Faujasite structure

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. 

Figure 12. The structure of zeolite-A formed by linking truncated octahedra through double four-membered rings (a), the sodalite structure formed by direct face-sharing of four-membered rings in the neighboring truncated octahedra (b), and the faujasite structure formed by linking the truncated octahedra through double six-membered rings (c).

Thomas et al. (39,41) recorded the Si-NMR spectrum of an aluminum-deficient Y zeolite prepared by reacting NaY zeolite with SiCl vapors. The spectrum showed a single sharp peak, characteristic of Si(0 Al) groupings, and indicative of an essentially luminum-free faujasite structure. 

Zeolite structures are designated by a three capital-letter code, for example, FAU stands for the faujasite structure, to which the well-known X and Y zeolites belong. A very useful short notation is used for the description of the pore system(s) each pore network is characterized by the channel directions, the number of atoms (in bold type) in the apertures, the crystallographic free diameter of the apermre (in A), asterisks (1, 2, or 3) indicating whether the systems is one-, two-, or three-dimensional. To completely specify the pore system, the eventual presence of cages (or channel intersections) should be indicated, along with their... 

Katranas,T.K.,Triantafyllidis, K.S., Vlessidis, A.G., and Evmiridis, N.P. (2007) Propane reactions over faujasite structure zeolites type-X and USY effect of zeolite silica over alumina ratio, strength of acidity and kind of exchanged metal ion. Catal. Lett., 118,79-85. 

Pi0 and P32 have parallel isotherms, as expected for samples of identical structure. The isotherms of Po and P5 have the same slope as those of P10 and P32 which are rather well crystalline. However, this observation is a necessary but not a sufficient condition to affirm that these materials, amorphous to X-rays, already contain microcrystallites having a Faujasite structure. 

FIGURE 7.14 The cubic faujasite structure, illustrating the channels which lie parallel to each of the face diagonals (oxygen bridges are included in this model). 

In X- and Y-type zeolites which exhibit the faujasite structure, the cations can reside in four basically different types of site which are located on the threefold axes of the cubic faujasite structure (Fig. 18). 

Fig. 18. The different cation sites in the faujasite structure (zeolites X and Y) (25S). The figure has been simplified, but the oxygen ions of the windows are not equivalent (see text).

Due to the larger size of sorbates and the lower diffusion rates in liquids, larger pore sizes are needed to treat liquid effluents, normally in the range 30 A (Noble and Terry, 2004). For gas-phase effluents, the pores sizes are in the range 10 to 25 A. For example, zeolite Y (Figure 4.3) exhibits the FAU (faujasite) structure. It has a three-dimensional pore structure with pore diameter 7.4 A and cavity of diameter 12 A. 

In the preceding decade, microporous silicoaluminophosphates have drawn increasing interest as solid catalysts in chemical technology, because of their acidic and shape-selective properties. H-SAPO-34 with the chabasite structure, for example, is a suitable catalyst for the conversion of MTO (210). H-SAPO-37 with the faujasite structure was applied for the isomerization of -decane (211) and the isobutylene/2-butene alkylation (212). 

Structural Determinations. Si-0 and Al-0 vibrations at 1200-350 cm-1 give information on zeolite structure (21-25). Qualitatively, the resolution of the bands around 1150 and 1050 cm-1, the intensity and sharpness of the bands around 580 and 390 cm-1, and the presence of a shoulder at ca. 500 cm-1 are characteristic of the faujasite structure. 

Figure 4 shows a series of spectra of the La-7 material heated at various temperatures. Compared with the spectrum of the Unheated sample, that of the sample heated at 550° C differs only by a slight increase in the frequencies of the bands around 1150 and 1050 cm-1. This difference may be caused by the removal of the NH4+ ions. The spectrum of the sample heated at 900°C still presents the main features of the faujasite structure, but the occurrence of defects is reflected in the lessened resolution of the high frequency bands as well as in the decline of the ca. 390 cm 1 band and the ca. 500 cm-1 shoulder. Frequency shifts are also found. Heating at 950°C results in the destruction of the faujasite structure as inferred from the disappearance of the characteristic bands. 

Most of the published information regarding surface acidity and its relation to catalytic activity has involved zeolites of the faujasite structure as found in zeolites X and Y. A smaller number of investigations of mor-denite have been reported. This discussion will concentrate on studies of these two types of zeolites because their acidic and catalytic properties have been most widely investigated, and because they are both of significant industrial importance. 

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. 

Analysis of vanadium-loaded model materials (such as EuY, amorphous aluminosilicate gels and EuY-gel mixtures) by electron paramagnetic resonance (EPR) has provided information concerning metal oxidation state and stereochemistry (67). EPR data has indicated that when vanadyl cations are introduced in the form of vanadyl naphthenate, they were stabilized in a zeolite with the faujasite structure as pseudo-octahedral V02+ even after calcination at 540°C. Upon steaming, these V02+ cations were then converted almost entirely to V+5 species (67). The formation of EuV04 was verified but the concentration of this vanadate was never proportional to the total rare-earth content of the zeolite. In EuY-gel mixtures the gel preferentially sorbed vanadium where it was stabilized mainly in the form of V205. 

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... 

It is clear that the Wacker cycle in a CuPdY zeolite incorporates the traditional features of the homogeneous catalysis combined with typical effects of a zeolite (303, 310). It also follows that whereas other cation exchangers in principle will show Wacker activity after cation exchange with Cu/Pd ions, the cage and pore architecture will probably be less suitable for Wacker chemistry than those of the faujasite structure. This is the case for fluoro-tetrasilicic mica, a synthetic layer silicate that swells under reaction conditions and allows access to the interlayer space (311). 

---