Oxidative coupling of methane
Encyclopedia
The oxidative coupling of methane (OCM) is a type of chemical reaction
discovered in the 1980s for the direct conversion of natural gas
, primarily consisting of methane
, into value-added chemicals. Direct conversion of methane into other useful products is one of the most challenging subjects to be studied in heterogeneous catalysis
. Methane activation is difficult because of its thermodynamic stability with a noble gas
-like electronic configuration. The strong tetrahedral 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 its conversion products, and limits efficient utilization of natural gas, the world’s most abundant petrochemical resource.
, the world’s largest commodity chemical and the fundamental building block of chemical industry. While the ability to convert methane to ethylene is highly attractive from the economic point of view, this is a major scientific challenge. Thirty years of research failed to produce a commercial OCM catalyst, preventing this promising process from advancing beyond experimental stage.
Ethylene
is the world’s most valuable commodity chemical with an estimated value of $160 billion/year. As a fundamental building block of chemical industry and the world’s largest chemical intermediate (annual demand estimated at 120–140 million ton), ethylene derivatives are found in products as diverse as food packaging, eyeglasses, cars, medical devices, lubricants, engine coolants and liquid crystal displays. While indispensable to our daily lives, ethylene production by steam cracking consumes more energy than any other chemical process, uses valuable oil fractions, such as naphtha
, and is the largest contributor to GHG emissions in the chemical industry.
The oxidative coupling of methane to ethylene (known as OCM) reaction is written below:
The reaction is exothermic
(∆H = -67kcals/mole) and occurs at high temperatures (750–950 ˚C). In the reaction, methane is activated heterogeneously on the catalyst surface, forming methyl free radicals, which then couple in the gas phase to form ethane . The ethane
subsequently undergoes dehydrogenation
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.
rendered the process economically unattractive.
From further mechanistic studies, it became apparent that the lack of selectivity was related to the poor properties of conventional catalysts. Specifically, known catalysts have poor C-H activation properties, resulting in the need for high reaction temperatures (750 ˚C and 950 ˚C) to activate the C-H bond. This high reaction temperature leads to 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 . This undesired and non-selective competing pathway negatively impacts selectivity.
The high temperature of the conventional catalysts is not only detrimental to the product selectivity, but also presents a challenge for the reaction engineering. Among the process engineering challenges are the requirements for expensive metallurgy
, lack of industry experience with operating high temperature catalytic processes, and the potential need for new reactor design to manage heat transfer efficiently.
Due to lack of selective and robust catalysts that function in an acceptable temperature regime, none of the OCM efforts have advanced beyond exploratory stage. It has even been postulated that there exists an inherent limit to OCM selectivity, which is fundamental and cannot be overcome, with the apparent conclusion that “expecting substantial improvements in the OCM performance might not be wise”.
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.
Chemical reaction
A chemical reaction is a process that leads to the transformation of one set of chemical substances to another. Chemical reactions can be either spontaneous, requiring no input of energy, or non-spontaneous, typically following the input of some type of energy, such as heat, light or electricity...
discovered in the 1980s for the direct conversion of natural gas
Natural gas
Natural gas is a naturally occurring gas mixture consisting primarily of methane, typically with 0–20% higher hydrocarbons . It is found associated with other hydrocarbon fuel, in coal beds, as methane clathrates, and is an important fuel source and a major feedstock for fertilizers.Most natural...
, primarily consisting of methane
Methane
Methane is a chemical compound with the chemical formula . It is the simplest alkane, the principal component of natural gas, and probably the most abundant organic compound on earth. The relative abundance of methane makes it an attractive fuel...
, into value-added chemicals. Direct conversion of methane into other useful products is one of the most challenging subjects to be studied in heterogeneous catalysis
Heterogeneous catalysis
In chemistry, heterogeneous catalysis refers to the form of catalysis where the phase of the catalyst differs from that of the reactants. Phase here refers not only to solid, liquid, vs gas, but also immiscible liquids, e.g. oil and water. The great majority of practical heterogeneous catalysts...
. Methane activation is difficult because of its thermodynamic stability with a noble gas
Noble gas
The noble gases are a group of chemical elements with very similar properties: under standard conditions, they are all odorless, colorless, monatomic gases, with very low chemical reactivity...
-like electronic configuration. The strong tetrahedral C–H bonds (435 kJ/mol) offer no functional group
Functional group
In organic chemistry, functional groups are specific groups of atoms within molecules that are responsible for the characteristic chemical reactions of those molecules. The same functional group will undergo the same or similar chemical reaction regardless of the size of the molecule it is a part of...
, magnetic moments, or polar distributions to undergo chemical attack. This makes methane less reactive than nearly all its conversion products, and limits efficient utilization of natural gas, the world’s most abundant petrochemical resource.
Ethylene production
The principal product of OCM is ethyleneEthylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
, the world’s largest commodity chemical and the fundamental building block of chemical industry. While the ability to convert methane to ethylene is highly attractive from the economic point of view, this is a major scientific challenge. Thirty years of research failed to produce a commercial OCM catalyst, preventing this promising process from advancing beyond experimental stage.
Ethylene
Ethylene
Ethylene is a gaseous organic compound with the formula . It is the simplest alkene . Because it contains a carbon-carbon double bond, ethylene is classified as an unsaturated hydrocarbon. Ethylene is widely used in industry and is also a plant hormone...
is the world’s most valuable commodity chemical with an estimated value of $160 billion/year. As a fundamental building block of chemical industry and the world’s largest chemical intermediate (annual demand estimated at 120–140 million ton), ethylene derivatives are found in products as diverse as food packaging, eyeglasses, cars, medical devices, lubricants, engine coolants and liquid crystal displays. While indispensable to our daily lives, ethylene production by steam cracking consumes more energy than any other chemical process, uses valuable oil fractions, such as naphtha
Naphtha
Naphtha normally refers to a number of different flammable liquid mixtures of hydrocarbons, i.e., a component of natural gas condensate or a distillation product from petroleum, coal tar or peat boiling in a certain range and containing certain hydrocarbons. It is a broad term covering among the...
, and is the largest contributor to GHG emissions in the chemical industry.
The oxidative coupling of methane to ethylene (known as OCM) reaction is written below:
- 2 + = + 2
The reaction is exothermic
Exothermic
In thermodynamics, the term exothermic describes a process or reaction that releases energy from the system, usually in the form of heat, but also in the form of light , electricity , or sound...
(∆H = -67kcals/mole) and occurs at high temperatures (750–950 ˚C). In the reaction, methane is activated heterogeneously on the catalyst surface, forming methyl free radicals, which then couple in the gas phase to form ethane . The ethane
Ethane
Ethane is a chemical compound with chemical formula C2H6. It is the only two-carbon alkane that is an aliphatic hydrocarbon. At standard temperature and pressure, ethane is a colorless, odorless gas....
subsequently undergoes dehydrogenation
Dehydrogenation
Dehydrogenation is a chemical reaction that involves the elimination of hydrogen . It is the reverse process of hydrogenation. Dehydrogenation reactions may be either large scale industrial processes or smaller scale laboratory procedures....
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.
Considerations
The economic promise of OCM has attracted significant industrial interest. In the 1980s and 1990s multiple research efforts were pursued by the world’s leading academic investigators and petrochemical companies. Hundreds of catalysts have been tested, and several promising ones identified and extensively studied. Following the initial excitement, it eventually became obvious that the reaction did not proceed with the selectivities that were required for an economically viable operation. Instead of converting into ethylene, the majority of methane was being non-selectively oxidized to carbon dioxide. This lack of chemoselectivityChemoselectivity
Chemical reactions are defined usually in small contexts , such generalizations are a matter of utility. The preferential outcome of one instance of a generalized reaction over a set of other plausible reactions, is defined as chemoselectivity...
rendered the process economically unattractive.
From further mechanistic studies, it became apparent that the lack of selectivity was related to the poor properties of conventional catalysts. Specifically, known catalysts have poor C-H activation properties, resulting in the need for high reaction temperatures (750 ˚C and 950 ˚C) to activate the C-H bond. This high reaction temperature leads to 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 . This undesired and non-selective competing pathway negatively impacts selectivity.
The high temperature of the conventional catalysts is not only detrimental to the product selectivity, but also presents a challenge for the reaction engineering. Among the process engineering challenges are the requirements for expensive metallurgy
Metallurgy
Metallurgy is a domain of materials science that studies the physical and chemical behavior of metallic elements, their intermetallic compounds, and their mixtures, which are called alloys. It is also the technology of metals: the way in which science is applied to their practical use...
, lack of industry experience with operating high temperature catalytic processes, and the potential need for new reactor design to manage heat transfer efficiently.
Due to lack of selective and robust catalysts that function in an acceptable temperature regime, none of the OCM efforts have advanced beyond exploratory stage. It has even been postulated that there exists an inherent limit to OCM selectivity, which is fundamental and cannot be overcome, with the apparent conclusion that “expecting substantial improvements in the OCM performance might not be wise”.
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.