Controlling high-temperature exposure and product selectivity in pulsed microwave plasma methane coupling

Abstract

Plasma can couple methane to higher hydrocarbons thereby providing opportunities for carbon circularity. Efficiency and control over ethylene and acetylene selectivity versus undesired soot formation are keys to success and the result of a delicate balance between reactor temperature and high-temperature exposure time. Here, we demonstrate that microwave power pulsing provides direct control over this balance. Pulse frequency and duty cycle were tuned and shifted hydrocarbon selectivity from 70% ethane (< 50 μs pulses), through an ethylene-selective window with a maximum selectivity of ∼ 35% and < 3% carbon deficit at 510 Hz, to approximately 70% acetylene selectivity (> 150 μs pulses). Methane conversion increased with duty cycle, reaching approximately 12% within the investigated operating range. Time-resolved in situ H 2 Raman spectroscopy yielded the gas temperature, which cycled between 1000 K and 3000 K. Gas temperature evolution was used to relate reactor performance to methane coupling chemistry based on 0D chemical kinetics modelling. These results show that power pulsing can control product distribution by modifying the combined temperature and high-temperature exposure history, providing a reactor-engineering strategy for tuning methane coupling selectivity and disentangling transport and reaction-kinetic effects.