Zeolite channel-type framework structures

Figure 7.7. Channel-type framework structure and channel dimension (with 10-rlng) of ZSM-5 (MEI) compared with the cage structure and window dimension (with 12-ring) of zeolite Y.

The framework structures and pore cross-sections of two types of zeolites are shown. (Top) A Faujasite-type zeolite has a three-dimensional channel system with pores of at least 7.4 A in diameter. A pore is formed by 12 oxygen atoms in a ring. (Bottom) ZSM-5 zeolite has interconnected channels running in one direction, with pores 5.6 A in diameter. ZSM-5 pores are formed by 10 oxygen atoms in a ring. Reprinted with permission from Chemical Engineering Progress, 84(2), February 1988, 32. 

One of the most signiflcant variables affecting zeolite adsorption properties is the framework structure. Each framework type (e.g., FAU, LTA, MOR) has its own unique topology, cage type (alpha, beta), channel system (one-, two-, three-dimensional), free apertures, preferred cation locations, preferred water adsorption sites and kinetic pore diameter. Some zeolite characteristics are shown in Table 6.4. More detailed information on zeolite framework structures can be found in Breck s book entitled Zeolite Molecular Sieves [21] and in Chapter 2. 

Offretite Type. The synthetic offretite-type zeolite, TMA-O, consists of a framework structure formed by linked cancrinite-type units in columns and enclosing a large C-axis channel (18). These columns are further joined by gmelinite-type units. The calculated total void space including the cancrinite units is 0.244 cm3/gram. The measured adsorption pore volumes shown in Table VI show that even a hydrocarbon such as n-butane... 

Three examples will suffice to demonstrate this information Figure 3 shows the polyhedral units in the synthetic zeolite Linde Type A, which link to provide a three-dimensional interconnecting array of channels, Figure 4 illustrates the essentially two-dimensional system of channels in the mordenite framework, and Figure 5 shows the major channels in synthetic zeolite Linde Type L arranged as parallel one-dimensional channels and shown as a stereo pair. Table 6 lists the Atlas notations for these structures with explanations, including the symbols used in Tables 2-5. 

Figure 10-2 Framework structures and pore cross sections of two types of zeolites. (Top) Faujasite-type zeolite has a three-dimensional channel system with pores at least 7.4 8, in diameter. A pore is formed by 12 oxygen atoms in a ring.

Y. Zhou, H. Zhu, Z. Chen, M. Chen, Y. Xu, H. Zhang, and D. Zhao, A Large 24-Membered-ring Germanate Zeolite-type Open-framework Structure with Three-dimensional Intersecting Channels. Angew. Chem., Int. Ed., 2001, 40, 2166-2168. 

The feature that is common to zeolite or zeolite-like materials is that they all have a 3-dimensional, 4-connectcd framework structure constructed from comer-sharing T04 tetrahedra (basic building unit or BBU), where T is any tetrahedrally coordinated cation. This framework structure is relatively open and characterized by the presence of channels and cavities. A description of a zeolite structure almost always begins with a description of the framework type in terms of the size of the pore openings and the dimensionality of the channel system. 

In the interior of these channels, which are characteristic of zeolites, are water molecules and mobile alkali metal ions, which can be exchanged with other cations. These compensate for the excess negative charge in the anionic framework resulting from the aluminum content. The interior of the pore system, with its atomic-scale dimensions, is the catalytically active surface of the zeolites. The inner pore structure depends on the composition, the zeolite type, and the cations. The general formula of zeolites is... 

Zeolites are crystalline alumina-silica-based open anionic framework structures with uniformly sized, rigid pores and channels. Incorporation of porphyrins into zeolites has been explored as means to biomimetic oxidation catalysts. " The initial efforts to combine porphyrins with zeolites resulted in materials in which the porphyrins were supported on the external zeolite surfaces rather than encapsulated inside the pores. The first of this type of material was reported by Bedioui and coworkers which the... 

Now the term zeolite is commonly used in a much broader sense to include all microporous materials that have three-dimensional networks made of comer-sharing TO4 tetrahedra in which T is an electropositive component, typically Si, Al, or P. Most of these materials have been synthesized in the laboratory. The large variety of materials arises from (i) the many ways the tetrahedra can be connected to form networks with channels and cavities of different sizes, (ii) the different type of cations present for chaige compensation in extra-framework positions, and (iii) the different combination of cations in the tetrahedral framework positions (T = Si, Al, P,. ..). The structure types and the examples found or synthesized are collected in the Atlas of Zeolite Stmcture Types which is currently updated and also available on the Internet. ... 

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