Non-oxidative methane coupling to C₂ hydrocarbons in a microwave plasma reactor

Non-oxidative methane activation is carried out in a microwave plasma reactor for coupling to higher hydrocarbons. Fourier transform infrared spectroscopy (FTIR) was used to measure absolute concentrations of the major hydrocarbon species. Hydrogen concentration was also independently inferred from pressure-based change in molar flow measurements. By closing both the carbon and hydrogen balance, from stoichiometry of the reactions, the amount of deposits was obtained as well. Additionally, core gas temperatures up to 2500 K were measured with Raman scattering when nitrogen acted as probing molecule in sample mixture discharges. At low gas temperatures, ethane and ethylene were significant products based on plasma chemistry, with ethane selectivities reaching up to 60%. At higher gas temperatures, thermal effects become stronger shifting the selectivity toward acetylene and deposits, resembling more with equilibrium calculations. The energy efficiency of the methane conversion reached up to 15% from which 10% represented coupling efficiency to higher hydrocarbons. It is concluded that there is an interplay between plasma and thermal chemistry where plasma generates radicals and final distribution is set by thermodynamics.