A study on the reaction mechanism of non-oxidative methane coupling in a nanosecond pulsed discharge reactor using isotope analysis

We present a study on the reaction pathways involved in non-oxidative methane coupling in a nanosecond pulsed discharge reactor using isotope analysis. Specifically, plasma-assisted reactions with isotopes, serving as tracers, are performed in two ratios (CH4:D-2 = 1:1 and CH4:D-2 = 1:3) and elevated pressures (up to 5 bar). Acetylene hydrogenation reactions are also performed in a tubular reactor under conventional furnace heating to simulate post plasma zone conditions. Depending on the pressure applied the major product formed changes. At ambient pressure, acetylene is the major product through thermally driven methane coupling to ethane in gas phase followed by stepwise dehydrogenation of ethane to acetylene (C2H6 -> C2H5 -> C2H4 -> C2H3 -> C2H2). At higher pressures (> 3 bar), acetylene is formed via the same sequence of paths. However, the higher bulk gas temperature attained when operating at overpressure also activates gas phase methane coupling to ethylene and acetylene hydrogenation to ethylene catalyzed by the copper-based electrode of the reactor. These two reaction paths shift product selectivity from acetylene to ethylene at higher pressures.