Methane oxidative condensation

The key initiation step in cationic polymerization of alkenes is the formation of a carbocationic intermediate, which can then interact with excess monomer to start propagation. We studied in some detail the initiation of cationic polymerization under superacidic, stable ion conditions. Carbocations also play a key role, as I found not only in the acid-catalyzed polymerization of alkenes but also in the polycondensation of arenes as well as in the ring opening polymerization of cyclic ethers, sulfides, and nitrogen compounds. Superacidic oxidative condensation of alkanes can even be achieved, including that of methane, as can the co-condensation of alkanes and alkenes. 

In a variation of this method, isolation of the ben2hydrol derivative is not required. The methane base undergoes oxidative condensation in the presence of acid with the same or a different arylamine direcdy to the dye. New fuchsine [3248-91 -7] Cl Basic Violet 2 (16), is prepared by condensation of two moles of o-toluidine with formaldehyde in nitrobenzene in the presence of iron salts to give the corresponding substituted diphenylmethane base. This base is also not isolated, but undergoes an oxidative condensation with another mole of o-toluidine to produce the dye. 

More recently, direct catalytic oxidative condensation of methane to ethane (with metal oxides),57,82-84 as well as to ethylene and acetylene (via high-temperature chlorinative conversion) was explored.76 In all these processes, however, a significant portion of methane is lost by further oxidation and soot formation. The selectivity in obtaining ethane and ethylene (or acetylene), respectively, the first C2 products, is low. There has, however, been much progress in metal-oxide-catalyzed oxidative condensation to ethane. 

Any condensation of methane to ethane and subsequently to higher hydrocarbons must overcome the unfavorable thermodynamics. This can be achieved in condensation processes of oxidative nature, where hydrogen is removed by the oxidant. SbF5- or FS03H-eontaining superacid systems also act as oxidants. The oxidative condensation of methane was subsequently found to take place with more economical cooxidants such as halogens, oxygen, sulfur, or selenium 91... 

DIRECT COUPLING OF METHANE 3.4.1. Catalytic Oxidative Condensation... 

Hydrocarbon formation from methyl chloride can be catalyzed by ZSM-5482 483 or bifunctional acid-base catalysts such as W03 on alumina.420,447 The reaction on ZSM-5 gives a product distribution (43.1% aliphatics and 57.1% aromatics at 369°C) that is very similar to that in the transformation of methanol, suggesting a similar reaction pathway in both reactions.483 W03 on A1203 gives 42.8% C2-C5 hydrocarbons at 327°C at 36% conversion.447 When using methyl bromide as the feed, conversions are comparable. However, in this case, HBr can be very readily air-oxidized to Br2 allowing a catalytic cycle to be operated. Since bromine is the oxidant, the reaction is economical. The one step oxidative condensation of methane to higher hydrocarbons was also achieved in the presence of chlorine or bromine over superacidic catalysts.357... 

It is also significant that the direct oxidative condensation of methane to higher hydrocarbons takes place in the presence of S8 over superacidic catalysts, such as TaF5 and the like.357... 

Beside methanol and formaldehyde, the oxidation of methane may be directed to another route, leading to the formation of its condensation products, for example, ethane, ethylene and benzene. This route may provide an alternative way for the chemical use of natural sources of methane. Here, various catalysts were also tested using both 02 and N20 as the oxidants [22], The general picture observed by most authors was similar to that with methane oxidation to oxygenates. The conversion of methane was always higher with 02 than with N20. However, the selectivity to the coupling products showed an opposite trend. 

Figure 7. Time dependent product formation for Pd(ll)-catalyzed oxidative condensation of methane directly to acetic acid.

This direct, oxidative condensation of methane to acetic acid in one-pot could be competitive with the current three-step, capital intensive process for the production of acetic acid based on methane reforming to CO, methanol synthesis from CO, and generation of acetic acid by carbonylation of methanol. Key improvements required with the PdS04/H2S04 system, however, will be to develop more stable, faster, and more selective catalysts. Although it is possible sulfuric acid could be utilized industrially as a solvent and oxidant for this reaction, it would be desirable to replace sulfuric acid with a less corrosive material. This chemistry has recently been revisited, verified, and extended by Bell et al., who used Cu(II)/02 as the oxidizing system [22],... 

Pd (II) Catalyzed Oxidative Condensation of Methane to Acetic acid ... 

A great number of papers have been devoted to the oxidative condensation (coupling) of methane (see reviews [70], recent original reports [71] and patents... 

The protolytic oxidative condensation of methane in Magic Acid solution at 60 °C is evidenced by the formation of higher alkyl cations such as t-butyl and t-hexyl cations (equation 42). It is not necessary to assume a complete cleavage of [CH5] to a free, energetically unfavorable methyl cation. The carbon-carbon bond formation can indeed be visualized through reaction of the C—H bond of methane with the developing methyl cation. 

In the combustion of methane, oxidation is accompanied by a thermal decomposition. Reactions of hydrocarbon radicals lead to unsaturated compounds such as gaseous olefins and acetylene which are then cyclized into condensed aromatic hydrocarbons. The aromatic compounds are subjected to further dehydrogenation in the reaction zone to produce carbon particles. This carbon black is a random aggregate of crystalline graphite embedded at a certain proportion in an amorphous matrix. Some hydrogen is always present in this carbon deposit. 

The data on the kinetics of the oxidative condensation of methane are more numerous than on the other reactions of its oxidation. They show that the yield of C2 hydrocarbons in this reaction does not exceed 25%. As for the aforementioned processes, reports of higher yields, up to 30% or even higher, have not been confirmed. It has been shown that the reaction proceeds via a redox mechanism involving the interaction of methane with 0 or C>2 oxygen centres on the catalyst surface and the formation of CH3 radicals and surface OHg... 

The influence of a number of heterogeneous catalysts (SrCOs, BaCOs, and 7% li/MgO), active in the oxidative condensation of methane at low pressures, on the process at 500—750 °C was studied in [80]. However, at pressures above 3 atm, no contribution from... 

Fig. 7.40 Proposed tandem catalysis mechanism for the oxidative condensation of methane to acetic acid with Pd(ll)/H2S04.

Scheme 55) (235. 236). -The product obtained (77) is probably formed via the protonated form of the thiazole, whose reactivity is treated in Section IV, 1. The light-yellow leucobase (77) is reported to be oxidized by PbOj to the red-black carbinol (78) (236). This condensation reaction is also successful when benzaidehyde is replaced by formaldehyde, bis(2-amino-4-phenylthiazolyl-5)methane (79 i beine obtained (Scheme 56) (237). 

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