Alkane metathesis
Encyclopedia
Alkane metathesis is a chemical reaction
in which alkane
s are rearranged to give longer or shorter alkane products. It is similar to olefin metathesis
, except that olefin metathesis cleaves and recreates a carbon-carbon double bond, but alkane methathesis operates on a carbon-carbon single bond.
by metal-containing compounds or complexes. One such class of catalyst systems, discovered and developed by the group of Jean-Marie Basset
, comprises tantalum
hydride
supported on silica (SiO2)TaH at temperatures of 25°C to 200°C. These systems catalyze the conversion of ethane
to give methane
, propane
and traces of butane
. Cross metathesis can also take place, in which, for example methane and propane molecules react to give two molecules of ethane. Ethane reacts with toluene
to give ethylbenzene
and xylene
. The reaction involves metallocyclobutane intermediates just as in olefin metathesis
.
Dual-catalyst systems have also been developed which are effective for the metathesis of higher alkanes. These systems are based on the tandem operation of two catalysts, one for transfer-dehydrogenation and one for olefin metathesis.
A heterogeneous system consisting of Pt on alumina mixed with W oxide on silica was reported by Burnett and Hughes. Systems based on pincer-iridium dehydrogenation catalysts and either "Schrock
-type" Mo-based olefin metathesis catalysts have been reported by Brookhart
and Goldman. Because the iridium-based catalysts show selectivity for the termini of n-alkanes, these systems have the unique ability to effect alkane metathesis with some selectivity for product molecular weight. Thus for example n-hexane can be converted to ethane and n-decane as the major products.
Numerous applications for alkane metathesis involving petrochemicals and fuels can be envisaged. For example, the conversion of n-hexane to n-decane and ethane has been proposed for the purpose of improving the overall yield of diesel grade n-alkane (C9H20 to C19H40) from Fischer-Tropsch reactors, which convert syngas
to a broad range of n-alkanes.
Scott, Goldman and Brookhart have reported the metathesis of cycloalkanes with the pincer-Ir/Mo system. Cyclooctane (cyclo-C8H16), for example is converted to a mixture of cyclohexadecane (cyclo-C16H32) and higher rings as well as polymers.
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...
in which alkane
Alkane
Alkanes are chemical compounds that consist only of hydrogen and carbon atoms and are bonded exclusively by single bonds without any cycles...
s are rearranged to give longer or shorter alkane products. It is similar to olefin metathesis
Olefin metathesis
Olefin metathesis or transalkylidenation is an organic reaction that entails redistribution of alkylene fragments by the scission of carbon - carbon double bonds in olefins . Its advantages include the creation of fewer sideproducts and hazardous wastes. Yves Chauvin, Robert H. Grubbs, and Richard R...
, except that olefin metathesis cleaves and recreates a carbon-carbon double bond, but alkane methathesis operates on a carbon-carbon single bond.
Examples and catalysts
Alkane metathesis is catalyzedCatalysis
Catalysis is the change in rate of a chemical reaction due to the participation of a substance called a catalyst. Unlike other reagents that participate in the chemical reaction, a catalyst is not consumed by the reaction itself. A catalyst may participate in multiple chemical transformations....
by metal-containing compounds or complexes. One such class of catalyst systems, discovered and developed by the group of Jean-Marie Basset
Jean-Marie Basset
Jean-Marie Basset is a French chemist, currently the director of KAUST catalysis research center.Professor Basset received his PhD in 1969 from the University of Lyon, France...
, comprises tantalum
Tantalum
Tantalum is a chemical element with the symbol Ta and atomic number 73. Previously known as tantalium, the name comes from Tantalus, a character in Greek mythology. Tantalum is a rare, hard, blue-gray, lustrous transition metal that is highly corrosion resistant. It is part of the refractory...
hydride
Hydride
In chemistry, a hydride is the anion of hydrogen, H−, or, more commonly, a compound in which one or more hydrogen centres have nucleophilic, reducing, or basic properties. In compounds that are regarded as hydrides, hydrogen is bonded to a more electropositive element or group...
supported on silica (SiO2)TaH at temperatures of 25°C to 200°C. These systems catalyze the conversion of 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....
to give 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...
, propane
Propane
Propane is a three-carbon alkane with the molecular formula , normally a gas, but compressible to a transportable liquid. A by-product of natural gas processing and petroleum refining, it is commonly used as a fuel for engines, oxy-gas torches, barbecues, portable stoves, and residential central...
and traces of butane
Butane
Butane is a gas with the formula C4H10 that is an alkane with four carbon atoms. The term may refer to any of two structural isomers, or to a mixture of them: in the IUPAC nomenclature, however, butane refers only to the unbranched n-butane isomer; the other one being called "methylpropane" or...
. Cross metathesis can also take place, in which, for example methane and propane molecules react to give two molecules of ethane. Ethane reacts with toluene
Toluene
Toluene, formerly known as toluol, is a clear, water-insoluble liquid with the typical smell of paint thinners. It is a mono-substituted benzene derivative, i.e., one in which a single hydrogen atom from the benzene molecule has been replaced by a univalent group, in this case CH3.It is an aromatic...
to give ethylbenzene
Ethylbenzene
Ethylbenzene is an organic compound with the formula C6H5CH2CH3. This aromatic hydrocarbon is important in the petrochemical industry as an intermediate in the production of styrene, which in turn is used for making polystyrene, a common plastic material....
and xylene
Xylene
Xylene encompasses three isomers of dimethylbenzene. The isomers are distinguished by the designations ortho- , meta- , and para- , which specify to which carbon atoms the two methyl groups are attached...
. The reaction involves metallocyclobutane intermediates just as in olefin metathesis
Olefin metathesis
Olefin metathesis or transalkylidenation is an organic reaction that entails redistribution of alkylene fragments by the scission of carbon - carbon double bonds in olefins . Its advantages include the creation of fewer sideproducts and hazardous wastes. Yves Chauvin, Robert H. Grubbs, and Richard R...
.
Dual-catalyst systems have also been developed which are effective for the metathesis of higher alkanes. These systems are based on the tandem operation of two catalysts, one for transfer-dehydrogenation and one for olefin metathesis.
A heterogeneous system consisting of Pt on alumina mixed with W oxide on silica was reported by Burnett and Hughes. Systems based on pincer-iridium dehydrogenation catalysts and either "Schrock
Richard R. Schrock
Richard Royce Schrock is an American chemist and Nobel laureate recognized for his contributions to the metathesis reaction used in organic chemistry.-Biography:...
-type" Mo-based olefin metathesis catalysts have been reported by Brookhart
Maurice Brookhart
Maurice S. Brookhart is the William R. Kenan, Jr. Professor of Chemistry in the Department of Chemistry at the University of North Carolina....
and Goldman. Because the iridium-based catalysts show selectivity for the termini of n-alkanes, these systems have the unique ability to effect alkane metathesis with some selectivity for product molecular weight. Thus for example n-hexane can be converted to ethane and n-decane as the major products.
Numerous applications for alkane metathesis involving petrochemicals and fuels can be envisaged. For example, the conversion of n-hexane to n-decane and ethane has been proposed for the purpose of improving the overall yield of diesel grade n-alkane (C9H20 to C19H40) from Fischer-Tropsch reactors, which convert syngas
Syngas
Syngas is the name given to a gas mixture that contains varying amounts of carbon monoxide and hydrogen. Examples of production methods include steam reforming of natural gas or liquid hydrocarbons to produce hydrogen, the gasification of coal, biomass, and in some types of waste-to-energy...
to a broad range of n-alkanes.
Scott, Goldman and Brookhart have reported the metathesis of cycloalkanes with the pincer-Ir/Mo system. Cyclooctane (cyclo-C8H16), for example is converted to a mixture of cyclohexadecane (cyclo-C16H32) and higher rings as well as polymers.