Methane

Methane (US: /ˈmɛθeɪn/ MEH-thayn, UK: /ˈmiːθeɪn/ MEE-thayn) is a chemical compound with the chemical formula CH4 (one carbon atom bonded to four hydrogen atoms). It is a group 14 hydride, the simplest alkane, and the main constituent of natural gas. The relative abundance of methane in the earth makes it a suitable economic fuel, although its absorption and storage poses many technical challenges due to its gaseous state under normal conditions for temperature and pressure. Natural methane is found both underground and under the seabed and is formed by geological and biological processes. The largest reservoir of methane under the seabed is in the form of methane chlorate. When methane reaches the surface and atmosphere, it is known as atmospheric methane. The concentration of methane in the Earth's atmosphere has increased by about 150% since 1750 and accounts for 20% of the total radiative forcing from all long-lived and mixed greenhouse gases globally. It has also been detected on other planets, including Mars, which has implications for astrobiology research.

Methane is a tetrahedral molecule with four equivalent C-H bonds. Its electronic structure is described by four bonding molecular orbitals (MOs) derived from the overlap of valence orbitals on C and H. The low-energy MO is the result of the overlap of the 2s orbital on carbon with the in-phase composition. Above this energy level, there is a set of triplet degenerate MOs consisting of overlapping 2p orbitals on carbon with various linear combinations of 1s orbitals on hydrogen. The resulting "three-on-one" bonding scheme is consistent with photoelectron spectroscopy measurements. Methane gas is odorless and colorless. It absorbs visible light, especially at the red end of the spectrum because of the color 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 methane-rich atmospheres and is then scattered outward. The familiar smell of natural gas used in homes is achieved by adding an odorant, usually compounds containing tert-butyl thiol, as a safety measure. The boiling point of methane is -161.5 degrees Celsius at a pressure of one atmosphere. As a gas, it is flammable in a wide range of concentrations (5.4-17%) in air at standard pressure. Solid methane exists in several forms. Currently, 9 cases are known. Cooling methane at normal pressure leads to the formation of methane I. This material crystallizes in the cubic system (space group Fm3m). The position of the hydrogen atoms in methane I is not fixed, that is, the methane molecules may rotate freely. Therefore, it is a plastic crystal.

Chemical reactions

Selective oxidation

The partial oxidation of methane to methanol, the easier liquid fuel, is peculiar because the reaction usually proceeds to carbon dioxide and water even with an insufficient supply of oxygen. The enzyme methane monooxygenase produces methanol from methane, but it cannot be used for industrial-scale reactions. Some homogeneous and heterogeneous catalyst systems have been developed, but all have significant drawbacks. These usually work by producing protected products that are protected against excessive oxidation. For example, we can mention the Catalytica system, copper zeolites and iron zeolites that stabilize the active site of alpha-oxygen. A group of bacteria catalyzes the oxidation of methane with nitrite as an oxidant in the absence of oxygen, creating the so-called anaerobic oxidation of methane. Acid-base reactions Like other hydrocarbons, methane is a very weak acid, with an estimated pKa of 56 in DMSO. It cannot be deprotonated in solution, but the conjugate base is known in forms such as methyllithium.

 Methane bubbles can be burned on a wet hand without harm. The heat of combustion of methane is 55.5 MJ/kg. Methane combustion is a multi-step reaction that is summarized as follows

 CH4 + 2 O2 → CO2 + 2 H2O (ΔH = -891 kJ/mol)

Under standard conditions (Peters' four-step chemistry is a systematically reduced four-step chemistry that describes the combustion of methane. Methane Radical Reactions Under appropriate conditions, methane reacts with halogen 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). The mechanism for this process is called free radical halogenation. It starts when UV light or some other radical initiator (such as peroxides) produces a halogen atom.

Applications

Fuel

 Methane is used in industrial chemical processes and may be shipped as a refrigerated liquid (liquefied natural gas or LNG). While leaking from a container of refrigerated liquid is initially heavier than air due to the increased density of the cold gas, the gas is lighter than air at ambient temperature. Gas pipelines distribute large quantities of natural gas, of which methane is the main component. Methane fuel is used as fuel for furnaces, homes, water heaters, furnaces, cars, turbines, etc. Activated carbon is used to store methane. Purified liquid methane is used as rocket fuel when combined with liquid oxygen, such as in the BE-4 and Raptor engines.

The chemical raw material of natural gas, which consists mostly of methane, is used to produce hydrogen gas on an industrial scale. Steam methane reforming (SMR), or simply known as steam reforming, is the industry standard method for producing commercial bulk hydrogen gas. More than 50 million metric tons are produced annually worldwide (2013), mainly from natural gas SMR. Most of this hydrogen is used in oil refineries, in the production of chemicals and in food processing. Very large amounts of hydrogen are used in the industrial synthesis of ammonia.

History

In November 1776, methane was first detected by the Italian physicist Alessandro Volta in the marshes of Lake Maggiore between Italy and Switzerland. Volta was inspired to search for this substance after reading an article written by Benjamin Franklin about "flammable air". Volta collected the exhaust gas from the lagoon and separated pure methane by 1778. He also showed that an electric spark could ignite gas. After the Fling Mine disaster of 1812 in which 92 men lost their lives, Sir Humphrey Davy proved that the dreaded fire was in fact mostly methane. The name "methane" was coined in 1866 by the German chemist August Wilhelm von Hoffmann. This name is derived from methanol.

Toxicity

Methane is a non-toxic gas, but it is highly flammable and may form explosive mixtures with air. Methane is also suffocating if the oxygen concentration is reduced to less than 16% by displacement, as most people can tolerate a reduction of 21% to 16% without adverse effects. The concentration of methane at which the suffocation hazard is significant is much higher than the concentration of 5 to 15% in a flammable or explosive mixture. Methane gas can seep into buildings near landfills, exposing residents to significant levels of methane. Some buildings have specially engineered recovery systems in their basements to actively capture this gas and remove it from the building. Methane explosions are responsible for many fatal mine disasters. A methane gas explosion was the cause of the Upper Big Branch coalmine disaster in West Virginia on April 5, 2010, which killed 29 people.

Refrence

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