(396a) Towards Rational Design of Metal-Embedded Zeolite Catalyst for Improved Catalytic Performance in Microwave-Assisted Methane Dehydro-Aromatization

Due to the limitations in the transportation of abundantly available natural gas to point-of-use locations, large amount of natural gas is flared causing significant environmental pollution. This motivates interest to convert natural gas to easily transportable value-added liquids. 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, oligomerized, and further aromatization takes place at the Bronstedt acid sites (BAS) of the zeolite where the zeolite provides shape selectivity towards the desired product benzene. However, coking and lengthy catalyst induction times are major challenges for this reaction.

Microwave-assisted heterogeneous catalysis may offer a way to mitigate these problems by selectively heating the solid catalyst, thus reducing unwanted side-reactions in the—comparatively cold—gas phase, reducing coking, and accelerating catalyst induction times. We had previously identified Fe-ZSM-5 as a promising catalyst for microwave-assisted MDA. Fe-ZSM-5 showed strongly increased methane conversion and shortened induction time under microwave catalysis conditions. However, increased conversion was obtained at the cost of lower selectivity to the desired product benzene. The results could be explained by the formation of strong metal hot spots which lead to deep dehydrogenation of methane to coke, while the relative weak microwave susceptibility – and hence low temperature - of the zeolite would result in low activity of the BAS on the zeolite and hence in poor aromatics selectivity.

Based on these results, in the present work we are exploring the design of SiC@Fe-ZSM-5 core@shell catalysts to reduce these intra-catalyst temperature gradients. Goal of the catalyst design is to combine the activity of the metal-exchanged zeolite with the well-known high microwave susceptibility of SiC while keeping the two components in intimate contact and hence maximizing heat transfer during microwave irradiation. By directly growing the zeolite phase onto a SiC, which is much more strongly heated by microwave irradiation, we aim to counter the low microwave absorption of the zeolite phase and hence reduce the intensity of the metal hot spot.

SiC@Fe-ZSM-5 core@shell catalysts with well-defined zeolite shells were successfully synthesized and characterized, and then evaluated in MDA at conventional thermos-catalytic and microwave-assisted conditions. Catalytic performance was evaluated in comparison between different catalyst configurations, including pure Fe-ZSM-5 and a corresponding physical mixture of SiC and Fe-ZSM-5 as a reference. We find indeed that the core@shell catalyst has improved catalytic performance in comparison to a pure Fe-ZSM-5 catalyst and the physical mixture, in agreement with the hypothesis that this configuration would reduce the intensity of the metal hot spot. Differences in reactive performance of the different catalyst configurations (conversion, selectivity, and induction time) can be explained consistently based on expected temperatures of the active sites (metal vs BAS in zeolite) and the intensity of metal hot spot attained in these catalysts. Ongoing work is extending these studies onto further tuning of the intensity of metal hot spots by tuning the core@shell structure, i.e. changing the relative size of SiC core vs Fe-ZSM-5 shell and thus changing the intensity/effectiveness of the indirect heating via the SiC core.

Overall, our work proposes an effective way to control internal catalyst temperature distribution under microwave-assisted catalytic conditions via rational catalyst design. We expect that such an intentional design of catalysts for use in microwave-assisted heterogeneous catalysis will enable taking full advantage of this promising reactor concept.