Side reaction controls in carbon nanotube synthesis from associated petroleum flare gas: Impact of ethane content in methane-rich feed

This study presents a systematic investigation of side reactions triggered by ethane (C 2 H 6 ) co-presence in methane-rich feed gas, a representative of associated petroleum flare gas, under catalytic chemical vapor deposition (CCVD) using a 10 wt% FeMo/MgO catalyst for synthesis of value-added carbon nanotubes (CNTs). Ethane co-existence with methane (CH 4 ) at 10–30 vol% in feed for CNT synthesis via CCVD reaction at 900 °C demonstrated side reactions synergistically induced through a gas-phase radical cascade: C 2 H 6 thermal cracking generated reactive intermediates that drove undesired polyaromatic hydrocarbons (PAHs) condensation and liquid oily by-product formation. This synergistic behavior occurred selectively in the binary CH 4 -C 2 H 6 system under improper reaction conditions. Mitigation strategies used in this study, which are (i) N 2 dilution, (ii) H 2 co-flow, and (iii) temperature reduction to 800 °C, effectively suppressed these side reactions while preserving CNT synthesis selectivity. Under controlled conditions at 800 °C, ethane exerted a net beneficial promoting effect: the CNT yield increased from 0.53 to 1.36 g-CNT/g-Cat with no observation of liquid deposits as C 2 H 6 increased from 0 to 30 vol%. These findings provide a systematic basis for advancing pure methane feeds toward real multicomponent associated petroleum flare gas in CNT production.