Active Sites and Induction Period of Fe/ZSM-5 Catalyst in Methane Dehydroaromatization

Fe/ZSM-5 is a potential methane dehydroaromatization (MDA) catalyst in addition to Mo/ZSM-5. In this study, Fe/ZSM-5 catalysts are prepared via a grinding synthesis method (GSM) using Na2FeO4 as the iron precursor. Iron species in the obtained catalysts are presented mainly as isolated or low-polymerized sites in zeolite cationic exchange positions and pores/channels, generating strong metal–support interactions. These iron species are found to be highly resistant to the reduction and carburization of methane in the MDA reaction, which contradicts common Fe/ZSM-5 catalysts. The rapid formation of active iron suboxides (highly dispersed FeO, Fe3O4 or FeOx) together with the fast generation of hydrocarbon pool species inside zeolite channels considerably shorten the induction period. Owing to the strong metal–support interaction, the formed iron suboxides could also withstand sintering/agglomeration and inhibit coke accumulation on the external catalyst surface during reaction. While for Fe/ZSM-5 catalysts prepared by the conventional impregnation method, the iron oxides remain principally at the external surface as big clusters/particles with weak metal–support interaction, which then undergo complete reduction and carburization during MDA reaction. The slow formation of active iron carbides and subsequent slow generation of hydrocarbon pool species result in a longer induction period. Moreover, complete dehydrogenation of methane takes place on the iron carbides at the outmost surface of the catalyst, causing massive coke accumulation. Hence, the shortened induction period together with higher coking resistance of GSM Fe/ZSM-5 provide more thought to the design of iron-based MDA catalysts.