Boehmite pseudoboehmite

Alhydrogeh, aluminium hydroxide adjuvant aluminium oxy-hydroxide poorly crystalline boehmite pseudoboehmite Rehydragel. 

Aluminium substrates are frequently pre-treated by anodic oxidation to generate a porous surface, which may serve as the catalyst support itself or as an adhesion layer for a catalyst support [122]. The surface area of the obtained alumina layer may reach 25 m g , with a thickness of 70 pm or higher. Assembled alumina micro-channel reactors can also be oxidised. The layer generated by anodic oxidation of aluminium and aluminium alloys is amorphous hydrated alumina. It is believed to contain boehmite, pseudoboehmite and physically adsorbed water [123]. It has the morphology of a packed array of hexagonal cells, each containing a pore in the centre. Thus the layer has a highly ordered porous structure and uniform thickness, when an... 

Stmctural order varies from x-ray indifferent (amorphous) to some degree of crystallinity. The latter product has been named pseudoboehmite or gelatinous boehmite. Its x-ray diffraction pattern shows broad bands that coincide with the strong reflections of the weU-crystallized boehmite. 

In addition to gibbsite there are other routes to manufacture Al(OH>3 and the consecutive transition oxides. One is the precipitation of Al(OH)3 from aluminum salts by adjusting the pH between 7 and 12 by adding bases. Precipitation at elevated temperatures and high pH leads to formation of bayerite, whereas at lower pH pseudoboehmite and subsequently boehmite are formed. By heating, these materials can be converted to the active transition aluminas. 

The procedure for the preparation of alumina spheres is as follows. Pseudo-boehmite powder (AIOOH.2H2O) is dispersed in an aqueous solution of urea and a monovalent inorganic acid, e.g. HNO3. The type of powder or powder mixture may exert considerable influence on the properties of the sol and the end product [12]. The type of acid which is used for the peptization of the pseudoboehmite powder is not very important [12], so nitric acid is used, being the most suitable one. 

Boehmite is of considerable interest to the surface scientist. It was pointed out by Lippens and Steggerda (1970) that a clear distinction should be made between crystalline boehmite and the gelatinous forms of pseudoboehmite, which always contains some non-stochiometric, interlamellar water. Pseudoboehmite is the main constituent of European bauxites and can be easily prepared by the neutralization of aluminium salts, but hydrothermal conditions are required for the formation of crystalline boehmite. 

Even though the differences between the Al NMR spectra of the transition aluminas are subtle, the technique has been used to smdy the thermal transformation sequences of the hydrated aluminas gibbsite, Al(OH)3 (Slade et al. 1991, Meinhold et al. 1993), boehmite, 7-AIOOH (Slade e a/. 1991a, Meinhold era/. 1993, Pecharromm eta/. 1999), pseudoboehmite (Meinhold et al. 1993) and bayerite, Al(OH)3 (Meinhold et al. 1993, Pecharroman et al. 1999). 

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. 

The initial step of the process is the formation of aluminum triethyl from aluminum metal, ethylene and hydrogen. In a second step ethylene is added to the aluminum triethyl causing the carbon chains to grow in increments of two carbon atoms. After the chain growth reaction the aluminum alkyl is oxidized to an aluminum alkoxide. The alkoxide is then hydrolyzed with water, forming fatty alcohols and alumina slurry. The alcohols and the alumina slurry can be separated from each other and processed into the final products. After drying of the slurry the alumina is obtained in the form of a high purity aluminum oxide monohydrate of boehmite or pseudoboehmite structure. 

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. 

The impact of crystallinity, however, goes beyond surface area and porosity properties. Phase transition temperatures and thermal stability of a product also change with the size of the primary crystals. This is demonstrated in figure 8, showing a typical phase transition diagram for a material of small and large crystallites, representing pseudoboehmite and boehmite alumina. 

At temperatures higher than about 80 °C, the oxyhydroxides become thermodynamically more stable than the trihydroxides (54) and thus tend to form. Poorly crystallized solids designated as pseudoboehmite form at lower temperatures, whereas weU-crystaUized boehmite forms at higher temperatures or after longer aging. The typical conditions for the formation of poorly crystallized, gelatinous pseudoboehmite are pH values of about... 

Figure 3.5 Proposed sequence for the growth of plate-like well-crystallized boehmite crystals in acetic acid solution. Growth starts from gibbsite or pseudoboehmite precursors. Top Micrographs showing the crystal shapes of well-crystallized synthetic boehmite prepared by the method described in Ref. (60) (full size images with scale available in original paper). Bottom proposed growth sequence with crystallographic relationships. Reprinted from Ref. (60).

In the preparation of a B-free support, it was observed that the crystallite size of the pseudoboehmite, measured on dried extrudates, increases by kneading, see Table 3. However, when boric add is added, the average size of the crystallites decreases. In B-containing supports, the crystallite size of the boehmite increased with the kneading time before addition of boric acid, see Table 3, as did the average pore diameter. 

From XRD and HT-XRD data, it appears that the gels of SOL-IA and SOL-3 are 7-boehmite or pseudoboehmite (JCPDS file no. 21-1307) and possibly a small amount of tohdite, 5AI2O3.H2O (JCPDS file no. 22-1119). After calcination the solids from SOL-1, SOL-2H and SOL-3 all contain y- and/or 5-AI2O3 (JCPDS file no. 10-425 and 16-394). The alumina (ex-SOL-3) is less crystallized in accordance with its higher surface area per gram of alumina and with the observation in Table 4. 

Among the wide variety of the phases formed by the thermal decomposition of aluminum hydroxides we shall consider only alumina forms which derive from pseudoboehmite, boehmite and bayerite. Information about these structures is rather poor. With the exception of the spinel form (t -, y-), the species are mainly characterized by the dimensions of the unit cell and by some indications on their crystallographic system [20]. The structure of y-AbOs was considered as a tetragonal distorted spinel lattice (a superstructure with one unit cell parameter tripled in the case of 8-AI2O3) and is depicted by the formula A18/3 [ ] 1/3 O4, where [ ] is a cation vacancy. This formula is consistant with the constraints of stoichiometry and electrical neutrality. It is difficult to know the exact distribution of the cations in the octahedral or tetrahedral sites despite an abundance of work on this subject [21-23]. The 0-phase, isomorphous with... 

This accounts perhaps for the tendency to form pseudoboehmite (or gelatinous boehmite) as the initial precipitation product over wide ranges of temperatures and pressures. 

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