Oxidative coupling of methane

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The oxidative coupling of methane (OCM) is a chemical reaction discovered in the 1980s for the direct conversion of natural gas, primarily consisting of methane, into value-added chemicals. The OCM process has not been commercially practiced.

Contents

Ethylene production

The principal desired product of OCM is ethylene, 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 consumes large amounts of energy and uses oil and natural gas fractions such as naphtha and ethane.

The oxidative coupling of methane to ethylene is written below: [1] [2]

2CH
4 + O
2 → C
2H
4 + 2H
2O

The reaction is exothermic (∆H = -280 kJ/mol) and occurs at high temperatures (750–950 ˚C). [3] In the reaction, methane (CH
4) is activated heterogeneously on the catalyst surface, forming methyl free radicals, which then couple in the gas phase to form ethane (C
2H
6). The ethane subsequently undergoes dehydrogenation to form ethylene (C
2H
4). The yield of the desired C
2 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. [4] Methane activation is difficult because of its thermodynamic stability with a noble gas like electronic configuration. The tetrahedral arrangement of strong C–H bonds (435 kJ/mol) offer no functional group, 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 catalysts have been tested, and several promising candidates were extensively studied. Researchers were unable to achieve the required chemoselectivity for economic operation. Instead of producing ethylene, the majority of methane was non-selectively oxidized to carbon dioxide.

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 C
2 products (leading to ethylene) strongly competes with COx side reactions. [3]

The high temperature also presents a challenge for the reaction engineering. Among the process engineering challenges are the requirements for expensive metallurgy, lack of industry experience with high temperature catalytic processes and the potential need for new reactor design to manage heat transfer efficiently. [5]

Labinger postulated an inherent limit to OCM selectivity, concluding that "expecting substantial improvements in the OCM performance might not be wise". [6] 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. [7] 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 O
2 atmosphere. [8]

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 in 2019. [9]

Related Research Articles

Catalysis chemical process

Catalysis is the process of increasing the rate of a chemical reaction by adding a substance known as a catalyst. Catalysts are not consumed in the catalyzed reaction but can act repeatedly. Often only very small amounts of catalyst are required. The global demand for catalysts in 2010 was estimated at approximately US$29.5 billion.

Hydrocarbon Organic compound consisting entirely of hydrogen and carbon

In organic chemistry, a hydrocarbon is an organic compound consisting entirely of hydrogen and carbon. Hydrocarbons are examples of group 14 hydrides. Hydrocarbons from which one hydrogen atom has been removed are functional groups called hydrocarbyls. Hydrocarbons are generally colourless and hydrophobic with only weak odours. Because of their diverse molecular structures, it is difficult to generalize further. Most anthropogenic emissions of hydrocarbons are from the burning of fossil fuels including fuel production and combustion. Natural sources of hydrocarbons such as ethylene, isoprene, and monoterpenes come from the emissions of vegetation.

Petrochemical Chemical product derived from petroleum

Petrochemicals are the chemical products obtained from petroleum by refining. Some chemical compounds made from petroleum are also obtained from other fossil fuels, such as coal or natural gas, or renewable sources such as maize, palm fruit or sugar cane.

Ethane is an organic chemical compound with chemical formula C
2H
6. At standard temperature and pressure, ethane is a colorless, odorless gas. Like many hydrocarbons, ethane is isolated on an industrial scale from natural gas and as a petrochemical by-product of petroleum refining. Its chief use is as feedstock for ethylene production.

Ethylene oxide Chemical compound

Ethylene oxide is an organic compound with the formula C
2H
4O. It is a cyclic ether and the simplest epoxide: a three-membered ring consisting of one oxygen atom and two carbon atoms. Ethylene oxide is a colorless and flammable gas with a faintly sweet odor. Because it is a strained ring, ethylene oxide easily participates in a number of addition reactions that result in ring-opening. Ethylene oxide is isomeric with acetaldehyde and with vinyl alcohol. Ethylene oxide is industrially produced by oxidation of ethylene in the presence of silver catalyst.

Alkylation Transfer of an alkyl group from one molecule to another

Alkylation is the transfer of an alkyl group from one molecule to another. The alkyl group may be transferred as an alkyl carbocation, a free radical, a carbanion or a carbene (or their equivalents). An alkyl group is a piece of a molecule with the general formula CnH2n+1, where n is the integer depicting the number of carbons linked together. For example, a methyl group (n = 1, CH3) is a fragment of a methane molecule (CH4). Alkylating agents use selective alkylation by adding the desired aliphatic carbon chain to the previously chosen starting molecule. This is one of many known chemical syntheses. Alkyl groups can also be removed in a process known as dealkylation. Alkylating agents are often classified according to their nucleophilic or electrophilic character.

Propene, also known as propylene or methyl ethylene, is an unsaturated organic compound with the chemical formula . It has one double bond, and is the second simplest member of the alkene class of hydrocarbons. It is a colorless gas with a faint petroleum-like odor.

The Fischer–Tropsch process is a collection of chemical reactions that converts a mixture of carbon monoxide and hydrogen or water gas into liquid hydrocarbons. These reactions occur in the presence of metal catalysts, typically at temperatures of 150–300 °C (302–572 °F) and pressures of one to several tens of atmospheres. The process was first developed by Franz Fischer and Hans Tropsch at the Kaiser-Wilhelm-Institut für Kohlenforschung in Mülheim an der Ruhr, Germany, in 1925.

Heterogeneous catalysis

In chemistry, heterogeneous catalysis is catalysis where the phase of catalysts differs from that of the reactants or products. The process contrasts with homogeneous catalysis where the reactants, products and catalyst exist in the same phase. Phase distinguishes between not only solid, liquid, and gas components, but also immiscible mixtures, or anywhere an interface is present. Catalysts are useful because they increase the rate of a reaction without themselves being consumed and are therefore reusable.

Wacker process

The Wacker process or the Hoechst-Wacker process refers to the oxidation of ethylene to acetaldehyde in the presence of palladium(II) chloride as the catalyst. This chemical reaction was one of the first homogeneous catalysis with organopalladium chemistry applied on an industrial scale.

Alkane metathesis is a class of chemical reaction in which an alkane is rearranged to give a longer or shorter alkane product. It is similar to olefin metathesis, except that olefin metathesis cleaves and recreates a carbon-carbon double bond, but alkane metathesis operates on a carbon-carbon single bond.

Carbon–hydrogen bond functionalization is a type of reaction in which a carbon–hydrogen bond is cleaved and replaced with a carbon–X bond. The term usually implies that a transition metal is involved in the C-H cleavage process. Reactions classified by the term typically involve the hydrocarbon first to react with a metal catalyst to create an organometallic complex in which the hydrocarbon is coordinated to the inner-sphere of a metal, either via an intermediate "alkane or arene complex" or as a transition state leading to a "M−C" intermediate. The intermediate of this first step can then undergo subsequent reactions to produce the functionalized product. Important to this definition is the requirement that during the C–H cleavage event, the hydrocarbyl species remains associated in the inner-sphere and under the influence of "M".

Huntsman Chemical Company of Australia Pty Ltd (HCCA) operated a complex chemical manufacturing plant in Somerville Rd Brooklyn in Melbourne. The site is 35 hectares in size and is located in the City of Brimbank. HCCA was partially owned by the Huntsman Corporation.

Roy A. Periana American organometallic chemist (born 1957)

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Scripps Energy & Materials Center

The Scripps Energy & Materials Center (SEMC) is an American research center that focuses on research in the basic energy and materials sciences. Located in Jupiter, Florida, the center is home to scientists, graduate students, and administrative staff. It is a part of the Scripps Research Institute (TSRI), one of the largest non-profit research institutes in the world.

Methane functionalization is the process of converting methane in its gaseous state to another molecule with a functional group, typically methanol or acetic acid, through the use of transition metal catalysts.

Karen Ila Goldberg is an American chemist, currently the Vagelos Professor of Energy Research at University of Pennsylvania. Goldberg is most known for her work in inorganic and organometallic chemistry. Her most recent research focuses on catalysis, particularly on developing catalysts for oxidation, as well as the synthesis and activation of molecular oxygen. In 2018, Goldberg was elected to the National Academy of Sciences.

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Heterogeneous gold catalysis

Heterogeneous gold catalysis refers to the catalysis of chemical reactions by gold, typically supported on metal oxide substrates. Despite the well known inertness of bulk gold, decreasing the diameter of supported gold clusters to c. 2 to 5 nm result in high catalytic activities towards low-temperature carbon monoxide (CO) oxidation. Several other industrially relevant reactions are also observed such as H2 activation, water gas shift, and hydrogenation.

References

  1. ↑ Zhang, Q. (2003). "Recent Progress in Direct Partial Oxidation of Methane to Methanol". J. Natural Gas Chem. 12: 81–89.
  2. ↑ Olah, G., Molnar, A. "Hydrocarbon Chemistry" John Wiley & Sons, New York, 2003. ISBN   978-0-471-41782-8.
  3. 1 2 Lunsford, J.H. (1995). "The catalytic coupling of methane". Angew. Chem. Int. Ed. Engl. 34: 970–980. doi:10.1002/anie.199509701.
  4. ↑ Naito, S. (2000). "Methane conversion by various metal, metal oxide and metal carbide catalysts". Catalyst Surveys from Japan. 4: 3–15. doi:10.1023/A:1019084020968.
  5. ↑ Mleczko L, Baerns M (1995). "Catalytic oxidative coupling of methane—reaction engineering aspects and process schemes". Fuel Processing Technology. 42: 217–248. doi:10.1016/0378-3820(94)00121-9.
  6. ↑ Labinger, J.A. (1988). "Oxidative Coupling of Methane: An inherent limit to selectivity". Catal. Lett. 1: 371–375. doi:10.1007/BF00766166.
  7. ↑ Mazanec TJ, Cable TL, Frye JG (1992). "Electrocatalytic cells for chemical reaction". Solid State Ionics. 53–56: 111–118. doi:10.1016/0167-2738(92)90372-V.
  8. ↑ Kwapien K, Paier J, Sauer J, Geske M, Zavyalova U, Horn R, Schwach P, Trunschke A, Schlögl R (August 2014). "Sites for methane activation on lithium-doped magnesium oxide surfaces". Angew. Chem. Int. Ed. Engl. 53 (33): 8774–8. doi:10.1002/anie.201310632. PMID   24757026.
  9. ↑ "McDermott buys Siluria for oxidative methane-coupling technology". Chemical & Engineering News.
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