Aluminosilicates tetrahedral sites

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),... 

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... 

Figure 4 shows the27 A1 NMR spectrum of a calcined sample. There are three peaks visible, a peak due to octahedrally coordinated aluminum (Oh), a peak due to tetrahedrally coordinated aluminum (Td) and a peak in between due to highly distorted tetrahedral sites. The tetrahedrally coordinated aluminum can be assumed to be incorporated into the aluminosilicate network while the octahedrally coordinated aluminum is occluded in the pores or exists as an amorphous by product. 

Trivalent iron is an effective activator when it is free of the effects of ferrous iron. Because of the larger value of Dq, the emission is shifted to 700 - 750 nm in the deep red when Fe + is present on tetrahedral sites. Fe + in octahedral coordination is predicted to emit in the near infrared at about 900 to 1000 nm but is rarely observed and is thus indicated by a dashed line in Figure 5. Fe luminescence in minerals is mainly observed when Fe + substitutes for Al3+ in tetrahedral sites in aluminosilicates. Observations on feldspars (15) provide the transition sketched on Figure 5. In the orthoclase structure the Stokes shift is 1500 wavenumbers. Trivalent iron is the most likely activator for luminescence seen in many terrestrial feldspars and feldspar-containing rocks. The characteristic 700-750 nm band is weak or absent in lunar feldspars (16,17). 

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],... 

Aluminosilicates form an extensive family of compounds that include layered compounds (such as clays, talc, and micas), 3-D compounds, (e.g. feldspars, such as granite), and microporous solids known as molecular sieves. The structural diversity of these materials is contributed to by aluminum s ability to occupy both tetrahedral and octahedral holes as it also does in y-Al203. Thus, aluminum substitution for silicon in silicate minerals may lead to replacement of silicon in tetrahedral sites or the aluminum can occupy an octahedral environment external to the silicate lattice. Replacement of Si with Al requires the presence of an additional cation such as H+, Na+, or 0.5 Ca + to balance the charge. These additional cations have a profound effect on the properties of the aluminosilicates. This accounts for the many types of layered and 3-D structures (see Silicon Inorganic Chemistry). 

Although distorted tetrahedral sites associated with tricluster oxygen defects may provide one possible explanation for the broad Al NMR peak in amorphous aluminosilicates, the resonance occurs in other systems where there is no obvious mechanism for tricluster formation. Because of its broad featureless characteristics, the peak cannot be simulated to obtain unique values of xq and t. In such cases, it is possible to derive a combined parameter, the quadrupolar product Pq (sometimes called the composite quadrupolar coupling constant ) which contains both xo and tq... 

The catalytic activity of aluminosilicate zeolites and aluminas appears to be directly related to the concentration of 30 ppm sites present, leading to the development of super-five materials displaying large NMR signals at this position (Wood et al. 1990). Since catalysis depends on the chemical nature of the Al at the surface, which may not be the same as in the bulk, cross-polarisation experiments between H and Al have been used to distinguish between the surface and bulk species (Coster et al. 1994). Since chemisorption of water provokes extensive surface reconstruction, a more suitable proton source for the CP experiments was found to be ammonia adsorbed on the surface. The results showed the presence of two kinds of surface Lewis sites associated with the non-framework Al (a tetrahedral site at ca.58 ppm with a xq of about 6 MHz, and an Af site at ca. 40 ppm with a slightly smaller xq)- Lewis sites either... 

Geopolymers are useful inorganic aluminosilicate framework compounds which form and harden at room temperature. Both the aluminium and silicon atoms in their structure occupy tetrahedral sites, with charge balance achieved by the presence of hydrated monovalent ions. NMR has been used to study the changes occurring when a potassium sialate geopolymer is heated to high temperatures. The cog positions... 

Using Table 22.1, predict the structure of each of the following aluminosilicate minerals (network, sheets, double chains, and so forth). In each case, the A1 atoms grouped with the Si and O in the formula substitute for Si in tetrahedral sites. Give the oxidation state of each atom. 

For aluminosilicates, Si NMR has proven to be extremely useful, because it gives naturally narrow peaks and exchange of an Si for Al in an adjacent tetrahedral site results in a large change in the Si chemical shift, about -5 ppm, that is linearly additive (Fig. 25). Thus, for framework structures with Al and Si disordered on a crystallographic... 

Borosilicates have been prepared via hydrothermal synthesis in alkaline solutions (1.16-24). Alternatively, synthesis has been successful from neutral or slightly acidic media in the presence of fluoride anions (22). Borosilicate molecular sieves have been prepared through secondary synthesis techniques as reported by Derouane, et al. (26.), in which the aluminosilicate ZSM-5 was treated with boron trichloride to replace aluminum with boron in tetrahedral sites. 

Barrie and Klinowski [109] discussed ordering in the magnesium alumi-nophosphate network on the basis of aluminum and phosphorus NMR. MgAPO-20 crystallizes in a structure similar to naturally occurring mineral sodalite, with Mg, P, and Al occupying the tetrahedral sites. Barrie and Klinowski postulated that the alternation of the Mg and Al in the tetrahedral sites adjacent to phosphorus creates a situation similar to that in zeolitic aluminosilicates and that the phosphorus spectrum can be interpreted in terms of P(0-A1) where n ranges from 0 through 4 (Fig. 20). 

The diversity of the inclusion of molecules of any type into host lattices which are distinctly inorganic is now becoming apparent [31], Apart from the oxidic zeolites, aluminosilicates and derivatives with other tetrahedral sites, there are the layered silicates and minerals, pillared clays and numerous other intercalation and inclusion systems, including graphite, metal sulfides and metal cyanides [23, 32, 33]. 

Si in Si(OSi)n(OAl)4 n environments, with n = 4 to 1). In another well-characterised example, SAPO-35, a small-pore aluminophosphate with two different tetrahedral sites, incorporation of silicon at low levels (Si/(A1 + Si + P) < 0.1) is found to be preferred in one of the sites. At higher silicon levels aluminosilicate islands are formed. 

Thomas JM, Klinowski J, Ramdas S, Hunter BK, Tennakoon DTB. The evaluation of non-equivalent tetrahedral sites in zeolites and related aluminosilicates. Chem Phys Lett 1983 102 158-62. 

Figure 2 (Left) shows the 27Al NMR spectra for the aluminosilicates. All of them displayed a tetrahedral incorporation of aluminum inside the silica network. That is corroborated by the signal at 55 ppm [9, 10] which also become more intense with the decreasing of Si/Al ratio. Octahedral aluminum was observed just for the samples with the lowest Si/Al ratio. Tetrahedral aluminum gives place to strong Bronsted acid sites, which were identified by the interaction of these groups with pyridine that generates a...

Wuest et al. have also prepared a related tetrahedral tecton 8.63, which also produces a diamondoid polymeric framework. In this case, the solid-state network is seven-fold interpenetrated, with one diamondoid lattice filling much of the large cavities in those adjacent. It is possible that the interpenetration in this instance is a result of the self-complementary nature of the host, which contains an equal number of hydrogen bond donor and acceptor sites. However, even in this case small cavities exist, which are filled by two molecules of butyric acid per host formula unit. The formation of these kinds of framework materials opens entirely new possibilities for tailor-made porous materials with very large cavities, although it is unlikely that purely organic frameworks will ever rival aluminosilicate-based materials for sheer mechanical strength. 

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