Abiotic methanogenesis from organosulphur compounds under ambient conditions

Abstract

Methane in the environment is produced by both biotic and abiotic processes. Biomethanation involves the formation of methane by microbes that live in oxygen-free environments. Abiotic methane formation proceeds under conditions at elevated temperature and/or pressure. Here we present a chemical reaction that readily forms methane from organosulphur compounds under highly oxidative conditions at ambient atmospheric pressure and temperature. When using iron(II/III), hydrogen peroxide and ascorbic acid as reagents, S-methyl groups of organosulphur compounds are efficiently converted into methane. In a first step, methyl sulphides are oxidized to the corresponding sulphoxides. In the next step, demethylation of the sulphoxide via homolytic bond cleavage leads to methyl radical formation and finally to methane in high yields. Because sulphoxidation of methyl sulphides is ubiquitous in the environment, this novel chemical route might mimic methane formation in living aerobic organisms.

Evidence for methane production by the marine algae <i>Emiliania huxleyi</i>

“…2d). However, this observation is in line with some previous findings (Althoff et al, 2010(Althoff et al, , 2014, who showed that the abiotic formation of CH 4 due to the degradation of methionine or ascorbic acid by light or oxidants such as iron minerals is possible. In the case of methionine, it was shown that the sulfur-bound methyl group of Met was the carbon precursor of CH 4 (Althoff et al, 2014).…”

Iron-catalysed oxidation and halogenation of organic matter in nature

“…An interesting recently described reaction, related to published work on demethylation and well-studied sulfoxidation reactions, is methane formation from methionine under oxidative conditions. [118] As an example of these biomimetic studies with well-defined systems and their complexes, we describe a few processes of bispidine-type ligands relevant to environmental transformations, also because some of the environmentally relevant mechanistic work has been done with these ligands. Note that in soils, the relevant organic matter is humic substances, and these are believed to coordinate to Fe II and Fe III to enable chemical reactions similar to the structurally well-defined model systems described here.…”

“…[108][109][110][111][112][113] A wide variety of published work on the stoichiometric and catalytic oxidation of organic molecules with low-molecularweight non-heme iron model complexes is available, [57,114,115] and this includes oxygen-transfer reactions, e.g. sulfoxidation, [91,116] oxidative demethylation, [117] and methane formation [118] as well as the oxidation of alkanes and alkenes, [91] which are of specific interest in the degradation of organic matter in nature. An interesting recently described reaction, related to published work on demethylation and well-studied sulfoxidation reactions, is methane formation from methionine under oxidative conditions.…”

“…Cl À or a species derived from H 2 O, e.g. OH À ): (1) olefin epoxidation and cis-dihydroxylation, [84,121] (2) cyclohexane oxidation, [92,120] and (3) halogenation, [89,118] (4) sulfoxidation, [116] (5) catechol oxidation [113] and (6) furan formation. [169] …”

“…Based on studies with several phenolic derivatives (catechol, hydroquinone, and recorcinol), [166,167] [159] (2) chloroethene and chloroalkanes, [160] (3) chlorinated acetic acid, [118,161] (4) muconic, formic, acetic, oxalic, malonic, fumaric and maleic acids, [162][163][164] (5) chloroethyne, [165] (6) carbon suboxide, [166] (7) trihalomethanes, [167] (8) furan. [168,169] In the corresponding enzymes and model systems, the iron species are transformed to the resting state, top left.…”