In alkane metathesis alkanes are rearranged (i.e. redistributed or disproportionated) to give higher and lower homologues.
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 metallacyclobutane 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 comprised 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 gives cyclohexadecane (cyclo-C16H32) as well as higher rings and polymers.
References
- Metathesis of Alkanes Catalyzed by Silica-Supported Transition Metal Hydrides Véronique Vidal, Albert Théolier, Jean Thivolle-Cazat, Jean-Marie Basset, Science, Vol 276, Issue 5309, 99-102 4 April 1997 Abstract
- Primary Products and Mechanistic Considerations in Alkane Metathesis Jean Marie Basset, Christophe Copéret, Laurent Lefort, Barry M. Maunders, Olivier Maury, Erwan Le Roux, Guillaume Saggio, Sophie Soignier, Daravong Soulivong, Glenn J. Sunley, Mostafa Taoufik, and Jean Thivolle-Cazat J. Am. Chem. Soc.., 127 (24), 8604 -8605, 2005. Abstract
- "Mechanism and poisoning of the molecular redistribution reaction of alkanes with a dual-functional catalyst system." Burnett, R. L.; Hughes, T. R. Journal of Catalysis 1973, 31, 55-64. Abstract
- "Catalytic Alkane Metathesis by Tandem Alkane Dehydrogenation-Olefin Metathesis." Goldman, A. S.; Roy, A. H.; Huang, Z.; Ahuja, R.; Schinski, W.; Brookhart, M. Science 2006, 312, 257-261. Abstract
- "Catalytic ring expansion, contraction, and metathesis-polymerization of cycloalkanes." Ahuja, R.; Kundu, S.; Goldman, A. S.; Brookhart, M.; Vicente, B. C.; Scott, S. L. Chem. Commun. 2008, 253-255. Abstract