(508e) Towards Elucidating and Accelerating Catalyst Activation in Methane Dehydroaromatization over Fe-ZSM-5

The abundance of domestic natural gas reserves continues to fuel interest in converting methane, the main component of natural gas, into higher-value chemicals. One such route is the upgrading of methane to ethylene, benzene, and hydrogen via a direct methane conversion process, methane dehydroaromatization (MDA). The main challenge of MDA lies in the methane activation step. Due to the stable tetrahedral structure of methane molecules, high temperature is thermodynamically required for the reaction to proceed efficiently, which inevitably leads to coke formation, selectivity loss, and the deactivation of the catalyst. The catalysts of choice for this reaction are metal-exchanged ZSM-5 zeolites, which require lengthy initial activation and again after each regeneration step for coke removal. Depending on the type of metal and its speciation, the induction period can take excessively long times and hence constitutes a significant hurdle toward the commercialization of MDA processes.

In the present work, the activation of Fe-based ZSM-5 catalyst was studied. Iron speciation was carefully controlled and characterized to examine its effect on the MDA induction period. We find that the induction period decreased systematically with increasing Fe loading in isomorphous substituted Fe-ZSM-5 catalysts. This suggests a correlation with an increasing amount of aggregated iron oxide clusters in the catalyst. The role of aggregated Fe species is verified by evaluating MDA on regenerated samples, where isolated Fe species aggregate to iron oxide clusters due to high local temperatures during carbon burn-off. The regenerated catalyst shows a systematic decrease in induction period with increasing regeneration cycles, confirming the acceleration of activation process by aggregated Fe species. This suggests that the presence of iron oxide nanoparticles or clusters via catalyst design could result in strongly accelerated catalyst activation. As a next step, we are now preparing Fe₂O₃@ZSM-5 catalysts via a synthetic route previously developed in our lab. In these core@shell type catalysts, iron oxide nanoparticles are encapsulated in mesopores of ZSM-5 which should enable accelerated activation of the catalyst as-synthesized. Overall, this study aims to improve our understanding of the impact of iron speciation on activation of Fe-ZSM-5 and lay the foundation for a rational design of a more efficient catalyst for MDA.