Photochemical oxidation of methane

With the addition of CO caused by photochemical oxidation of methane, a significant flux enters the atmosphere annually, but the principal global contributions are terrestrial, anthropogenic and from atmospheric photolysis of methane. 

Carbon monoxide (CO) strongly influences the concentration of the radical OH in the tropical atmosphere. CO oxidation can lead to either production or destruction of ozone, depending on the NOx mixing ratio. Tropical soils are either a sink or a weak source of CO, where photochemical oxidation of methane and other hydrocarbons and biomass burning emissions are the predominant CO sources. 

The photochemical oxidation of methane is the most important source of formaldehyde. Atmospheric formaldehyde is also produced by the photochemical oxidation of non-methane hydrocarbons. The kinetics of the reaction HCHO + OH has been studied both experimentally and theoretically.151"162 Kinetic isotopic effects for some deuterated formaldehyde isotopomers have been reported.153"155 Results of experimental and theoretical studies151"162 indicate a complex reaction mechanism consisting of three competitive reaction channels... 

In the very cold tropopause layer at the boundary between the troposphere and the stratosphere, water vapor in the atmosphere is condensed and forms ice crystals, a phenomenon that serves as a barrier to the movement of water into the stratosphere. Thns, little water is transferred from the troposphere to the stratosphere, and the main source of water in the stratosphere is the photochemical oxidation of methane ... 

Methane in the troposphere contributes to the photochanical production of carbon monoxide and ozone. The photochemical oxidation of methane is a major source of water vapor in the stratosphere. 

A number of possibilities exist for the direct one-step oxidation of methane. These include homogeneous gas-phase oxidation, heterogeneously initiated homogeneous reaction, heterogeneous catalytic oxidation, and photochemical and electrophilic oxidations. Several review articles give detailed treatment of the homogeneous and catalytic oxidations.18-27 Some important conclusions are as follows. 

Ogura et al. [124] studied the photochemical and electrochemical oxidation of methane to methyl chloride and methanol. This room-temperature process evolved chlorine at a platinum anode followed by photochemical generation of CT (chloride radical) from CI2. The chloride radical subsequently oxidized methane gas (fed to the anode) to the methyl radical (CH3). The methyl radical reacted with chlorine gas to form methyl chloride which was immediately hydrolyzed to methanol. The formation of methyl chloride and methanol began at t.l V vs. SCE, which corresponds to the onset of chlorine evolution. 

Photochemical oxidation of acetone at room temperature yields peroxide [119,120], acids [119—121] (acetic acid [121]), aldehydes [119—121] (in particular formaldehyde [121]), and C02 [120]. Methane and ethane are produced in small amounts [120], Under pressure at 180— 200°C, acetone is oxidized to peroxide (apparently CH3COCH2OOH), methylglyoxal, formaldehyde, acetic and formic acids, H20, and C02 [122], The oxidation produces, after 400 min, 0.25 mole l"1 methylglyoxal, 6 X 10"3 mole l-1 formaldehyde, 1.05 mole l-1 acetic acid, and 0.14 mole l-1 formic acid at 190°C and a pressure of 40 atm. The conversion to oxidation products occurs by two parallel routes [122]... 

In the lower atmosphere CH3O is an intermediate in the photochemical oxidation of hydrocarbons and may be important in the conversion of NO into NOg. Thus, in an extension of the work of Wiebe et al., CH3ONO was photolysed in the presence of NO, NO plus NO2, and NO plus Og. Nitro-methane has also been photolysed in the gas phase at 313 nm, the major products being CH3ONO = 0.22), HCHO = 0.20), CH3NO = 0.06), and NO = 0.10). When nitromethane decomposes thermally the same basic reaction occurs ... 

Photochemical Processes Basic Gas-Phase Free Radical Chemistry The Oxidation of Methane Hydrocarbon Oxidation Mechanisms Ozone... 

In this model, for every methane molecule which reacts, the sequence leads to 4 ozone and 2 hydroxyl radicals, extra. Formation of ozone in the lower troposphere is therefore catalysed by photochemical oxidation of organic molecules, but it does require comparatively high levels of NO (mixing ratio > 5 — 10 x 10 ) to be present. If it goes to completion, OH can react further with CO to make CO2 thus completing the oxidation of methane (Scheme 5.2). At low NO levels, the net reaction is the destruction of ozone via the reaction with CO [Scheme 5.2b]. 

In addition to these principal commercial uses of molybdenum catalysts, there is great research interest in molybdenum oxides, often supported on siHca, ie, MoO —Si02, as partial oxidation catalysts for such processes as methane-to-methanol or methane-to-formaldehyde (80). Both O2 and N2O have been used as oxidants, and photochemical activation of the MoO catalyst has been reported (81). The research is driven by the increased use of natural gas as a feedstock for Hquid fuels and chemicals (82). Various heteropolymolybdates (83), MoO.-containing ultrastable Y-zeoHtes (84), and certain mixed metal molybdates, eg, MnMoO Ee2(MoO)2, photoactivated CuMoO, and ZnMoO, have also been studied as partial oxidation catalysts for methane conversion to methanol or formaldehyde (80) and for the oxidation of C-4-hydrocarbons to maleic anhydride (85). Heteropolymolybdates have also been shown to effect ethylene (qv) conversion to acetaldehyde (qv) in a possible replacement for the Wacker process. 

As mentioned in Chapter 2, methane is a one-carhon paraffinic hydrocarbon that is not very reactive under normal conditions. Only a few chemicals can he produced directly from methane under relatively severe conditions. Chlorination of methane is only possible by thermal or photochemical initiation. Methane can be partially oxidized with a limited amount of oxygen or in presence of steam to a synthesis gas mixture. Many chemicals can be produced from methane via the more reactive synthesis gas mixture. Synthesis gas is the precursor for two major chemicals, ammonia and methanol. Both compounds are the hosts for many important petrochemical products. Figure 5-1 shows the important chemicals based on methane, synthesis gas, methanol, and ammonia. ... 

Actually, a similar approach was used in studying the oxidative addition of methane to an iridium complex. Hydrocarbon solvents would have reacted faster than methane with the photochemically produced unsaturated iridium species, therefore J.K. Hoyano et al chose perfluorinated hexane as being an inert solvent. The elevated pressure was necessary in order to increase the concentration of the methane in the solution sufficiently to shift equilibrium (15) to the right /20/. 

For the measurement of the hydrocarbon precursors of photochemical oxidants, the naturally occurring methane must be separated from the other so-called nonmethane hydrocarbons. Only one procedure, gas chromatography coupled with flame ionization detection, is available for this separation and measurement. Although instrumentation for routinely accomplishing this process is commercially available, its maintenance (continued operation) requires a degree of operational know-how that may be too costly for most public agencies in the United States to support. Consequently, the data currently are insufficient to relate the occurrence of photochemical oxidants and ozone accurately to some of their most important precursors, the nonmethane hydrocarbons. 

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