Al substitution

As structural Al increases, the dehydroxylation temperature, the OH stretching frequency and both OH bending vibrations as well as their separation (50H - yOH) 

The magnetic hyperfine field (Bhf) decreases as structural Al increases. This is to be expected because the diamagnetic Al does not contribute to the magnetic field. Since, however, Bhf (T) is also reduced as the crystal size decreases (even at 4 K), both Al substitution and crystal size (here represented by mean coherence length perpendicular to (111), MCLm, in nm) must be combined to account for the variation of Bhf satisfactorily, (Schwertmann Murad, 1983) i.e. 

Al-for-Fe substitution in natural goethites was originally discovered, with the aid of X-ray diffraction, in marine, oolithic, iron ores from the Jurassic era by Correns von Engelhardt as early as in 1941 and twenty years later found in soils by Norrish and Taylor (1961). Since then, a large number of studies has revealed that Al substitution in goethites from the weathered zone, e.g. in soils (see chap. 16), appears to be the rule rather than the exception. It should be noted that Al located in the struc- 

Oxidation of FeCl2 solutions at pH 7-8 in the presence of soluble Al led to Al substituted lepidocrocite with an Al/(Fe-i-Al) of up to 0.14 the a, h and c unit cell edge lengths fell linearly as the Al content increased (Schwertmann Wolska, 1990). The Bhf at 4.2K also decreased linearly over the same range of Al substitution, i. e. 

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In the most economically efficient route to indoles reported this past year, Al-substituted anilines 68 were reacted with triethanolamine in the presence of a homogeneous ruthenium catalyst to give the corresponding 1-substituted indoles 69 <96SC1349>. 

The high catalytic activity also enabled aza-Claisen rearrangements to form Al-substituted quaternary stereocenters (Fig. 26) [71]. The catalyst does not need to distinguish between differently sized substituents on the double bond of 49 (e.g., R = CDa, R = CHs, ee = 96%), indicating that coordination of the olefin is the stereoselectivity predetermining step. The imidate-N-atom subsequently attacks intermediate 47-1 from the face remote to the Pd-center totally resulting in a... 

Two types of imidazolium salts must be distinguished depending on the M.Al -substitution symmetrically (R = R ) and unsymmetricaUy substituted (R R ) versions. For the first type (which is also the predominant one, see Fig. 1.4), two main strategies are viable reaction of imidazole with RX in the presence of a base... 

The guest molecules experience different potential depending on the nature and the spatial distribution of the ions and the structural modifications in the aluminosilicate framework associated with the Si-Al substitution. Accordingly, the diffusive process can be different [1], The efficiency of migration of guest molecules depends on several factors the Si/Al ratio, the nature of the extra framework cations, the presence of sorbed water molecules, the temperature, and the sorbate concentration [1]. 

Layered nickel hydroxide can be used as an electrode for alkaline secondary cells. To improve its properties, modification has been carried out by incorporation of other metal elements to form Ni/M LDHs, including Co [221], Zn [222], Al [223], Cr or Mn [224] and Fe [225]. For example, Chen et al. reported the electrochemical performance of Al-substituted layered a-... 

KemI (2000). Non-toxic Environment. See www.kemi.se Lohse, Joachim et.al. Substitution of Hazardous Chemicals in Products and Processes, Report compiled for the Directorate General Environment, Nuclear Safety and Civil Protection of the Commission of the European Communities, Contract No. B3-4305/2000/293861/MAR/E1,... 

Bossio R, Marcaccini S, Pepino R, Polo C, Torroba T (1992) Synthesis of Al-substituted 2-alkyl-4-aryloxazole-5-carboxamides. Org Prep Proced Int 24 188-191... 

Fig. 3.4 Electron micrographs of goethites with Al substitution increasing from 0 to 0.167 mol mol" (produced from 2-line ferri-hydrite in alkaline solution at 70°C.

Fig. 3.9 Effect of Al-substitution in synthetic hematites on (Left) the unit cell edge length a of hematites synthesized at various temperatures (Stanjek Schwertmann, 1992, with permission), and (Right) the magnetic hyperfine field Bhf of hematites formed at 70 °C and 1000°C dotted lines indicate 95% confidence limits (Murad Schwertmann 1986 with permission).

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