Aluminium oxyhydroxide

Direct methods for determining the combinational form of an element or its oxidation state include infrared absorption spectrometry, X-ray diffraction and, more recently, electron paramagnetic resonance - nuclear magnetic resonance -and Mossbauer spectrometry. With such techniques the combinational forms of major elements in soil components such as clay minerals, iron, manganese and aluminium oxyhydroxides and humic materials and the chemical structures of these soil components have been elucidated over the past 50 years. These direct, mainly non-destructive, methods for speciation are dealt with in some detail in Chapter 3 and are not further discussed here. 

A key mineralogical problem with aluminates is the question of cation disordering in spinels. Numerous Al MAS NMR studies have been carried out on MgAl204. One O MAS NMR study of spinels quenched from between 700 and 1400°C showed an increase in disorder with temperature. Both the and Al MAS NMR spectra showed a similar increase in disorder but the value determined from O NMR was systematically lower (Millard et al. 1992). O MAS has been used to monitor the solid state NMR reaction of sol-gel formed LaAlOs (Figure 6.22). Heating to 450°C produced O signals from LaO(OH) and an aluminium oxyhydroxide, but peaks from... 

Several authors [1, 18-20] have surveyed the chemical and crystallographic changes which occur during the dehydroxylation of the three forms of aluminium hydroxide, (Al(OH)3 gibbsite (y), bayerite (a) and norstrandite) and the two forms of aluminium oxyhydroxide, (AlOOH diaspore (a) and boehmite (y)). These phases... 

Section 1 considers the methods of synthesis and physico-chemical properties of new types of inorganic sorbents (complex carbon-mineral sorbents, co-precipitated hydroxides, functional polysiloxane sorbents, porous glasses with controlled porosity, colloidal silicas, aluminium oxyhydroxide colloids, apatites). These sorbents are widely used in scientific investigations, in chemical practice and are important from a technological point of view. The presented results provide additional possibilities for the preparation of inorganic sorbents possessing unique adsorption and catalytic properties. Moreover, Section 1 presents the possibilities of the computational studies on the design of synthetic materials for selective adsorption of different substances. 

When considering results for AU/AI2O3 catalysts, it has to be remembered that the oxides, oxyhydroxides and hydroxides of aluminium can exist in many crystalline forms and can have a wide range of surface areas, these can both change in response to pretreatments, structure, surface area and changes during preparation are not always reported. There has been no systematic study of the importance of these variables on catalytic activity. 

Hydrous Oxides. This term is generally taken to include the oxides, hydroxides, and oxyhydroxides of aluminium, iron and manganese, which form in soil when these elements are released from primary minerals by weathering. They exist mainly as small particles in the claysized fraction of a soil (<2 pm), and also as coatings on other soil minerals or as components of larger aggregates. 

The most widespread of the secondary minerals formed during the development of a laterite weathering profile are iron and aluminium ses-quioxides (Table 3.1). These may form either directly from the alteration of primary minerals, or else via a series of pathways involving the formation of intermediary sheet silicate minerals and clays (e.g. chlorite, illite, smectite, vermiculite and halloysite), which are then themselves broken down, stripped of their mobile ions and silica, and eventually converted to alumina and ferric oxyhydroxide residua (Figure 3.9). It is not possible to describe these mineral transformations in detail, but the key issue is that under tropical-type weathering conditions these transformation pathways lead to... 

Only two simple oxides have the ability to become microporous alumina and silica. Alumina exists in a number of different phases, derivable by calcination from different oxyhydroxides, the nature of which depends upon the procedure for precipitation form solution of an aluminium salt. The most commonly used forms of alumina are the y (area 100 m g ) and the low-area a (l-10 m g ),formed by high-temperature calcination. The chemistry of silica is simpler amorphous silica can be made in a number of ways, with areas in the range 500-600 m g . Several low-area forms occur naturally, or may be obtained by high-temperature calcination (quartz, cristobalite). 

The synthesis of silicates that contain metal cations other than aluminium in framework positions results in solids with modified adsorptive and catalytic properties. The criteria for successful incorporation of cations into tetrahe-drally coordinated silicate frameworks are that they should exhibit solubility under synthesis conditions without the formation of an insoluble oxide or oxyhydroxide and that the substituting species should be able to adopt tetrahedral coordination. Preparation of a phase that is shown to contain other metals (by selected area chemical analysis in an electron microscope, for example) is no guarantee that the metal has adopted a framework site. Physicochemical methods must be used to determine whether this is the case. This is complicated when the metal adopts more than one site or is present at very low levels. Confirmatory evidence for framework substitution may be available from unit cell size determination (substitution of a cation larger than Si" will in general result in an increase in unit cell dimension), and determination of coordination geometry by NMR, UV-visible and EXAFS spectroscopies (Chapter 3), which are able to distinguish whether the metal is within the... 

Boehmite (y-AlOOH(s)) becomes the stable solid oxyhydroxide phase of aluminium at a temperature of around 100 C. However, solubiUty data at zero ionic strength are available to lower temperatures (on the basis of the reported temperature dependence of the solubility). The relationship between the solubility constants and temperature is illustrated in Figure 13.2. As was the case with gibbsite. 

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