Page 324 - Adsorption by Powders and Porous Solids

CHAPTER 10. ADSORPTION BY METAL OXIDES 313
between two adjacent A (or B) layers is 0.28 nrn whereas the A-B distance is
0.20 nm.
Bayerite does not occur in nature, but it can be made in a number of different ways
(e.g. by the hydrolysis of an aluminium alkoxide). The OH layers in bayerite appear
to be stacked in the order ABABAB.. . . Within the double layer the A-B distance is
0.21 nrn and between the double layers the A-B distance is 0.26 xun. The density of
bayerite is correspondingly a little higher than that of gibbsite.
Although deposits of nordstrandite have been found, this modification is not easy
to prepare in a relatively pure form. For this reason, the exact structure is still under
discussion. However, the layer stacking is likely to be made up of a combination of
both bayerite and gibbsite.
In its most common industrial form, gibbsite is a sandy material, of c. 50-100 pm
grain size. Each grain is itself a dense agglomerate of smaller hexagonal crystals, typ-
ically 5-15 pn in size, and the BET-nitrogen surface area is usually not more than
0.2 mZ g-'. Other fonns of gibbsite have also been subjected to physisorption studies.
These include loose, thin hexagonal crystals with BET-nitrogen areas of 5 and
15 m2 g-' corresponding to mean crystal sizes of 1 and 0.2 pn, respectively
(Rouquerol et al., 1975), and also porous aggregates. For example, Ramsay and
Avery (1979) found that a batch of very pure gibbsite powder gave a Type TV nitro-
gen isotherm with an H1 hysteresis loop at high p/pO. It appeared that the gibbsite
was mesoporous and possibly also macroporous, the effective pore width being
mainly > 20 nm. The BET-nitrogen area was 41 m2 g-l, which was consistent with
the mean thickness of c. 25 nrn of the thin hexagonal platelets, as determined by elec-
tron microscopy and X-ray line broadening. This rather ill-defined porosity was
ascribed to the space between the gibbsite crystallites and was found to persist after
heat treatment at 40°C. A batch of gibbsite of lower BET-nitrogen area (5.6 m2 g-')
was used by Stacey (1987). In this case, the polycrystalline grains of mean diameter
75 pn were composed of 0.3 pn platy crystallites.
A sample of microcrystalline nordstrandite was found to be somewhat mesoporous
by Aldcroft and Bye (1967). The nitrogen hysteresis loop was Type H3, which indi-
cated the existence of slit-shaped pores between the crystallites. The BET-nitrogen
area of 34 m2 g-l appeared to represent the external area of the crystallites.
A relatively low-area (8 m2 g-') aged sample of bayente (Bye and Robinson,
1964) which gave a reversible Type I1 nitrogen isotherm, was shown to be essentially
non-porous (Payne and Sing, 1969). Electron microscopy revealed that this sample
was composed of discrete conical crystals.

10.3.3. Aluminium oxide-hydroxides
There are two well-known oxide-hydroxides (Al00H) with closely related struc-
tures: diaspore and boehmite. Diaspore occurs in some types of clay and bauxite. It
has been produced by the hydrothermal treatment of corundum, a-A120,. Whereas
boehmite is characterized by cubic close-packing of the anions, diaspore has a hexag-
onal close-packed structure. This difference probably accounts for the direct thermal
transformation of diaspore to corundum at relatively low temperatures (450-600°C).