Reactions in Methane

At thermal energies, the CH4 ion reacts with methane according to the relation 

The reaction was observed by means of a photoionization technique by Poschenrieder and Warneck, who used a wavelength of 922 A (13.45 eV). The occurrence of this reaction at thermal energies has been used to get an upper limit on the heat of formation of the CH5 ion. However, such a limit based on electron-impact data is not reliably precise, particularly in such a case as methane. Both the photoionization cross section as a function of wavelength for production of CH4 from and the 

More recently,photoionization of CH4 at liquid-nitrogen temperature has established that its ionization potential is 12.61 eV (or possibly still lower) and that the thermal reaction to form occurs 

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Both this and previous studies demonstrate the existence of rather long chains of consecutive ion-molecule reactions in methane, ethylene, and acetylene, and thus they provide direct evidence for ionic mechanisms of condensation or polymerization in these gases. Polymers have been found in relatively high yields among the radiolysis products of these... 

Engleman, V. S. "Survey and Evaluation of Kinetic Data on Reactions in Methane/Air Combustion" Report No. EPA-600/2-76-003, 1976. 

Zhang, X.-Y., and Lipscomb, J. D., 1999, Kinetic studies on electron transfer reactions in methane monooxygenase from M. trichosporium OB3b, J. Inorg. Biochem. 74 349. 

TABLE 12.1 Estimated Gibbs Free Energy Changes of Selected Biological Reactions in Methane Fermentation under Physiological Conditions ... 

It was difficult to distinguish between associative and dissociative oxygen adsorption with MvK reactions. It is, however, clear that oxygen adsorption was an important factor in the reaction in methane rich conditions. In methane rich feed... 

Blundell, R. V. W. G. A. Cook, D. E. Hoare, and G. S. Milne (1965). Rates of radical reactions in methane oxidation. Int. Symp. Combustion, 10th, pp. 445-452. The Combustion Institute, Pittsburgh, Pennsylvania. 

The Winchester mechanism (10-7)-(10-ll) of methane conversion into lydrogenwith growth of the carbon cluster (Section 2.2.6) can be interpreted as a chain reaction with multiple uses of a positive ion. Decrease of the ionization potentials in the seqnence Cn 2n+ makes chain propagation (10-9) exothermic (Winchester mechanism) and fast becanse the ion-molecular reactions have no significant activation barriers. Additional information regarding the kinetics of ion-molecttlar reactions in methane and its derivatives can be found, in particular, in Hamlet and Moss (1969) Davis and Libby (1966) Sheridan, Greer, and Libby (1972) Olah and Schlosberg (1968) Adams and Babcock (2001) and Harioka and Kebarle (1976, 1977). 

D. A. Durden, P. Kebarle, and A. Good, Thermal ion-molecule reaction rate constants at pressures up to 10 Torr with a pulsed mass spectrometer. Reactions in methane, krypton, and oxygen, J. Chem. Phys. 50, 805-813 (1969). 

What is the difference between the a-bond metathesis and the electrophilic reaction in methane activation ... 

If air (or oxygen) and steam are both passed through a high-temperature bed of coal or coke these reactions can be balanced, thus controlling the bed temperature and the fusion of the ash. In the higher pressure Lurgi process the gas obtained is high in methane, formed in reactions such as... 

This reaction is an undesirable side reaction in the manufacture of hydrogen but utilised as a means of removing traces of carbon monoxide left at the end of the second stage reaction. The gases are passed over a nickel catalyst at 450 K when traces of carbon monoxide form methane. (Methane does not poison the catalyst in the Haber process -carbon monoxide Joes.)... 

Methane oxidations occur only by intermediate and high temperature mechanisms and have been reported not to support cool flames (104,105). However, others have reported that cool flames do occur in methane oxidation, even at temperatures >400 ° C (93,94,106,107). Since methyl radicals caimot participate in reactions 23 or 24, some other mechanism must be operative to achieve the quenching observed in methane cool flames. It has been proposed that the interaction of formaldehyde and its products with radicals decreases their concentrations and inhibits the whole oxidation process (93). 

Aldehydes are important products at all pressures, but at low pressures, acids are not. Carbon monoxide is an important low pressure product and declines with increasing pressure as acids increase. This is evidence for competition between reaction sequence 18—20 and reaction 21. Increasing pressure favors retention of the parent carbon skeleton, in concordance with the reversibiUty of reaction 2. Propylene becomes an insignificant product as the pressure is increased and the temperature is lowered. Both acetone and isopropyl alcohol initially increase as pressure is raised, but acetone passes through a maximum. This increase in the alcohoLcarbonyl ratio is similar to the response of the methanoLformaldehyde ratio when pressure is increased in methane oxidation. 

Commercial-scale processes have been developed for the production of hydrogen sulfide from heavy fuel oils and sulfur as well as from methane, water vapor, and sulfur. The latter process can be carried out in two steps reaction of methane with sulfur to form carbon disulfide and hydrogen sulfide followed by hydrolysis of carbon disulfide (116). 

For equation 26, starting with methane and soHd sulfur at 25°C, and ending with gaseous products at 600°C, the reaction is endothermic and requires 2.95 MJ /kg (705 kcal/kg) of CS2. The reaction of methane and sulfur vapor in the diatomic form is actually exothermic (23,78). Superheating of the sulfur is claimed to be preferable (79), and series operation of reactors offers a means of reducing process temperatures at which the sulfur dissociates (80). 

Chlorination of Methane. Methane can be chlorinated thermally, photochemicaHy, or catalyticaHy. Thermal chlorination, the most difficult method, may be carried out in the absence of light or catalysts. It is a free-radical chain reaction limited by the presence of oxygen and other free-radical inhibitors. The first step in the reaction is the thermal dissociation of the chlorine molecules for which the activation energy is about 84 kj/mol (20 kcal/mol), which is 33 kJ (8 kcal) higher than for catalytic chlorination. This dissociation occurs sufficiendy rapidly in the 400 to 500°C temperature range. The chlorine atoms react with methane to form hydrogen chloride and a methyl radical. The methyl radical in turn reacts with a chlorine molecule to form methyl chloride and another chlorine atom that can continue the reaction. The methane raw material may be natural gas, coke oven gas, or gas from petroleum refining. 

When one of the elements is solid, as in tire case of carbon in the calculation of the partial pressures of tire gaseous species in the reaction between methane and air, CO(g) can be used as a basic element together widr hydrogen and oxygen molecules, and thus the calculation of the final partial pressure of methane must be evaluated using the equilibrium constant for CH4 formation... 

The reaction shown above for the steam reforming of methatie led to die formation of a mixture of CO and H2, die so-called synthesis gas. The mixture was given this name since it can be used for the preparation of a large number of organic species with the use of an appropriate catalyst. The simplest example of this is the coupling reaction in which medrane is converted to ethane. The process occurs by the dissociative adsorption of methane on the catalyst, followed by the coupling of two methyl radicals to form ethane, which is then desorbed into the gas phase. 

Fig. 1. Examples of temperature dependence of the rate constant for the reactions in which the low-temperature rate-constant limit has been observed 1. hydrogen transfer in the excited singlet state of the molecule represented by (6.16) 2. molecular reorientation in methane crystal 3. internal rotation of CHj group in radical (6.25) 4. inversion of radical (6.40) 5. hydrogen transfer in halved molecule (6.16) 6. isomerization of molecule (6.17) in excited triplet state 7. tautomerization in the ground state of 7-azoindole dimer (6.1) 8. polymerization of formaldehyde in reaction (6.44) 9. limiting stage (6.45) of (a) chain hydrobromination, (b) chlorination and (c) bromination of ethylene 10. isomerization of radical (6.18) 11. abstraction of H atom by methyl radical from methanol matrix [reaction (6.19)] 12. radical pair isomerization in dimethylglyoxime crystals [Toriyama et al. 1977].

Stable intermediates are those where concentration and lifespan are comparable to those of stable reactants and products. An example is the reaction between methane and oxygen in the gas phase at 700 K and 1 atmosphere. The overall reaction is ... 

The chemical reactions that occnr in flames transform an initial reactant mixtnre into final reaction prodncts. In the case of fnel-oxygen combns-tion, the final prodncts are principally water vapor and carbon dioxide, althongh nnmerons other prodncts snch as carbon monoxide may be formed, depending on the reactant composition and other factors. If the ratio of fnel-to-oxygen is stoichiometric, the final reaction prodncts, by definition, contain no excess fnel or oxygen. Theoretically, this means that partial oxidation prodncts snch as CO (itself a fnel) are not formed. In reality, partial oxidation prodncts snch as CO or OH are formed by high tem-peratnre reactions. For example, the molar stoichiometric reaction of methane is written ... 

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