Structures boehmite

While silica-aluminas, as represented in figure 11, still to a larger smaller or degree maintain a basic boehmite structure, there are other examples of alumina mixed oxides where a totally different crystal structure is obtained, such as in the case of aluminum/magnesium hydrotalcites (figure 12). 

The structural evolution of the nanocomposites upon thermal treatment is very interesting, as formation of the alloy takes place concurrently with crystallization and phase transition of the alumina matrix. Pure AI2O3 aerogels obtained by this procedure exhibited a layered pseudo-boehmite structure (in particular, the ethyl derivative of boehmite was formed as a consequence of esterification during supercritical drying), which upon thermal treatment was converted to nanocrystalline 7-AI2O3, and finally at 1,000°C, the thermodynamically stable polymorph a-Al203 started to crystallize. 

Data on the mechanism of lithium sorption from liquors with freshly precipitated aluminium hydroxide are limited and contradictory. For example, Goodenough reports on the isolation of lithium-aluminate complex [17]. Some authors [11] suggest that lithium is adsorbed on the surface of aluminium hydroxide, and high adsorption coefficient is provided by the presence of salt background which prevents aluminium hydroxide from crystallization when the temperature of the process is raising. Other authors assume that lithium coprecipitation with aluminium hydroxide leads to intercalation of lithium into the structure of the latter and to the formation of lithium-containing aluminium hydroxide characterized by pseudo-boehmite structure [12]. The authors of [13] conclude that the overall interaction of lithium with amorphous aluminium hydroxide can be represented by the following scheme ... 

Figure 9-1. Boehmite structure (illustration by Dr. Steven I. Dutch).

Fig. 130. The orthorhombic structures of boehmite y-AlO(OH) (left) and FeOCl (right) viewed parallel to the layers. The asymmetric hydrogen bonds between the (OH) ions in the boehmite structure are indicated by broken lines.

It is noteworthy that the amides ZrN(NH2) and possibly FeO(NH2) also crystallize in the FeOCl structure whereas substitution of the halogen ion by the hydroxyl ion converts the structure to the boehmite type. The structure of orthorhombic y-AlO(OH), boehmite, is made up of the same kind of puckered sheets as FeOCl but these sheets are stacked in a different manner. In the boehmite structure planar zig-zag chains link the sheets by means of H bonds. OH—OH = 2.70 A. In addition to the true boehmite representatives we have listed in Table 142 also the data for Cu(OH)2 which crystallizes in a closely related structure type. The structure of Cu(OH)2, which contains the same... 

Tlie structure of boehmite consists of double layers in wliich the oxygen ions exliibit cubic packing. Hydroxyl ions of one double layer are located over the depression between OH ions in the adjacent layer such that the double layers are linked by hydrogen bonds between hydroxyls in neighboring planes. Tliere is some technical production and use of synthetically produced boehmite. 

Cs NMR results for Cs on the surfaces of illite, kaolinite, boehmite and silica gel (Figure 3) show that for this large, low charge cation the surface behavior is quite similar to the interlayer behavior. They also illustrate the capabilities of NMR methods to probe surface species and the effects of RH on the structural environments and dynamical behavior of the Cs. The samples were prepared by immersing 0.5 gm of powdered solid in 50 ml of O.IM CsCl solution at 2 5°C for 5 days. Final pHs were between 4.60 and 7.77, greater than the zero point of charge, except for boehmite, which has a ZPC... 

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. 

When the gibbsite is dehydrated a structural collapse occurs with a large increase in surface area. The boehmite sample has a nominal surface area of 325 m /g. The infrared spectrum of the boehmite shows distinct structure in the OH stretching region, with two peaks located at 3090 and 3320 cm". There are three features at 1648, 1516 and 1392 cm" that are due to adsorbed water and carbonate, which are removed upon heating the boehmite to 350 0 in hydrogen. 

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. 

The results obtained for the various aluminum oxides and hydroxides indicate that infrared photoacoustic spectroscopy may be useful in characterizing structural transformations in these species. Very clear differences between a-alumina and y-alumina were noted in the region of the lattice vibrations. The monohydrate, boehmite, showed a very distinct Al-OH stretching feature at 1070... 

According to El-Mashri et al.,190 the A106 A104 ratio determines the hydration capacity of anodic oxides. Tetrahedral sites are hydrated easily to form a boehmite-like structure, which is known to be composed of double layers of Al-centered octahedra, weakly linked by water molecules to other layers.184 As the oxide formed in H3P04 contains about 70% tetrahedral aluminum bonds, its hydration ability should be higher than that of the oxide formed in tartrate solution. However, this has not been found in practice, which is interpreted by El-Mashri et al. as being due to some reduction of A104 by incorporated phosphate species. 

Norbornene-ethene copolymer, 16 113 Norbornene-ethylene copolymers, 20 433 physical properties of, 20 420-422 Norbornenodiazetine derivatives, 13 306 Nordel IP (metallocene), 7 637 Nordihydroguaiaretic acid, antioxidant useful in cosmetics, 7 830t Nordstrandite, 2 421, 425 activation, 2 394 classification, 2 422 decomposition sequence, 2 392 from gelatinous boehmite, 2 427 structural properties of, 2 423t NO-reduction reactions, TWC catalyst, 10 49... 

The gel layer thickness increases linearly with the square root of dipping time indicating that indeed a slip-casting process is operative. The rate constant depends on gel structure and pore size of the support. If the modal pore size of the support is increased from 0.12 /im (type 1 support) to 0.34 /xm (type 2 support) the casting rate is decreased in accordance with theory. Typical casting rates for type 1 and type 2 supports are 4.4 /xm/s and 2.8 /xm/s, respectively for HNO3-stabilized sols with a concentration of 1.22 mol boehmite/L. 

Figure X7. Idealized model of the boehmite membrane structure, d is the distance between the 2 boehmite crystals A and B, r is the thickness of the boehmite plates (Leenaars and Burggraaf 1985).

Lippens (48) has studied the texture of the catalytically active aluminas by means of diffraction and adsorption techniques. He concluded that the structure of tj-alumina formed from bayerite consists of lamellae with an average thickness of about 15 A and a distance of about 25 A. y-Alumina prepared from gelatinous boehmite is composed of fibrillar-shaped particles of about 30 X 30 A. Both structures can easily account for the pseudosolvent effect of alumina, which will be referred to in more detail in the forthcoming discussion. 

The orange coloured lepidocrocite, y-FeOOH, is named after its platy crystal shape (lepidos scale) and its orange colour (krokus = saffron). It occurs in rocks, soils, biota and rust and is often an oxidation product of Fe ". It has the boehmite (y-AlOOH) structure which is based on cubic close packing (ccp) of anions. 

Lepidocrocite (Greek lepidos = scale, flake and krokoeis = saffron-coloured) is iso-structural with boehmite (Tab. 2.1). Unlike goethite and akaganeite which have a turmel structure, lepidocrocite is a layered compound. The orthorhombic unit cell contains four formula units and has the edge lengths, a = 1.2520(6) nm h = 0.3873(2) nm and c = 0.3071(6) nm (Ewing, 1935 Oles et al., 1970 Christensen Norlund-Christensen, 1978). 

Lastly, the positions of hydrogen atoms have been located in a series of low-surface-area aluminium oxide-hydroxides. Hence, the long known solid-state structures of diaspore (a-AlOOH) and boehmite (y-AlOOH) have been recently probed using H CRAMPS [45] along with X-ray and neutron diffraction techniques [46]. For diaspore results point to the presence of Alaf/Xa-OH) groups with 6-fold and 4-fold coordination at aluminium and oxygen, respectively, while Al2(/x-OH) moieties with 6-coordinate aluminium and 3-coordinate oxygen are revealed in boehmite [46]. 

The structure of boehmite contains double sheets of octahedra with A1 ions at their centers, and the sheets themselves are composed of chains of octahedra. In diaspor the oxygens are in a hexagonal close packed layer those within the double octahedral layers in boehmite are in a cubic pacldng relationship. Luminescence center Cr characterizes steady-state spectra (Solomonov et al. 1994 Shoval et al. 1999). The natural boehmite and diaspor in our study consisted of twelve samples. The laser-induced time-resolved technique enables us to detect Cr emission centers (Figs. 4.62-4.63). 

---