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 a flow micro-reactor at a 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 Brønsted acid sites (BAS) was measured by 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 the Lewis acid site position (Zn²⁺ and [Zn–O–Zn]²⁺) can perform aromatization. Small ZnO clusters can only catalyze dehydrogenation reaction, and on their own did not catalyze aromatization, as 5% Zn-silicalite-1 catalyzed ethane dehydrogenation to ethylene while it showed slow reaction rates for ethylene conversion. The catalytic properties of Zn-ZSM-5 for ethane/ethylene aromatization depend on the balance between the number of metal/Lewis acid sites and BAS (Zn/BAS). For ethane as a reactant, Zn/BAS < 0.8 favored ethylene formation while by increasing Zn/BAS both conversion and formation of aromatics were increased, implying the essential role of Zn sites in ethane aromatization. For ethylene as a 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 rapidly on BAS.