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Methane ( or ) is a chemical compound with the chemical formula (one atom of carbon and four atoms of
hydrogen
hydrogen
). It is a group-14 hydride and the simplest
alkane , the simplest alkane In organic chemistry, an alkane, or paraffin (a historical trivial name In chemistry, a trivial name is a nonsystematic name for a chemical substance. That is, the name is not recognized according to the rules of any form ...
, and is the main constituent of
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carb ...
. The relative abundance of methane on
Earth Earth is the third planet from the Sun and the only astronomical object known to harbor life. About 29% of Earth's surface is land consisting of continent A continent is one of several large landmasses. Generally identified by con ...
makes it an economically attractive fuel, although capturing and storing it poses technical challenges due to its gaseous state under normal conditions for temperature and pressure. Naturally occurring methane is found both below ground and under the
seafloor
seafloor
, and is formed by both geological and biological processes. The largest reservoir of methane is under the seafloor in the form of methane clathrates. When methane reaches the surface and the
atmosphere An atmosphere (from the greek words ἀτμός ''(atmos)'', meaning 'vapour', and σφαῖρα ''(sphaira)'', meaning 'ball' or 'sphere') is a layer or a set of layers of gases surrounding a planet or other material body, that is held in ...
, it is known as atmospheric methane. The Earth's atmospheric methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed
greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The pr ...
es. Methane has also been detected on other planets, including
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
, which has implications for astrobiology research.


Properties and bonding

Methane is a tetrahedral molecular geometry, tetrahedral molecule with four equivalent Carbon–hydrogen bond, C–H bonds. Its electronic structure is described by four bonding molecular orbitals (MOs) resulting from the overlap of the valence orbitals on Carbon, C and Hydrogen, H. The lowest-energy MO is the result of the overlap of the 2s orbital on carbon with the in-phase combination of the 1s orbitals on the four hydrogen atoms. Above this energy level is a triply degenerate set of MOs that involve overlap of the 2p orbitals on carbon with various linear combinations of the 1s orbitals on hydrogen. The resulting "three-over-one" bonding scheme is consistent with photoelectron spectroscopic measurements. At room temperature and standard pressure, methane is a colorless, odorless gas. The familiar smell of natural gas as used in homes is achieved by the addition of an odorizer, odorant, usually blends containing tert-butylthiol, as a safety measure. Methane has a boiling point of −161.5 Degree Celsius, °C at a pressure of one Atmosphere (unit), atmosphere. As a gas, it is flammable over a range of concentrations (5.4–17%) in air at standard pressure. Solid methane exists in several Polymorphism (materials science), modifications. Presently nine are known. Cooling methane at normal pressure results in the formation of methane I. This substance crystallizes in the cubic system (space group Fmm). The positions of the hydrogen atoms are not fixed in methane I, i.e. methane molecules may rotate freely. Therefore, it is a plastic crystal.


Chemical reactions

The primary chemical reactions of methane are combustion, steam reforming to syngas, and halogenation. In general, methane reactions are difficult to control.


Selective oxidation

Partial Redox, oxidation of methane to methanol is challenging because the reaction typically progresses all the way to carbon dioxide and water even with an insufficient supply of oxygen. The enzyme methane monooxygenase produces methanol from methane, but cannot be used for industrial-scale reactions. Some homogeneously Catalysis, catalyzed systems and heterogeneous systems have been developed, but all have significant drawbacks. These generally operate by generating protected products which are shielded from overoxidation. Examples include the Methane functionalization#The Catalytica system, Catalytica system, copper zeolites, and iron zeolites stabilizing the alpha-oxygen active site. One group of bacteria drive methane oxidation with nitrite as the Oxidizing agent, oxidant in the absence of oxygen, giving rise to the so-called anaerobic oxidation of methane.


Acid–base reactions

Like other hydrocarbons, methane is a very weak acid. Its PKa, pKa in Dimethyl sulfoxide, DMSO is estimated to be 56. It cannot be Deprotonation, deprotonated in solution, but the conjugate base is known in forms such as methyllithium. A variety of cation, positive ions derived from methane have been observed, mostly as unstable species in low-pressure gas mixtures. These include methenium or methyl cation , methane cation , and methanium or protonated methane . Some of these have been list of interstellar and circumstellar molecules, detected in outer space. Methanium can also be produced as diluted solutions from methane with superacids. Cations with higher charge, such as and , have been studied theoretically and conjectured to be stable. Despite the strength of its C–H bonds, there is intense interest in catalysts that facilitate C–H bond activation in methane (and other lower numbered alkanes).


Combustion

Methane's heat of combustion is 55.5 MJ/kg. Combustion of methane is a multiple step reaction summarized as follows: :CH4 + 2 O2 → CO2 + 2 H2O (, at standard conditions) Peters four-step chemistry is a systematically reduced four-step chemistry that explains the burning of methane.


Methane radical reactions

Given appropriate conditions, methane reacts with halogen Radical (chemistry), radicals as follows: :X• + CH4 → HX + CH3• :CH3• + X2 → CH3X + X• where X is a halogen: fluorine (F), chlorine (Cl), bromine (Br), or iodine (I). This mechanism for this process is called free radical halogenation. It is initiated when UV light or some other radical initiator (like peroxides) produces a halogen atom. A two-step chain reaction ensues in which the halogen atom abstracts a hydrogen atom from a methane molecule, resulting in the formation of a hydrogen halide molecule and a methyl radical (CH3•). The methyl radical then reacts with a molecule of the halogen to form a molecule of the halomethane, with a new halogen atom as byproduct. Similar reactions can occur on the halogenated product, leading to replacement of additional hydrogen atoms by halogen atoms with dihalomethane, trihalomethane, and ultimately, tetrahalomethane structures, depending upon reaction conditions and the halogen-to-methane ratio.


Uses

Methane is used in industrial chemical processes and may be transported as a refrigerated liquid (liquefied natural gas, or LNG). While leaks from a refrigerated liquid container are initially heavier than air due to the increased density of the cold gas, the gas at ambient temperature is lighter than air. Pipeline transport, Gas pipelines distribute large amounts of natural gas, of which methane is the principal component.


Fuel

Methane is used as a fuel for ovens, homes, water heaters, kilns, automobiles, turbines, and other things. Activated carbon is used to store methane. Refined liquid methane is Liquid rocket propellants#Bipropellants, used as a rocket fuel, when combined with liquid oxygen, as in the BE-4 and Raptor (rocket engine family), Raptor engines. As the major constituent of
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carb ...
, methane is important for electricity generation by burning it as a fuel in a gas turbine or Boiler (power generation), steam generator. Compared to other fossil fuel, hydrocarbon fuels, methane produces less carbon dioxide for each unit of heat released. At about 891 kJ/mol, methane's heat of combustion is lower than that of any other hydrocarbon. However, it produces more heat per mass (55.7 kJ/g) than any other organic molecule due to its relatively large content of hydrogen, which accounts for 55% of the heat of combustion but contributes only 25% of the molecular mass of methane. In many cities, methane is piped into homes for domestic heating and cooking. In this context it is usually known as
natural gas Natural gas (also called fossil gas; sometimes just gas) is a naturally occurring hydrocarbon gas mixture consisting primarily of methane, but commonly including varying amounts of other higher alkanes, and sometimes a small percentage of carb ...
, which is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 British thermal unit, BTU per standard cubic foot. Liquefied natural gas (LNG) is predominantly methane (CH4) converted into liquid form for ease of storage or transport. As a rocket fuel, methane offers the advantage over kerosene of producing small exhaust molecules. This deposits less soot on the internal parts of rocket motors, reducing the difficulty of booster re-use. The lower molecular weight of the exhaust also increases the fraction of the heat energy which is in the form of kinetic energy available for propulsion, increasing the specific impulse of the rocket. Liquid methane also has a temperature range (91–112 K) nearly compatible with liquid oxygen (54–90 K).


Chemical feedstock

Natural gas, which is mostly composed of methane, is used to produce hydrogen gas on an industrial scale. Steam reforming, Steam methane reforming (SMR), or simply known as steam reforming, is the most common method of producing commercial bulk hydrogen gas. More than 50 million metric tons are produced annually worldwide (2013), principally from the SMR of natural gas. Much of this hydrogen is used in petroleum Refinery, refineries, in the production of chemicals and in food processing. Very large quantities of hydrogen are used in the Ammonia production, industrial synthesis of ammonia. At high temperatures (700–1100 °C) and in the presence of a metal-based catalyst (nickel), steam reacts with methane to yield a mixture of Carbon monoxide, CO and Dihydrogen, H2, known as "water gas" or "syngas": : CH4 + water, H2O ⇌ carbon monoxide, CO + 3 hydrogen, H2 This reaction is strongly endothermic (consumes heat, Δ''H''r = 206 kJ/mol). Additional hydrogen is obtained by the reaction of carbon monoxide, CO with water via the water-gas shift reaction: : CO + H2O ⇌ carbon dioxide, CO2 + H2 This reaction is mildly exothermic (produces heat, Δ''H''r = −41 kJ/mol). Methane is also subjected to free-radical chlorination reaction, chlorination in the production of chloromethanes, although methanol is a more typical precursor.Rossberg, M. ''et al.'' (2006) "Chlorinated Hydrocarbons" in ''Ullmann's Encyclopedia of Industrial Chemistry'', Wiley-VCH, Weinheim. .


Generation


Geological routes

The two main routes for geological methane generation are (i) organic (thermally generated, or thermogenic) and (ii) inorganic (Abiotic component, abiotic). Thermogenic methane occurs due to the breakup of organic matter at elevated temperatures and pressures in deep sedimentary Stratum, strata. Most methane in sedimentary basins is thermogenic; therefore, thermogenic methane is the most important source of natural gas. Thermogenic methane components are typically considered to be relic (from an earlier time). Generally, formation of thermogenic methane (at depth) can occur through organic matter breakup, or organic synthesis. Both ways can involve microorganisms (methanogenesis), but may also occur inorganically. The processes involved can also consume methane, with and without microorganisms. The more important source of methane at depth (crystalline bedrock) is abiotic. Abiotic means that methane is created from inorganic compounds, without biological activity, either through magmatic processes or via water-rock reactions that occur at low temperatures and pressures, like Serpentinite, serpentinization.


Biological routes

Most of Earth's methane is Biogenic substance, biogenic and is produced by methanogenesis,Lessner, Daniel J(Dec 2009) Methanogenesis Biochemistry. In: eLS. John Wiley & Sons Ltd, Chichester. http://www.els.net [doi: 10.1002/9780470015902.a0000573.pub2] a form of anaerobic respiration only known to be conducted by some members of the domain Archaea. Methanogens occupy landfills and other soils, ruminants (for example cows or cattle), the guts of termites, and the Anoxic waters, anoxic sediments below the seafloor and the bottom of lakes. Rice fields also generate large amounts of methane during plant growth. This multistep process is used by these microorganisms for energy. The net reaction of methanogenesis is: :CO2 + 4 H2→ CH4 + 2 H2O The final step in the process is catalyzed by the enzyme Coenzyme-B sulfoethylthiotransferase, methyl coenzyme M reductase (MCR).


Ruminants

Ruminants, such as cattle, belch methane, accounting for ~22% of the U.S. annual methane emissions to the atmosphere. One study reported that the livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane. A 2013 study estimated that livestock accounted for 44% of human-induced methane and ~15% of human-induced greenhouse gas emissions. Many efforts are underway to reduce livestock methane production, such as medical treatments and dietary adjustments, and to trap the gas to use as energy.


Seafloor sediments

Most of the subseafloor is Anoxic waters, anoxic because oxygen is removed by Aerobic respiration, aerobic microorganisms within the first few centimeters of the seafloor sediment, sediment. Below the oxygen replete seafloor, methanogens produce methane that is either used by other organisms or becomes trapped in Clathrate hydrate, gas hydrates. These other organisms which utilize methane for energy are known as methanotrophs (methane-eating), and are the main reason why little methane generated at depth reaches the sea surface. Consortia of Archaea and Bacteria have been found to oxidize methane via Anaerobic oxidation of methane, Anaerobic Oxidation of Methane (AOM); the organisms responsible for this are Methanotroph, Anaerobic Methanotrophic Archaea (ANME) and Sulfate-reducing microorganisms, Sulfate-Reducing Bacteria (SRB).


Industrial routes

There is little incentive to produce methane industrially. Methane is produced by hydrogenation, hydrogenating carbon dioxide through the Sabatier process. Methane is also a side product of the hydrogenation of carbon monoxide in the Fischer–Tropsch process, which is practiced on a large scale to produce longer-chain molecules than methane. Example of large-scale coal-to-methane gasification is the Great Plains Synfuels plant, started in 1984 in Beulah, North Dakota as a way to develop abundant local resources of low-grade lignite, a resource that is otherwise difficult to transport for its weight, Coal assay#Ash, ash content, low calorific value and propensity to spontaneous combustion during storage and transport. Power to gas#Power to methane, Power to methane is a technology that uses electricity, electrical power to produce hydrogen from water by electrolysis and uses the Sabatier reaction to combine hydrogen with carbon dioxide to produce methane. As of 2016, this is mostly under development and not in large-scale use. Theoretically, the process could be used as a buffer for excess and off-peak power generated by highly fluctuating wind turbines and solar arrays. However, as currently very large amounts of natural gas are used in power plants (e.g. Combined cycle, CCGT) to produce electric energy, the losses in efficiency are not acceptable.


Laboratory synthesis

Methane can be produced by protonation of methyl lithium or a methyl Grignard reagent such as methylmagnesium chloride. It can also be made from anhydrous sodium acetate and dry sodium hydroxide, mixed and heated above 300 °C (with sodium carbonate as byproduct). In practice, a requirement for pure methane can easily be fulfilled by steel gas bottle from standard gas suppliers.


Occurrence

Methane was discovered and isolated by Alessandro Volta between 1776 and 1778 when studying marsh gas from Lake Maggiore. It is the major component of natural gas, about 87% by volume. The major source of methane is extraction from geological deposits known as natural gas fields, with coal seam gas extraction becoming a major source (see Coal bed methane extraction, a method for extracting methane from a coal deposit, while enhanced coal bed methane recovery is a method of recovering methane from non-mineable coal seams). It is associated with other hydrocarbon fuels, and sometimes accompanied by helium and nitrogen. Methane is produced at shallow levels (low pressure) by anaerobic organism, anaerobic Decomposition, decay of organic matter and reworked methane from deep under the Earth's surface. In general, the sediments that generate natural gas are buried deeper and at higher temperatures than those that contain Petroleum, oil. Methane is generally transported in bulk by Pipeline transport, pipeline in its natural gas form, or LNG carriers in its liquefied form; few countries transport it by truck.


Atmospheric methane

In 2010, methane levels in the Arctic were measured at 1850 nmol/mol. This level is over twice as high as at any time in the last 400,000 years. Keeling Curve, Historic methane concentrations in the world's atmosphere have ranged between 300 and 400 nmol/mol during glacial periods commonly known as ice ages, and between 600 and 700 nmol/mol during the warm interglacial periods. The Earth's oceans are a potential important source of Arctic methane. Methane is an important
greenhouse gas A greenhouse gas (sometimes abbreviated GHG) is a gas that Absorption (electromagnetic radiation), absorbs and Emission (electromagnetic radiation), emits radiant energy within the thermal infrared range, causing the greenhouse effect. The pr ...
with a global warming potential of 34 compared to CO2 (potential of 1) over a 100-year period, and 72 over a 20-year period.Intergovernmental Panel on Climate Change, IPCC Fifth Assessment Report
Table 8.7, Chap. 8, p. 8–58
(PDF; 8,0 MB)
The Earth's atmospheric methane concentration has increased by about 150% since 1750, and it accounts for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases (these gases don't include water vapor which is by far the largest component of the greenhouse effect). From 2015 to 2019 sharp rises in levels of atmospheric methane have been recorded. In February 2020, it was reported methane emissions from the fossil fuel industry may have been significantly underestimated. Climate change can increase atmospheric methane levels by increasing methane production in natural ecosystems, forming a Climate change feedback.


Clathrates

Methane clathrates (also known as methane hydrates) are solid cages of water molecules that trap single molecules of methane. Significant reservoirs of methane clathrates have been found in arctic permafrost and along continental margins beneath the Seabed, ocean floor within the Gas hydrate stability zone, gas clathrate stability zone, located at high pressures (1 to 100 MPa; lower end requires lower temperature) and low temperatures (< 15 °C; upper end requires higher pressure). Methane clathrates can form from biogenic methane, thermogenic methane, or a mix of the two. These deposits are both a potential source of methane fuel as well as a potential contributor to global warming.Miller, G. Tyler (2007). ''Sustaining the Earth: An Integrated Approach''. U.S.A.: Thomson Advantage Books, p. 160. The global mass of carbon stored in gas clathrates is still uncertain and has been estimated as high as 12,500 Gigaton, Gt carbon and as low as 500 Gt carbon. The estimate has declined over time with a most recent estimate of ~1800 Gt carbon. A large part of this uncertainty is due to our knowledge gap in sources and sinks of methane and the distribution of methane clathrates at the global scale. For example, a relatively newly discovered source of methane was discovered in an Mid-ocean ridge, ultraslow spreading ridge in the Arctic. Some climate models suggest that today's methane emission regime from the ocean floor is potentially similar to that during the period of the Paleocene–Eocene Thermal Maximum (PETM) around 55.5 million years ago, although there are no data indicating that methane from clathrate dissociation currently reaches the atmosphere. Arctic methane release from permafrost and seafloor methane clathrates is a potential consequence and further cause of global warming; this is known as the clathrate gun hypothesis. Data from 2016 indicate that Arctic permafrost thaws faster than predicted.


Extraterrestrial methane


Interstellar medium

Methane is abundant in many parts of the Solar system and potentially could be harvested on the surface of another solar-system body (in particular, using In situ resource utilization, methane production from local materials found on
Mars Mars is the fourth planet from the Sun and the second-smallest planet in the Solar System, being larger than only Mercury (planet), Mercury. In English, Mars carries the name of the Mars (mythology), Roman god of war and is often referred to ...

Mars
or Titan (moon), Titan), providing fuel for a return journey.


Mars

Methane has been detected on all planets of the solar system and most of the larger moons. With the possible exception of Life on Mars, Mars, it is believed to have come from Abiogenic petroleum origin, abiotic processes. The Curiosity (rover), ''Curiosity'' rover has documented seasonal fluctuations of Atmosphere of Mars, atmospheric methane levels on Mars. These fluctuations peaked at the end of the Martian summer at 0.6 parts per billion. Methane has been proposed as a possible rocket propellant on future Human mission to Mars, Mars missions due in part to the possibility of synthesizing it on the planet by In situ resource utilization#Mars, in situ resource utilization. An adaptation of the Sabatier reaction, Sabatier methanation reaction may be used with a mixed catalyst bed and a Water-gas shift reaction#Reverse water-gas shift, reverse water-gas shift in a single reactor to produce methane from the raw materials available on Mars, utilizing water from the Martian soil, Martian subsoil and carbon dioxide in the Atmosphere of Mars, Martian atmosphere. Methane could be produced by a non-biological process called serpentinite, serpentinization involving water, carbon dioxide, and the mineral olivine, which is known to be common on Mars.


History

In November 1776, methane was first scientifically identified by Italian people, Italian physicist Alessandro Volta in the marshes of Lake Maggiore straddling Italy and Switzerland. Volta was inspired to search for the substance after reading a paper written by Benjamin Franklin about "flammable air".Volta, Alessandro (1777
''Lettere del Signor Don Alessandro Volta ... Sull' Aria Inflammable Nativa Delle Paludi''
[Letters of Signor Don Alessandro Volta ... on the flammable native air of the marshes], Milan, Italy: Giuseppe Marelli.
Volta collected the gas rising from the marsh, and by 1778 had isolated the pure gas. He also demonstrated that the gas could be ignited with an electric spark. The name "methane" was coined in 1866 by the German chemist August Wilhelm von Hofmann. The name was derived from Methanol#History, methanol.


Etymology

Etymologically, the word "''methane''" is coined from the chemical suffix "''-ane''", which denotes substances belonging to the alkane family; and the word "''methyl''", which is derived from the German "''methyl''" (1840) or directly from the French "''méthyle''", which is a back-formation from the French "''méthylène''" (corresponding to English "methylene"), the root of which was coined by Jean-Baptiste Dumas and Eugène Péligot in 1834 from the Greek "''methy''" (wine) (related to English "mead") and "''hyle''" (meaning "wood"). The radical is named after this because it was first detected in methanol, an alcohol first isolated by distillation of wood. The chemical suffix "''-ane''" is from the coordinating chemical suffix "''-ine''" which is from Latin feminine suffix "''-ina''" which is applied to represent abstracts. The coordination of "-ane", "-ene", "-one", etc. was proposed in 1866 by German chemist August Wilhelm von Hofmann (1818–1892).


Abbreviations

The abbreviation CH4-C can mean the mass of carbon contained in a mass of methane, and the mass of methane is always 1.33 times the mass of CH4-C. CH4-C can also mean the methane-carbon ratio, which is 1.33 by mass. Methane at scales of the atmosphere is commonly measured in teragrams (Tg CH4) or millions of metric tons (MMT CH4), which mean the same thing. Other standard units are also used, such as nanomole (nmol, one billionth of a mole), Mole (unit), mole (mol), kilogram, and gram.


Safety

Methane is nontoxic, yet it is extremely flammable and may form explosive mixtures with air. Methane is also an asphyxiant gas, asphyxiant if the oxygen concentration is reduced to below about 16% by displacement, as most people can Cabin pressurization#Need for cabin pressurization, tolerate a reduction from 21% to 16% without ill effects. The concentration of methane at which asphyxiation risk becomes significant is much higher than the 5–15% concentration in a flammable or explosive mixture. Methane off-gas can penetrate the interiors of buildings near landfills and expose occupants to significant levels of methane. Some buildings have specially engineered recovery systems below their basements to actively capture this gas and vent it away from the building. Methane gas explosions are responsible for many deadly mining disasters. A methane gas explosion was the cause of the Upper Big Branch Mine disaster, Upper Big Branch coal mine disaster in West Virginia on April 5, 2010, killing 29.


See also


Notes


References


Cited sources

*


External links


Methane
at ''The Periodic Table of Videos'' (University of Nottingham)
International Chemical Safety Card 0291


– United States Geological Survey *
CDC – Handbook for Methane Control in Mining
{{Authority control Methane, Anaerobic digestion Fuel gas Fuels Greenhouse gases Industrial gases Gaseous signaling molecules