Boehmite fibrillar

Figure 22-4. TEM photograph of boehmite fibrillar sols (Ishiguro et al., 1990).

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. 

A fibrillar type of boehmite (i.e. a du Pont product named Baymal colloidal alumina) was first described by Bugosh etal. (1962). The BET area was reported to be 275 m2g-1. The discrete boehmite fibres of about 5 ran diameter and 100 nm length were protected by adsorbed acetate groups and were thus easily dispersible. 

At our laboratory we have studied aluminum-oxide montmorillonite complexes prepared from ACH-solutions hydrothermally treated at temperatures up to 160 0 (9). Hydrothermal treatment of ACH at temperatures above about 120 0 yields positively charged, fibrillar boehmite in colloidal suspension (1 ). The size of the boehmite fibrils increases with increasing temperature and time of hydrothermal treatment. Ion-exchange of montmorillonite with these positively charged fibrils resulted in AMCs with... 

Coated glass beads consisting of a single layer of finely divided diatomaceous earth particles bonded to the glass beads with fibrillar boehmite have also been described as a chromatographic support (Kirkland reference as above Kirkland, J. J., Anal. Chem., 37, 1458-1461, 1965). 

A good illustration of the importance of the order in which reactants are mixed is given in Figure 7.4, which shows the differences in XRD for aluminum hydroxide precipitation at pH 9 [18]. Each procedure led to a different compound microporous boehmite (aluminum nitrate added to sodium carbonate), bayerite (sodium carbonate added to aluminum nitrate), or fibrillar boehmite (simultaneous addition). 

The Yoldas [164,165] process outlined schematically in Fig. 22 is the most common method of forming transparent, monolithic alumina gels. (See Fig. 23.) It consists of hydrolyzing an aluminum alkoxide, Al(OR)j, normally AI(OBu )3, in a large excess of water (r = 100-200) at 80-100°C, resulting in precipitation of fibrillar boehmite, followed by peptization with a mineral acid (HNO3) to stable particulate sol. Cold-water hydrolysis... 

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