Toward Accelerated Activation of Fe-ZSM-5 in Methane Dehydroaromatization

Fe-ZSM-5 is a promising catalyst for methane dehydroaromatization (MDA) but currently suffers from long activation periods that exacerbate a comparatively low activity vs the state-of-the-art Mo-ZSM-5 catalysts. Iron can exist in ZSM-5 in many forms, and while it is well-known that metal speciation impacts the activation period, the underlying process is poorly understood to date. The present study aimed to fill this gap by elucidating the effect of Fe speciation on catalyst activation as a guide for rational catalyst design. Iron speciation was carefully controlled via different synthesis pathways (wet impregnation, ion exchange, and isomorphous substitution), and its effect on the activation period in MDA was examined. A higher degree of aggregation of iron oxide was found to be beneficial for the acceleration of the activation period, which could be explained by the involvement of reduction and carburization steps in the formation of the active phase. Aggregated iron oxide is more readily carburized than atomically dispersed iron in the framework, which cannot be carburized directly and needs to be transformed into aggregated iron oxide before being reduced and carburized into the active phase. This suggests a fine balance required for synthesizing an optimal Fe-ZSM-5 catalyst for MDA, which balances fast activation enabled by high Fe agglomeration against a large active surface area for high reactivity.