Boehmite hydrolysis

Lewis, D.G. Cardde, C.M. (1989) Hydrolysis of Fe(III) solution to hydrous iron oxides. Aust. J. Soil Res. 27 103-115 Lewis, D.G. Farmer,V.C. (1986) Infrared absorption of surface hydroxyl groups and lattice vibrations in lepidocrodte (y-FeOOH) and boehmite (y-Al-OOH). Clay Min. 21 93-100... 

Pawlowski and Schartel92 have added 1 or 5 wt % of boehmite to blends of PC/ABS with PTFE and RDP or bisphenol A bis(diphenylphosphate). The release of water from AlOOH influences the decomposition of the material by enhancing the hydrolysis of PC and RDP. Consequently, the condensed action of RDP or BDP is perturbed. The reaction of the arylphosphate with boehmite replaces both the formation of anhydrous alumina and alumina phosphate on the one hand, and the cross-linking of arylphosphate with PC on the other hand, since less phosphate is available to perform condensed-phase action. The reaction with arylphosphate therefore decreases the char formation, but the formation of aluminum phosphate could enhance barrier properties. On the whole, even high levels of fire retardancy can be achieved (V-0 ratings) the combination of boehmite with arylphosphates acting in the condensed phase seems very complex, particularly when the host polymer can undergo hydrolysis reactions due to water release. 

The siloxides of aluminum can be prepared in the same maimer as the alkoxides and exhibit similar stractural features. Interest in these compounds arose from a desire for sol-gel or polymeric precursors for aluminosilicates. Unfortunately, these compounds extensively hydrolyze, losing most of the silicon as the silanol. However, the investigation of the hydrolysis of (Et3SiO)3Al has generated considerable insight into the stracture and composition of aUcoxy- and siloxy-substitutedalumoxanepolymers, (ROAIO) These macromolecules are now believed to have a structure related to that of boehmite. 

Gamma-alumina membranes were the first and most investigated mesopor-ous membranes to follow the colloidal preparation method. Based on a sol-gel process developed by Yoldas [13], a boehmite sol can be prepared by hydrolysis... 

Alumina from the hydrolysis of alcoholates is typically obtained in the form of boehmite or pseudoboehmite. It is important to mention that both processes give products of equivalent quality. 

Boehmite type aluminas from hydrolysis of aluminum alkoxides are typically not fully crystallized. This is reflected in a broader X-ray pattern as well as in an overstoihcimetric water content. These so-called pseudoboehmites are of the same structure as boehmite, but have additional water incorporated in the crystal. There are different theories about the exact structure of pseudoboehmite [7]. However, the widely accepted model is that the additional water molecule are located between the boehmite layers (figure 6). The amount of additional water uptake can be adjusted by the processing conditions and determines the properties of an alumina to a great extent. 

Figure 3 SEM micrographs of the boehmite coating on the zirconia surface precipitated using AIN powder hydrolysis at 90 °C (a) after deposition (b) after heat treatment at 900°C in air.

The second example is the synthesis of -y-alumina nanorods from boehmite nanofibers using a modified sol-gel process (42). First, a solution of aluminum iso-propoxide in anhydrous ethanol is prepared. To this, ethanol with 4% water is added leading to a viscous liquid after 15 h. The viscous hquid heated at 600°C leads to y-alumina nanorods (diameter < 10 nm length 50 to 200 nm). Since in the synthesis, less water is taken, only partial hydrolysis takes place. The removal of one water molecule from two A10(0H) octahedra leads to the formation of AI2O3 nanorods, (CH3CH2CH20)3A1 + 2H2O A10(0H) + 3CH3CH2CH2OH. 

Aerogels are materials prepared by supercritical drying of gels produced by hydrolysis ofalkoxides (14). High-surface-area boehmite (75,76) or pse-udoboehmite (77) can be prepared by this method, and these materials can subsequently be transformed into high-surface-area aluminas. 

Commercial boehmites supplied by Condea GmbH (synthesis from hydrolysis of alkoxides) Boehmite gels synthesized according to Ref. 5 by neutralization of A1(N03)3 with NH4OH. Peicent relative to the highly crystalline boehmite P-200, assumed as 100% crystalline (0% amorphous). 

The formation of the sol can be a critical part of the process. While boehmite and the amorphous hydroxide prepared by cold water hydrolysis can be peptized to a clear sol, bayerite will not form a sol and its formation during hydrolysis should therefore be avoided. In addition, the nature of the acid has a significant effect on the peptization step. Table 5.8 shows the peptizing effect of various acids on the precipitate formed by hydrolysis of aluminum s ec-butoxide. The results are similar when aluminum isopropoxide is used. It appears that only strong or fairly strong acids, which do not form chemical complexes (or form only very weak complexes) with aluminum ions, are effective for achieving peptization. For these acids, the concentration of the acid also has an effect. Peptization... 

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