Isomorphously Substituted [Fe,Al]ZSM-5 Catalysts for Methane Dehydroaromatization

Dehydroaromatizationof methane (MDA) under non-oxidativeconditionsis a promising reaction for direct valorization of natural gas andbiogas. Typically, Fe-modified ZSM-5 catalysts display low aromaticproductivity and high coke selectivity in the MDA reaction. Herein,we show the benefit of starting from isomorphously substituted Fe-sitesin [Fe,Al]ZSM-5 zeolites prepared by direct hydrothermal synthesis.Upon calcination, these samples contain predominantly isolated Fe3+ species, either atomically dispersed within the zeoliteframework or anchored at exchange sites inside zeolite channels. Interms of the integral hydrocarbon productivity, [Fe,Al]ZSM-5 catalystsoutperform Fe/ZSM-5, prepared by impregnation, as well as Mo/ZSM-5catalysts with the same Si/Al ratio and molar metal loading. OperandoX-ray absorption spectroscopy coupled with mass spectrometry (XANES-MS)demonstrates that the initial tetrahedral Fe3+ within thezeolite framework or at exchange sites are transformed into octahedralextraframework Fe2+ active sites during the MDA reactionand form small Fe2O3 clusters during oxidativeregeneration. Combining activity measurements and operando thermogravimetryshows that the duration of the induction period, related to the formationof active hydrocarbon pool intermediates, strongly depends on theFe dispersion and loading and can be used as a suitable descriptorfor the MDA activity of [Fe,Al]ZSM-5. The shorter induction periodof [Fe,Al]ZSM-5 in comparison to impregnated Fe/ZSM-5 can be linkedto the higher methane conversion rate over highly dispersed Fe-sitesand faster formation of active hydrocarbon pool intermediates.

Automated summary

Dehydroaromatization of methane (MDA) under non-oxidative conditions is a promising reaction for direct valorization of natural gas and biogas. Fe-modified ZSM-5 catalysts prepared by direct hydrothermal synthesis, containing isomorphously substituted Fe-sites, exhibit superior MDA activity compared to impregnated Fe/ZSM-5 catalysts. The higher activity of [Fe,Al]ZSM-5 can be attributed to the transformation of tetrahedral Fe3+ into octahedral Fe2+ active sites during the MDA reaction.