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Ethane and Ethylene Aromatization on Zinc-Containing Zeolites

Abstract

The conversion of ethane on zinc containing zeolites SSZ-13 (CHA), ZSM-5 (MFI) and beta (*BEA) and ethylene on Zn-containing ZSM-5 was investigated using flow micro-reactor at temperature of 773 K and atmospheric pressure. Zinc was incorporated in the zeolites (Si/Al=11-25) and in silicalite-1 by ion exchange (Zn< 2.7% w/w, Zn/BAS< 0.8) or by wet impregnation (Zn> 5% w/w and Zn/BAS> 3). Drying of wet-impregnated samples by freeze-drying led to better dispersion of subnano-sized clusters of ZnO in the zeolite pores, compared to drying in an oven. The samples were characterized by solid-state NMR spectroscopy, X-ray powder diffraction, nitrogen physisorption and chemical analysis; the density of BrĂ˜nsted acid sites (BAS) was measured using TPD-TGA of 2-propanamine. It was shown that several zinc sites (Zn2+, [Zn-O-Zn]2+ dimers at ion exchange positions, and (ZnO)n clusters) can catalyze ethane activation and higher zinc content resulted in higher reaction rates. However, Zn in Lewis acid site position (Zn2+ and [Zn-O-Zn]2+) can perform aromatization. Small ZnO clusters can only catalyze the dehydrogenation reaction, and on their own did not catalyze aromatization, as 5% Zn-silicalite-1 catalyzed the ethane dehydrogenation to ethylene while it showed low reaction rates for ethylene conversion. Zn-ZSM-5 catalytic properties for ethane/ethylene aromatization depends on the balance between number of metal/Lewis acid sites and BAS (Zn/BAS). For ethane as reactant, Zn/BAS< 0.8 favored ethylene formation while by increasing Zn/BAS both conversion and formation of aromatics were increased, implying essential role of Zn sites for ethane aromatization. For ethylene as reactant, Zn/BAS=3.8 resulted in predominant ethane formation while at Zn/BAS=0.8 aromatics formation was predominant. The difference in product selectivity for ethane/ethylene conversion with respect to Zn/BAS could be due to the fact that ethylene, unlike ethane, can react quickly on BAS.

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Publication details

The article was received on 15 May 2017, accepted on 07 Jul 2017 and first published on 07 Jul 2017


Article type: Paper
DOI: 10.1039/C7CY00890B
Citation: Catal. Sci. Technol., 2017, Accepted Manuscript
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    Ethane and Ethylene Aromatization on Zinc-Containing Zeolites

    A. Mehdad and R. F. Lobo, Catal. Sci. Technol., 2017, Accepted Manuscript , DOI: 10.1039/C7CY00890B

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