Functionalization of methane

Fig. 3 Methane conversion and hydrogen yield as a function of methane flow rates...

The carbon particle size varied from 80 to 120 run. The BET surface areas of carbon as a function of methane flow rate were compared with those of commercial carbon blacks in Table 3. The BET surface area ranges from 81 to 193 m /g with methane flow rates and this decrease is due to the increase of particle size. Carbon black which has lower surface area of 30 to 100 m /g can be used in rubber industry, while high surface area (> 700 m /g) carbon black is applied to activated carbon. 

This cycle, often referred to as the Shilov-cycle converts methane into methanol and chloromethane in homogeneous aqueous solution at mild temperatures of 100-120 °C (11). However, while Pt(II) (added to the reaction as PtCl ) serves as the catalyst, the system also requires Pt(IV) (in the form of PtCle-) as a stoichiometric oxidant. Clearly, this system impressively demonstrates functionalization of methane under mild homogeneous conditions, but is impractical due to the high cost of the stoichiometric oxidant used. A recent development by Catalytica Advanced Technology Inc., often referred to as the Catalytica system used platinum(II) complexes as catalysts to convert methane into methyl-bisulfate (12). The stoichiometric oxidant in this case is S03, dissolved in concentrated H2S04 solvent. This cycle is depicted in Scheme 3. 

For the catalytic functionalization of methane 0.21 mmol of compoimd 18 and 100 equivalents K2S2O8 are suspended in a mixture of 60 mL trifluoroacetic acid and 10 mL trifluoroacetic acid anhydride and are transferred into an autoclave. With an initial methane pressure of 20 atmospheres and a temperature of 80 °C the only product is trifluoroacetic acid methylester (510% yield relative to palladium). 

Some of the first catalytic model systems for the simulation of the function of methane monooxygenase comprise monomeric as well as dimeric iron-containing model complexes bearing hydro-tris(pyrazolyl)borate ligands [6]. These complexes, e.g. 3, catalyze the oxidation of aromatic and aliphatic carbon-hydrogen bonds in the presence of oxygen (1 atm), acetic acid and zinc powder at room temperature (Scheme 2). 

Shilov s catalytic process using a mixture of Pt(II) and Pt(IV) salts for the conversion of methane into methanol and methyl chloride in aqueous solution makes a chemical paradigm for alkane functionalization. One of the new approaches finds oxidative functionalization of methane is catalyzed by a (bipyrimidine)platinum(II) complex in concentrated sulfuric acid at 100 In... 

Fig. 11. Rates of products formation as a function of methane pressure in the COS-...

Figure 11 Selectivities toward CO and H2 formation, obtained over Ru/Ti02(D) catalysts, as a function of methane conversion at 1073° K under conditions of partial oxidation of methane. (From Ref. 95.)...

Methane and ethane are the most abundant and the least reactive members of the hydrocarbon family, and their selective conversion to useful chemical products is of great scientific, as well as practical, interest. This review highlights some of the recent advances in the area of low temperature, catalytic, activation and functionalization of methane and ethane. Particular emphasis has been placed on C-H and C-C activation processes leading to the formation of oxygenates. 

Below, we describe catalytic systems for the activation and functionalization of methane and ethane. Rather than a comprehensive review, the account highlights some of the recent advances in the area. 

The radical-initiated functionalization of methane proceeds even more readily in fuming sulfuric acid (27-33% S03 content by weight was employed) [25]. Thus, a variety of radical-initiators were found to convert methane to CH3S03H at 90°C. For every initiator examined, the product concentration was many times the concentration of the initiator (>700 times in the case of K2S208 ). The... 

Scheme 14 Functionalization of methane and ethane using silver-based catalysts and supercritical carbon dioxide as the solvent...

Function. An obvious function of methane monooxygenase is the oxidation of methane to methanol. The membrane-bound form could fulfill this function within an electron-transport chain. A further, possible function could be derived from the observation that, in methanotrophic bacteria grown in copper-enriched medium, the main mass of membrane-bound proteins is methane... 

Figure 2.7. C2 yields in a membrane and a co-feed reactor as function of methane conversion. From Ma et al. [2.190], with permission from Elsevier Science.

Figure 6. Abundance of the CHS and CtHs radicals produced by the radiolysis of methane with 100 e.v. electrons as a function of methane pressure...

Figure A2.5.29. Peak positions of the liquid-vapour heat capacity as a function of methane coverages on graphite. These points trace out the liquid-vapour coexistence curve. The full curve is drawn for p = 0.127. Reproduced from [31] Kim H K and Chan M H W Phys. Rev. Lett. 53 171 (1984) figure 2. Copyright (1984) by the American Physical Society.

Whereas the Mobil process starts with syn gas based methyl alcohol, Olah s studies were an extension of the previously discussed electrophilic functionalization of methane and does not involve any zeolite-type catalysts. It was found that bifunctional acidic-basic catalysts such as tungsten oxide on alumina or related supported transition metal oxides or oxyfluorides such as alumina or related supported transition metal oxides or oxyfluorides such as tantalum or zirconium oxyfluoride are capable of condensing methyl chloride, methyl alcohol (dimethyl ether), methyl mercaptan (dimethyl sulfide), primarily to ethylene (and propylene) (equation 65) . 

Several systems for selective catalytic reactions based on Shilov s system have been developed with oxidants more practical than platinum(IV). Periana reported two different systems for the oxidation of methane in sulfuric acid containing SO,. One of the catalysts is a simple mercuric halide, and reactions catalyzed by this mercury compound generated methyl sulfate with turnover frequencies of 10" s" . The second system is more reactive and is based on a platinum complex containing a bipyrimidine ligand (Equation 18.7). In this case, methane is converted to methyl bisulfate with 81% selectivity, greater than 500 turnovers, and a turnover frequency of 10 s" . These reactions are selective for the functionalization of methane to this methanol derivative because the electron-withdrawing... 

Figure 15W6.1 The heat released per of methane-air mixtures as a function of methane concentration Cp.

Table 17. Comparison of extrapolated and tabulated values of the G-function of methane...

Puentes MA, Olmos A, Munoz BK, et al. Catalytic functionalization of methane and light alkanes in supercritical carbon dioxide. Chem Eur J. 2014 20 11013-11018. 

Direct oxidative functionalization of methane to C2 hydrocarbons and oxygenates has some inherent obstacles, which are mainly due to the low reactivity of methane molecule compared to the desired products. Thus, reaction products are further oxidized to carbon oxides more easily than methane. This problem may be overcome when using soft oxidants like N2O, NO, or H2O2. However, their price in comparison to the price of the desired products should be taken into account. The application of solar energy for methane activation was not often and thoroughly tested to draw final conclusions and, therefore, deserves further investigating. 

Wang, Y., An, D. and Zhang, Q. (2010). Catalytic Selective Oxidation or Oxidative Functionalization of Methane and Ethane to Organic Oxygenates, Sci. China Chem., 53, pp. 337-350. 

Lorkovic, I., Yilmaz, A., Yilmaz, G., et al. (2004). A Novel Integrated Process for the Functionalization of Methane and Ethane Bromine as Mediator, Catal Today, 98, pp. 317-322. 

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