Stable Alumina Polymorphs

Under ambient conditions, only two stable modifications of AI2O3 exist. The thermodynamically stable polymorph between room temperature and the melting point at 2050 °C is a-AhOs (corundum), which forms a lattice ofhexagonally close-packed oxygen atoms with stacking order AB-AB. in which two-thirds of the 

Recently, the structure of gibbsite was re-evaluated by Balan et al. (2006) using ah initio density functional theory (DFT) calculation with local density approximation (LDA), and the following unit ceU constants were reported a = 874.2 pm, b = 511.2pm, c = 980.1pm, P = 94.54° S.G. P2j/n (14). 

The so-called P-AI2O3 is, strictly speaking, not an alumina polymorph but rather an alkali aluminate (see Section 7.1.4.2). 

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To explore this question, the adsorption of glycine on AlOOH was recently investigated at the interface with water [78, 97]. This study confirmed the occurrence of an inner sphere Al-O-C-0 bond that was predicted from calculations at the interface with vacuum, and from numerous experimental studies of carboxylic acid adsorption on alumina polymorphs (see Ref. [78] and references therein). It was found that inner sphere adsorption (shown in Fig. 5.14, right), with -161.6 and -113.6 kJ moF for the anionic and zwittetionic species, respectively is significantly more stable as compared to outer sphere adsorption (-20.5 kJ mol (Fig. 5.14, left)). Beyond the result itself, it is important to note that complex events can now he handled with ab initio methods because they are able to model aU types of forces in reasonable agreement with experiment. 

Due to recent developments in synthesis, the preparation of nanocrystalline polymorphs, which are usually unstable as bulk phases, has been achieved for several materials such as ZrC>2, Ti02 and various perovskites. The appearance of these exotic materials does not necessarily mean that they are thermodynamically stable, since the kinetics (templates and surfactants) are probably more important for the processes than the thermodynamics. Adsorption of water may also play an important role as in the case of alumina, but in the data given in Figure 6.19 the effect of water has been accounted for [25]. 

Alumina is a widespread component of siliceous minerals. It occurs as single crystals in the form of sapphire, and with chromium impurity as ruby, and in large deposits as the hydrated oxide bauxite (A1203-2H20). The dehydration of this and other hydrated oxides at temperatures below 1000°C leads to the formation of y-Al203 which is converted to a-Al203 above 1000 °C. The transformation is irreversible and the a-polymorph is stable from absolute zero to its melting point at 2050 °C. 

Polymorphism of Ferric oxide.—The yellow colour in certain bricks is stated to be due to a yellow modification of anhydrous ferric oxide rendered stable by alumina.6... 

Hydrothermal synthesis of a-alumina has been well studied. Since the hydro-thermal reaction of aluminum compound yields boehmite at relatively low temperatures (approximately 200°C), transformation of boehmite was examined and it was reported that more than 10 hours is required for complete conversion into a-alumina, even with a reaction at 445°C in a 0.1 N NaOH solution and in the presence of seed crystals. On the other hand, under glycothermal conditions, a-alumina is formed at 285°C for 4 h. The equilibrium point between diaspore (another polymorph of AlOOH) and a-alumina under the saturated vapor pressure of water was determined to be 360°C. However, near the equilibrium point, the transformation rate is very sluggish, and only a small conversion of diaspore is observed. Therefore complete conversion of diaspore into a-alumina requires a much higher temperature. Since boehmite is slightly less stable than diaspore, the hypothetical equilibrium point between boehmite and a-alumina would be lower than that for diaspore-alumina. However, a-alumina would not be formed by a hydrothermal reaction at such a low temperature as has been achieved in the glycothermal reaction. 

In Chapter 3, Busca summarizes the current state of knowledge of aluminas, the various polymorphs of which constitute some of the most commonly used catalyst components. The author starts with a discussion of the bulk structures of transition aluminas, which are the intermediate phases formed in the thermal transformation of aluminum oxyhydroxides into the thermodynamically most stable modification, a-alumina. Crucial are the definitions of the various phases, which are based on the methods of preparation rather than on the structural properties. The understanding of many alumina structures is incomplete, and progress, even with modem analytical methods and theory, is hampered by the defective and disordered nature of these materials. The stabilities of the various phases are governed by both thermodynamics and kinetics, either of which can be affected by impurities. The uncertainties in the surface stmctures are even greater than those of the bulk stmctures. Numerous models of alumina surface stmctures have been formulated over decades, but the tme stmctures seem to become even more elusive. Busca concludes his chapter with a list of research needs. 

The structural evolution of the nanocomposites upon thermal treatment is very interesting, as formation of the alloy takes place concurrently with crystallization and phase transition of the alumina matrix. Pure AI2O3 aerogels obtained by this procedure exhibited a layered pseudo-boehmite structure (in particular, the ethyl derivative of boehmite was formed as a consequence of esterification during supercritical drying), which upon thermal treatment was converted to nanocrystalline 7-AI2O3, and finally at 1,000°C, the thermodynamically stable polymorph a-Al203 started to crystallize. 

Thus the form of the shrinkage curve of the compacts from the nanopowders of the metastable forms of alumina is determined by the processing of the polymorph transitions of alumina to stable a-AbOs. At the staring stage (t < Ta) the material consists of the y, 5, and 0 phases mixture. The polymorph transition (y + 8 + 0) a starts at Ta temperature and can... 

Commercially, the silica-alumina system is an important one because the principal constituents of many ceramic refractories are these two materials. Figure 12.25 shows the Si02-Al203 phase diagram. The polymorphic form of silica that is stable at these... 

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