The role of zeolite beta nanoparticles solutions in the synthesis of zeolite-functionalised materials with bimodal porosity.

摘要

The last decade, much attention has been devoted to the development of bimodal materials with zeolitic properties, with the intention to combine the beneficial properties of zeolites with these of mesoporous structures. Various synthesis methods have been developed, of which several are using zeolite nanoparticles solutions as silica-alumina source to form the mesoporous material. Extensive research has been conducted to the properties and the formation mechanism of both the nanoparticles solution and the final bimodal materials. However, still little is known about the correlation between the characteristics of the initial nanoparticles solution and the structural, chemical and physico-chemical properties of the final materials. The focus of this research is to acquire this lacking knowledge, which would allow tailoring of the final material characteristics, depending on the application requirements.;In this PhD work, two types of bimodal materials with zeolitic features are investigated, i.e. a mesotemplate-free method and an impregnation on a mesoporous substrate, both using a zeolite beta nanoparticles solution as precursor. The main focus is put on how the synthesis conditions of the zeolite beta nanoparticles solution influence the final material properties. It is shown that the hydrothermal synthesis temperature (373K--393K--413K--423K/24h) of the zeolite beta nanoparticles solution has a direct impact on the porosity and zeolitic features of the final materials. Moreover, a clear threshold is observed: Synthesis temperatures below or equal to 413K result in bimodal materials without clear zeolitic properties, while a higher temperature of 423K gives materials with clear zeolitic features. Furthermore, this difference in material properties gives better results for the materials above the threshold regarding the acidity, stability and catalytic activity.;A transmission electron microscopy and advanced electron tomography study showed that the mesotemplate-free structures are possessing a wormhole-like mesoporous structure, formed by the condensation of nanoparticles via neck formation. Furthermore, the study revealed a 2D growth of the nanoparticles under the threshold and a change towards 3D growth at the threshold temperature of 413K. This suggests a standard sol-mechanism below and at the threshold, and indicates an interruption of this mechanism by the start of the crystallisation process above 413K.