Characterization and Reactivity of External Surface OH Species in Zeolites

A key characteristic of zeolites is their pores which are typically near molecular size. The pores enable size-selective reactivity, as molecules larger than the pores cannot diffuse into the zeolite. In some cases this can be a disadvantage, as diffusion through the zeolite may be slow and limit the overall reactivity. Large or bulky molecules may also be excluded from interacting with internal reaction sites. Much current research therefore focuses on synthesizing novel zeolite structures and frameworks with increased surface reaction sites. One approach is the synthesis of zeolites with both micro- and mesopores[1,2,3]. The creation of a mesopore network (within zeolites such as MFI or LTA) helps overcome pore size limitations. Other work has led to the synthesis of “two-dimensional” zeolites[4,5], where the thickness of the zeolite in one direction is small, and diffusion lengths are thus small. These latter morphologies can be described as nanosheets or thin film structures. A common feature of both two-dimensional and mesoporous zeolites is that the reactivity is enhanced significantly by external reaction sites (those not in micropores) on mesopore walls or on external surfaces. Bulky molecules may thus react significantly with the zeolite[6].

In an effort to understand and characterize external reaction sites we have modeled (001) surfaces of LTA, a model zeolite. Mesoporous LTA has been experimentally synthesized[3] and the current work is a method to probe the chemistry at the walls of the mesopores. The current work examines the reactivity of OH groups on the external surface using density functional theory (DFT). Because of the unsaturated coordination, surfaces of zeolites typically terminate with hydroxyls due to water dissocation. These hydroxyls show some acidic properties and may have some reactive properties[7,8]. OH groups may also exist as Brønsted acid sites (Al-O-H) or defects like silanol nests. We calculated the stability of three potential surface terminations of the [001] surface at various degrees of hydration. Identifying these surface models allows the modeling of surface reactivity. We calculated the acidity of different OH groups and Brønsted acid sites (both at the surface and within the zeolite bulk) through interactions with probe molecules. We performed molecular dynamics simulations to obtain information on the mobility and structure of surface OH groups. Finally, we considered representative reactions, such as reactions of simple hydrocarbons to form carboniums, as another means to examine these surface sites. Our study provides fundamental insight on potential reaction sites (OH groups) at zeolite surfaces, and compares such sites with reaction sites within the internal pores. Such information lays the basis for understanding the reactivity of both mesoporous and thin film zeolites, and also for future studies involving other zeolites such as the MFI framework.

References

[1] M. Holm, E. Taarning, K. Egeblad, and C. Christensen. “Catalysis with hierarchical zeolites.” Catalysis Today 168, no. 1 (June 2011): 3-16.

[2] D. Liu, A. Bhan, M. Tsapatsis, and S. Al Hashimi. “Catalytic Behavior of Brønsted Acid Sites in MWW and MFI Zeolites with Dual Meso- and Microporosity.” ACS Catalysis 1, no. 1 (January 7, 2011): 7-17.

[3] H. Chen, J. Wydra, X. Zhang, P. Lee, Z. Wang, W. Fan, and M. Tsapatsis. “Hydrothermal synthesis of zeolites with three-dimensionally ordered mesoporous-imprinted structure.” Journal of the American Chemical Society 133, no. 32 (August 17, 2011): 12390-3.

[4] W. Roth, and J. Čejka. “Two-dimensional zeolites: dream or reality?” Catalysis Science & Technology 1, no. 1 (2011): 43.

[5] M. Choi, K. Na, J. Kim, Y. Sakamoto, O. Terasaki, and R. Ryoo. “Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts.” Nature 461, no. 7261 (September 10, 2009): 246-9.

[6] F. Liu, T. Willhammar, L. Wang, L. Zhu, Q. Sun, X. Meng, W. Carrillo-Cabrera, X. Zou, and F. Xiao. “ZSM-5 zeolite single crystals with b-axis-aligned mesoporous channels as an efficient catalyst for conversion of bulky organic molecules.” Journal of the American Chemical Society 134, no. 10 (March 14, 2012): 4557-60.

[7] T. Armaroli, M. Bevilacqua, M. Trombetta, F. Milella, A. Alejandre, J. Ramı́rez, B. Notari, R. Willey, and G. Busca. “A study of the external and internal sites of MFI-type zeolitic materials through the FT-IR investigation of the adsorption of nitriles.” Applied Catalysis A: General 216, no. 1–2 (August 2001): 59-71.

[8] T. Kawai, and K. Tsutsumi. “Reactivity of silanol groups on zeolite surfaces.” Colloid & Polymer Science 276, no. 11 (November 27, 1998): 992-998.