Identification of atomically dispersed Fe-oxo species as new active sites in HZSM-5 for efficient non-oxidative methane dehydroaromatization

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• Atomically dispersed Fe-oxo species was recognized in the Fe-exchanged ZSM-5 by Mössbauer spectrum. • The Fe-oxo species are highly active and selective for conversion of CH 4 into aromatics. • Metallic Fe and Fe carbide (at least χ-Fe 5 C 2 ) in zeolite are favorable for CH 4 decomposition. • Divalent Fe salts like FeSO 4 is welcome for preparation of the Fe-exchanged zeolite catalyst. • Excess S residual in catalyst could greatly enhance CH 4 decomposition. Non-oxidative methane dehydroaromatization (MDA) provides a promising and efficient technology for the valorization of huge amount of CH 4 resources. Fe-based HZSM-5 zeolite as the second most researched catalyst system for MDA, however, commonly showed a much lower activity or/and selectivity for aromatics’ formation, probably due to an unsuitable Fe phase in the zeolite. Different from the reported Fe/HZSM-5 catalysts by the traditional impregnation method with Fe(NO 3 ) 3 , a Fe/HZSM-5 catalyst was specially designed for the title reaction which was prepared by ion-exchange method with a stable, soluble and divalent Fe salt, i.e. , FeSO 4 . Interestingly, the Fe 2+ -exchanged HZSM-5 catalyst can show an extremely higher aromatics formation activity with a maximum selectivity of 75–80% at a CH 4 conversion of about 15% than any other Fe 3+ -exchanged and Fe-impregnated HZSM-5 zeolite catalysts at 1073 K. With various characterizations (UV-vis, UV-Raman, CO-DRFIT, HADDF-STEM and, especially, 57 Fe Mössbauer spectroscopy), atomically dispersed Fe-oxo species anchored at framework Al sites are recognized to be responsible for selectively activating CH 4 into CH x species available for aromatics’ formation. By comparison, the common Fe/HZSM-5 catalysts prepared by impregnation method with Fe(NO 3 ) 3 had a large amount of reducible Fe 3 O 4 clusters on zeolite surface, and as they were reduced and carburized, the resulted metallic Fe and Fe carbide were both favorable for CH 4 decomposition only, probably due to their poor dispersion. Fortunately, the limited S residuals (about 0.05 wt%) in the Fe 2+ -exchanged HZSM-5 catalyst from FeSO 4 did not show its negative side on strongly promoting C–H cleavage. By comparison, the two impregnated Fe/HZSM-5 catalysts from FeSO 4 and Fe 2 (SO 4 ) 3 both showed a complete CH 4 decomposition due to considerable SO 4 2- remained. Thanks to the application of the ion-exchange method and FeSO 4 as Fe precursor, a highly active and selective Fe-based HZSM-5 catalyst has been successfully modeled and Fe-oxo species are recognized as new active sites for MDA, which may give us a new insight into constructing a catalyst with better catalytic performance for MDA and other reactions.