Boehmite high-surface-area

Co-condensation of oxides. The stoichiometric spinel MgAl204 can be prepared (4) by reacting boehmite type reactive alumina with high surface area MgO at a temperature higher than 1200°C (equation 1). 

Similar experiments were carried out with pseudo-boehmite. The conversion was higher, as pseudo boehmite was more active than gibbsite (Table 1). The final product was indistinguishable from that produced from gibbsite as a starting material for the preparation of high surface area alumina. 

Aerogels are materials prepared by supercritical drying of gels produced by hydrolysis ofalkoxides (14). High-surface-area boehmite (75,76) or pse-udoboehmite (77) can be prepared by this method, and these materials can subsequently be transformed into high-surface-area aluminas. 

AI2O3 is customarily obtained by calcination of boehmite, bayerite, Y-AI2O3, or TI-AI2O3 at temperatures above 650—800 °C, and its formation is associated with a decrease of the surface area to 70—120 m or less. Apphcations of 0-AI2O3 as a catalyst support are mainly motivated by its stabihty at high temperatures (500—800 °C) namely, it retains a medium-to-high surface area of about 100 in g. ... 

Increasing the attrition lesistance of the few fluid-bed catalysts brings to an appreciable reduction of catalyst costs due to a decreased consumption. High surface area alumina in its various forms (boehmite, bayerite, gamma, delta) and silica-alnmina are the preferred supports for fluid bed catalysts. However in mai cases their attrition resistance is not satisfactory, like, for instance, in the case of pure and silica containing boehmites. A remarkable improvement of the attrition resistance of these supports can be obtained by fluorination withHF (Table 1). 

High surface area alumina was obtained by calcining (under static air) a cormnercial boehmite (Versal 200, Euro Support) at 500 °C (5 h). AI2O3 textural properties were... 

Alumina occurs in nature as corundum, a-Al203, which is noted for its great hardness, high electrical resistance and low chemical reactivity. It can be made by the high temperature treatment (at > 1200°C) of boehmite or gibbsite and normally has a low specific surface area (< 5m2 g-1). 

Boehmite is itself decomposed at c. 400—450°C. As expected, the calcined products have much lower specific surface areas than the activated aluminas produced from the trihydroxides. However, the results of de Boer and his co-workers (de Boer, 1972) indicate that a sample prepared at 580°C was highly microporous and that up to this temperature there was only a small change in the external area. 

The washcoat provides a high, stable surface area for dispersion of the precious metals [2,3]. The bulk of the washcoat consists of alumina, stabilised by the addition of small amounts of baria or lanthana. Thermal stabihsation is essential, since temperatures in the catalyst bed can rise to over 1000°C. The choice of alumina precursors (boehmite or gibbsite) and the preparation/stabihsation procedures are vital to the stability of the washcoat [2,3]. 

Rapid activation of Bayer hydrate up to 800 C results in a fine particle material nearly free of boehmite. This product can be fomied into spheres due to the tendency of the material to rehydrate. The formed product is reactivated at 400°C. The final product exhibits 300-350 m /g surface area and pore volumes as high as 0.4 cm /g. 

Recently, a fume pyrolysis with the sol-gel technique was examined to prepare aluminum oxide powders from boehmite sol suspension derived from Al-alkoxide. Aluminum oxide was spherical shape and crude structure. In addition, the aluminum oxide possessed a high thermal resistance at 1473K as high as 50 m g- l in surface area[5]. The fume pyrolysis method was also an effective preparation for thin films composed of mixed oxides because one of the advantages is that the atomic composition in source solution is the same as the formed oxide particles. A Yi Ba2Cu307.x film was successfully prepared[6]. These suggest that the combination of the fume pyrolysis and the sol-gel method seem to be an interesting preparation method. 

Rouquerol [9,10] studied the effect of water vapour pressure on the decomposition, at low pressure, of 1 pm crystallites of gibbsite using a vacuum thermobalance and constant reaction rate regimes. He showed that, at pressures of less than 1,33 Pa, the formation of boehmite is minimised and the gibbsite decomposes directly to produce a highly microporous alumina. He found that the BET surface areas could be varied from 40 to 430 m g" by changing the pressure of water vapour over the solid from 5 to 130 Pa. 

---