Photocoupling of Methane in Water Vapor to Saturated Hydrocarbons
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Photocoupling of Methane in Water Vapor to Saturated Hydrocarbons JunePyo Oh Æ Taketoshi Matsumoto Æ Junji Nakamura
Received: 4 February 2008 / Accepted: 6 March 2008 / Published online: 19 April 2008 Ó Springer Science+Business Media, LLC 2008
Abstract Methane can be converted into alkanes (from C2 to C6) continuously by ultraviolet (185 nm) irradiation in the presence of water vapor. The products from this reaction are alkanes, which is different from the comparable heterogeneous catalytic reactions, where alkene formation is also observed. The mechanism involves the coupling of alkyl radicals formed by hydrogen abstraction with OH radicals produced following the UV irradiation of water. Keywords Methane coupling UV irradiation Natural gas Alkane
1 Introduction Methane is a major component of natural gas and is regarded as a promising resource in the development of new natural gas reserves, such as methane hydrate and biomass. The direct conversion of methane into hydrocarbons is an attractive technique for transporting and storaging liquid fuels. The formation of other hydrocarbons, all of which are less stable than methane at 1273 K, have unfavorable free energies of reaction and are strongly limited by equilibrium. These reactions typically need considerable energy input. Indeed, temperatures higher than 1273 K are required to transform methane into benzene, acetylene, ethylene, and ethane by the oxidative coupling of methane (OCM) with oxide catalysts [1, 2], while the combination of reforming with H2O or CO2 over Ni catalysts [1, 3, 4] and the Fischer– J. Oh T. Matsumoto J. Nakamura (&) Graduate School of Pure and Applied Science, University of Tsukuba, Tsukuba, Ibaraki, Japan e-mail: [email protected]
Tropsch (FT) synthesis, are all energy-intensive methods. Unlike most OCM reactions, such as the carbocationic OCM reaction with super-acids at low temperature [5, 6], the OCM involving photoreactions typically save hydrogen or wasted heating energy [7–14]. As a unique photoreaction, Crabtree et al. have reported the mercury photosensitized dehydrodimerization using low-pressure mercury lamp [12–14]. They showed that the excited state of mercury by a 254 nm photon homolyzes a C–H bond of alkanes (cyclopentane, methyl butane, pentane, and so on) or alcohols (methanol, ethanol, propanol, and so on) to give a carbon radical and a hydrogen atom. These carbon radicals recombine to give dimer. We report here that water catalyzes the formation of C2–C6 or higher saturated hydrocarbons in the photocoupling of methane with a low-pressure mercury lamp in a flow reactor at 1 atm. Water is a convenient and harmless molecule, so that the photoreaction is attractive in practical use. Furthermore, this method is free from catalyst preparation [7–10] and light with high energy for the direct excitation of methane [10].
2 Method The photochemical reactions using methane (10% methane diluted with Ar and 99.9% purity) and water were carried out in a quartz tubular reactor (inner diameter of 16 mm, length of
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