(349ai) The Role of Surface Barriers in ZSM-5 Zeolite Catalyzed Benzene Alkylation

Many applications of zeolites in catalysis are impeded by transport limitations, which could result from diffusion limitations in micropores, but also from transport resistance across external crystal surfaces or through internal grain boundaries. The microporous diffusion resistance has been extensively discussed, and spurred great interest in synthesizing hierarchically structured and nanosized zeolites.¹ Although receiving increasing attention in recent years, the fundamental nature of the external or internal surface barriers remains unclear and controversial. Moreover, the possible influence of these surface barriers on zeolite-catalyzed reactions is much less well documented.^{2, 3}

Here, we take ZSM-5 zeolite-catalyzed benzene alkylation as the model system to explore the effects of external surface barriers and interfacial grain boundaries on catalytic performance.² A series of polycrystalline hierarchical, conventional microporous, and nanosized ZSM-5 zeolites were synthesized with identical acidities but different particle sizes and external surface areas. Additionally, a polycrystalline hierarchically structured zeolite was compared to an alkaline-treated hierarchical zeolite consisting of single crystals. These two hierarchical zeolites were characterized and selected to possess similar acidities, particle sizes, and mesopore volume; therefore, the significant difference was the number of internal grain boundaries. Finally, we analyzed the apparent activity, selectivity, and rate-limiting step of the above-mentioned ZSM-5 zeolites to probe the role of surface barriers.

References

1. M.-O. Coppens, T. Weissenberger, Q. Zhang, G. Ye, 2020, Nature-inspired, computer-assisted optimization of hierarchically structured zeolites. Adv. Mat. Int. 2001409, 1-26.

2. M. Rao, E. Saraci, R. Gläser, M.-O. Coppens, 2017, Surface barriers as dominant mechanism to transport limitations in hierarchically structured catalysts – Application to the zeolite-catalyzed alkylation of benzene with ethylene. Chem. Eng. J. 329, 45-55.

3. Guo, X. Li, S. Hu, G. Ye, X. Zhou, M.-O. Coppens, 2020, Understanding the role of internal diffusion barriers in Pt/Beta zeolite catalyzed isomerization of n-heptane. Angew. Chem. Int. Ed. 132(4), 1564-1567.