Hydrothermal boehmite

The results of development work on processes indicate that the two main methods of preventing the duplex microstructure from forming appear to be fast-firing, or increasing the amount of / "-alumina at low temperatures. Based on these results, Duncan et al. [21] and Zyl et al. [22] have described production processes starting from aluminum oxyhydroxides or aluminum hydroxides as precursors for the synthesis of the solid electrolyte "-alumina. Duncan et al. described an alumina precursor which substitutes in part or wholly for a-alumina in an established slurry solution spray-drying process. As a precursor, hydrothermal boehmite, Cera hydrate, has been used. A calcination step is important at the begirming of the process. Boehmite was used both in the as-received condition and after calcination. The effect of the calcination temperature on the fired properties of /S"-alumina can be seen in Table 21.5. 

A comparison of boehmite with other raw materials is included in Table 6. In this table the soda and lithia contents of compositions based on a range of raw materials and the resultant properties are detailed. The level of / "-alumina was always higher with the hydrate-type raw material the hydrothermally prepared raw materials gave the highest content of / " -alumina. 

Alumina - Alumina forms a variety of oxides and hydroxides whose structures have been characterized by X-ray diffraction (16). From the catalytic viewpoint y-alumina is the most important. This is a metastable phase that is produced from successive dehydration of aluminum trihydroxide (gibbsite) to aluminum oxide hydroxide (boehmite) to y-alumina, or from dehydration of boehmite formed hydrothermally. y-alumina is converted into a-alumina (corundum) at temperatures around 1000 C. 

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). 

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. 

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. 

It is now clear that if the hydrothermal formation of boehmite is avoided (e g. by using low-pressure CRTA and small crystals), then the three hydroxides lose their structural water at quite low temperature ( 200°C) to give the almost amorphous form, p-Al203. Complex changes occur as the temperature is increased to 250-800°C with the formation of certain members of the y-series aluminas y or rj and 9. At temperatures 1200°C the conversion to the dense, low-area a-Al203 normally takes place. 

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... 

Solvothermal reaction of gibbsite in ethanol gives boehmite (AlOOH a polymorph of alnminnm oxyhydroxide), which is also obtained by hydrothermal reaction of the same starting material. The prodnct is comprised of randomly oriented, thin, small crystals of boehmite. This resnlt snggests that a dissolntion-recrystallization mechanism takes place dnring the conversion. 

Intraparticle hydrothermal reaction originally proposed by de Boer et al. - for the formation of boehmite during thermal dehydration of coarse gibbsite. 

Hydrothermal synthesis of a-alumina has been well studied. Since the hydro-thermal reaction of aluminum compound yields boehmite at relatively low temperatures (approximately 200°C), transformation of boehmite was examined and it was reported that more than 10 hours is required for complete conversion into a-alumina, even with a reaction at 445°C in a 0.1 N NaOH solution and in the presence of seed crystals. On the other hand, under glycothermal conditions, a-alumina is formed at 285°C for 4 h. The equilibrium point between diaspore (another polymorph of AlOOH) and a-alumina under the saturated vapor pressure of water was determined to be 360°C. However, near the equilibrium point, the transformation rate is very sluggish, and only a small conversion of diaspore is observed. Therefore complete conversion of diaspore into a-alumina requires a much higher temperature. Since boehmite is slightly less stable than diaspore, the hypothetical equilibrium point between boehmite and a-alumina would be lower than that for diaspore-alumina. However, a-alumina would not be formed by a hydrothermal reaction at such a low temperature as has been achieved in the glycothermal reaction. 

The difference between two reactions may be attributed to the activity of water present in the reaction system, since the overall reaction is the dehydration reaction (Equation 2.1). However, intentional addition of a small amount of water caused enhancement of a-alumina formation rather than the retardation expected from the equilibrium point of view. Another important factor is the difference in the thermodynamic stabilities of the intermediates between glycothermal and hydrothermal reactions that is, the glycol derivative of boehmite vs. well-crystallized boehmite. The latter compound is fairly stable and therefore conversion of this compound into a-alumina has only a small driving force. On the other hand, the glycol derivative of boehmite has Al-O-C bonds and therefore is more unstable with respect to a-alumina. Thus conversion of this compound into a-alumina has a much larger driving force. The smaller crystallite size of the glycol derivative of boehmite also contributes to the instability of the intermediate. 

The titanium mordenite was synthesized hydrothermally from a system with molar composition IR 1.35 Na20 AI2O3 7.5 Si02 110 H2O 1.2 NaCl, where R represents a template DABCO. Titanium tetrabutoxide was used as a source of titanium. A typical preparation was as follows Boehmite (catapal B, 2g) was slurried in water (5g) and solution of NaOH (1.4g in lOg water) was added to the above slurry. This mixture was then added to a 30% silica sol in water (25g) under stirring. A solution of DABCO in water (1.86g in 5g... 

The boehmite system (y-AlOOH), originally studied by Zocher and Torok [63] and Bugosh [64] was further developed by Lekkerkerker and coworkers [65]. They extended the hydrothermal preparation pioneered by Bugosh [64] by starting from an aqueous aluminum alkoxide mixture acidified with hydrochloric acid [65a]. They studied the phase behavior of both charge stabilized aqueous dispersions of colloidal boehmite rods [65b,c] as well as sterically stabilized colloidal boehmite rods in an organic solvent (cyclohexane) [65d-f]. 

Diaspore, which is thermodynamically more stable than boehmite, can also be obtained by hydrothermal transformation of gibbsite or boehmite, but its formation is slow. Higher temperatures (i.e., >200 °C) and pressures (>15 MPa) are required for the synthesis, and the presence of diaspore seed crystals helps to avoid boehmite formation. Methods to produce weU-crystaUized diaspore have been reported these include hydrothermal synthesis at 300 °C and 3.45 x 10 Pa (5000 psi) over a 72-h period (67) or high-pressure calcination of boehmite (68). 

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. 

A sodium metasilicate modified high-alumina cement may be used in geothermal wells at temperatures up to 300°C (Sugama and Garciello, 19%). Sodium calcium silicate hydrate and boehmite are formed as products of reaction in the hydrothermal reaction. 

The transformation sequences of gibbsite and bayerite are affected with a particle size and partial pressure of water in the heating atmosphere. The transition forms %- and ri-AUOa are usually developed from gibbsite and bayerite, respectively. In the coarse particles of these hydrates, from which the water of dehydroxylation cannot evaporate rapidly, boehmite is formed as a result of local hydrothermal conditions. If dehydroxylation of gibbsite takes place in vacuum or in rapidly moving air the amorphous form (p-AhOs) is obtained at the temperature above 1000 K. 

Lu et al. [31] synthesized alumina nanotubes via the hydrothermal technique. For a typical synthesis, AKNOsjs and camphor sulfonic acid, as surfactant, were dissolved in water. An ammonia aqueous solution was added to adjust the pH value to 5.4. A Teflon-lined stainless steel autoclave heated to 160 °C was used to grow the nanotubes for 24 h. A solid precipitate was collected by centrifugation, washed with ethanol and dried in air at room temperature followed by grinding. The product of this synthesis exhibits a one-dimensional morphology with a length of 500 nm and a diameter of 50 nm, and a boehmite phase. 

Yang synthesized boehmite (y-AlOOH) nanorods using aluminum nitrate and sodium hydroxide as starting materials. The results show that the nanorods had regular shape with a diameter of 10-30 nm, length of 200-300 nm, when produced hydrothermically at 200-220 °C for 24 h at pH = 5. Figure 6.5 depicts the influence of the pH in the structure of the boehmite. 

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