Aluminum substitution

Figure 3. Geometry optimized aluminum substituted cluster with an electron hole Si4A10,6>2-...

Fig. 4.6 Protein adsorption model on aluminum-substituted mesoporous silica. Adapted from [37], A. Vinu et al.,J. Nanosci. Nanotechnol. 2006, 6, 1510.

In the structures cited in Table 12.3, except for pure silicon dioxide, metal ions are required for overall electrical neutrality. These metal ions are positioned in tetrahedral, octahedral, etc. positions in the silicate-like lattice. Sometimes they replace the silicon atom. Kaolinite asbestos has aluminum substituted for silicon in the Gibbosite sheet. Further, sites for additional anions, such as the hydroxyl anion, are available. In ring, chain, and sheet structures neighboring rings. 

IV. Yttrium basic carbonate and oxide on hematite. J. Coll. Interf Sd. 126 645-649 Ainsworth, C.C. Sumner, M.E. (1985) Effect of aluminum substitution in goethite on phosphorus adsorption II. Rate of adsorption. Soil Sd. Soc. Am. J. 49 1149-1153... 

Barron, V. Torrent, J. (1984) Influence of aluminum substitution on the color of synthetic hematites. Clays Clay Min. 32 157-158 Barron, V. Torrent, J. (1986) Use of the Ku-belka-Munk theory to study the influence of iron oxides on soil colour. Soil Sci. 37 499-510... 

Agresti D. G., ShelferT. D. (1992) Reflectivity (visible and near IR), Mossbauer, static magnetic, and X ray diffraction properties of aluminum-substituted hematites. J. Geophys. 

Schulze, D.G. (1982) The identification of iron oxides by differential X-ray diffraction and the influence of aluminum substitution on the structure of goethite. Dissertation, Techn. Univ. Miinchen, Freising-Weihenstephan, Germany. University Microfilms Int., Ann Arbor, Michigan... 

Schwertmann, U. Carlson, L. (1994) Aluminum influence on iron oxides XVII. Unitcell parameters and aluminum substitution of natural goethites. Soil Sci. Soc. Am. J. 58 256-261... 

G.M. De Grave, E. (2001) About the Morin transition in hematite in relation with particle size and aluminum substitution. Czech J. 

Using the cell volume-decomposition pressure correlation, one predicts that aluminum substitutions in LaNis should lower the hydrogen decomposition pressures by one order of magnitude for every two-A3 increase in cell volume (12). 

The output test using an aluminum block is performed in accordance with Test 303 of MIL-STD-331A. This test is identical in all respects with the steel dent test except that dent block is made of aluminum. Substitution of the softer metal allows testing of components whose output is insufficient to dent steel. ]... 

Here we report the synthesis and catalytic application of a new porous clay heterostructure material derived from synthetic saponite as the layered host. Saponite is a tetrahedrally charged smectite clay wherein the aluminum substitutes for silicon in the tetrahedral sheet of the 2 1 layer lattice structure. In alumina - pillared form saponite is an effective solid acid catalyst [8-10], but its catalytic utility is limited in part by a pore structure in the micropore domain. The PCH form of saponite should be much more accessible for large molecule catalysis. Accordingly, Friedel-Crafts alkylation of bulky 2, 4-di-tert-butylphenol (DBP) (molecular size (A) 9.5x6.1x4.4) with cinnamyl alcohol to produce 6,8-di-tert-butyl-2, 3-dihydro[4H] benzopyran (molecular size (A) 13.5x7.9x 4.9) was used as a probe reaction for SAP-PCH. This large substrate reaction also was selected in part because only mesoporous molecular sieves are known to provide the accessible acid sites for catalysis [11]. Conventional zeolites and pillared clays are poor catalysts for this reaction because the reagents cannot readily access the small micropores. 

The effects of the partial aluminum substitution on the resistivity dependence of x were also examined at 700 °C. The conduction mechanism of SrTi03 itself was a p—type in an atmospheric environment as stated previously, therefore, the enhancement of positive hole concentration will be expected by the partial substitution of aluminum. All specimens showed similar resistivity characteristics as shown in Figure 11. With increasing the additional amount of alumina, the resistivity of specimens tended to increase with de-... 

Although they are rare in sediments, there is a wide spectrum of trioctahedral expanded clays. The most common in the low-charge range (0.3—0.5) are hectorite, which contains magnesium and lithium in the octahedral sheet, and saponite, which has considerable magnesium in the octahedral sheet and some aluminum substitution in the tetrahedral sheet. 

Lewis Acid Sites. Many other mechanisms (66, 85) are best described in terms of the more general concept of aprotic, or Lewis acidity which is defined in terms of the capacity to donate or share pairs of electrons. Aprotic acid sites are commonly derived from the coordinatively unsaturated cations at crystal edges or adsorbed on crystal faces, from deydration of hydroxylated surfaces, and from deamination or deamination and dehydration of silica-aluminum catalysts or similarly treated clays having extensive tetrahedral substitution (130, 132). Formation of Lewis acid sites by deamination or deamination-dehydration is dependent on inversion of the basal oxygens of the aluminum-substituted tetrahedron away from the surface, in order to expose the aluminum (131). 

Note 2 -- Aluminum substitutes for sub-surface Ga atoms. For 0.5 monolayers second layer Ga atoms are replaced, for 1.0 monolayers second and third layer Ga atoms are replaced. Above 1.5 monolayers all near surface Ga atoms are replaced by Al, forming an epitaxial AlAs(llO) surface. In all cases the first interlayer spacing contracts by 0.10 A. 

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). 

The energetics of molecule sorption depend on the guest molecule or atom, the specific cations present in the framework, and the Si Al ratio. Frameworks with low levels of aluminum substitution and few nonframework cations have relatively low affinities for polar molecules. 

Nontronite is essentially a ferric, dioctahedral smectite with significant amounts of aluminum substituting for iron. A very general formula could be written as... 

Barron, V and Torrent, I (1984) Influence of aluminum substitution on the color of synthetic hematites. Clays Clay Min. 32 157-158. 

DeGrave, E., Bowen, L. H. and Weed, S. B. (1982) M5ssbauer study of aluminum-substituted hematites. J. Magnetism Magnetie Mat. 27 98-108. 

Stanjek, H. and Schwertmann, U. (1992) The influence of aluminun on iron oxides Part XVI Hydroxyl and aluminum substitution in synthetic hematites. Clays Clay Min. 40 347-354. 

Ainsworth, C. C., Girvin, D. C., Zachara, J. M., and Smith, S. C., 1989, Chromate adsorption ongoethite Effects of aluminum substitution Soil Science Society of America Journal, v. 53, p. 411-418. 

Perry EC Jr (1971) Imphcations for geothermometry of aluminum substitution in quartz from Kings Mormtairr, North Carolina. Contrib Mineral Petrol 30 125-128... 

Figure 6.14 Plot of mechanism of gallium-aluminum substitution. Reproduced with permission from [32], Copyright (1991) Royal Society of Chemistry...

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