Aluminum-oxygen bridges

Figure 5. Schematic representation of the ft -alumina structure. The aluminum (green) and oxygen (red) ions form spinel blocks which are separated from each other by oxygen bridges. The mobile sodium ions (blue) are located in the layer. The unit cell is indicated.

OH group becomes more acidic the less aluminum atoms are bound to the respective silicon atom via oxygen bridges. 

Figure 4.33 Si MAS NMR chemical shift ranges for aluminosilicates. This figure is redrawn based on references [152-155], Here Q" is the tetrahedral silicon connected to n aluminum atoms n = 0 ) via oxygen bridges.

Different results are obtained when NH4Y is activated at higher temperatures (600°C). Vedrine et al. (266) showed that y irradiation in vacuo of such an activated zeolite leads to two types of signals. The signal with g = 2.0125, g = 2.0030, and a 12-line hyperfine structure with A aiso = 10.0 G was attributed to a positive hole (V center) trapped on an oxygen bridging two aluminum atoms ... 

Fig. 16.2 Stereoviews of (al sodafile. Na Al Sv0.4-2H 0 [ring sizes are 4 and 6 1220 pml lb) ZSM-5. NasAI Siy)0,ra-l6H,0 [ring sizes are 4. 5. 6. 7. 8. 10 (J80 pml] lc> mordenile. Na AIHSiwOw-24H.O [ring sizes are 4. 5. 6. 8. 12 (760 pm>. See also Fig. 1.3. Lines represent oxygen bridges intersections of lines show positions of the aluminum and silicon atoms. Note increasing size of pore aperture (largest diameter given in parentheses). [From Meier. W. M. Olson. D. H. Atlas af Zeolite Structure Types, 2nd ed. Buttcrworths London. 1987. Reproduced with permission.]...

In particular, Alberti et al. (1991) proposed zeolite-based sensors for detection of hydrocarbons such as butane and Balkus et al. (1997) used thin film aluminophosphate (AlPO)-5 molecular sieve as the dielectric phase in a capacitance-type chemical sensor for CO and CO. AlPO-n is a family of phosphorus molecular sieves which, similar to zeolites, have ordered molecular-sized pores. The AlPO-5 structure used for the dielectric layer consists of four- and six-membered rings of alternating phosphate and aluminum ions bridged by oxygen. These rings are arranged to produce one-dimensional channels 0.73 nm in diameter. The properties of AlPO-n are reviewed in detail by Ishihara and Takita (1996), and one of the attractive properties of these materials is their heat stability. The properties of zeolites as they relate to zeolite-based gas sensors are discussed in a special section in Vol. 2. 

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