(324b) Decoupling MW Sensitivity and Reactivity: Towards Understanding Fe-ZSM-5@SiC As Effective Microwave Catalyst for Methane Dehydro-Aromatization

Due to the limitations in transportation of abundantly available natural gas to point-of-use locations, large amounts of natural gas are flared causing significant environmental pollution. Decentralized conversion of natural gas to value-added chemicals could help address this issue. One such conversion route is the direct conversion of methane to benzene via methane dehydro-aromatization (MDA). Typically, metal/zeolite catalysts are used for this reaction, where methane is activated on a metal site and then oligomerized on the Bronstedt acid sites of the zeolite where the zeolite provides shape selectivity towards the desired product benzene. Thermodynamically, MDA requires high temperatures to attain sufficient conversion. However, at these high temperatures, carbon deposition becomes favorable and results in rapid deactivation of the catalyst, rendering the process uneconomic. This motivates the search for novel ways of conducting this reaction. One such approach is based on microwave-assisted heterogeneous catalysis.

In microwave-assisted heterogeneous catalysis, microwave radiation is used to selectively heat the (solid) catalyst. By delivering the energy directly to the active site, this not only promises higher energy efficiency than conventional thermal heating, but also might mitigate coke formation by keeping the gas phase relatively cold and hence suppressing undesired side-reactions that result in coke forming. 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. We had previously identified Fe-ZSM-5 as a promising catalyst for microwave-assisted MDA and had systematically explored the effect of catalyst properties, including acidity, metal loading, and particle size, on conventional reactivity, microwave absorption, and microwave reactivity.

Building on this work, we are now exploring Fe-ZSM-5@SiC core@shell catalysts which combine the activity of the metal-exchanged zeolite with the well-known high MW absorption of SiC. By designing these core@shell structures, we aim decouple MW heating and catalytic activity via separation into different material components while keeping the two components in intimate contact and hence maximizing heat transfer. We developed a synthetic route for these core@shell catalysts and systematically explored key synthesis parameters, such as solvent amount and ZSM-5/SiC ratio, to obtain homogeneous, well-defined Fe-ZSM-5@SiC structures while minimizing undesired homogeneous nucleation of ZSM-5 in the synthesis mixture. The Fe-ZSM-5@SiC samples were carefully characterized via SEM, EDX, XRD, XPS and BET to confirm the synthesis of desired catalyst structure and composition. Comparative reactive evaluation at conventional thermal heating and microwave-assisted conditions are currently on-going in comparison between various catalyst configurations, including core@shell structures and physical mixtures. Overall, we expect that such intentional design of catalysts for use in microwave-assisted heterogeneous catalysis will enable taking full advantage of this promising reaction concept.