Boehmite oxygen atoms

Figure 5. Structure of boehmite. Oxygen atoms are to be imagined at the vertices of each octahedron, which has an aluminum atom at the center. The double lines indicate hydrogen bonding.

Alkali leach methods axe exemplified by the Bayer process for the preparation of pure a-A C for electrolysis (Section 17.5) from the mineral bauxite. Bauxite consists mainly of a-AlO(OH) (diaspore) and/or 7-A10(0H) (boehmite), the difference between these being essentially that the oxygen atoms form hep and ccp arrays, respectively. The chief contaminants are silica, some clay minerals, and iron(III) oxides/hydroxides, which impart a red-brown color to the mineral. Aluminum (III) is much more soluble than iron(III) or aluminosilicates in alkali, so that it can be leached out with aqueous NaOH (initially 10-15 mol L 1) at 165 °C under approximately 0.6 MPa pressure, leaving a red mud of iron (and other transition metal) oxides/hydroxides and aluminosilicates ... 

Figure 2.41. Comparison of the core structure of (a) siloxy- substituted alumoxane gels with (b) diaspore, and (c) boehmite minerals. The aluminum and oxygen atoms are shown in blue and red, respectively. Reproduced with permission from Chem. Mater. 1992, 4, 167. Copyright 1992 American Chemical Society.

NUCLEAR MAGNETIC RESONANCE SPECTROSCOPY Nuclear magnetic resonance (NMR) spectroscopy can be applied to aqueous samples and can distinguish between inner- and outer-sphere ion surface complexes. The adsorption behavior of the cations Cs+ and Na" " was studied on the surfaces of silica, boehmite, kaolinite, and illite (Kim and Kirkpatrick, 1997). Cesium was adsorbed both as inner-and outer-sphere surface complexes and in the diffuse layer, while Na was adsorbed only as outer-sphere surface complexes and in the diffuse layer. The adsorbed Na ions were fully hydrated, while the Cs ions had direct contact with the surface oxygen atoms. 

For the silica gel (Figure 3A), the solution was removed slightly less effectively, and more Cs was left (ca. 0.0020 atoms/A2). The spectral behavior is quite similar to that of boehmite, except that there is a peak due to surface Cs coordinated by only water molecules and not in contact with the surface oxygens (so-called outer sphere complexes)at 30% RH. Complete dynamical averaging among sites at frequencies greater than ca. 10 kHz occurs at 70% RH and greater. 

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]. 

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