Biological oxidation of methane

Lieberman, R.L. and Rosenzweig, A.C. (2005) Crystal structure of a membrane-bound metalloenzyme that catalyses the biological oxidation of methane, Nature, 434, 177-182. 

There are only two important sinks that serve to destroy methane. The first is the oxidation of methane by aerobic bacteria in soils whereas the second and the most important sink is reaction (oxidation) with hydroxyl radicals in the atmosphere. Biological oxidation of methane in soils is responsible for 6-10% of the global source strength. Oxidation dne to the reaction of methane with hydroxyl radicals in the atmosphere, however, accounts for the remaining 90% (Cicerone and Oremland, 1988). An estimated 500 Tg year is removed from the atmosphere each year over 95% of the annual emission is removed through these two primary sinks (Khalil et al., 1992). 

Methane in coal is only released in significant volumes when a void is made in the coal (e.g. workings or a borehole). The methane then desorbs from the coal into the void. This can continue for up to 50 years. Chemical and biological oxidation of methane then forms carbon dioxide. Coal bed methane can also desorb into open fissures and faults. 

Rosenzweig AC, Frederick CA, Lippard SJ, Nordlund P. 1993. Crystal structure of a bacterial non-heme iron hydroxylase that catalyzes the biological oxidation of methane. Nature 366 537-543. 

Biological oxidation of methanol and ethanol in the body produces the corresponding aldehyde followed by the acid. At times the alcoholics, by mistake, drink ethanol, mixed with methanol also called denatured alcohol. In the body, methanol is oxidised first to methanal and then to methanoic acid, which may cause blindness and death. A methanol poisoned patient is treated by giving intravenous infusions of diluted ethanol. The enz5mie responsible for oxidation of aldehyde (HCHO) to acid is swamped allowing time for kidneys to excrete methanol. 

Balasubramanian R, Smith SM, Rawat S, Yatsunyk LA, Stemmier TL, Rosenzweig AC. Oxidation of methane by a biological dicopper centre. Nature. 2010 465 115-9. 

Transition metal catalysts, specifically those composed of iron nanoparticles, are widely employed in industrial chemical production and pollution abatement applications [67], Iron also plays a cracial role in many important biological processes. Iron oxides are economical alternatives to more costly catalysts and show activity for the oxidation of methane [68], conversion of carbon monoxide to carbon dioxide [58], and the transformation of various hydrocarbons [69,70]. In addition, iron oxides have good catalytic lifetimes and are resistant to high concentrations of moisture and CO which often poison other catalysts [71]. Li et al. have observed that nanosized iron oxides are highly active for CO oxidation at low tanperatures [58]. Iron is unique and more active than other catalyst and support materials because it is easily reduced and provides a large number of potential active sites because of its highly disordered and defect rich structure [72, 73]. Previous gas-phase smdies of cationic iron clusters have included determination of the thermochemistry and bond energies of iron cluster oxides and iron carbonyl complexes by Armentrout and co-workers [74, 75], and a classification of the dissociation patterns of small iron oxide cluster cations by Schwarz et al. [76]. 

The escape of natural gas from the gas distribution networks also exerts a considerable effect on physical, chemical and biological processes in the soil. The extent of the gas zone depends greatly on the rate of its escape, depth of the groundwater level, type of pipeline, the nature and moisture of the soil, treatment of its surface, etc. The composition of the gaseous phase in the soil is affected by the microbiological oxidation of methane. The rate of this depends on the soil temperature, presence of oxygen and the content of nutrients. At low temperatures, this microbial process is restricted, which also restricts the anaerobic zone. 

Nevertheless, direct use of dioxygen as a terminal oxidant would be very attractive for both economic and ecologic reasons. For example, easy and safe procedures for the partial oxidation of methane according to Reactions 3 and 4 would offer tremendous possibilities for the on-site conversion of CH4, stemming from biological or geological resources, into more valuable feedstocks. 

Thus, as in the case of alkenes in the preceding chapter, we start with the radical type of activation that is much older. Transition-metal compounds play a key role in radical activation, because they provide very strong oxidants that can oxidize hydrocarbons either by (reversible) electron transfer or H-atom transfer (more rarely by hydride transfer). Biological oxidation of hydrocarbons involves reactive metal-0X0 species in methane mono-oxygenases and many related synthetic models, and a number of simple metal-oxo complexes also work. The clear criterion of distinction between an organometallic C-H activation and a radical activation is the above selectivity in activated C-H bonds. 

The C-H activation of alkanes is known as a difficult chemical process [11]. By the chemical method, severe conditions and/or expensive catalysts have been used to activate the C-H bond [12]. However, the biological system achieves such a difficult reaction very easily at room temperature [13]. Methane monooxygenase (MMO) catalyzes an oxidation of methane to methanol in methanotrophs, a methane-utilizing bacteria [eq. (2)] [14,15]. In the catalytic reaction of MMO, one oxygen atom of O2 is incorporated into a substrate and another oxygen atom is transformed into water. 

The production of reduced products like methane and ammonia by the gut microbial flora has important environmental consequences, as such compounds contribute to the chemical and biological oxygen demand. The detection of dissolved oxygen in the gut of piglets led to tests to show whether methane and other reduced products could be oxidized in the pig gut. The production of C-labelledCOj from C-labelledmethane has been demonstrated however, it is calculated that at most only a very small proportion of methane produced is likely to be oxidized using O 2 as electron acceptor. Methane may also be oxidized anaerobically, but only an extremely small amount of methane is likely to... 

Results discussed above show in several lines a distinct biomimetic-type activity of iron complexes stabilized in the ZSM-S matrix. The most important feature is their unique ability to coordinate a very reactive a-oxygen form which is similar to the active oxygen species of MMO. At room temperature a-oxygen provides various oxidation reactions including selective hydroxylation of methane to methanol. Like in biological oxidation, the rate determining step of this reaction involves the cleavage of C-H bond. 

Natural sources of CO include CO from biomass burning and the oxidation of organics such as methane and isoprene, CO from biological processes in soils, CO from vegetation and termites, and CO from the ocean. 

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