Pseudoboehmite oxide, 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 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. 

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... 

The other area where cerium cations may have an influence is in complex formation in the solution immediately adjacent to the surface, in the dissolution and precipitation phase via replacement of Al(OH)[ " species with Ce(OH) " species. For example, moderate levels of Ni have been observed on the surface of boehmite-like stractures developed after treatment in solutions containing NiCl and NaCl, and it was proposed that oxide formation involved Ni/Al complex formation in the solution adjacent to the developing oxide. Although hydrolysis of cerium ions in solution is well known, and mixed AFCe precipitates may form at the metal/solution interface, the low concentration of cerium observed on the surface of the alloys treated in CeCl solution suggests that substitution of Al(OH) species with Ce(OH) species does not occur under the conditions of pseudoboehmite growth. Certainly at room temperature Ce(OH)j forms at higher... 

In aluminum alloy (2024-T3)-epoxy joints, for exanqrle, the initial oxide produced on the aluminum substrate is usually amorphous AI2O3. Upon exposure to moisture, AI2O3 is converted to aluminum hydroxide with a chemical composition between that of boehmite (AI2O3H2O) and pseudoboehmite (A1203-2H20). Failure surface analysis reveals that the hydroxide layer is normally attached to the adhesive side, suggesting that adhesion of the hydroxide to aluminum is very weak. Thus, once a hydroxide is formed, it is separated easily from the substrate, causing failure of the joint. 

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