Boehmite hydrothermal treatment

There are two well-known oxide-hydroxides (AlOOH) with closely related structures diaspore and boehmite. Diaspore occurs in some types of clay and bauxite. It has been produced by the hydrothermal treatment of corundum, a-Al203. Whereas boehmite is characterized by cubic close-packing of the anions, diaspore has a hexagonal close-packed structure. This difference probably accounts for the direct thermal transformation of diaspore to corundum at relatively low temperatures (450-600°C). 

Diaspore, o -AlO(OH), occurs in some types of clay and bauxite and can be synthesized by hydrothermal treatment of boehmite, y-AlO(OH), in 0.4% aqueous NaOH at 380 °C and 500 atm. Crystalline boehmite is easily obtained upon warming the amorphous, gelatinous precipitate that forms when cold solutions of aluminum salts are treated with ammonia. 

At our laboratory we have studied aluminum-oxide montmorillonite complexes prepared from ACH-solutions hydrothermally treated at temperatures up to 160 0 (9). Hydrothermal treatment of ACH at temperatures above about 120 0 yields positively charged, fibrillar boehmite in colloidal suspension (1 ). The size of the boehmite fibrils increases with increasing temperature and time of hydrothermal treatment. Ion-exchange of montmorillonite with these positively charged fibrils resulted in AMCs with... 

Hydrothermal treatment of Y-AI2Q3 is commonly used for alteration of the porous structure parameters (Chertov et al., 1982). Our study demonstrated that this technique is efficient for controlling the state of the oxide surface. Hydrothermal treatment of Y-AI2Q3 was carried out in a temperature range of 50-200 °C with the treatment time varying from 0.5 to 12 h. This produced a hydroxide phase of boehmite AIO(OH) on the Y-AI2O3 surface, which amount can be readily controlled by the treatment conditions. After hydrothermal treatment, the samples were calcined at 550 °C to reduce the oxide phase. 

Burning PA, Pathmamanoharan C, Jansen JBH, Lekkerkerker HNW (1991) Preparation of colloidal boehmite needles by hydrothermal treatment of aluminum alkoxide precursors. J Am Ceram Soc 74 1303-1307... 

In the 1950s, de Boer and his co-workers (de Boer et al., 1954, 1956 de Boer, 1957) used a variety of techniques in their studies of the thermal dehydration of gibbsite and bayerite and a more detailed picture was obtained of the conditions under which the two decomposition routes were manifested. For example, it was shown that relatively well-crystallized boehmite could be produced by the treatment of gibbsite or bayerite in saturated steam at temperatures of c. 165°C. These and other findings provided qualitative confirmation that the formation of boehmite involved an intragranular hydrothermal transformation. 

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