Plasma-Catalyst Synergy in the One-Pot Nonthermal Plasma-Assisted Synthesis of Aromatics from Methane

Abstract

Electrification of the methane dehydroaromatization reaction with the use of nonthermal plasmas could alleviate the high-temperature requirement for this process while promoting the formation of valuable aromatics. Here, we evaluate the use of nonthermal plasma to investigate methane activation and conversion to aromatics by systematically varying bulk gas temperature in a one-pot, plasma-stimulated catalytic reactor over Mo/H-ZSM-5 and metal-free H-ZSM-5 catalysts. We report that Mo is not required for methane activation under low-temperature plasma conditions (573–773 K), and methane conversions up to ∼15% with a 1:1 methane/N2 feed are obtained under a 10 W plasma. However, Mo contributes to the formation of aromatics in the presence of a plasma at 773 K, achieving close to a 2-fold increase in the production of aromatics when compared to unmodified H-ZSM-5. Further, the exposure of as-prepared Mo/H-ZSM-5 to the methane plasma feed induces the formation of Mo-carbide phases in the temperature range studied. These findings highlight the complex roles of nonthermal plasmas in the direct activation of methane and the importance of plasma-catalyst design to facilitate aromatization reactions under plasma-assisted reaction conditions.

Nonthermal Plasma-Stimulated C–N Coupling from CH4 and N2 Depends on the Presence of Surface CHx and Plasma-Phase CN Species

“…In a review paper by Chung and Chang, it is reported that high temperature (>1000 °C) drives carbide formation . Rivera-Castro et al exhibited the formation of molybdenum carbide and oxycarbide as low as at 573 K (300 °C) under nonthermal plasma exposure, where thermal carburization typically takes place at 973 K (700 °C). Silver and gold are not known to form carbide.…”

“…Catalysts then play an important role in driving the selectivity of plasma-activated chemistry toward the production of target products. The integration of heterogeneous catalysis and reactive NTPs shows particular promise for the chemical transformation of hard-to-activate molecules such as N 2 and CH 4 . , …”

“…Laboratory simulations of Titan’s atmosphere using a CH 4 /N 2 mixture NTP at reduced pressure with have been reported to form a solid organic material, termed tholin. , Tholins have been analyzed to show that they are composed of multiple different C–N coupled chemical species. − Also, plasma-stimulation of CH 4 /N 2 mixtures has been used for the deposition of hydrogenated carbon nitride films on nonmetal surfaces, − and the production of nitrogen-doped diamond films. − However, a detailed understanding of the reaction mechanisms and the intermediate species involved is critical for optimizing and harnessing the potential of the C–N reactions. Despite the existence of previous studies on plasma-stimulated C–N coupling, a systematic exploration of the underlying mechanism and identification of key intermediates is lacking, especially within the context of producing valuable C–N products …”

Plasma Catalysis for Hydrogen Production: A Bright Future for Decarbonization

“…In order to elucidate temperature effects for CH 4 conversion to aromatics under plasma stimulation, Rivera-Castro et al 255 investigated the influence of the bulk gas temperature over Mo/H-ZSM-5. Although not quantified, stoichiometric amounts of H 2 were purportedly produced for the plasma-assisted CH 4 conversion in the temperature range of 573–773 K, where CH 4 conversions up to 15% were obtained, and a 2-fold increase in the benzene production was achieved over Mo/H-ZSM-5 at 773 K. 255 At a bulk gas temperature of 873 K, a decrease in CH 4 conversion for the plasma-catalytic system was obtained. The remaining activity was largely due to thermal catalytic pathways taking place at the higher temperature conditions.…”