Hexagonal faujasite

The linking pattern of two zeolites is shown in Fig. 16.24. They have the /I-cage as one of their building blocks, that is, a truncated octahedron, a polyhedron with 24 vertices and 14 faces. In the synthetic zeolite A (Linde A) the /3-cages form a cubic primitive lattice, and are joined by cubes. j3-Cages distributed in the same manner as the atoms in diamond and linked by hexagonal prisms make up the structure of faujasite (zeolite X). 

New Zeolitic Structures. Multiply twinned faujasitic zeolites (typically zeolite-Y) have recently been shown (30, 31) to be capable, by recurrent twinning on 111 planes, to generate a new, hexagonal zeolite in which tunnels replace the interconnected cages of the parent cubic structure. 

Zeolites are crystalline porous solids with pore dimensions at the molecular level. Some zeolite types, such as the faujasites (zeolite X and Y or their hexagonal isomer EMT), possess large supercages with an internal diameter of approximately 1.2 nm, connected by pores with a diameter of approximately 0.75 nm. A metal complex will be confined in the supercage, when its size exceeds 0.8 nm. 

Delprato, F., Delmotte, L., Guth, J.L, and Huve, L. (1990) Synthesis of new silica-rich cubic and hexagonal faujasites using crown-ether based supramolecules as templates. Zeolites, 10, 546. 

The crystal structure of the faujasite is built up by linking the sodalite cages tetrahedrally through their hexagonal faces forming connecting hexagonal prisms. 

Hedron (hexagonal prism) 2 6-rings 6 4-rings 12 2.3 in plane of 6-rings faujasite, ZK-5, chabazite erionite, offretite, levynite... 

Nature of acidic sites. The location of the acidic hydroxyl groups in the faujasite structure has been the subject of numerous investigations and much discussion. The results of adsorption experiments with several molecules led Eberly (170) to conclude that the 3550-cm-1 hydroxyl absorption band represented hydroxyl groups located in the hexagonal prisms of the faujasite framework [(Si sites (171)], where they were relatively inac-... 

Experiments to further demonstrate the critical role of extraframework Al, or another polyvalent cation, have recently been carried out in our laboratory (19.20). A series of faujasite-type zeolites was prepared that had Alf concentrations between 21 and 54 per u.c. At the low end of the range, AHF was used to remove the framework Al, and an H-ZSM-20 zeolite with 42 Alf/u.c. was synthesized. ZSM-20 is an intergrowth of the cubic faujasite structure and the hexagonal variant know as Breck s structure six (BSS) (21). Thus, it is a faujasite-like material. The catalytic activities of these zeolites for hexane cracking are compared in Figure 5 (lower data set) with the activities of zeolites prepared by steaming or by treatment with SiClA (upper data set). The solid lines represent N(0) distributions. The samples without extraframework Al exhibited very modest activity, even though some of them had a favorable N(0) concentration. 

Fig. 3 Stereoscopic view of hexagonal prism atom distribution in nickel faujasite. Reproduced with permission from reference 17. (Copyright 1968 American Chemical Society)...

Faujasite or zeolite X and Y exists in nature. Its framework can be formed as follows. The cubo-octahedra are placed at the positions of the carbon atoms in the diamond structure, and they are joined by hexagonal prisms through four of the eight hexagonal faces of each cubo-octahedron, as shown in Fig. 14.4.11(b). 

Mossbauer spectral data[136] allowed the respective amounts of complexed Fe11, free Fe11 and Fe ions in the hexagonal prism of the faujasite structure to be... 

Fig. 8. Sodalite and hexagonal prism site I and II cation locations in Ag/faujasites. Configuration (a) shows the normal cation locations with occupied sites at SI, SI and SII. Configuration (b) shows cation sites that have resulted from cation and/or cluster migration upon vacuum thermal treatment. This configuration shows occupied sites at SI, SII, and SII (Hutson and Yang, 2000).

Meta-diisopropylbenzene is reacted with propylene over the acid form of the molecular sieves SAPO-5, mordenite, offretite, beta, hexagonal and cubic faujasite (EMT and FAU), L, SAPO-37, and an amorphous silica-alumina at temperatures around 463 K in a flow-type fixed-bed reactor. A small amount of cracking is observed. However, the main reactions of meta-diisopropylbenzene are isomerization and alkylation. It is proposed that this alkylation can be used as a new test reaction to characterize the effective size of the voids in larger pore (12 T-atom rings or above) molecular sieves by measuring the weight ratio of 1,3,5- to 1,2,4-triisopropylbenzene formed. In most cases, this ratio increases with die increasing effective void size of the molecular sieves in the order SAPO-5 < mordenite < offretite < beta < EMT FAU < L < SAPO-37 < amorphous silica-alumina. 

When Aerosil is used instead of tetraethylorthosilicate a novel faujasite polytype material having a different hexagonal packing is obtained. 

From the topological point of view, with the truncated octahedra of faujasite, a large number of structures can be derived considering a close packing of hexagonal layers of sodalite cages (171. 

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