Hydrothermal stability of hierarchically-structured two-dimensional MFI zeolite nanosheets

Abstract

Two-dimensional (< two-unit-cell-thick) zeolites are an important class of two-dimensional nanoporous materials. They possess the intrinsic properties of the conventional three-dimensional crystalline silicate/aluminosilicate zeolites and when synthesized in a pillared or intergrown form they exhibit a hierarchical porosity. Despite the tremendous effort directed towards their synthesis, very limited knowledge is available on their hydrothermal stability under industrially-relevant conditions. The hydrothermal stability of the thin crystalline domains governs their enhanced use as selective catalysts, adsorbents and in thin membranes’ preparation. This dissertation focuses on developing a fundamental understanding of the response of these novel thin crystallites under hydrothermal environments towards an advanced design and engineering of their synthesis and industrial applications. Self-Pillared Pentasil zeolite was utilized as a model system of hierarchically-structured two-dimensional MFI zeolite nanosheets in the structural and catalytic stability investigations explored in this dissertation. The nanostructural evolution of purely-siliceous nanosheets in presence of water under different atmospheres was studied via a thorough structural characterization analysis which involved the use of high-resolution and three-dimensional tomography bright-field transmission electron microscopy (the tilt series acquired in this analysis are enclosed as supplementary media files in this dissertation). The kinetic and thermodynamic driving forces behind the captured structural changes were investigated. The acidity modifications and the catalytic consequences resultant from the hydrotreatment structural transformation were explored using the aluminosilicate nanosheets counterparts.