Faujasite-type zeolites

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. 

Extraframework cations in faujasite type zeolites analytical methods and general distribution trends... 

Many experimental and, more recently, simulation methods have been put to use to try to localise the cations in faujasite (figure 1) in different situations hydrated or dehydrated zeolites, zeolites saturated with organic molecules, e.g. benzene, toluene, xylene. The four techniques that are described below have been used in more than 90% of all published works to detect and localise extraframework cations in faujasite type zeolites. 

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. 

The frameworks of zeolites are formed by fully connected Si04 and A104 tetrahedra linked by shared oxygen atoms as shown in Figure 4.1 (top) for a Faujasite-type zeolite. Faujasite is a zeolitic mineral, which can be found in nature. Synthetic Faujasite-type zeolites are of particular importance in zeolite catalysis as we will see below. 

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. 

Catalytic cracking is a process that is currently performed exclusively over fluidized catalyst beds. The fluid catalytic cracking (FCC) process was introduced in 1942 and at that time replaced the conventional moving bed processes. These early processes were based on acid-treated clays as acidic catalysts. The replacement of the amorphous aluminosilicate catalysts by Faujasite-type zeolites in the early-1960s is regarded as a major improvement in FCC performance. The new acidic catalysts had a remarkable activity and produced substantially higher yields than the old ones. 

Ozin GA, Flugues F, Mattar SM, McIntosh DF (1983) Low nuclearity silver clusters in faujasite-type zeolites optical spectroscopy, photochemistry and relationship to the photodimerization of alkanes. J Phys Chem 87 3445-3450... 

Arika,)., Aimoto, M., and Miyazaki, H. (1986) Process for preparation of high-silica faujasite type zeolite. US Patent 4,587,115. 

Maher, P.K. and Scherzer, J. (1970) Method of preparing microcrystalline faujasite-type zeolite. US Patent 3,516,786. 

Heidler, R., janssens, G.O.A., Mortier, W.J., and Schoonheydt, R.A. (1995) Charge sensitivity analysis of intrinsic basicity of faujasite-type zeolites using the electronegativity equalization method... 

Stocker, M., Mostad, H., and Rorvik, T. (1994) Isobutane/2-butene alkylation on faujasite- type zeolites. Catal. Lett., 28, 203-209. 

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). 

NMR Investigations of the Framework Cations of Various Faujasite-Type Zeolites and Their Interpretation by Model Calculations... 

A different variation in size for water adsorption is observed for the faujasite-type zeolites (Figure 5). These zeolites at first contract, reaching a limit of 0.3-0.45 % at 0 = 0.7-0.8, and then they expand. Except for zeolite CaY, their sizes, even at p/ps 0.85, remain below their initial values. The maximum contraction is observed for zeolite NaY, and the minimum for NaX. 

Carbon dioxide adsorption causes changes in the sizes of all the zeolites studied similar to the variation observed for faujasite-type zeolites after water adsorption (Figure 6). For all zeolites, an increase in the adsorption of carbon dioxide leads to contraction this reached a minimum in the adsorption range 3-5.5 mM/gram. The final length of the pellets is below the initial value up to a relative pressure of p/ps 0.7 for zeolites CaA, CaY, and NaY while for NaA and NaX the contraction passes to an expansion, reaching 0.11% of the initial length at p/ps = 0.66 for NaX and 0.32 for NaA. 

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. 

Faujasite type zeolites were prepared from two NaY commercial samples (Si-to-Al ratio 2.4) with average crystal size of 0,80 (HY-100) or 0.30 Un by exchanging with solutions of ammonium acetate... 

Both reactions of alkynes and nitriles were studied over mordenite, B and Y-faujasite type zeolites in their H-form. 

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. 

Faujasite is a rare aluminosilicate mineral. Its synthetic high-aluminia counterpart, Linde X, was synthesized by Milton (2), and Linde Y, a high-silica/alumina synthetic faujasite, was synthesized by Breck (3). The framework of faujasite-type zeolites has been well established by Bergerhoff et al. (4) and Broussard and Shoemaker ( 5). 

As long as one is dealing with faujasite-type zeolites, there is a reasonable separation between the spectral regions for Si(OAl), Si(lAl), etc. However, for other types of zeolites there can be cause for ambiguity. This has led in the past to the conclusion that in some zeolites there is violation of Loewenstein s rule,62 which, briefly stated, says that Al-O-Al bridges are absent in zeolites. Current evidence, however, provides strong support that the rule is obeyed.56 57 63... 

METALLO-PHTHALLOCYANINES ENCAPSULATED IN THE CAGES OF FAUJASITE-TYPE ZEOLITES... 

The flexible ligand method, consisting of the tetramerization of pyrrole and an aldehyde, can be used for the entrapment of metallo-porphyrins (MePOR) in situ in the supercages of faujasite-type zeolites (Scheme 10.7).[74]... 

The transport and adsorption properties of hydrocarbons on microporous zeolites have been of practical interest due to the important properties of zeolites as shape-selective adsorbents and catalysts. The system of benzene adsorbed on synthetic faujasite-type zeolites has been thoroughly studied because benzene is an ideal probe molecule and the related role of aromatics in zeolitic catalysts for alkylation and cracking reactions. For instance, its mobility and thermodynamic properties have been studied by conventional diffusion 1-6) and adsorption 7-9) techniques. Moreover, the adsorbate-zeolite interactions and related motion and location of the adsorbate molecules within the zeolite cavities have been investigated by theoretical calculations 10-15) and by various spectroscopic methods such as UV (16, 17), IR 17-23), neutron 24-27), Raman 28), and NMR 29-39). 

Adsorption Properties of Benzene in Faujasite-type Zeolites... 

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