Boehmite crystallites

Structure of Surface °f Boehmite, Crystallite Attrition at 600°C, Attrition at 1000PC... 

Further dehydration of boehmite at 600 0 produces y-alumina, whose spectrum is shown in Figure 3b. There is a loss in surface area in going from boehmite to y-alumina. The sample shown here has a surface area of 234 m /g (this sample was obtained from Harshaw A23945 the calcined Kaiser substrate gave an identical infrared spectrum). The y-alumina sample shows two major differences from o-alumina. First, there is a more intense broad absorption band at 3400 cm" due to adsorbed water on the y-alumina. Second, the y-alumina does not show splitting of the phonon bands between 400 and 500 cm" as was observed for o-alumina. The y-alumina is a more amorphous structure and has much smaller crystallites so the phonon band is broader. The y-alumina also shows three features at 1648, 1516 and 1392 cm" due to adsorbed water and carbonate. 

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. 

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. 

Although the glycol derivative of boehmite is also prepared from aluminum alkoxides (see Section 111.B.9), this product cannot transform into a-alumina because of the absence of a-alumina nuclei and water. However, the reaction of a mixture of gibbsite (microcrystalline) and aluminum alkoxide in the presence of a small amount of water gives a-alumina with larger crystallite sizes than those obtained from gibbsite alone. 

SEM and TEM characterizations have shown that B1 boehmite is a "dense" agglomerate of platelet like crystallites developing a small dimension porosity (Fig. 2). B2,... 

The impact of crystallinity, however, goes beyond surface area and porosity properties. Phase transition temperatures and thermal stability of a product also change with the size of the primary crystals. This is demonstrated in figure 8, showing a typical phase transition diagram for a material of small and large crystallites, representing pseudoboehmite and boehmite alumina. 

The pore size of the membrane is determined by the type of acid and its concentration during dispersion (typically 0.03 to 0.15 mol acid per mol of AIO-OH) along with the final thermal treatment, both of which affect the crystallite size of the gamma-alumina. The porous support is dipped into the sol and capillary action pulls the sol into the pores increasing the concentration of Boehmite at the entrance of the pores to form a gel. The calcination of this gel film (above 400°C) yields the final film of gamma-alumina. 

Third, the homogeneity of the starting hydroxide is a critical factor, even when the crystallite structure is boehmite. For example, with various boehmites, the products were more attrition resistant and more stable as the fineness and homogeneity of the crystallites of the precursor hydroxide increased (Table II). 

In the preparation of a B-free support, it was observed that the crystallite size of the pseudoboehmite, measured on dried extrudates, increases by kneading, see Table 3. However, when boric add is added, the average size of the crystallites decreases. In B-containing supports, the crystallite size of the boehmite increased with the kneading time before addition of boric acid, see Table 3, as did the average pore diameter. 

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. 

XRD data reveal that alumina particles in the sol are of boehmite crystalline structure and the particles in zircornia and titania sols are of amorphous structure [34], The alumina, titania and zirconia samples obtained from the sols after gelation and calcination at 450°C are respectively in the phases of y-alumina, tetragonal zirconia and anatase. These are thermodynamically metastable phases, and may transform to the thermodynamically stable phases, which are a-alumina, monoclinic zirconia and rutile. The crystallite structure and lattice parameters of these phases are listed in Table 1. 

Bokhimi X, Toledo-Antonio JA, Guzman-Castillo ML, Hemandez-Beltian F (2001) Relationship between crystallite size and bond lengths in boehmite. J Solid State Chem 159 32—40... 

Pseudo-boehmite is poorly crystalline and is often the first crystalline form to appear when hydrous alumina gels are aged. Certain of the XRD peaks in pseudo-boehmite are shifted relative to boehmite and pseudo-boehmite is characterizedby higher water content. These differences have been explained by the exceedingly small crystallite size of pseudo-boehmite (Baker and Pearson, 1974) and the presence of interlayer water (Tettenhorst and Hoffman, 1980). In many cases commercial boehmite samples contain both well crystallized boehmite and pseudo-boehmite. 

Some kinetic work has been carried out on the decomposition of (pseudo)-boehmite. Callister et al. [19] reported on the effect of the water pressure. Tsuchlda et al. [20] reported on the effect of crystallite size and confirmed the effect of water pressure as determined by Callister et al. [19]. A consistent kinetic model for the decomposition of pseudoboehmite, which is applicable over the entire temperature range of interest in calcination (20 - 800 °C) and which takes into account the effects of particle size and water pressure is not available in the literature. Therefore the model of Leyko et al. [21] has been fitted to the data of Fig. 6 as a first approximation ... 

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