hydrocarbons, ethane and ethylene. Numerous investigations have followed this seminal work and a large number of catalysts have been found which give total selectivity to C2 hydrocarbons higher than 90% at low (<2%) methane conversion [2-6]. 

To demonstrate their approach, the preparation of the parent system 76 and the so-called bow-type hydrocarbon 78 is discussed here [42]. To prepare the trimer of 47 (which was not among the oxidative dimerization products, see above), the triyne 82 was coupled with ortho-... 

Kuchynka et al. [125] studied the electrochemical oxidative dimerization of methane to C2 hydrocarbon species using perovskite anode electrocatalysts. Three designs of solid oxide fuel cells were used, including tubular and flat plate solid electrolytes. The maximum current density for the dimerization reaction at these electrocatalysts was related to the oxygen binding energies on the catalyst surface. The anodic reaction was ... 

Methane can be catalytically oxidized in the fuel cell mode to simultaneously generate electricity and C2 hydrocarbons by dimerization of methane using a yttria-stabilized zirconia membrane. A catalyst, used as the anode, is deposited on the side of the membrane that is exposed to methane and the cathode is coated on the other side of the membrane. When the catalyst Ag>Bi2C>3 is used as the anode for the reaction at 750> 900X and atmospheric total pressure, the selectivity to ethane and ethylene exceeds 90%. But this high selectivity is at the expense of low power output and low overall methane conversion (less than about 2%). 

The oxidative dimerization of methane in the presence of oxygen or air often gives substantial amounts of carbon oxides as byproducts [1]. Conversion of methane to methylchloride and further condensation to higher hydrocarbons and hydrogen chloride avoids this problem. The reoxidation of hydrogen chloride to chlorine, as well as the formation of methylchloride from methane is known technology. 

Surprisingly perhaps, oxidation of C3 and C4 olefins over bismuth oxide alone leads mainly to oxidative dimerization to Cg or Cg dienes, and small amounts of cyclic hydrocarbons. The surface is possibly highly reduced, providing many Lewis acid centres for olefin coordination but few oxide ions for hydrogen abstraction and transfer. (The supported gallium oxide catalyst, developed by BP, leads to further hydrogen abstractions and significant yields of aromatics from lower olefins). 

In the second mechanism proposed as an alternative to the previous one, radical-cations are involved as active intermediate species [11]. In this case it is postulated that a Lewis acid (A), considered as an electron acceptor, reacts with the aromatic hydrocarbon and its derivatives to give a radical-cation species (9), which couples with another aromatic substrate to produce a dimer (10). Such a mechanism was proposed in order to interpret the oxidative dimerization of anisole by Lewis acids, involving addition of a radical-cation to the substrate (Scheme 6.3) [12,13]. 

Any substance capable of reacting with free radicals to form products that do not reinitiate the oxidation reaction could be considered to function as free-radical traps. The quinones are known to scavenge alkyl free radicals. Many polynuclear hydrocarbons show activity as inhibitors of oxidation and are thought to function by trapping free radicals [25]. Addition of R to quinone or to a polynuclear compound on either the oxygen or nitrogen atoms produces adduct radicals that can undergo subsequent dimerization, disproportionation, or reaction with a second R to form stable products. 

Faraday, in 1834, was the first to encounter Kolbe-electrolysis, when he studied the electrolysis of an aqueous acetate solution [1], However, it was Kolbe, in 1849, who recognized the reaction and applied it to the synthesis of a number of hydrocarbons [2]. Thereby the name of the reaction originated. Later on Wurtz demonstrated that unsymmetrical coupling products could be prepared by coelectrolysis of two different alkanoates [3]. Difficulties in the coupling of dicarboxylic acids were overcome by Crum-Brown and Walker, when they electrolysed the half esters of the diacids instead [4]. This way a simple route to useful long chain l,n-dicarboxylic acids was developed. In some cases the Kolbe dimerization failed and alkenes, alcohols or esters became the main products. The formation of alcohols by anodic oxidation of carboxylates in water was called the Hofer-Moest reaction [5]. Further applications and limitations were afterwards foimd by Fichter [6]. Weedon extensively applied the Kolbe reaction to the synthesis of rare fatty acids and similar natural products [7]. Later on key features of the mechanism were worked out by Eberson [8] and Utley [9] from the point of view of organic chemists and by Conway [10] from the point of view of a physical chemist. In Germany [11], Russia [12], and Japan [13] Kolbe electrolysis of adipic halfesters has been scaled up to a technical process. 

Polycyclic aromatic hydrocarbons Silica gel G Monovalent oxidation of the iodine complexes via radical cations yields dimeric or tetrameric aromatics [16]... 

The dimeric p-oxo diferric complex is then reduced and reacts with H02 to produce a terminal oxo-iron(V) complex which oxidizes hydrocarbons. 

At least for ethylene hydrogenation, catalysis appears to be simpler over oxides than over metals. Even if we were to assume that Eqs. (1) and (2) told the whole story, this would be true. In these terms over oxides the hydrocarbon surface species in the addition of deuterium to ethylene would be limited to C2H4 and C2H4D, whereas over metals a multiplicity of species of the form CzH D and CsHs-jD, would be expected. Adsorption (18) and IR studies (19) reveal that even with ethylene alone, metals are complex. When a metal surface is exposed to ethylene, selfhydrogenation and dimerization occur. These are surface reactions, not catalysis in other words, the extent of these reactions is determined by the amount of surface available as a reactant. The over-all result is that a metal surface exposed to an olefin forms a variety of carbonaceous species of variable stoichiometry. The presence of this variety of relatively inert species confounds attempts to use physical techniques such as IR to char-... 

As an example we may consider the Kolbe reaction, the oxidation of carboxylic acid and carboxylates of the form R-COOH or R-COO- to form coupled hydrocarbon products of the form R2. Investigation of this reaction in aqueous and non-aqueous solvents has revealed that the processes taking place are very complex indeed. In general, the product R2 is only formed at high current densities on smooth electrodes. At lower current densities, alkenes and non-dimeric products such as R-H are found, and, especially in alkaline solutions, the product R-OH can be formed in good... 

The activation of hydrocarbons on the catalyst surface was also discussed in the literature [255]. There are no clear experimental evidences of this activation with free radical generation [270]. However, examples of dimer (RR) formation as a result of oxidation of RH on the surface of Mn02 are known [270],... 

Anodic oxidation in inert solvents is the most widespread method of cation-radical preparation, with the aim of investigating their stability and electron structure. However, saturated hydrocarbons cannot be oxidized in an accessible potential region. There is one exception for molecules with the weakened C—H bond, but this does not pertain to the cation-radical problem. Anodic oxidation of unsaturated hydrocarbons proceeds more easily. As usual, this oxidation is assumed to be a process including one-electron detachment from the n system with the cation-radical formation. This is the very first step of this oxidation. Certainly, the cation-radical formed is not inevitably stable. Under anodic reaction conditions, it can expel the second electron and give rise to a dication or lose a proton and form a neutral (free) radical. The latter can be either stable or complete its life at the expense of dimerization, fragmentation, etc. Nevertheless, electrochemical oxidation of aromatic hydrocarbons leads to cation-radicals, the nature of which is reliably established (Mann and Barnes 1970 Chapter 3). 

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