Page 322 - Adsorption by Powders and Porous Solids

CHAPTER 10. ADSORPTION BY METAL OXIDES 31 1

of hydrophobicity shown by all dehydroxylated silicas. The surface rehydroxylation
was very slow atp/pO < 0.3, but became rapid in the multilayer range; this phenome-
non was evidently responsible for the low-pressure hysteresis. Indeed, it was found
that the desorprion branches of the isotherms on gels E and J were remarkably similar
to the corresponding parts of the water isotherms on the two hydroxylated gels
(Baker and Sing, 1976). We may conclude that the micropores in the rehydroxylated
gel E regained their high affmity for water.

103. Aluminas: Structure, Texture and Physisorption

10.3.1. Activated alumina
The name 'activated alumina' is generally applied to an adsorbent alumina (usually
an industrial product) prepared by the heat treatment of some form of hydrated
alumina (i.e. a crystalline hydroxide, oxide-hydroxide or hydrous alumina gel). It has
been known for many years that certain forms of activated alumina can be used as
powerful desiccants or for the recovery of various vapours. It was apparent at an early
stage that the adsorbent activity was dependent on the conditions of heat treatment.
For example, in 1934 Bayley reported that the adsorption of H,S by a commercial
sample of activated alumina was affected by prior heating of the adsorbent at differ-
ent temperatures, the maximum uptake being obtained after heat treatment at 550°C.
During an investigation of the catalytic dehydration of alcohols, Alekseevskii (1930)
found that a calcination temperature of c. 400°C was required to optimize the adsorp-
tion of the alcohol reactants, whereas calcination at 600°C was preferable for the
adsorption of the olefine products.
Somewhat later it began to appear that there was a lack of agreement between the
recorded dependence of surface area on the temperature of calcination (e.g. in the
work of Krieger, 1941; Feachem and Swallow, 1948; Taylor, 1949; Gregg and Sink,
1951; de Boer, 1957). In fact, such differences are not really surprising. To obtain
reproducible adsorbent properties it is necessary to control: (a) the chemical and phys-
ical nature of the starting material (i.e. its st~ucture, crystal/particle size, amount of
sample purity); (b) the conditions of heat treatment (type of furnace, atmosphere, time
- temperature profile - preferably by a controlled rate thermal analysis (CRTA)
heating procedure); and (c) the methods used to interpret the adsorption data (BET,
BJH, etc.).

10.3.2. Aluminium trihydroxides
Although various modifications of aluminium trihydroxide, Al(OH),, have been
described in the literature, there are only three common forms: gibbsite (originally
also called hydrargillite) bayerite and nordstrandite. Gibbsite is the best known and
most abundant. It is the main constituent of North and South American bauxite and
is obtained as an intermediate product (i.e. 'Bayer Hydrate') in the Bayer process for
the production of aluminium from bauxite.