methane ch4

Properties and bonding

Methane is a tetrahedral molecule with four equivalent C-H bonds. Its electronic structure is described by four bonding molecular orbitals (MOs) 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 level in energy is a triply degenerate set of MOs that involve overlap if 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.^[4] The familiar smell of natural gas as used in homes is a safety measure achieved by the addition of an odorant, usually blends containing tert-butylthiol. Methane has a boiling point of −161 °C (−257.8 °F) at a pressure of one atmosphere.^[5] As a gas it is flammable only over a narrow range of concentrations (5–15%) in air. Liquid methane does not burn unless subjected to high pressure (normally 4–5 atmospheres).^[6]

[edit]Chemical reactions

Main reactions with methane are: combustion, steam reforming to syngas, and halogenation. In general, methane reactions are difficult to control. Partial oxidation to methanol, for example, is challenging because the reaction typically progresses all the way to carbon dioxide and water even with incomplete amounts of oxygen. The enzymes methane monooxygenase can produce methanol from methane, but they cannot be used for industrial scale reactions.^[7]

[edit]Acid-base reactions

Like other hydrocarbons, methane is a very weak acid. Its pKa in DMSO is estimated to be 56.^[8] It cannot be deprotonated in solution, but the conjugate base with methyllithium is known. Protonation of methane can be achieved with super acids to give CH₅⁺, sometimes called the methanium ion. Despite the strength of its C-H bonds, there is intense interest in catalysts that facilitate C–H bond activationin methane (and other low alkanes).^[9]

[edit]Combustion

In the combustion of methane, multiple steps are involved. The following equations are part of the process, with the net result being:

CH₄ + 2 O₂ → CO₂ + 2 H₂O (ΔH = −891 kJ/mol (at standard conditions))

1.    CH₄+ M^* → CH₃ + H + M

2.    CH₄ + O₂ → CH₃ + HO₂

3.    CH₄ + HO₂ → CH₃ + 2 OH

4.    CH₄ + OH → CH₃ + H₂O

5.    O₂ + H → O + OH

6.    CH₄ + O → CH₃ + OH

7.    CH₃ + O₂ → CH₂O + OH

8.    CH₂O + O → CHO + OH

9.    CH₂O + OH → CHO + H₂O

10. CH₂O + H → CHO + H₂

11. CHO + O → CO + OH

12. CHO + OH → CO + H₂O

13. CHO + H → CO + H₂

14. H₂ + O → H + OH

15. H₂ + OH → H + H₂O

16. CO + OH → CO₂ + H

17. H + OH + M → H₂O + M^*

18. H + H + M → H₂ + M^*

19. H + O₂ + M → HO₂ + M^*

The species M^* signifies an energetic third body, from which energy is transferred during a molecular collision. Formaldehyde (HCHO orH₂CO) is an early intermediate (reaction 7). Oxidation of formaldehyde gives the formyl radical (HCO) (reactions 8, 9 & 10), which then give carbon monoxide (CO) (reactions 11, 12 & 13). Any resulting H₂ oxidizes to H₂O or other intermediates (reaction 14 & 15). Finally, the CO oxidizes, forming CO₂ (reaction 16). In the final stages (reactions 17, 18 & 19), energy is transferred back to other third bodies. The overall speed of reaction is a function of the concentration of the various entities during the combustion process. The higher the temperature, the greater the concentration of radical species and the more rapid the combustion process. ^[10]

[edit]Reactions with halogens

Methane reacts with halogens given appropriate conditions as follows:

X₂ + UV → 2 X•

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 with UV light or some other radical initiator. A chlorine atom is generated from elemental chlorine, which abstracts a hydrogen atom from methane, resulting in the formation of hydrogen chloride. The resulting methyl radical, CH₃·, can combine with another chlorine molecule to givemethyl chloride (CH₃Cl) and a new chlorine atom.^[11] Similar reactions can produce dichloromethane (CH₂Cl₂), chloroform (CHCl₃), and, ultimately, carbon tetrachloride (CCl₄), depending upon reaction conditions and the chlorine to methane ratio.

[edit]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. Gas pipelines distribute large amounts of natural gas, of which methane is the principal component.

[edit]Fuel

Main article: natural gas

Methane is important for electrical generation by burning it as a fuel in a gas turbine or steam boiler. Compared to other hydrocarbon fuels, burning methane produces less carbon dioxide for each unit of heat released. At about 891 kJ/mol, methane's heat of combustion is lower than any other hydrocarbon but the ratio of the heat of combustion (891 kJ/mol) to the molecular mass (16.0 g/mol, of which 12.0 g/mol is carbon) shows that methane, being the simplest hydrocarbon, produces more heat per mass unit (55.7 kJ/g) than other complex hydrocarbons. In many cities, methane is piped into homes for domestic heating and cooking purposes. 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.

Methane in the form of compressed natural gas is used as a vehicle fuel and is claimed to be more environmentally friendly than other fossil fuels such as gasoline/petrol and diesel.^[12] Research into adsorption methods of methane storage for use as an automotive fuel has been conducted.^[13]

[edit]Developing technologies

Research is being conducted by NASA on methane's potential as a rocket fuel.^[14] The assembly of a 5,500-pound-thrust liquid oxygen/liquid methane rocket engine has been completed. This propulsion technology is under consideration as the way off the Moon for human explorers. One advantage of methane is that it is abundant in many parts of the solar system and it could potentially be harvested on the surface of another solar-system body, providing fuel for a return journey.^[15] Current methane engines in development produce a thrust of 7,500 pounds-force (33 kN), which is far from the 7,000,000 lbf (31 MN) needed to launch the Space Shuttle. Instead, such engines will most likely propel voyages from the Moon or send robotic expeditions to other planets in the solar system.^[16]

[edit]Chemical feedstock

Although there is great interest in converting methane into useful or more easily liquified compounds, the only practical processes are relatively unselective. In the chemical industry, methane is converted to synthesis gas, a mixture of carbon monoxide and hydrogen, by steam reforming. This endergonic process (requiring energy) utilizes nickelcatalysts and requires high temperatures, around 700–1100 °C:

CH₄ + H₂O → CO + 3 H₂

Related chemistries are exploited in the Haber-Bosch Synthesis of ammonia from air, which is reduced with natural gas to a mixture of carbon dioxide, water, and ammonia.

Methane is also subjected to free-radical chlorination in the production of chloromethanes, although methanol is a more typical precursor.^[17]

[edit]Production

[edit]Biological routes

Main article: methanogenesis

Naturally occurring methane is mainly produced by the process of methanogenesis. This multistep process is used by microorganisms as an energy source. The net reaction is:

CO₂ + 8 H⁺ + 8 e^- → CH₄ + 2 H₂O

The final step in the process is catalysed by the enzyme methyl-coenzyme M reductase. Methanogenesis is a form of anaerobic respiration used by organisms that occupy landfill,ruminants (e.g., cattle), and the guts of termites.

It is uncertain if plants are a source of methane emissions.^[18][19][20]

[edit]Industrial routes

Methane can be produced by hydrogenating carbon dioxide through the Sabatier process. Methane is also a side product of the hydrogenation of carbon monoxide in the Fischer-Tropsch process. This technology is practiced on a large scale to produce longer chain molecules than methane.

Natural gas is so abundant that the intentional production of methane is relatively rare. The only large scale facility of this kind 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 which is otherwise very hard to transport for its weight, ash content, low calorific value and propensity to spontaneous combustion during storage and transport.

[edit]Laboratory synthesis

Methane can also be produced by the destructive distillation of acetic acid in the presence of soda lime or similar. Acetic acid is decarboxylated in this process.

[edit]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. The gas at shallow levels (low pressure) forms by anaerobic decayof organic matter and reworked methane from deep under the Earth's surface. In general, sediments buried deeper and at higher temperatures than those that contain oil generate natural gas.

It is generally transported in bulk by pipeline in its natural gas form, or LNG carriers in its liquefied form; few countries transport it by truck.

[edit]Alternative sources

Apart from gas fields, an alternative method of obtaining methane is via biogas generated by the fermentation of organic matter including manure, wastewater sludge, municipal solid waste (including landfills), or any other biodegradable feedstock, under anaerobic conditions. Rice fields also generate large amounts of methane during plant growth. Methane hydrates/clathrates (ice-like combinations of methane and water on the sea floor, found in vast quantities) are a potential future source of methane. Cattle belch methane accounts for 16% of the world's annual methane emissions to the atmosphere.^[21] One study reported that the livestock sector in general (primarily cattle, chickens, and pigs) produces 37% of all human-induced methane.^[22] Early research has found a number of medical treatments and dietary adjustments that help slightly limit the production of methane inruminants.^[23] [24] A more recent study, in 2009, found that at a conservative estimate, at least 51% of global greenhouse gas emissions were attributable to the life cycle and supply chain of livestock products, meaning all meat, dairy, and by-products, and their transportation.^[25]

Paleoclimatology research published in Current Biology suggests that flatulence from dinosaurs may have warmed the Earth.^[26]

[edit]Atmospheric methane

Main article: Atmospheric methane

2011 methane concentration in the uppertroposphere^[27]

Methane is created near the Earth's surface, primarily by microorganisms by the process of methanogenesis. It is carried into thestratosphere by rising air in the tropics. Uncontrolled build-up of methane in the atmosphere is naturally checked — although human influence can upset this natural regulation — by methane's reaction with hydroxyl radicals formed from singlet oxygen atoms and with water vapor. It has a net lifetime of about 10 years,^[28] and is primarily removed by conversion to carbon dioxide and water.

Methane also affects the degradation of the ozone layer.^[29][30]

In addition, there is a large (but unknown) amount of methane in methane clathrates in the ocean floors as well as the Earth's crust. Most methane is the result of biological process called methanogenesis.

In 2010, methane levels in the Arctic were measured at 1850 nmol/mol, a level over twice as high as at any time in the 400,000 years prior to the industrial revolution. Historically, 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 to 700 nmol/mol during the warm interglacial periods. It has a high global warming potential: 72 times that of carbon dioxide over 20 years, and 25 times over 100 years,^[31] and the levels are rising.

A Bristol University study published in Nature claims that methane under the Antarctic Ice Sheet may yet play an important role globally. Researchers believe these sub-ice environments to be biologically active, in that microbes are converting organic carbon to carbon dioxide and methane.^[32]

Methane in the Earth's atmosphere is an important greenhouse gas with a global warming potential of 25 compared to CO₂ over a 100-year period (although accepted figures probably represent an underestimate^[33]). This means that a methane emission will have 25 times the effect on temperature of a carbon dioxide emission of the same mass over the following 100 years. Methane has a large effect for a brief period (a net lifetime of 8.4 years in the atmosphere), whereas carbon dioxide has a small effect for a long period (over 100 years). Because of this difference in effect and time period, the global warming potential of methane over a 20 year time period is 72. 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).^[34] Usually, excess methane from landfills and other natural producers of methane is burned so CO₂ is released into the atmosphere instead of methane, because methane is a more effective greenhouse gas. Recently, methane emitted from coal mines has been successfully utilized to generate electricity.