Zeolites tetrahedral sites

Tin incorporated mesoporous Sn-MFI catalysts with different Si/Sn ratio using microwave were synthesized with carbon as hard template. These tin MFI catalysts were characterized using various physicochemical techniques XRD reviled the formation of more crystalline MFI structures which was further supported by the SEM and TEM imaging which clearly showed well ordered zeolite single crystals with mesoporosity. The N2 sorption isothers reviled the formation of bimodal mesoporous zeolites and the presence of tin in tetrahedral site was confirmed by FTIR (970 cm 1) and XPS (3ds/2 and 3 dj 2 electronic states). The thus synthesized mesoporous Sn-MFI catalysts with different Si/Sn ratios were used in studying the catalytic Baeyer-Villiger Oxidation (BVO) of cyclic ketones... 

Figure 10a, Scheme of the aluminosilicate framework of a typical faujasitic zeolite Si/Al ratio of LI8 (arbitrarily chosen to illustrate the ordering among tetrahedral sites) before (left half) and after (right half) exposure to SiCls, which dealuminates the zeolite (see Figure 10b),... 

The effect of probe molecules on the 27A1 NMR has attracted some attention recently. In particular, the determination of the quadrupole coupling constant, Cq, is a sensitive means to learn more about the bonding situation at the aluminum in acid sites, and how it reflects the interaction with basic probe molecules. If one of the four oxygen atoms in an AIO4 tetrahedral coordination is protonated, as in a zeolitic acid site, the coordination is somewhat in between a trigonal and a tetrahedral A1 environment [232]. The protonated oxygen decreases its bond order to A1 to approximately half of its size compared to an unprotonated zeolite. 

To summarize, the assessment of Al-content in the tetrahedral sites of zeolites via diffraction data is strongly influenced by ... 

Table 2 - Percent of At in tetrahedral sites, average long-range order coefficient Sj, average S and Si/Al ratio for zeolites with an ideal Si/Al=1.0.

Table 3 - Percent of A1 in tetrahedral sites, "average long-range order coefli-cient S, and Si/Al ratio for zeolite A.

A lot of work [4,68,69] has been done using A1-NMR and Si-NMR spectra on the isostructural synthetic zeolite fl to determine Si/Al distribution in the two tetrahedral sites. These data clearly indicate that Si and Al distribution in zeolite (I is not random in nature, with an A1tz/A1ti ratio in the range 0.9-1.6, depending on the different Si/Al ratios and different synthesis conditions, and with a preferential location in the 6-memberedring, as in natural mazzite. 

Zeolites are intrinsically microporous aluminosilicates of the general formula [(A102) t(Si02) ] mH20 and may be considered as open structures of silica in which aluminium has been substituted in a fraction x/(x + y) of the tetrahedral sites. The net negative charge of the aluminosilicate framework is neutralized by exchangeable... 

A few cases have come to light in which crystallographic nonequivalence of tetrahedral sites for silicon has been reflected in the 29Si MAS NMR spectra of zeolites. The most complex and the best resolved example is the spectrum of silicalite which was discussed in Section III,F another is the spectrum of the well-ordered natural zeolite scolecite (57), which contains two signals corresponding to nonequivalent Si(3 Al) units (see Fig. 23). The correctness of... 

The intensities of the two signals in the spectrum of the siliceous zeolite omega are in the 2 1 ratio. While zeolite omega is more appropriately called synthetic mazzite (94), both the previously suggested structure (95) and the structure of mazzite (96) call for two nonequivalent tetrahedral sites in a 2 1 population ratio. Accordingly, Thomas et al. (91) were able to assign the... 

Fig. SO. Projection drawing, viewed along [001] of the structure of zeolite omega (synthetic mazzite). There are two distinct tetrahedral sites, one more (A) and one less (B) accessible via large channels. The unit cell is enclosed within the dashed lines.

The Na ZK-4 and Y zeolites have Si to A1 ratios of 1.65 and 2.61, respectively, but although the ZK-4 zeolite is a variant of zeolite A, powder neutron diffraction data61 showed that no superlattice reflection could be found at the angle where it occurs in the T1 zeolite A.59 60 The small unit cell implied that in this zeolite the Si and A1 atoms were no longer preferentially located on alternate tetrahedral sites. A similar result was found for the Y zeolite. 

From the obtained data, it was possible to conclude that iron found in the natural zeolite is high-spin Fe3+ in place of Al3+ in the framework tetrahedral sites. Besides, Fe3+ is located in extraframework octahedral sites as Fe(H20)6+. Finally, Fe2+ is present in the octahedral coordination in extra-framework sites or in other aluminosilicates present in the zeolite rock [44],... 

When applied to zeolites the term "isomorphous substitution" refers to the replacement of silicon or aluminum atoms by elements with ionic radii and coordination requirements which are compatible with the T (tetrahedral) sites of the zeolite structure. One method of preparing isomorphously substituted zeolites is to include a reactive source of the replacement... 

Properties of zeolites are intimately related to the type of occupancy of the tetrahedral sites. Modification of the composition of the framework by increasing the silicon content increases the thermal stability of the samples. The catalytically active centres in zeolites are the acidic (Bronsted) hydroxyl groups associated with tetrahedrally coordinated framework aluminium atoms. Catalytic activity is thus strongly dependent on the concentration and location of aluminium in the framework. 

It is clearly desirable to be able to alter the Si/Al ratio of the framework, particularly in the case of zeolites X, Y and ZSM-5. This can be conveniently achieved by "secondary synthesis", i.e. by isomorphous substitution of Si or A1 on the tetrahedral sites after the completion of the original zeolite crystallization (1-4). We shall demonstrate that ... 

The distribution of Si and A1 among the tetrahedral sites in the treated material is very different from that in as-prepared zeolite Y with the same Si/Al ratio. 

Figure 2. The structures of the zeolites listed in Table I may be regarded as having been derived from various, regular stacking sequences of the single sheet as shown. Thus an AAB sequence yields offretite, AB cancrinite, and so on. Each vertex is a tetrahedral site (T = Si4+ or Al3+ surrounded by four oxygens).10...

In the zeolites each lattice cation is tetrahedrally coordinated to four oxygen anions (see Fig. 4.57). Each oxygen anion shares two lattice cations. In smectite clays an octahedral layer, usually containing Al3+ or Mg2+, connects two tetrahedral layers. The tetrahedral layer can be considered neutral when the tetrahedral site contains a four valent cation. This is usually a Si4+ ion. A top view and side view are shown in Fig. 4.59. 

The proton is always bonded to one of the oxygens belonging to the substituted T-site A1 in alumino-silicates or Si in silico-alumino-phosphates. Since the two vicinal tetrahedral sites are practically never substituted, the immediate neighborhood of the protonated framework oxygen is identical in both alumino-silicates and SAPOs, as indicated in the above scheme. This bridging OH group or Br0nsted acid site is at the heart of the catalytic activity of zeolites and SAPOs. 

Employing a classical model, Monte Carlo calculations were performed to investigate the A1 distribution in zeolites. " Si and A1 atoms were randomly distributed over the tetrahedral sites, and in each step an attempt was made to interchange each A1 with a nearby Si. Loewenstein s rule, which forbids Al-O-Al bridging, was investigated, and results from the simulation indicate that the A1 distribution in zeolite structures is generally in accordance with this rule. 

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