Oxidative Coupling Of Methane
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The oxidative coupling of methane (OCM) is a potential
chemical reaction A chemical reaction is a process that leads to the chemistry, chemical transformation of one set of chemical substances to another. When chemical reactions occur, the atoms are rearranged and the reaction is accompanied by an Gibbs free energy, ...
studied in the 1980s for the direct conversion of
natural gas Natural gas (also fossil gas, methane gas, and gas) is a naturally occurring compound of gaseous hydrocarbons, primarily methane (95%), small amounts of higher alkanes, and traces of carbon dioxide and nitrogen, hydrogen sulfide and helium ...
, primarily consisting of
methane Methane ( , ) is a chemical compound with the chemical formula (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 abundance of methane on Earth makes ...
, into value-added chemicals. Although the reaction would have strong economics if practicable, no effective catalysts are known, and thermodynamic arguments suggest none can exist.


Ethylene production

The principal desired product of OCM is
ethylene Ethylene (IUPAC name: ethene) is a hydrocarbon which has the formula or . It is a colourless, flammable gas with a faint "sweet and musky" odour when pure. It is the simplest alkene (a hydrocarbon with carbon–carbon bond, carbon–carbon doub ...
, the world's largest commodity chemical and the chemical industry's fundamental building block. While converting methane to ethylene would offer enormous economic benefits, it is a major scientific challenge. Thirty years of research failed to produce a commercial OCM catalyst, preventing this process from commercial applications. Ethylene derivatives are found in food packaging, eyeglasses, cars, medical devices, lubricants, engine coolants and liquid crystal displays. Ethylene production by
steam cracking Steam cracking is a petrochemical process in which saturated hydrocarbons are broken down into smaller, often unsaturated, hydrocarbons. It is the principal industrial method for producing the lighter alkenes (or commonly olefins), including ethen ...
consumes large amounts of energy and uses oil and natural gas fractions such as
naphtha Naphtha (, recorded as less common or nonstandard in all dictionaries: ) is a flammable liquid hydrocarbon mixture. Generally, it is a fraction of crude oil, but it can also be produced from natural-gas condensates, petroleum distillates, and ...
and
ethane Ethane ( , ) is a naturally occurring Organic compound, organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is List of purification methods ...
. The oxidative coupling of methane to ethylene is written below: : 2 + → + 2 The reaction is
exothermic In thermodynamics, an exothermic process () is a thermodynamic process or reaction that releases energy from the system to its surroundings, usually in the form of heat, but also in a form of light (e.g. a spark, flame, or flash), electricity (e ...
(∆H = -280 kJ/mol) and occurs at high temperatures (750–950 ˚C). In the reaction, methane () is activated heterogeneously on the catalyst surface, forming
methyl In organic chemistry, a methyl group is an alkyl derived from methane, containing one carbon atom bonded to three hydrogen atoms, having chemical formula (whereas normal methane has the formula ). In formulas, the group is often abbreviated as ...
free radicals In chemistry, a radical, also known as a free radical, is an atom, molecule, or ion that has at least one unpaired electron, unpaired valence electron. With some exceptions, these unpaired electrons make radicals highly chemical reaction, chemi ...
, which then couple in the gas phase to form ethane (). The
ethane Ethane ( , ) is a naturally occurring Organic compound, organic chemical compound with chemical formula . At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is List of purification methods ...
subsequently undergoes
dehydrogenation In chemistry, dehydrogenation is a chemical reaction that involves the removal of hydrogen, usually from an organic molecule. It is the reverse of hydrogenation. Dehydrogenation is important, both as a useful reaction and a serious problem. At ...
to form ethylene (). The yield of the desired products is reduced by non-selective reactions of methyl radicals with the surface and oxygen in the gas phase, which produce (undesirable) carbon monoxide and carbon dioxide.


Catalysis

Direct conversion of methane into other useful products is one of the most challenging subjects to be studied in
heterogeneous catalysis Heterogeneous catalysis is catalysis where the Phase (matter), phase of catalysts differs from that of the reagents or product (chemistry), products. The process contrasts with homogeneous catalysis where the reagents, products and catalyst exis ...
. Methane activation is difficult because of its
thermodynamic Thermodynamics is a branch of physics that deals with heat, work, and temperature, and their relation to energy, entropy, and the physical properties of matter and radiation. The behavior of these quantities is governed by the four laws of th ...
stability with a
noble gas The noble gases (historically the inert gases, sometimes referred to as aerogens) are the members of Group (periodic table), group 18 of the periodic table: helium (He), neon (Ne), argon (Ar), krypton (Kr), xenon (Xe), radon (Rn) and, in some ...
like electronic configuration. The tetrahedral arrangement of strong C–H bonds (435 kJ/mol) offer no
functional group In organic chemistry, a functional group is any substituent or moiety (chemistry), moiety in a molecule that causes the molecule's characteristic chemical reactions. The same functional group will undergo the same or similar chemical reactions r ...
, magnetic moments or polar distributions to undergo chemical attack. This makes methane less reactive than nearly all of its conversion products, limiting efficient utilization of natural gas, the world's most abundant petrochemical resource. The economic promise of OCM has attracted significant industrial interest. In the 1980s and 1990s multiple research efforts were pursued by academic investigators and petrochemical companies. Hundreds of
catalyst Catalysis () is the increase in rate of a chemical reaction due to an added substance known as a catalyst (). Catalysts are not consumed by the reaction and remain unchanged after it. If the reaction is rapid and the catalyst recycles quick ...
s have been tested, and several promising candidates were extensively studied. Researchers were unable to achieve the required
chemoselectivity Chemoselectivity is the preferential reaction of a chemical reagent with one of two or more different functional groups. In a chemoselective system, a reagent in the presence of an aldehyde and an ester would mostly target the aldehyde, even if it ...
for economic operation. Instead of producing ethylene, the majority of methane was non-selectively oxidized to
carbon dioxide Carbon dioxide is a chemical compound with the chemical formula . It is made up of molecules that each have one carbon atom covalent bond, covalently double bonded to two oxygen atoms. It is found in a gas state at room temperature and at norma ...
. The lack of selectivity was related to the poor C-H activation of known catalysts, requiring high reaction temperatures (750 ˚C and 950 ˚C) to activate the C-H bond. This high reaction temperature establishes a secondary gas-phase reaction mechanism pathway, whereby the desired reaction of methyl radical coupling to products (leading to ethylene) strongly competes with COx side reactions. The high temperature also presents a challenge for the reaction engineering. Among the process engineering challenges are the requirements for expensive
metallurgy Metallurgy is a domain of materials science and engineering that studies the physical and chemical behavior of metallic elements, their inter-metallic compounds, and their mixtures, which are known as alloys. Metallurgy encompasses both the ...
, lack of industry experience with high temperature catalytic processes and the potential need for new reactor design to manage heat transfer efficiently. Labinger postulated an inherent limit to OCM selectivity, concluding that "expecting substantial improvements in the OCM performance might not be wise". Labinger's argument, later demonstrated experimentally by Mazanec et al., is based on the mechanism of methane activation, which is a radical mechanism, forming H and CH3 radicals by the homolytic cleavage of the C-H bond. Ethylene and ethane that are proposed products have C-H bonds of similar strength. Thus, any catalyst that can activate methane can also activate the products. The yield of ethylene (and/or ethane) is limited by the relative rates of the methane and ethylene reactions, and these rates are very similar. Reactions of the products lead to higher homologues, and eventually to aromatics and coke. The same limitation applies to direct pyrolysis of methane, which is also a radical process. Nevertheless, some recent work have shown that the mechanism of the OCM could be initiated by an heterolytic cleavage of the C-H bond on magnesium oxide in the presence of atmosphere. Eventually, the inability to discover a selective catalyst led to a gradual loss of interest in OCM. Beginning in the mid-1990s, research activity in this area began to decline significantly, as evidenced by the decreasing number of patents filed and peer-reviewed publications. The research company Siluria attempted to develop a commercially viable OCM process, but did not succeed. The company sold their OCM technology to
McDermott McDermott or MacDermott is an Irish surname and the anglicised version of Mac Diarmada (also spelled Mac Diarmata), the surname of the ruling dynasty of Moylurg, a kingdom that existed in Connacht from the 10th to 16th centuries. The last ruling ...
in 2019.


References

{{Reflist Chemical reactions Methane