(308c) Catalyst Design for Microwave Assisted Methane Dehydroaromatization

The abundance of domestic natural gas reserves has resulted in increasing interests in generating higher value chemicals from methane, the main component of natural gas. While methane upgrading is currently achieved through an indirect route via syngas, direct upgrading offers conceptually a less energy intensive and costly alternative. One promising direct upgrading route is the non-oxidative conversion of methane to aromatics via dehydroaromatization (DHA). However, at the very high temperatures thermodynamically required for this reaction to proceed, carbon deposition becomes inevitable and results in rapid deactivation of the catalyst, rendering the process uneconomic. In the present project, we are exploring a novel process for methane upgrading by applying microwave radiation. Microwave radiation could provide a much more efficient way than conventional heating to couple energy into the reaction system, and is expected to do so more selectively by selectively heating the active (metal) site of the catalyst while leaving the bulk catalyst and gas phase at a lower temperature to avoid or reduce carbon formation. However, due to their complexity, microwave-assisted heterogeneous reactions are poorly understood to-date and systematic studies on how microwaves interact with catalytic materials are largely lacking. Here, we present results from an on-going study that aims to improve the understanding of catalyst design for microwave catalysis by systematically exploring and identifying catalyst properties that affect microwave absorption.

The catalyst for methane DHA is typically composed of metal nanoparticles dispersed in zeolites. In these catalyst systems, parameters including zeolite particle size, the concentration of Brønsted acid sites, speciation and dispersion of metals, and the nature of the metal can be expected to not only affect the catalytic reactivity but also to influence the ability of the catalyst to absorb and interact with microwave radiation. The main objective of our study is hence to obtain an improved understanding of what gives metal/zeolite-based catalyst higher microwave sensitivity. Towards this goal, we synthesized carefully defined metal/ZSM-5 catalysts to individually vary the size and Si/Al ratio of the zeolite ZSM-5, as well as speciation and type of metal in the catalyst. We find that incorporating more Al into the ZSM-5 framework increases the microwave sensitivity, while the size of ZSM-5 has a comparatively small effect. On the metal side, Fe-ZSM-5 was prepared via three different synthetic approaches to control metal dispersion and speciation. We find that both metal speciation and metal loading have significant impact on microwave sensitivity. Further studies regarding the nature of the metal are currently on-going. The observed effects will be presented and discussed in detail in the presentation.

Overall, through these studies, we aim to develop a fundamental understanding of metal/zeolite catalysts in microwave catalysis as a basis for rational catalyst design for microwave-assisted methane DHA.