Modelling of the catalytic initiation of methane coupling under non-oxidative conditions

The experimentally observed interplay between catalytic activation of methane on Fe©SiO2 and gas-phase free radical methane coupling under non-oxidative conditions is analyzed by mechanistic modeling as well as by experiments. For the modeling, an off-the shelf gas-phase model, AramcoMech 3.0, was used unaltered to keep the number of adjustable parameters as low as possible. It was complemented by surface reactions specifically accounting for methane activation to methyl radicals. The model was validated against an independent set of experimental data and exhibited good accordance. The model accurately captured the significant contribution of gas-phase reactions responsible for methane conversion in the post-catalytic zone, indicative of gas-phase autocatalytic methane coupling. The low-activity induction period in gas-phase methane pyrolysis can effectively be overcome by adequate catalytic activation. Results show that the catalytic reaction only influences the activity of the system, with gas-phase reactions dictating the selectivity distribution. Simulations demonstrated that the optimum catalytic conversion roughly amounts to 4 % at 1000 °C and 1 atm. An equivalent effect can be reached by adding ca. 2 % of ethane or ethylene to the feed. Detailed reaction-path analyses were employed to corroborate these phenomena. Gas-phase reactions were found to be very rapid at 1000 °C, hence determining the product selectivity, without impact from either catalyst or C2 hydrocarbon addition. Current, freely available gas-phase models lack the required accuracy for detailed kinetic modeling of the product distribution, showing the requirement for the development of a dedicate non-oxidative methane coupling model.