Authors: Peng Dong | Lin Zhu | Longhua Luo | Tiesen Li | Tinghai Wang | Chan Wang | Qingyan Cui | Jie Shi | Yuanyuan Yue
Source: Chinese Science Bulletin (Chinese Version) (article, 2024-05-01)
Field: Chemistry — Inorganic Chemistry

Research progress in the synthesis and application of hierarchical FAU-type zeolites <p indent="0mm">FAU-type zeolites are an important class of porous aluminosilicate materials with a 3-dimiensional network of accessible micropores <sc>(0.74 nm),</sc> which primarily encompass X and Y zeolites. Typically, FAU zeolite with a framework SiO<sub>2</sub>/Al<sub>2</sub>O<sub>3</sub> molar ratio between 2.0 and 3.0 is called X zeolite, and that is greater than 3.0 is classified as Y zeolite. Due to their distinctive structure and pore characteristics, FAU-type zeolites have emerged as one of the most widely used catalysts, catalyst carriers, and adsorbents in the chemical industry. However, the sole microporous channels of the conventional FAU-type zeolites not only make them difficult for the reactions involving bulky molecules to occur within pore channel, leading to insufficient contact between reactants with active sites of the zeolite, but also restrict the diffusion of product within the zeolite, thus affecting the activity and lifetime of the catalyst. Therefore, the key to improving the performance of FAU-type zeolites lies in alleviating the adverse effects of micropore diffusion on mass transfer. Introducing mesopores and/or macropores into microporous zeolites to construct hierarchical zeolites with a multi-modal pore structure is believed to be a promising approach. This modification not only can preserve the active sites from the microporous structure, but also significantly enhances the diffusion rate of guest molecules within the zeolites. Consequently, researchers have concentrated on the preparation and application of hierarchical zeolites. This article focuses on the current research status on the preparation of hierarchical X and Y zeolites based on “bottom-up” direct synthesis and “top-down” post-treatment methods. The direct synthesis method mainly comprises template and template-free methods. In terms of the template method, the pore structure of FAU zeolites can be tailored by altering the nature of the templating agent, synthesis conditions and so on. Differently, the template-free method enables the preparation of hierarchical FAU zeolites without the addition of any template. In particular, hierarchical X and Y zeolites have been successfully synthesized directly from natural aluminosilicate minerals without the use of organic templates, which not only reduces the synthesis cost of hierarchical zeolites from raw materials, but also contributes to environmental protection. This method provides a novel synthetic pathway for the preparation of hierarchical zeolites in a low-cost and green way. On the other hand, post-treatment method based on desilication and dealumination is the main method for the preparation of hierarchical Y zeolite, whereas few researchers have made use of post-treatment method to introduce meso/macroporous structures into X zeolite because of the poor chemical stability of X zeolite. In addition, the article introduces the application performance of hierarchical X and Y zeolites in adsorption, catalysis as well as other reaction processes. Despite the superior diffusion capabilities of hierarchical FAU-type zeolites, there are several problemsincluding high cost of organic templates and pollution emissions from the removal of template in the process of synthesizing hierarchical zeolites based on template method, limiting the industrial application of this method. As a result, designing cheap and recyclable templates is the key to realizing green and low-cost preparation of hierarchical FAU-type zeolite. Furthermore, post-treatment method cannot achieve the regulation of mesoporous size in hierarchical zeolites. Future research should deeply investigate the structure-activity relationship between the pore structure and reaction performance of hierarchical FAU-type zeolites, so as to further open pore channels, and enhance diffusion, thus promoting the development of hierarchical FAU-type zeolites in industrial applications. Computer science Zeolite Catalysis and Synthesis ; Mesoporous Materials and Catalysis ; Catalysis and Oxidation Reactions