Structures faujasite-type

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. 

Chatteijee, A., Ebina, T., Iwasaki, T., and Mizukami, F. 2003. Cholorofluorocarbons adsorption structures and energetic over faujasite type zeolites—a first principle study. THEOCHEM 630 233-242. 

Figure 4.1 The framework structure of a Faujasite-type zeolite and simplified structure representations thereof (top ball and stick model, middle simplified stick model, and bottom comparison stick model and polyhedra model).

The connectivity (topology) of the zeolite framework is characteristic for a given zeolite type, whereas the composition of the framework and the type of extra-framework species can vary. Each zeolite structure type is denoted by a three-letter code [4], As an example, Faujasite-type zeolites have the structure type FAU. The pores and cages of the different zeolites are thus formed by modifications of the TO4 connectivity of the zeolite framework. 

The nomenclature of zeolites is rather arbitrary and follows no obvious rules because every producer of synthetic zeolites uses his/her own acronyms for the materials. However, as mentioned before, at least the structure types of the different zeolites have a unique code. For example, FAU represents Faujasite-type zeolites, LTA Linde Type A zeolites, MFI Mobile Five, and BEA Zeolite Beta. The structure commission of the International Zeolite Association (IZA) is the committee granting the respective three-letter codes [4], Some typical zeolites, which are of importance as catalysts in petrochemistry, will be described in the following sections. 

Caglione, A.)., Cannan, T.R., Greenlay, N., and Hinchey, R.). (1994) Process for preparing low silica forms of zeolites having the faujasite type structure. US Patent 5,366,720. 

Some vibrational modes of zeolites are sensitive to the amount of aluminum in the framework [93]. The substitution of aluminum for silicon atoms in the zeolite framework changes the T-O-T bond angles (where T is a tetrahedral atom that can be either Si or Al). This is primarily due to the smaller size and different charge density of the aluminum atoms compared to silicon. This results in a shift in frequency for vibrational modes in the zeolite due to external linkages. The T-O-T asymmetric (1100-980 cm ) and symmetric (800-600 cm ) stretching modes as well as structural unit vibrations Mke double four- and double six-rings exhibit a shift to lower frequencies as the aluminum content of the framework is increased. Figure 4.19 shows this relationship for a faujasite-type framework. 

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. 

Zeolites have attracted much attention as cobalt catalyst supports ]151-155]. Co2(CO)8 reacts rapidly from the vapor phase with X and Y faujasite type zeolites Co4(CO)i2, subcarbonyl species and [Co(CO)4] species form inside the pores. Further migration of Co4(CO)i2 carbonyl is inhibited because of pore size hmita-tions, and subsequent decarbonylation can take place only above 150 °C. In contrast, the reaction of Co2(CO)g with an A-type zeolite is limited to the surface due to the inability of the carbonyl precursors to pass through the apertures of the cavities of the structure. 

Faujasite-type zeolite structures have maximum symmetry Fd3m, and all the 192 T atoms per unit cell of the A structure are symmetrically equivalent. The observed Si/Al ratios of synthetic faujasite-type species vary within a range from slightly over 1 up to 2.5 (and occasionally above). Unmodified species thus normally contain between 48 and almost 96 A1 atoms per unit cell. The almost continuous range in A1 content does not by itself rule out any kind of Si, A1 order. Discontinuities in the plot of the cell dimensions against the number of A1 atoms per unit cell have been reported by several investigators (11, 12). The observed discontinuity at around 64 Al, in particular, has been related to Si, A1 ordering (12). Full details and references on faujasite-type zeolite structures can be found in the comprehensive and critical review by Smith (13). 

Figure 4- Probable Si, Al ordering schemes in double 6-ring unit of faujasite-type structures. Al positions are marked by circles. B requires 96 AVs and C and D require 64 AVs per unit cell. The space group symmetry of the framework is given for each arrangement of the Al atoms. Numbering refers to the nonequivalent T atoms in the common subgroup F222...

McNicol et al. (49) used luminescence and Raman spectroscopy to study structural and chemical aspects of gel growth of A and faujasite-type crystals. Their results are consistent with a solid-phase transformation of the solid amorphous network into zeolite crystals. Beard (50) used infrared spectroscopy to determine the size and structure of silicate species in solution in relationship to zeolite crystallization. 

Smith, J. V., Faujasite-Type Structures Aluminosilicate Framework Positions of Cations and Molecules, Paper 15, presented at the Second International Conference on Molecular Sieve Zeolites, Worcester, Mass., 1970 Advan. Chem. Ser. (1971) 101, 171. 

The absence of an initial expansion in faujasite-type zeolite (Figure 5) is probably associated with the peculiarities of their structure and with the number and location of the cations. The heats of adsorption of water on zeolite NaX, in contrast with that of NaA, rapidly decrease in the... 

Correlations between structure and catalytic activity have been described for carbonium-ion type reactions (1). Much effort was also spent to establish a correlation between structural and compositional factors and the activity for redox type reactions (1, 9-12). Transition metal ions in zeolites were shown to be active in the oxidation and hydrogenation of hydrocarbons. In this connection various techniques were used to locate the cations in the framework of the faujasite-type zeolites (13-20). These ions migrate upon thermal treatment or by the adsorption of various substances. Thus, methods are needed to determine the location of the cations under reaction conditions. 

In these experiments, synthetic zeolites of the faujasite-type without binding substance were used. Calcium and nickel-calcium samples in ionic form were obtained by ion exchange under conditions ensuring stability of the crystal structure (5). Platinum addition was carried out by ion exchange with Pt(NH3)6Cl4 (6). 

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. 

We describe a systematic investigation of various synthesis variables that usually affect the crystallization of faujasite-type structures from Si, Al, Na, tetraethylammonium (TEA) hydrogels.A careful control of parameters such as the composition of the precursor hydrogel, temperature and crystallization time is needed to selectively prepare and stabilize pure zeolite ZSM-20 in high yield. 

With these ideas in mind, we used Aerosil to prepare a silica-richer ZSM-20. Surprisingly, under our reference conditions, we observed the formation of another open structure, here called "FAU-polytype", that could not be identified by XRD to any of the known faujasite-type polymotphs. "Figure 10 . The doublet of the... 

Faujasite type zeolites because of the size of their cavities and apertures were the most frequently used in this purpose. Their well known structure, acid-base and redox properties helped much in selecting those zeolites. 

Fig. 4. Three zeolites with the same structural polyhedron, cubo-octahedrons. (a) Type A, Na12[(Al02)12(Si02)12] 27H20 (b) sodalite [1302-90-5, (c) faujasite (Type X, Y), where X = Na86[(.-yO2)86(SiO2)106] 264H,O Y = Na56[(.A102)Kl i02)13(S]-250H20...

The distorted cubane features four metal atoms and four ti3-OH groups occupying the alternate vertices of the cube. This motif was identified as early as 1968 in the crystal structure of rare earth Faujasite-type zeolites [101]. Its prevalence has subsequently been suggested by a number of unexpected cluster complexes isolated under markedly different reaction conditions [38, 64, 65]. The large number of complexes bearing such a core structure, from the extensive work by Zheng and coworkers, clearly establish its ubiquity [69-71]. 

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.

---