Aluminas hydrothermal precipitation

One of the industrial applications of hydrothermal precipitation is ordinary alumina production. The Bayer process is shown in Figine 1.7. T... 

Membranes with extremely small pores ( < 2.5 nm diameter) can be made by pyrolysis of polymeric precursors or by modification methods listed above. Molecular sieve carbon or silica membranes with pore diameters of 1 nm have been made by controlled pyrolysis of certain thermoset polymers (e.g. Koresh, Jacob and Soffer 1983) or silicone rubbers (Lee and Khang 1986), respectively. There is, however, very little information in the published literature. Molecular sieve dimensions can also be obtained by modifying the pore system of an already formed membrane structure. It has been claimed that zeolitic membranes can be prepared by reaction of alumina membranes with silica and alkali followed by hydrothermal treatment (Suzuki 1987). Very small pores are also obtained by hydrolysis of organometallic silicium compounds in alumina membranes followed by heat treatment (Uhlhom, Keizer and Burggraaf 1989). Finally, oxides or metals can be precipitated or adsorbed from solutions or by gas phase deposition within the pores of an already formed membrane to modify the chemical nature of the membrane or to decrease the effective pore size. In the last case a high concentration of the precipitated material in the pore system is necessary. The above-mentioned methods have been reported very recently (1987-1989) and the results are not yet substantiated very well. 

AI2O3 were prepared by precipitating alumina onto silica gel, followed by hydrothermal treatment. These materials were characterized by 27A1NMR and ESCA and evaluated in gas oil cracking. NMR revealed the presence of tetrahedral, pentacoordinated and octahedral A1 species in the steamed Si02 - AI2O3 samples with 27 and 13% AI2O3. 

KA13(S04)2(0H)6. Based on an invention made by G.S. Tilley in 1924. The ore is first dehydrated at up to 600°C. It is then leached with a solution of sulfuric acid and potassium sulfate. After clarification of the leachate, potassium alum is crystallized out. Hydrothermal treatment of potassium alum precipitates a basic potassium alum, K2S04-3A1203 4S03-9H20, simultaneously regenerating potassium sulfate and sulfuric acid. Calcination of this potassium alum yields a mixture of alumina and potassium sulfate, which is leached out. Piloted by Kalunite in Salt Lake City, UT, in 1943 but later abandoned. See also Alumet. 

Ghanizadeh S, Bao X, Vaidhyanathan B, Burner J (2014) Synthesis of nano a-alumina powders using hydrothermal and precipitation routes a comparative study. Ceram Int 40 1311-1319... 

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. 

Previous work (13,14) has shown that the glycothermal synthesis process, a liquid phase precipitation at elevated temperatures under autogeneous pressure using a glycol as solvent, for a-Al203 is mediated by the formation of a precursor pseudo-boehmite phase (15). The pseudo-boehmite is thermodynamically unstable under the reaction conditions, and transforms to the stable a-Al203 (similar to the formation of alumina in the hydrothermal system (16)). Thus, the overall reaction is as follows. 

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