(544fy) Metal-Promoted Dehydroaromatization of Ethylene over ZSM-5 Catalysts

Industrial production of aromatics heavily relies on catalytic reforming in refineries.^1-2 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. Ethylene dehydroaromatization (DHA) over Ag-ZSM-5 is one of the promising technologies given the availability of ethylene feedstock. Ag-ZSM-5 was chosen for the ethylene DHA reaction, in part because Ag-ZSM-5 was reported to promote aromatic selectivity in the isobutane aromatization,³ and to facilitate non-oxidative methane upgrading at moderate reaction temperatures.⁴ Here, we will report the influence of Ag species in Ag-ZSM-5 on ethylene DHA without and with co-feeding methane. We observe that Ag-ZSM-5 greatly promotes aromatic selectivities by ~3-fold. In contrast, H-ZSM-5 tends to produce light olefins and other aliphatic hydrocarbons under otherwise identical conditions.⁵ With methane co-feed, Ag- and H-ZSM-5 both exhibit very similar catalytic behavior compared to the conditions without methane. Interestingly, methane is activated over Ag-ZSM-5 (≤ 5% conversion), but H-ZSM-5 is inactive for methane conversion.

Through systematic studies of Ag/H-ZSM-5 by a combination of experiments and density functional theory calculations at different reaction conditions, we have been able to differentiate the role of Ag species in methane/ethylene activation with enhanced product selectivities to value-added aromatics. We have also extended this study to several metals in an attempt to establish more robust structure-performance relationships in DHA catalysis. Collectively, this work provides new insights into the design of metal-containing zeolite catalysts for natural gas upgrading.

1. Franck, H.-G.; Stadelhofer, J. W., Industrial Aromatic Chemistry. Spring: 1988.

2. Arpe, H.-J.; Hawkins, S., Industrial Organic Chemistry. 5 ed.; 2010.

3. Ono, Y., Transformation of Lower Alkanes into Aromatic Hydrocarbons over ZSM-5 Zeolites. Catalysis Reviews 1992, 34 (3), 179-226.

4. Ono, Y.; Baba, T., Unique properties of silver cations in solid-acid catalysis by zeolites and heteropolyacids. Physical chemistry chemical physics : PCCP 2015, 17 (24), 15637-15654.

5. Hsieh, M.-F.; Zhou, Y.; Thirumalai, H.; Grabow, L. C.; Rimer, J. D., Silver-Promoted Dehydroaromatization of Ethylene over ZSM-5 Catalysts. ChemCatChem 2017, 9 (9), 1675-1682.