The role of different active sites (Lewis and BrØnsted acid sites) for dehydroaromatization of ethane in Zn/ZSM-5 was studied in a flow micro-reactor at 500 °C. Varying amounts of zinc were loaded on silicalite-1 and ZSM-5 (Si/Al=12-25) by ion exchange (Zn&lt; 2.7% w/w, Zn/H+&lt;0.8) or wet impregnation (Zn&gt;5% w/w, Zn/H+&gt;3). Different zinc sites in the form of Lewis acid sites (Zn2+, (ZnOZn)2+ and Zn(OH)+) and ZnxOy clusters can activate ethane. However, Zn in the form of Lewis acid site can catalyze the aromatization, since ethane conversion on 5% Zn/silicalite-1 merely formed ethene. The results revealed that by increasing the Zn/H+ ratio, rate of aromatics formation increased, implying the important role of Zn in the form of Lewis acid site compared to BrØnsted acid sites (BAS) for catalyzing ethane aromatization. This was further confirmed by the FTIR spectra showing stronger interaction of ethene with Zn/ZSM-5 than with H-ZSM-5. CO2 as a soft oxidant was fed into the reactor to scavenge the formed hydrogens. Zn sites in the zeolites can catalyze the reverse water-gas shift reaction forming water and CO. Hydrolysis of the Zn Lewis acid sites was observed by the FTIR spectra. Upon hydrolysis of Zn2+ sites to Zn(OH)+ and BAS, formation of methane and aromatics dropped by factors of 3 and 7. In fact, steam (via hydrolysis of Zn2+ sites) suppressed oligomerization/aromatization in favor of dehydrogenation. However, the change in product selectivity was reversible and by decreasing the vapor pressure of steam, aromatization was resumed. The results suggested that Zn(OH)+ sites were effective in ethane dehydrogenation but Zn2+ sites were required for aromatization. The loss of aromatization in presence of steam suggested that the Zn2+ sites catalyzed aromatization at faster rates than BAS.

Driving the ethane dehydroaromatization pathway in Zn/ZSM-5 via speciation of Zn sites

2019 Spring National Meeting

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