Methane
Methane is a chemical compound with the chemical formula . It is a group-14 hydride, the simplest alkane, and the main constituent of natural gas. The abundance of methane on Earth makes it an economically attractive fuel, although capturing and storing it is difficult because it is a gas at standard temperature and pressure. In the Earth's atmosphere methane is transparent to visible light but absorbs infrared radiation, acting as a greenhouse gas. Methane is an organic hydrocarbon, and among the simplest of organic compounds.
Naturally occurring methane is found both below ground and under the 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, it is known as atmospheric methane.
The Earth's atmospheric methane concentration has increased by about 160% since 1750, with the overwhelming percentage caused by human activity. It accounted for 20% of the total radiative forcing from all of the long-lived and globally mixed greenhouse gases, according to the 2021 Intergovernmental Panel on Climate Change report. Strong, rapid and sustained reductions in methane emissions could limit near-term warming and improve air quality by reducing global surface ozone.
Methane has also been detected on other planets, including Mars, which has implications for astrobiology research.
Properties and bonding
Methane is a tetrahedral molecule with four equivalent C–H bonds. Its electronic structure is described by four bonding molecular orbitals resulting from the overlap of the valence orbitals on C and 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.Methane is an odorless, colourless and transparent gas at standard temperature and pressure. It does absorb visible light, especially at the red end of the spectrum, due to overtone bands, but the effect is only noticeable if the light path is very long. This is what gives Uranus and Neptune their blue or bluish-green colors, as light passes through their atmospheres containing methane and is then scattered back out.
The familiar smell of natural gas as used in homes is achieved by the addition of an odorant, usually blends containing tert-butylthiol, as a safety measure. Methane has a boiling point of −161.5 °C at a pressure of one atmosphere. As a gas, it is flammable over a range of concentrations in air at standard pressure.
Solid methane exists in several modifications, of which nine are known. Cooling methane at normal pressure results in the formation of methane I. This substance crystallizes in the cubic system. 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 oxidation of methane to methanol, a more convenient, liquid fuel, 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 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 Catalytica system, copper zeolites, and iron zeolites stabilizing the alpha-oxygen active site.One group of bacteria catalyze methane oxidation with nitrite as the oxidant in the absence of oxygen, giving rise to the so-called anaerobic oxidation of methane.
Acid–base reactions
Like other hydrocarbons, methane is an extremely weak acid. Its pKa in DMSO is estimated to be 56. It cannot be deprotonated in solution, but the conjugate base is known in forms such as methyllithium.A variety of 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 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.
Combustion
Methane's heat of combustion is 55.5 MJ/kg. Combustion of methane is a multiple step reaction summarized as follows: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 radicals as follows:where X is a halogen: fluorine, chlorine, bromine, or iodine. This mechanism for this process is called free radical halogenation. It is initiated when UV light or some other radical initiator 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. 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.
This reaction is commonly used with chlorine to produce dichloromethane and chloroform via chloromethane. Carbon tetrachloride can be made with excess chlorine.
Uses
Methane may be transported as a refrigerated liquid. 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. 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, rockets, turbines, etc.As the major constituent of natural gas, methane is important for electricity generation by burning it as a fuel in a gas turbine or steam generator. Compared to other 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, but the ratio of the heat of combustion to the molecular mass shows that methane, being the simplest hydrocarbon, produces more heat per mass unit than other complex hydrocarbons. In many areas with a dense enough population, methane is piped into homes and businesses for heating, cooking, and industrial uses. In this context it is usually known as natural gas, which is considered to have an energy content of 39 megajoules per cubic meter, or 1,000 BTU per standard cubic foot. Liquefied natural gas is predominantly methane converted into liquid form for ease of storage or transport.
Rocket propellant
Refined liquid methane as well as LNG is used as a rocket fuel, when combined with liquid oxygen, as in the TQ-12, BE-4, Raptor, YF-215, and Aeon engines. Due to the similarities between methane and LNG such engines are commonly grouped together under the term methalox.As a liquid rocket propellant, a methane/liquid oxygen combination offers the advantage over kerosene/liquid oxygen combination, or kerolox, of producing small exhaust molecules, reducing coking or deposition of soot on engine components. Methane is easier to store than hydrogen due to its higher boiling point and density, as well as its lack of hydrogen embrittlement. 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. Compared to liquid hydrogen, the specific energy of methane is lower but this disadvantage is offset by methane's greater density and temperature range, allowing for smaller and lighter tankage for a given fuel mass. Liquid methane has a temperature range nearly compatible with liquid oxygen. The fuel currently sees use in operational launch vehicles such as Zhuque-2, Vulcan and New Glenn as well as in-development launchers such as Starship, Neutron, Terran R, Nova, and Long March 9.
Chemical feedstock
, which is mostly composed of methane, is used to produce hydrogen gas on an industrial scale. Steam methane reforming, or simply known as steam reforming, is the standard industrial method of producing commercial bulk hydrogen gas. More than 50 million metric tons are produced annually worldwide, principally from the SMR of natural gas. Much of this hydrogen is used in petroleum refineries, in the production of chemicals and in food processing. Very large quantities of hydrogen are used in the industrial synthesis of ammonia.At high temperatures and in the presence of a metal-based catalyst, steam reacts with methane to yield a mixture of CO and Dihydrogen|, known as "water gas" or "syngas":
This reaction is strongly endothermic.
Additional hydrogen is obtained by the reaction of CO with water via the water-gas shift reaction:
This reaction is mildly exothermic.
Methane is also subjected to free-radical chlorination in the production of chloromethanes, although methanol is a more typical precursor.
Hydrogen can also be produced via the direct decomposition of methane, also known as methane pyrolysis, which, unlike steam reforming, produces no greenhouse gases. The heat needed for the reaction can also be GHG emission free, e.g. from concentrated sunlight, renewable electricity, or burning some of the produced hydrogen. If the methane is from biogas then the process can be a carbon sink. Temperatures in excess of 1200 °C are required to break the bonds of methane to produce hydrogen gas and solid carbon. Through the use of a suitable catalyst the reaction temperature can be reduced to between 550 and 900 °C depending on the chosen catalyst. Dozens of catalysts have been tested, including unsupported and supported metal catalysts, carbonaceous and metal-carbon catalysts.
The reaction is moderately endothermic as shown in the reaction equation below.