461850 Dehydroaromatization of Ethane over Zinc Containing Zeolites

Thursday, November 17, 2016: 2:20 PM
Franciscan B (Hilton San Francisco Union Square)
Ali Mehdad and Raul F. Lobo, Chemical and Biomolecular Engineering, Center for Catalytic Science and Technology, University of Delaware, Newark, DE

Production of light alkanes such as methane or ethane has increased significantly due to the use of hydraulic fracturing for the extraction of shale gas. As a result, the cost of natural gas has plummeted creating an opportunity converting light alkanes to more valuable products such as olefins and aromatics. Metals such as Zn(II) or Ga(III) exchanged into aluminosilicate zeolites have showed promising properties for the aromatization of light alkanes [1-4].

Zn-exchanged zeolites (MFI, BEA) were investigated for the aromatization of ethane. Different amounts of zinc (0-10 %wt), incorporated by ion exchange or incipient wetness impregnation, have been exchanged into ZSM-5 and beta zeolites with different SiO2/Al2O3 (23 to 50) and investigated using a plug-flow microreactor at 500 °C and atmospheric pressure. The measurements indicate that lower ratios of SiO2/Al2O3 and higher zinc content lead to higher ethane conversion. Product selectivity is dependent of zinc content and zeolite composition and structure.

The dehydroaromatization of ethane requires a bifunctional catalyst, in which the initial ethane dehydrogenation to ethylene is catalyzed by the metal sites (H+/ZSM5 does not activate ethane at similar reaction conditions) and the oligomerizationd and aromatization of olefins is catalyzed by acid sites. However, extra-framework zinc increases the selectivity towards aromatic products. Based on the amounts of zinc, there appear to be different types of zinc sites, which have different activities towards aromatization: low zinc contents show higher stability, lower conversion with dominant selectivity to ethylene while samples with high zinc contents show lower stabilities, higher conversion with high selectivity (~ 50%) towards aromatics. The role of different zinc sites such as isolated Zn ions at the ion exchange positions or ZnxOy for the aromatization and methods to stabilize those sites will be further investigated.

References:

1. Biscardi, J.A. and E. Iglesia, Reaction Pathways and Rate-Determining Steps in Reactions of Alkanes on H-ZSM5 and Zn/H-ZSM5 Catalysts. Journal of Catalysis, 1999. 182(1): p. 117-128.

2. Biscardi, J.A., G.D. Meitzner, and E. Iglesia, Structure and Density of Active Zn Species in Zn/H-ZSM5 Propane Aromatization Catalysts. Journal of Catalysis, 1998. 179(1): p. 192-202.

3. Almutairi, S.M.T., et al., Structure and Reactivity of Zn-Modified ZSM-5 Zeolites: The Importance of Clustered Cationic Zn Complexes. ACS Catalysis, 2012. 2(1): p. 71-83.

4. Schulz, P. and M. Baerns, Aromatization of Ethane over Gallium-promoted H-ZSM-5 Catalysts. Applied Catalysis, 1991. 78(1): p. 15-29.


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