(509co) Effect of Water on Cumene Dealkylation over ZSM-5 Zeolites

Zeolites has been playing a significant role as solid acid catalysts in research and chemical industry for decades. We are only beginning to understand the important implications of Brønsted acid site proximity and consequences on a variety of industrially important reactions [1]. It is also known that water molecules facilitate proton delocalization at Brønsted acid sites and form hydronium ion clusters [2]. As a result, water can extend the ability of proximate sites to interact with transition states by improving proton mobility through water-bridge networks, especially in zeolites with high Brønsted acid site density [3]. In this study, the effect of water on cumene dealkylation over different H-ZSM-5 zeolites was investigated in a flow reactor under conditions where rates are controlled by the surface reaction. The products of cumene dealkylation are benzene and propylene, which are high-value chemicals, but this reaction also serves as a surrogate for the broader scope of alkylation and dealkylation chemistry as well as polystyrene decomposition. It was found that proximate paired sites lead to entropic gains in the kinetically relevant transition state. Addition of water further increases transition state entropies, with a clear correlation with the proximity of Bronsted sites in the zeolite. Water also has the beneficial effect of prolonging catalyst lifetimes under these conditions, so activation barriers reported here reflect true enhancements in ideal rates. These findings reflect the interesting direct influence that water can have on reactions with modest activation barriers.

References

1. Di Iorio, C. Nimlos, R Gounder, ACS Catalysis 2017, 7, 6663−6674
2. Vjunov, M. Wang, N. Govind, T. Huthwelker, H. Shi, D.Mei, J. Fulton, J. Lercher, Chemistry of Materials, 2017, 29, 9030−9042
3. K. Chen, A. Gumidyala, M. Abdolrhamani, C. Villines, S. Crossley, and J. L. White, Journal of Catalysis 351 (2017) 130-135