(425b) Microstructural Evolution of Self-Pillared Pentasil (SPP) Single-Unit-Cell Thick Siliceous Zeolite Under Steaming

Two-dimensional (2D) zeolites are an important class of 2D nanoporous materials. They possess the intrinsic properties of the conventional three-dimensional (3D) crystalline silicate/aluminosilicate zeolitesand when synthesized in a layered and/or intergrown form; they constrain within them a hierarchical architecture of multidimensional porous network. They proved enhancement in classical catalytic, adsorption and separation applications and found novel implementations in areas like film membranes’ fabrication. One example, is Self-Pillared Pentasil (SPP) zeolite. ^[1] SPP is composed of orthogonally interconnected nanosheets of the structure type MFI (0.52 nm microporosity) that are intergrown in a house-of-cards morphology, creating a mesoporosity of 2 nm – 7 nm within the interlamellar space.

Among the ultimate factors that determine the versatility of zeolites, in general, is their hydrothermal stability. Although, previous reports have established that 2D and nanocrystallite MFI can be stable under certain conditions; ^[2,3] a systematic study following the unavoidable microstructural evolution of single-unit-cell thick nanosheets does not exist!

We captured by high resolution transmission electron microscopy (HR-TEM), the evolution of single-unit-cell thick (~ 2 nm) SPP nanosheets into thicker crystalline domains (> 8 nm) along the [010] crystallographic direction. Interestingly, upon the applied steaming conditions; this morphological transformation occurs without noticeable changes in X-ray diffraction (XRD) patterns and Argon (Ar) physisorption isotherms which usually are the characterization techniques used to assess the structural and textural stability of 2D zeolites.This finding is attributed to domain coarsening by local rearrangement of zeolite nanosheets.

We verified the observation of SPP nanosheets thickening via two steaming treatments. One exposes SPP to superheated steam under flow conditions for extended durations up to 30 days providing the first long-term stability investigation on 2D zeolites. In fact, such prolonged studies are very rare ^[4] and to the best of our knowledge have never been reported at the single-unit-cell level before. A second treatment is conducted in a closed system under autogenic pressure of water vapor.

Upon steaming, SPP loses the characteristic thin dimension of the MFI nanosheets. Such a structural evolution is not associated with a globular morphological change. The steamed particles did not collapse into bulk dense form and preserved the intergrowth branching of the domains holding their sheet-like morphology. This structural transformation indicates that there is a redistribution of silica within the individual steamed SPP particle to evolve the nanosheets of MFI domains while maintaining the house of cards intergrowth hierarchy.

It is the ultrathin (< 2-unit-cell thick) nature of the starting SPP that enabled recognizing this nanoscale crystal evolution phenomenon. Such an establishment was deemed unfeasible in 3D micrometer-sized bulk zeolites.

The evolution that steamed SPP experiences is significant at the single unit cell scale to be visible in TEM, but minor to considerably affect the micro- and mesoporous volumes per mass of particle. This explains why XRD and Ar physisorption failed to catch the instability of 2D zeolites to steaming conditions.

The captured phenomenon has significant implications on the use of 2D zeolites as catalysts. It changes the characteristic diffusion length and may cause migration of the isomorphically substituted elements (Al, Sn, etc.) and/or form extra framework species!

[1] Zhang, X., Liu, D., Xu, D., Asahina, S., Cychosz, K. A., Agrawal, K. V., Tsapatsis, M. (2012). Synthesis of Self-Pillared Zeolite Nanosheets by Repetitive Branching. Science, 336(6089), 1684-1687

[2] Choi, M., Na, K., Kim, J., Sakamoto, Y., Terasaki, O., & Ryoo, R. (2009). Stable single-unit-cell nanosheets of zeolite MFI as active and long-lived catalysts. Nature, 461(7265), 828-828

[3] Kim, K., Ryoo, R., Jang, H., & Choi, M. (2012). Spatial distribution, strength, and dealumination behavior of acid sites in nanocrystalline MFI zeolites and their catalytic consequences. Journal of Catalysis, 288, 115-123

[4] Elyassi, B., Zhang, X., & Tsapatsis, M. (2014). Long-term steam stability of MWW structure zeolites (MCM-22 and ITQ-1). Microporous and Mesoporous Materials, 193, 134-144