Elucidating the Formation Mechanism and Catalytic Role of C1 Intermediates in the Initial Stages of Methane Dehydroaromatization over Mo/HZSM-5

Abstract

The methane dehydroaromatization (MDA) reaction is crucial for converting methane into valuable aromatics but involves harsh conditions and complex intermediates. This study utilized advanced in situ synchrotron radiation photoionization mass spectrometry (SR-PIMS) to monitor the 6 wt % molybdenum (Mo)-loaded catalyst and intermediate behaviors in real time. Significant insights include the direct detection of highly reactive radicals [methyl (^•CH₃), and specifically carbene (^•CH₂)] and organic oxygenated species [methanol (CH₃OH), methoxy radical (^•OCH₃), and formaldehyde (HCHO)], as well as insights into the two-step activation and deoxygenation processes at molybdenum (Mo) sites. The study also captured coke deposition and elimination dynamics, confirming significant acetylene production as a key precursor for aromatics. The Mo site activation and gas-phase species generation occur in two stages: initially, Mo oxides are deoxygenated and activated by methane with the production of methanol and formaldehyde as the organic oxygenated intermediates. Methane is then dehydrogenated by the active sites, yielding carbene and methyl radicals, which promote the further activation of Mo sites, driving coke elimination and promoting the formation of C2 hydrocarbons, including acetylene and ethylene. The work sheds light on the catalytic mechanism, offering valuable guidance for designing more efficient MDA catalysts.