Design hierarchical zeolites

L. Huang, Z. Wang, H. Wang, J. Sun, Q. Li, D. Zhao, and Y. Yan, Hierarchical Zeolite Structures with Designed Shape by Gel-casting of Colloidal Nanocrystal Suspensions. Chem. Commun., 2001, 1364—1365. 

Wang, H. Huang, L. Wang, Z. Mitra, A. Yan, Y. Hierarchical zeolite structures with designed shapes by gelcasting of colloidal nanocrystal suspensions. Chem. Comm. 2001, 1364-1365. 

As well as the crystal structure of the microporous catalysts, the secondary mesoporosity is also important, because molecular transport to and from the active sites is favoured in these materials. In steamed Y the mesoporosity and extra-framework aluminium results in a very active catalyst for cracking. Designed hierarchical structures, in which nanoparticles of zeolites are joined together to and connected by a secondary mesopore system for the same reason are discussed further in Chapter 10. 

Figure 8.4 Tailored design of zeolites for the enhancement of the accessibility to active sites, (a) Ultralarge-pore size zeolites, (b) nanocrystal zeolites, (c) delaminated zeolites, and (d) hierarchical zeolites.

SDAs are featured for their structure diversity and controllability to meet the objective of designed synthesis of zeolites with specific structures. Studies show that SDAs with proper shape, polarity, charges, and flexibility are the keys to achieve the design of novel zeolite structures [63]. Nowadays, many zeolite structures have been synthesized by using novel SDAs. Besides, hierarchical zeolites can be synthesized with bifunctional templates. Some representative works will be discussed later. 

Hierarchical porous materials are solids that are ordered at different length scales. Materials with multiple porosities are of high interest for applications in catalysis and separation, because these applications can take advantages of different pore structures. For example, microporous-mesoporous composites have shown superior catalytic activities by the combination of strong acidity from zeolites with high reactant or product mobility due to large uniform mesopores. Several approaches have been reported on the design and synthesis of hierarchical porous materials, as discussed below. 

Soler-Illia GJ, Sanchez C, Lebeau B, Patarin J (2002) Chemical strategies to design textured materieils from micropo-rous and mesoporous oxides, to nanonetworks and hierarchical stmctures. Chem Rev 102 4093 138 Varsani P, Afonja A, Williams DE, Parkin IP, Binions R (2011) Zeolite-modified WOj gas sensors-enhanced detection of NOj. Sens Actuators B Chem 160 475-482... 

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