Effect of Coexistent Hydrogen on the Selective Production of Ethane by Dehydrogenative Methane Coupling through Dielectric-Barrier Discharge under Ordinary Pressure at an Ambient Temperature

The effect of coexistence of hydrogen on the product selectivity to ethane from methane by dielectric-barrier discharge (DBD) reactor was examined experimentally under ordinary pressure without use of catalyst and external heating. By the dilution of methane with hydrogen, both the increase of methane conversion and the decrease of alkene production were observed, improving the selectivities to ethane by ca. 70%. 1. Introduction To make use of natural gas as an alternative feedstock for petrochemical products, we have tried to synthesize C2 hydrocarbons by nonoxidative, that is, dehydrogenative coupling of methane, because this process produces hydrogen as a sole byproduct. The dehydrogenative coupling could become one of the green chemistry processes generating no waste materials without use of catalysts. Our previous performances were the synthesis of acetylene from methane (reaction (1)) by the microwave plasma reaction [1] and ethylene from methane (reaction (2)) by the thermal diffusion column [2–8], both with the acceptable high selectivities of the main product, being the maximum acetylene selectivity of 97.4% with the methane conversion ( ) of 92.7% and also the ethylene selectivity of 91.5% with the of 9.4%, respectively, The high selectivity is principally attributable to the local high energy supply to the reactant molecule under a reduced pressure in case of the microwave plasma reaction and also the steep temperature gradient between the center and the wall of the reactor column in case of the thermal diffusion reaction. Although the highest selectivity should be possible for these techniques, the low energy efficiency and the peculiar reactor setup give rise to be a stumbling block to the further progress for practical usages. Subsequently, intensive investigations on dehydrogenative (nonoxidative) coupling of methane have been conducted in atmospheric nonthermal plasmas generated by spark discharge or dieletric-barrier discharge (DBD) without catalysts [9–11] and by DBD with special catalysts [12–15]. Ethane is the main product of these methane activation techniques, and the selectivity was as much as 60%, accompanying unsaturated and C3+ hydrocarbons. In our previous paper on the methane conversion by DBD [16], it has been clarified that the product distribution is greatly affected by the introduced gas pressure. Under a reduced pressure (12？kPa) acetylene was given in the highest selectivity of 40%, but it get down to 5% at the atmospheric pressure (101？kPa), resulting in the increase of the selectivity to ethane up to 57% and propane