Boehmite phase transformation

The sequence of phase transformations shown in Figure 2 is an approximationp largely because process variables such as time, atmosphere and properties of precursor hydroxides are not included. Thus, for example, bayerite and glbbslte may be converted to boehmite and thence to y-alumina during calcination if the particle size Is large and the precipitate Is moist [18]. 

The inherent stability of aluminas can be further improved by addition of other oxides (ref. 32). Base metals can act as promoters and in an ideal situation would fulfill a dual role. Fig. 3 shows the change in surface areas for boehmite derived activated aluminas as a function of temperature. It may be seen that addition of barium retards phase transformation and consequent loss of surface area to well above 1000°C. 

Fig. 2. Surface area thermal stability and phase transformations for transitional aluminas derived from Gibbsite and Boehmite.

Meanwhile this study reveals that a much shorter dwell time was needed to form a-alumina from the gel powder than the boehmite powder at 1000 °C. This is possibly due to the difference in the crystallinity and the morphology of the gel powder and the boehmite powder. The gel powder is amorphous, while the boehmite is nanocrystalline with several broad peaks in the XRD pattern. Amorphous state is less stable than the crystalline phase, therefore the activation energy required for the transformation to a-alumina will be lower for the amorphous gel powder. Meanwhile, the flaky morphology of the boehmite makes less available contact points which might help to retard the possible diffusion controlled reaction leading to grain growth followed by phase transformation. ... 

The crystalline phase transformation of alumina is shown in Figure 6.14. Transformation is also dependent on the gaseous atmosphere, as well as the other conditions, as detailed in Figure 6.14. In this figure, gibbsite and bayerite are trihydroxide, Al(OH)3, or AI2O3.3H2O. Boehmite and diaspore are AlOOH. 

Structural and morphological changes accompanying these phase transformations have been investigated by numerous researchers employing such methods as TEM [190,194-197], XRD (see, e.g., refs. [193-195]), Al MAS NMR [198], DTA/TGA [192,193] and gas adsorption-condensation [47,199]. Several TEM and XRD investigations have shown that boehmite... 

Having modeled the two phases, we now model the transformation from one phase to the other phase. Express the quasi-chemical reaction of change of the boehmite phase into the y-alumina phase by taking into account defects of each phase. Consider that there is a univariant system. Calculate the equilibrimn constant of this change of the phase as a function of water pressure and the equilibrium constant of boehmite (constants Xa and can be computed by extrapolation). 

Previous work (13,14) has shown that the glycothermal synthesis process, a liquid phase precipitation at elevated temperatures under autogeneous pressure using a glycol as solvent, for a-Al203 is mediated by the formation of a precursor pseudo-boehmite phase (15). The pseudo-boehmite is thermodynamically unstable under the reaction conditions, and transforms to the stable a-Al203 (similar to the formation of alumina in the hydrothermal system (16)). Thus, the overall reaction is as follows. 

Liquid phase modifications. Alternatively a porous membrane can be reduced in pore size by a liquid deposition prcx ess where the membrane is dipped into a solution or sol to form deposits inside the membrane pores. For example, a silicon nitride tube with a mean pore diameter of 0.35 pm is first immersed in a solution of aluminum alcoholate (aluminum isopropylate or 2aluminum tris(ethyl acetoacetate) or ethyl acetoacetate aluminum diisopropylate) in an organic solvent (hexane, cyclohexane, benzene, isopropanol, etc.). It is then treated with saturated water vapor to hydrolyze the alcoholate or chelate to form bochmite inside the pores, thus changing the pore diameter to as small as 20 nm [Mitsubishi Heavy Ind., 1984a and 1934b]. Upon calcining at 800X, boehmite transforms into transition-alumina. 

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