Despite the tremendous effort directed towards the synthesis of hierarchically-structured two-dimensional (2D) (&lt; two-unit-cell-thick) zeolites, systematic studies investigating their hydrothermal stability are rare in the literature. Their reactivity to steaming conditions under mild or severe thermal treatments determines their applicability and presets their operating conditions. In our study, a purely-siliceous Self-Pillared Pentasil (SPP) hierarchical zeolite which is composed of single-unit-cell-thick MFI nanosheets orthogonally-intergrown in a house-of-cards construction, is exposed to superheated steam at 350 oC in an equimolar flow with nitrogen (N2) for extended durations up to 30 days. We have captured by high-resolution transmission electron microcopy (HR-TEM) and three-dimensional (3D) TEM tomography, the evolution of the single-unit-cell-thick (2.1 nm) nanosheets into thicker crystalline domains (&gt; 8 nm ~ four-unit-cell-thick) along the [010] direction, while the hierarchical intergrown assembly remained intact. This nanostructural evolution was associated with a contraction along the [001] direction. Upon the applied steaming conditions; this morphological transformation occurs without noticeable changes in X-ray diffraction (XRD) patterns and Argon (Ar) physisorption isotherms. This shows that steamed SPP retains its full crystallinity and the MFI framework structure and preserves its micro- and mesoporosity intact. The observed morphological transformation is replacing one type of surface area by another via a local rearrangement that redistributes silica within the particle itself. The applied steaming treatment to result in the captured local rearrangement phenomenon, can be potentially applied to tune the thickness of the zeolitic domains in hierarchical catalysts and can also have surface passivation implications in thin films’ fabrication.

Nanostructural evolution of hierarchically-structured single-unit-cell-thick siliceous MFI zeolite nanosheets under steaming

2019 Fall National Meeting

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