Light alkane dehydroaromatization on MFI-type zeolites is an industrially significant process for creating value-added products. In particular, gallium modified H-MFI (Ga/H-MFI) is known to be a highly active and stable catalyst for this reaction, owing to its high dehydrogenation activity, which in turn increases selectivity for aromatization. However, the exact structure of the active gallium species in Ga/H-MFI and the mechanistic details of light alkane dehydrogenation are still a matter of debate. To this end, we have investigated these open questions using a hybrid quantum mechanical/mechanical (QM/MM) approach found to be highly accurate for predicting molecular heats of adsorption. Ethane was used to probe dehydrogenation pathways over three different sites: GaH2+, Ga+ and GaH2+. In total, seven different pathways were explored. The most competitive pathways are found to be a stepwise alkyl-mediated mechanism on GaH2+ and a carbenium-mediated pathway on GaH2+. Both pathways contribute to ethane dehydrogenation under aromatization conditions of 823 K and 1 atm. Though Ga+ formation is thermodynamically favored, we observe that GaH2+ is trapped kinetically, based on high activation barriers for GaH2+ decomposition to Ga+, whether the decomposition is direct or a consequence of ethane dehydrogenation on GaH2+. In addition, we find that ethane dehydrogenation on GaH2+ is mediated by GaH2+ and H+ sites, which facilitate H2 desorption. Overall, both GaH2+ and GaH2+ sites are found to be much more active for ethane dehydrogenation than H+ sites, which are also are also present in Ga/H-MFI. Our results are in very good agreement with the experimentally measured activation barrier for ethane dehydrogenation on Ga/H-MFI, in contrast to previously reported theoretical calculations. We attribute this better agreement to the incorporation of long-range framework effects (dispersion and electrostatics) in our model, interactions that can significantly impact the chemistry surrounding the active site.

Insights from a QM/MM Investigation of ethane dehydrogenation pathways on Ga/H-MFI

253rd National Meeting (2017)

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