Boehmite precursor

This transition produces an isomorphous phase and the resulting y-alumina has the same morphology and texture as its boehmite precursor. With increasing temperature and time the mean pore diameter increases gradually and other phases appear (S-, 6-alumina). Due to the broad XRD lines, the distinction between y- and S-alumina cannot be made 6-alumina occurs at about 900°C while the conversion to the chemically very stable a-alumina phase takes place at T> 1000°C. Some typical results for alumina membranes synthesized without binders are given in Table 2.4. When PVA was used as a binder, thermogravimetric analysis showed that, provided the appropriate binder type was used, the binder could be effectively removed at T > 400°C. The ash residue is of the order of 0.01 wt.%. Mean pore size and... 

Since then, HP became a widely used technique to prepare transparent spinel ceramics. For example, HP process was used to fabricate MgAl204 transparent ceramics, with a nearly 100 % densification [44]. Spinel powders for the production of transparent polyciystalline ceramic windows were produced by using both the traditional ceramic solid-state reaction and solgel methods. Among them, the powders produced from the reaction of organomagnesium compounds with surface modified boehmite precursors were best to produce high-quality transparent spinel ceramics. The powder synthesis method allowed fine control over the particle size. 

Hetero-phase additions can influence both the transition alumina sequence and the temperature at which alpha alumina forms. For example, magnesium oxide additions introduced via addition of a magnesium nitrate salt to the boehmite precursor sol induces the formation of eta alumina after heating to 500°C rather than gamma alumina. This eta phase has a very diffuse X-ray diffraction patem at 500°C which sharpens with heating to 1000°C. The magnesium alumina spinel precipitates as a distinct fine crystal upon finther heating. 

The surface area of the carboxylate-alumoxanes as determined by gas desorption was found to be significantly higher than the boehmite precursor (ca. 22 m2). While the exact value is dependent on the acid and treatment conditions, all the alumoxanes were found to have a surface area of 120 - 150 m (18). 

Only one paper that we are aware of explores a combined synthesis and processing route to aluminosilicates. Kemmitt and Milestone use precursors made by reaction of sodium hydroxide, boehmite [Al(0)OH] and silica in ethylene glycol in a 4 3 1 ratio160. The precursor structures are related to those shown above. On removal of solvent (ethylene glycol) a glycolate precursor is obtained that contains a pentacoordinated... 

The structure of y-alumina resembles that of the spinel-type MgAl204 t/-alumina has a very similar structure. The difference lies in the concentration of stacking faults which is caused by the difference in the structures of the precursors boehmite and bayerite [45]. The concentration of stacking faults is higher in the fj-alumina. 

The siloxides of aluminum can be prepared in the same maimer as the alkoxides and exhibit similar stractural features. Interest in these compounds arose from a desire for sol-gel or polymeric precursors for aluminosilicates. Unfortunately, these compounds extensively hydrolyze, losing most of the silicon as the silanol. However, the investigation of the hydrolysis of (Et3SiO)3Al has generated considerable insight into the stracture and composition of aUcoxy- and siloxy-substitutedalumoxanepolymers, (ROAIO) These macromolecules are now believed to have a structure related to that of boehmite. 

Contrary to SEM, TEM does not provide the striking three-dimensional images. Shown in Figure 4.5 are some well dispersed crystallites of boehmite as the precursor to a thin, unsupported partially calcined alumina membrane the TEM image of which is given in Figure 4.6. It is estimated from the TEM that many of the crystallites appear to be smaller than about 50 nm. The ordered suiicture of the very thin, partially calcined alumina membrane is evident in Figure 4.6. 

Figuic 4.5 Transmission electron microscopy image of well dispersed boehmite particles as a precursor to fine-pore alumina membranes... 

Al-sol can be prepared using various aluminum precursors, such as pseudo-boehmite, A10(0H) xH20, and aluminum alkoxide [48-50,55,60]. Xu et al. [55] described several possibilities. Additives, e.g., urea, or organic amines, e.g., hexamethylenetetramine (HMT), can be added to the sol in order to improve the quality of alumina obtained. The decomposition of these additives in heat treatment (calcination) may lead to a better porosity of the alumina. The reaction for urea decornposition is as follows ... 

The sequence of phase transformations shown in Figure 2 is an approximationp largely because process variables such as time, atmosphere and properties of precursor hydroxides are not included. Thus, for example, bayerite and glbbslte may be converted to boehmite and thence to y-alumina during calcination if the particle size Is large and the precipitate Is moist [18]. 

In addition, there Is pressure to reduce the cost of the catalyst, Vashcoat aluminas have desired surface areas and porosities and are thermally stable. They are best produced by calcination of particular precursors, and aluminium isopropDxlde or boehmite have been suggested to be useful materials to calcine [42]. Both of these precursors are not cheap, and less expensive raw materials would be desired,... 

A dispersion of solids can be prepared by dispersing a powder in a liquid or by synthesising the solid particles in situ in a liquid. Examples of the first method are suspensions of submicrometer alumina or zirconia powders in water. Examples of the second method are boehmite sols and titania sols prepared from organo-metallic precursors. ... 

A last complication results from the measurement of the thickness of supported layers. This can easily be done with SEM for calcined layers. Wet lyogel, or even dry xerogel, layers cannot be measured in this way. Reproducible thickness measurements on wet lyogel films could not be obtained with other easy-to-perform methods. Consequently layer thicknesses were measured after calcination. Estimates of the shrinkage in the thickness direction were made for supported alumina (boehmite) membranes dried at 40°C and 60% RH made with a standard precursor solution of 1 mol AlOOH/1 stabilised at pH = 4 by... 

It is well documented that the addition of organic polymeric additives to precursor sols promotes the formation of defect-free membranes [4,12,33]. To investigate this effect, stress measurements were performed on boehmite membranes obtained from standard sols (1 mole AlOOH/1) mixed with different amounts of PVA solutions (containing 35 ml PVA/1, molecular weight 72000). 

Fig. 8.19. Stress in the constant stress region of a drying boehmite membrane as a function of the amount of PVA added to the precursor solution. Drying conditions 40°C and 60% RH. From Kumar [13].

In comparison with pure "j alumina, mixing the precursor boehmite sol with 3% LaNOs or impregnation of calcined y-alumina with LaNOs solution stabilised the unsupported membrane. After 120 h at 800°C the La doped system had a pore diameter of about 5 nm compared with about 9 nm for the pure y-alumina. 

Peptization mechanisms of boehmite used as precursors for catalysts... 

A possible way to increase the amount of active phase deposited on hydrotreatment catalysts may consist of reacting the active metal solutions directly with boehmite (AlOOH), the precursor of y-alumina carrier. Experiments carried out on well defined systems such as boehmite sols provide a more precise insight into molybdenum/boehmite interaction. Therefore, this work gives some keys for modulating the catalytic properties in realistic systems. 

Laser Raman spectroscopy (LRS) The Raman spectra of the samples were recorded using a Raman microprobe (Infinity from Dilor), equipped with a photodiode array detector. The exciting light source was a YAG laser emitting the 532 nm line and the wavenumber accuracy was 2 cm. The laser power was around ImW at the samples. The identification by LRS of the oxomolybdate phases present on the boehmite surface has been established by comparison of the Raman features of the oxidic precursor with those of reference solids, their modifications being ascribed to the effect of the interaction with the carrier. 

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