Three-dimensional 12-ring channels

Figure 2.25 Eramework structures for two polymorphs of beta zeolite (a) BEA and (b) BEC. All beta polymorphs contain three-dimensional 12-ring channels.

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 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. 

Zeolite ITQ-21 is a recently discovered zeolite [1], containing Si, Ge and optionally A1 as framework cations. Its three-dimensional structure is formed by three linear 12 ring (12-R) channels that intersect to produce large inner cavities with a nearly spherical shape about 1.18 nm in diameter (Figure 1), similar to those present in the Faujasite structure. However, in the case of ITQ-21 these cavities are accessible through six circular 12-R windows of 0.74 nm wide. 

The best correlation of the observed isomerization selectivities was found in terms of the diameter of the intracrystalline cavity, determined from the known crystal structure (9) of these zeolites, as shown in Figure 2. While faujasite, mordenite and ZSM-4 all have 12-membered ring ports and hence should be similar in their diffusion properties, they differ considerably in the size of their largest intracrystalline cavity both mordenite and ZSM-4 have essentially straight channels, whereas faujasite has a large cavity at the intersection of the three-dimensional channel system. 

Pluth, J.J., and Smith, J.V. (1990) Crystal structure of boggsite, a new high-silica zeolite with the first three-dimensional channel system bounded by both 12- and 10-rings. Am. Mineral., 75, 501. 

Figure 2.11 The channel system in FAU is three-dimensional with 12-ring pores opening into large supercages.

The MOR framework (Table 2.6, Figure 2.13) is built from chains of five-mem-bered rings linked laterally to form the three-dimensional structure. The framework defines a one-dimensional 12-ring channel system coimected in one direction by eight-ring channels (Figure 2.14). 

Figure 2.17 Channel system in CHA. The three-dimensional eight-ring pore system opens into large cha cavities.

Virginia Polytechnic Institute number 5 (VPI-5) is a family of aluminophosphate based molecular sieves (refs. 1-5) which share a common three-dimensional topology and contain 18-membered rings (refs. 1-8). The extra-large pores of the VPI-5 materials are unidimensional channels circumscribed by rings containing 18 tetrahedral atoms and possess free diameters of approximately 12-13 A. 

Bell et al. (81) presented forced diffusion calculations of butene isomers in the zeolite DAF-1. DAF-1 (82) is a MeALPO comprising two different channel systems, both bounded by 12-rings. The first of these is unidimensional with periodic supercages, while the other is three-dimensional and linked by double 10-rings. The two channel systems are linked together by small 8-ring pores. It is a particularly useful catalyst for the isomerization of but-l-ene to isobutylene (S3) its activity and selectivity are greater than those of ferrierite, theta-1, or ZSM-5. 

Favorable sorption sites for the butene isomers were found to be the double 10-rings of the three-dimensional channel system. Thus, diffusion was investigated between adjacent 10-rings. Results showed that the diffusion was an activated process the lowest barrier was 17.5 kJ/mol (but-l-ene) and the largest 22.5 kJ/mol (m-but-2-enc). The authors concluded that all of the three-dimensional channel system is accessible by the four butene isomers, since the diffusion barriers are small enough to be overcome at ambient temperatures. 

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