Aluminium substitution

The omnipresence of aluminium in weathering environments results in most of the Fe oxides in soils, except lepidocrocite, being Al-substituted. The possible range of substitution as deduced from synthesis experiments (see Chap. 3) viz. up to Al/ (Fe Al) of ca. 0.33 in goethite and up to Al/(Fe Al) of ca. 0.16 in hematite is also found in soil goethites and hematites. Where the two oxides coexist on a small scale 

1991 Prasetyo Gilkes, 1994). Goethite has been found to contain about twice as much A1 as hematite in some cases (Schwertmann Kampf 1984 Singh Gilkes, 

1992 Da Motta Kampf, 1992), but no correlation has been found in others (Anand Gilkes, 1987 Zeese et ah, 1994). The ratio may depend on whether or not the two oxides were formed simultaneously in the same environment. Maghemites from tropical soils contained A1 up to an Al/(Fe -i- Al) ratio of ca. 0.15 as indicated by chemical analysis and reduction in unit cell size (Schwertmann Fechter, 1984 Fontes Weed, 1991). 

Owing to their extremely low solubilities in an aerobic environment, goethite and hematite remain unchanged over geological time spans. They may, therefore, store information about the environment in which they formed. Al substitution may be one such piece of information. Thus, medium to high Al substitution has been observed in goethites from tropical and subtropical soils, bauxites and saprolites (Fitzpatrick Schwertmann, 1982 Schwertmann Kampf, 1983 Curi Franzmeier, 

Sample group n Range of substitution mol moir Intercept nm Slope nm Reference 

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Jonas, K Solymar, K. (1970) Preparation, X-ray derivatographic and infrared study of aluminium-substituted goethites. Acta Chim. 

Murad, E. Schwertmann, U. (1983) The influence of aluminium substitution and crystallinity on the Mdssbauer spectra of goethite. Clay Min. 18 301-312... 

Wolska, E. (1990) Studies on the ordered and disordered aluminium substituted maghemites. Solid State Ionics 44 119-123 Wolska, E. Szajda.W. Piszora, P. (1992) Determination of solid solution limits based on the thermal behaviour of aluminium substituted iron hydroxides and oxides. J. Thermal Analysis 38 2115-2122 Wolski.W. (1985) Das Eisenoxidgelb. Farbe Lack 91 184-189... 

Hara and Inoue (H61) studied the lattice parameters of tobermorites. They confirmed an early observation (K62) that c increases with aluminium substitution, but found that several other variables affect the parameters to smaller extents. The a-axial lengths of highly crystalline, natural tobermorites appear to be lower (0.560-0.562 nm for the pseudocell) than those of the synthetic materials (M5I). 

G. Ricchiardi and J. M. Newsam,/. Phys. Chem. B, 101, 9943 (1997). Predicted Effects of Site-Specific Aluminium Substitution on the Framework Geometry and Unit Cell Dimensions of Zeolite ZSM-5 Materials. 

Ldwenstein s Rule and have both aluminium substitutions in the same D6R unit. A filled circle is aluminium on the upper ring, while an open circle is an aluminium on the lower ring. The silicon sites are numbered clockwise with site 7 directly behind site 1. 

Configurations with both aluminium substitutions on the different rings... 

Schwertmann, U. and Fechter, H. (1984) The influence of aluminium on iron oxides. XI. Aluminium-substituted maghemite in soils and its formation. Soil Sci. Soc. Amer. J. 48 1462-1463. 

In an effort to probe catalytic sites and their stability, the zeolite catalyst, ZSM-5 was Investigated by impedance and fourier transform infrared spectroscopies as a function of aluminium substitution and cation exchange. Samples were provided by Chemistry Division, DSIR, with (Si + A1)/A1 ratios of ao, 1000, 500, 200, 136 and 40. Crystallite size and morphology varied somewhat with aluminium content but typically the samples had crystal size distributions in the range 0.2 pus to 2 (im. 

Due to their particular catalytic properties, iron- and iron-aluminium-substituted zeolites are especially interesting for application in oxidation processes. In this paper the preparation procedure and the adsorption properties of Fe-Al-BEA and Fe-Al-MOR zeolites are described and the results are compared with the data obtained for the corresponding aluminium forms of both zeolites. 

Vetrivel, Catlow and Colbourn. A small cluster treated explicitly by ab initio molecular orbital methods was embedded in an 82-ion point ion block chosen so that the Madelung potential and electric field reproduced those from static lattice calculations. The effect of aluminium substitution on the proton binding energy was determined, and found to be stronger than when Al was present in the framework. 

The importance of this study is that it showed the existence of unique i.r. bands corresponding to the vibrations of the D6R and D6R units> and also investigated the effect that aluminium substitution has on the position of these bands. It was shown that the vibrational frequencies increased upon greater aluminium substitutionf and that this could be correlated with the fraction of aluminium in the structure. 

Derouane and Fripiat have also looked at the effect of aluminium substitution on the stability of two zeolite structures, mordenite and ZSM-5. In contrast to the other studies, much larger clusters (5T atoms), and the actual atomic positions from the x-ray studies have been used. This is in response to the suggestion that the different atomic positions may not be equivalent. By centring the cluster on different atomic positions, and by changing the other atoms correspondingly, it was shown that the most stable position of aluminium atoms in mordenite is at the A-rings. The relative stabilities of all the other positions were also evaluated, and were found to be at least 10 kcal/mol less stable. Using the same methods it was also possible to evaluate the most probable position... 

Suzuki, K., Namikawa, T. Yamazaki, Y. (1988). Preparation of zinc- and aluminium-substituted cobalt-ferrite thin films and their Faraday rotation. Japanese Journal of Applied Physics, 27, 361-5. 

Particular case of aluminium, substituting simnltaneonsly Ca and Si" in alite lattice is possible becanse the ionic radii ratio of Al and eqnal 0.36 (51/140) is... 

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