Transitional Alumina Polymorphs

A recent investigation by Whittington and IHevski (2004) charted somewhat different pathways of the thermal dehydration of gibbsite. At the high heating rates and temperatures relevant to the Bayer process, the sequence gibbsite X Y O— a was found. 

Recent studies have indicated that the mechanical activation of bayerite by soft grinding accelerates the phase transition to stable a-A Os. The onset of phase transition occurs in the presence of a-A Os seeds already at 300 °C, some 800 °C lower than in the non-ground state. The transition sequence is  

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I 7 Oxide Ceramics Structure, Technoiogy, and Appiications 7.1.3.2 Transitional Alumina Polymorphs... 

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 transition aluminas contain varying amount of hydroxyl ions and are not well defined stoichiometrically therefore, they cannot be considered as true polymorphs of alumina. The structures of the forms occurring at a given temperature range are determined by the structure of the starting materials, and they are different for gibbsite, bayerite, boehmite, or diaspore. The microcrystalline state of these substances and the poor character of their X-ray diffraction pattern preclude the application of refined methods of structure determination. 

Levin, I. and Brandon, D. (1998) Metastable alumina polymorphs crystal structures and transition sequences. J. Am. Ceram. Soc., 81 (8), 1995-2012. 

McHale JM, Auroux A, Perrotta AJ, Navrotsky A (1997) Surface energies and thermodynamic phase stability in nanocrystalline aluminas. Science 277 788-791 Molteni C, Martonak R, Parrinello M (2001) First principles molecular dynamics simulations of pressure-induced stiuctural transformations in silicon clusters. J Chem Phys 114 5358-5365 Murray CB, Norris DJ, Bawendi MG (1993) Synthesis and characterization of nearly monodisperse CdE (E = S, Se, Te) semiconductor nanocrystallites. J Am Chem Soc 115 8706-8715 Onodera A (1972) Kinetics of polymorphic transitions of cadmium chalcogenides under high pressure. Rev Phys Chem Japan 41 1... 

The published data concerning the structural, surface, and catalytic properties of aluminas are reviewed, and these properties are related to the preparation procedures. The experimental and computational investigations of the structural characteristics of the polymorphs most useful for applications in catalysis, which are y- q-, 8- and 0-AI2O3, are critically analyzed. The thermodynamics of the various polymorphs and the kinetics of the phase transitions are considered. The available information on Bronsted sites (i.e., hydroxyl groups), Lewis acid sites, and acid-base pairs on... 

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... 

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