Towards Elucidating Critical Properties of Fe-ZSM-5 for Methane DHA

The abundance of domestic natural gas reserves has resulted in an increased interest in converting methane, the main component of natural gas, into higher-value chemicals. One promising route is the non-oxidative conversion of methane to aromatics via dehydroaromatization (DHA). However, at the high temperatures thermodynamically required for this reaction to proceed, coke formation becomes inevitable and results in rapid catalyst deactivation. An alternative energy source, microwave irradiation, can be applied to aid the catalytic reaction of methane DHA. Compared to conventional heating methods, microwave irradiation allows for selective heating, i.e. the active metal sites can be heated selectively while keeping the bulk catalyst and the gas phase at a lower temperature and thus avoid or reduce undesired side reactions, such as coke formation. However, microwave-assisted heterogeneous reactions are poorly understood to-date due to their complexity. In this project, we aim to deconvolute the complexity into two aspects: Investigating the properties that control microwave sensitivity of the catalyst on one hand, and properties that control the (thermo-)catalytic reactivity on the other hand. The latter will be the focus of the current presentation.

The present study is focused on Fe-ZSM5 as a model catalyst for DHA and identification of its key properties that control reactivity for DHA. By carefully controlling the properties of Fe-ZSM5 catalysts, we found a strong correlation both for the Si/Al ratio and Fe loading with catalyst activity and selectivity. Interestingly, this is in good agreement with parallel work in which we had identified that Si/Al ratio and Fe loading are also strongly correlated with the microwave sensitivity of these catalysts. Specifically, we find that catalyst reactivity can be correlated with Fe species distribution and acid site concentration, and that both vary depending on the extent of both Fe and Al incorporation into ZSM-5. Further characterizations of the Fe speciation and acid sites of the samples and correlation with the catalyst performance are currently on-going.

Through this study, we aim to improve our understanding of how the catalytic reactivity of ZSM-5 can be affected by the Al and Fe incorporation and lay the groundwork for the rational catalyst design for thermocatalytic and microwave-assisted methane DHA.