(632e) Dehydroaromatization of Ethylene over Metal-Exchanged H-ZSM-5 Catalysts: The Nature and Role of (Extra)Framework Acid Sites

Industrial production of aromatics heavily relies on catalytic reforming in refineries.^[1] Driven by the limited supply of fossil fuels and the recent shale gas revolution, it is highly desirable to develop methane- and/or light olefin-based processes to meet the growing demand for aromatics. To this end, ethylene dehydroaromatization (DHA) over metal-exchanged ZSM-5 is promising given the worldwide availability of ethylene feedstock. ZSM-5 was chosen as a catalyst to study ethylene DHA, because it has been used commercially in the UOP/BP Cyclar process, which converts light alkanes to aromatics.^[2] Zeolites with extra-framework metals are conventionally prepared by aqueous ion exchange or incipient-wetness impregnation; however, these methods often result in large fractions of metal oxide deposits on external surfaces due to the slow diffusion of hydrated metal ions into the micropores. Alternative methods have been introduced by Bell and coworkers to incorporate Lewis acids in ZSM-5 pores by vapor-phase exchange.^[3] Jones and coworkers demonstrated a direct hydrothermal synthesis using functional silanes to prepare metal-exchanged MFI for propane dehydrogenation (PDH).^[4]

In this presentation, we will discuss the preparation and testing of metal-exchanged ZSM-5 catalysts using three different synthesis approaches, including the conventional aqueous ion exchange and direct hydrothermal synthesis (with and without the addition of inorganic structure-directing agents). Our findings reveal dramatic differences in catalyst performance depending on the method of synthesis, with aromatic selectivities enhanced by ~3-fold compared to H-ZSM-5. Collectively, this work provides new insights into the design of metal-containing zeolite catalysts for natural gas upgrading.

[1] aFranck, H.-G.; Stadelhofer, J. W., Industrial Aromatic Chemistry. Spring: 1988; bArpe, H.-J.; Hawkins, S., Industrial Organic Chemistry. 5 ed.; 2010.

[2] Gosling, C. D.; Hamm, D. A. Process for the production of benzene from light hydrocarbons 1992.

[3] Phadke, N. M.; Van der Mynsbrugge, J.; Mansoor, E.; Getsoian, A. B.; Head-Gordon, M.; Bell, A. T., ACS Catalysis 2018, 8, 6106-6126.

[4] Choi, S. W.; Kim, W. G.; So, J. S.; Moore, J. S.; Liu, Y. J.; Dixit, R. S.; Pendergast, J. G.; Sievers, C.; Sholl, D. S.; Nair, S.; Jones, C. W., Journal of Catalysis 2017, 345, 113-123.