(58f) Mechanistic Insight into Ethene Oligomerization over H-MFI

Increasing shale gas production in the U.S. has incentivized the conversion of natural gas liquids, primarily of ethane, to aromatics. Gallium-exchanged H-MFI zeolites (Ga/H-MFI) are highly active and selective catalysts for light alkane (e.g., ethane and propane) dehydroaromatization. This process begins with alkane dehydrogenation over Ga species to produce alkenes, which then undergo oligomerization over Brønsted acid OH groups. The resulting oligomers undergo subsequent dehydrogenation over the Ga species and cyclization over the Brønsted acid sites, ultimately producing aromatics. Given the complexity of the process, this investigation is focused on oligomerization of ethene over H-MFI. Previous studies of this reaction have shown that the primary products are butene and propene, but the mechanism by which these products are formed is not well established [1]. In this work, experimental and theoretical methods were used to investigate the mechanism of ethene oligomerization over H-MFI.

The products of ethene oligomerization over H-MFI at 250 °C and 1 atm are predominantly C₃-C₆ alkenes. This product distribution is consistent with that previously observed for ethene oligomerization over H-MFI [2]. Propene and butene are the major products under the conditions investigated; the combined carbon selectivity to these two products is > 75%. C₅⁺ alkenes are also observed as primary products of ethene oligomerization. Theoretical calculations of the Gibbs free energy landscape were performed and used together with the Energetic Span Model proposed by Kozuch [3] to guide the identify the rate-determining elementary steps involved in the formation of propene and butene during ethene oligomerization. The results indicate that the formation of propene as a primary product can occur via β-scission of a 2° butoxide surface intermediate, followed by ethene coupling with resultant surface methoxy group to form a second propene molecule. The calculated Gibbs free energy landscape indicates that this reaction is competitive with butene formation via ethene coupling, consistent with the both the observed product distribution and the results of spacetime studies.

A model of the reaction kinetics was developed based on the proposed mechanism. The rates of propene and butene formation measured at differential conversion as a function of temperature and ethene partial pressure are well described by the model. Reasonable agreement was obtained, as well, between the calculated and measured apparent activation energies for propene and butene formation, supporting the proposed mechanism. The present study provides a detailed understanding of the mechanism and kinetics of ethene oligomerization on Brønsted acid protons in H-MFI and, in particular, proposes plausible reaction pathways for the formation of both butene and propene directly from ethene.

References

1. Lin, B., Zhang, Q., and Wang, Y. Eng. Chem. Res. 48, 10788 (2009)
2. Sarazen, M. L., Doskocil, E., and Iglesia, E. ACS Catal. 6, 7059 (2016)
3. Kozuch, S. A. Wiley Interdiscip. Rev. Mol. Sci. 2, 795 (2012)