Crystalline hydroxides

Release of water from the crystalline hydroxides (dehydroxylation) differs from the dehydration of a crystalline hydrate (Sect. 1) in that product release must be preceded by chemical interaction between anions. 

The name activated alumina is generally applied to an adsorbent alumina (usually an industrial product) prepared by the heat treatment of some form of hydrated alumina (i.e. a crystalline hydroxide, oxide-hydroxide or hydrous alumina gel). It has been known for many years that certain forms of activated alumina can be used as powerful desiccants or for the recovery of various vapours. It was apparent at an early stage that the adsorbent activity was dependent on the conditions of heat treatment. For example, in 1934 Bayley reported that the adsorption of H2S by a commercial sample of activated alumina was affected by prior heating of the adsorbent at different temperatures, the maximum uptake being obtained after heat treatment at SS0°C. During an investigation of the catalytic dehydration of alcohols, Alekseevskii (1930) found that a calcination temperature of c. 400°C was required to optimize the adsorption of the alcohol reactants, whereas calcination at 600°C was preferable for the adsorption of the olefine products. 

The properties of calcined AI2O3 (particle size and shape, specific surface area, reactivity) also depend to a certain degree, particularly at the lower calcination temperatures (up to about 1400 °C), on the type of the initial hydroxide and on the sequence of the structural transformations. Apart from the gel-like form arising by precipitation of aluminium salts, the following four crystalline hydroxides are known ... 

In the aqueous solutions of metal salts commonly used for the preparation of oxides, metal ions are present mostly in the form of hydrated ions [M(H20)/0H)J + or complexes with other compounds occurring in the solution. Using radiation, in the process of their conversion to oxides two basic simultaneous reactions are employed—change of the metal oxidation state (most often radiation reduction) and/or precipitation of insoluble compounds due to the formation of the precipitant. Both reactions are initiated and controlled by the radiation absorbed in the aqueous solution as a result of the process, solid phase is formed in the solution. According to the solid phase composition, the preparations may be classified into two main categories—direct and indirect formation of oxides. Direct oxide formation means that after drying the solid phase consists of (nano)crystalline oxide phase, whereas indirect formation encompasses the formation of precursors to oxide(s). These precursors include amorphous or crystalline hydroxides, oxide hydroxides, carbonates, or basic carbonates. Oxides are obtained by calcination of the precursors in the oven, similar to other oxide preparation techniques. 

The chemistry of the precipitation of aluminum hydroxides and oxides is very complex (40). When the solubility is exceeded, gelatinous precipitates, which are found to be amorphous by X-ray diffraction, usually form initially. Al MAS NMR shows the predominance of octahedraUy coordinated Al ions in these amorphous hydroxides, as are present in the crystalline trihydroxides and oxyhydroxides. However, in the amorphous materials some pentacoordinated and tetracoordinated Al ions are also found (15). As discussed above, there are many different crystalline hydroxides or oxyhydroxides, and which of them will be formed depends on the conditions (4f). Primary factors are temperature and pH, as well as aging time however, the nature of the anions present and the possible presence of organic components (42,43) also play a role. At low temperature in an excess of water, the hydroxides are preferentially formed, specifically bayerite at pH values between 5.8 and 9 or gibbsite for pH values smaller than 5.8 or larger than 9. 

AH of these , values are cited as basic potentials (pH 14) in Fig. 17.2. From estimated activity products (e.g. for Ac(OH)3) and appropriate cycles, the free energies of formation of the crystalline hydroxides and of Np(vii) aquo ions were calculated (Table 17.14). 

This solubility constant is also retained. As expected, the crystalline hydroxide is less soluble than the amorphous phase. Moreover, the solubility constant for NpjOgCs), as determined by Pan and Campbell (1998), of log =3.91 0.20, is less soluble again. However, Lemire et al. (2001) selected an enthalpy for Np205(s) of = -(2162.7 9.5) kj mol . When a Gibbs energy value is determined from the solubility constant of Pan and Campbell (1998) and combined with the enthalpy selected by Lemire et al. (2001), an entropy that is much too negative is derived. As such, the solubility constant of Pan and Campbell is not retained in the present review. The solubility constant derived when using the entropy and enthalpy selected by Lemire et al. (2001) is... 

Savenko, V.S. (1998) Stabihty constants of M(0H)3° hydroxo complexes from their solubility products of the corresponding crystalline hydroxides. Russ. J. Inorg. Chem., 43, 461 -463 (English translation). 

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