(90h) Synthesis and Transport Property of Hierarchical Siliceous Zeolites Synthesized By Post-Synthetic Surfactant Templating

The microporous nature of the zeolites, composed of the narrow pore distribution of molecular dimensions and high internal surface area, result in shape selective properties for zeolite materials. These micropore structures (< 2 nm) however also impose slow diffusion rates within zeolites leading to transport limitation for the materials in the applications of catalysis and separation. This transport limitation can be tackled by introduction of mesopores (> 2 nm) in the zeolitic microporous structure and development of Hierarchical Molecular Sieves (HMSs).^1,2 A variety of strategies can be pursued to create mesoscale porosity, such as numerous templating techniques and post-synthetic methods. Post-synthetic modification of zeolites in the presence of surfactants under basic conditions is one of the easy, scalable, and low-cost approaches. The simplicity and efficiency of this “Surfactant Templating” strategy make it very attractive for industrial applications.³ However, the mechanism for the formation of mesopores in zeolites using the surfactant-templating method has not been fully understood.⁴ It is important to study the effects of various parameters including zeolite compositions and defects and create a base knowledge so that it can be extended to zeolites with different physical and chemical features.

In this presentation, we studied the surfactant templating process for the formation of intracrystalline mesoporosity in a series of zeolites with different framework structure, aluminum content and defect density. This study was performed at different concentrations of NaOH in presence of cetyltrimethylammonium ammonium bromide (CTAB). Crystallinity of the samples was investigated by X-ray diffraction (XRD). Presence of mesopores was confirmed by nitrogen adsorption-desorption experiments. Transport property of the hierarchical zeolites was evaluated using Zero-length Column technique (ZLC). It was found that the surfactant templating method is more effective for zeolites with large pore structure than small-pore zeolites. The presence of Al and defects in the zeolites play a critical role in the formation mesoporosity by the surfactant-templating method. Al containing zeolites require harsher conditions for the formation of mesopore which can be attributed to the stability of Si-O-Al bonds in basic conditions. In addition, we demonstrated that the formation of mesopores in zeolites can be controlled by the introduction of structural defects, leading to a method for the synthesis of hierarchical zeolites with controllable mesopore structures.

References:

1. Martínez-García, J., Li, K. & Davis, M. E. Mesoporous Zeolites: Preparation, Characterization and Applications. (Wiley-VCH, 2015).
2. Vattipalli, V., Qi, X., Dauenhauer, P. J. & Fan, W. Long Walks in Hierarchical Porous Materials due to Combined Surface and Configurational Diffusion. Chem. Mater. 28, 7852–7863 (2016).
3. Sachse, A. et al. Development of Intracrystalline Mesoporosity in Zeolites through Surfactant-Templating. Cryst. Growth Des. 17, 4289–4305 (2017).
4. Sachse, A. & García-Martínez, J. Surfactant-Templating of Zeolites: From Design to Application. Chem. Mater. 29, 3827–3853 (2017).