Bergaya F. Handbook of Clay Science
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the non-polar alpha-pinene. The yields, based on alpha-pinene, for the most active
catalysts are betw een 80 and 90%. These yields are directly comparable with those
obtained by others using zeolites and PILC although the acid-activated polycation-
treated clays are marginally less selective towards camphene (Breen and Watson,
1998).
Two montmorillonites STx- 1 and SWy-2 were activated with different amounts of
12 M HCl and then exchanged with a fixed amount of 1 M tetramethylammonium
(TMA
+
) chloride solution at room temperature, giving rise to H
+
/TMA
+
samples.
In addition, TMA
+
/H
+
samples were obtained by acid-activation of TMA
+
-
exchanged samples. The BET surface area of these samples is determined by N
2
adsorption, the acidity by adsorption of cyclohexylamine and the catalytic activity
by the isomerization of 1-butene at 300 1C (to yield cis- and trans-2-butene). The
total conversion for the isomerization of 1-butene is higher for the TMA
+
/H
+
-
samples than for the H
+
/TMA
+
-samples. TMA
+
cations adsorbed on the clays are
extremely resistant to exchange by protons, but protons are easily displaced by
TMA
+
cations (Moronta et al., 2002).
The preparation and characterization of Ti-pillared acid-activated clays and con-
ventional Ti-pillared clays were described by Bovey et al. (1996). These materials
have good catalytic properties for the dehydration of pentanol compared with PILC.
7.1.9. CONCLUSION
Acid activation of clays and clay minerals was used for decades both in labo-
ratories for basic and applied research and in industrial production for many ap-
plications. Even so, it still remains one of the most common chemical modifications
of clays for the future of clay science and applications.
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Handbook of Clay Science
Edited by F. Bergaya, B.K.G. Theng and G. Lagaly
Developments in Clay Science, Vol. 1
r 2006 Elsevier Ltd. All rights reserved.
289
Chapter 7.2
THERMALLY MODIFIED CLAY MINERALS
L. HELLER-KALLAI
Institute of Earth Sciences, The Hebrew University of Jerusalem, IL-91904 Jerusalem,
Israel
The structure and composition of clay minerals are modified by heating. The
concomitant changes in properties can be exploited for practical purposes. The ac-
tual temperatures at which changes occur vary greatly from one clay mineral group
to another, and even for different specimens within a given group. These temper-
atures also depend on the particle size and on the heating regime. Four principal
temperature ranges in which significant changes occur in the structures of clay min-
erals may be distinguished.
(1) Temperatures sufficiently low to cause partial freezing of clay suspensions or
pastes (5 1C): in this temperature range some of the water is converted into ice.
However, even at a temperature as low as –60 1C a significant amount of water
remains in a liquid or semi-liquid state, forming a film that separates the mineral
surface from the ice.
(2) Temperatures above dehydration but below dehydroxylation: when the temper-
ature is raised from ambient to that of the onset of dehydroxylation, clays lose
adsorbed an d hydration water. As a result, the interlayer spaces collapse, while
pore space is changed, and the acidity of the clay mineral surfaces and interlayers
is substantially altered.
(3) Temperatures abo ve dehyd roxylation, but below those leading to complete
destruction of the structure: the changes occurring in this temperature range
vary for different clay mineral groups. Dehydroxylation destroys the layer
structure of trioctahedral, 2:1 type (T-O-T) minerals, whereas that of their di-
octahedral counterparts is preserved. Kaolinite group minerals become amor-
phous to X-rays although some features of the structural framework are
preserved.
(4) Temperatures at which new phases crystallise: dehydroxylated clays that do not
turn amorphous to X-rays may become so on further heating, before high-
temperature phases crystallise. With trioctahedral clay minerals and palygorskite
the interval between stage s (3) and (4) is very short. Thus, intermediate phases, if
formed, may escape detection. When new phases crystallise, clay minerals lose
DOI: 10.1016/S1572-4352(05)01009-3
their origi nal identity although the crystallographic orientation of the products is
frequently related to that of the starting material.
Clay minerals can be heated in different forms: (i) witho ut any admixtures or
pretreatment; (ii) mixed with various reagents before heating; (iii) after pretreatment
such as acid-activation; and (iv) after preheating and pretreatment, for example, by
acid-activation, and subsequent reheating.
In view of the great variety of starting materials and the many variables involved
in the heating regime, a range of options is available for the thermal modification of
clays. The ubiquitous impurities in natural clays may also play a part. The pre-
activation temperatures required to develop desirable propert ies, or prevent unde-
sirable properties from developing, vary with different clay minerals. Some minerals
are preferably used in the dehydrate d, others in the dehydroxylated form. The fol-
lowing discussion is concerned with some properties of clay minerals that are
changed or modified by thermal treatment.
7.2.1. FREEZING OF CLAY MINERALS
Freezing affects pore wat er and water adsorbed on clay mineral surfaces, in the
interlayer spaces, and in channels. Not all the water is converted into ice. Some
remains in a mobile semi-liquid or liquid state, coating the surfaces (Anderson and
Tice, 1971).
A. Effect of Freeze-Drying on the Texture of Clay Minerals
On freeze-drying, water is expelled from the frozen mineral (under vacuum) by
evaporation of liquid water and sublimation of ice. This affects the texture of the
clay mineral. Freeze-drying tends to promote edge-to-face association resulting in
the formation of aggregates with more macropores than in air-dried samples
(Lagaly, 1 987)(Fig. 7.2.1 ). Freeze-drying may either increase or decrease the BET
surface area, depending on the original clay mineral (de Carvalho et al., 1996).
B. Changes in Acidity
The effect of freezing on the acidity of smectite interlayers is similar to that of drying.
As some of the hydration water is removed the interlayer spaces become more acidic
(see Section 7.2.2.C). The proportion of water that is frozen depends upon the
temperature. Changes in acidity in the interlayer spaces are clearly demonstrated by
the colour of a benzidine–montmorillonite assemblage, which depends upon the H
+
-
ion concentration. On cooling from –5 to 15 1C and below, the colour changes
from the original blue of the monovalent semiquinone to green or bluish-green. The
green colour is produced by a mixture of the blue monomer and the yellow dimer
that is formed at low pH (Lahav and Anderson, 1973).
Chapter 7.2: Thermally Modified Clay Minerals290
C. Changes in Mechanical and Rheological Properties
Freezing of water in pores and fissures causes expansion, which leads to cracking and
changes in the mechanical properties of the clay mineral. Repeated freezing–thawing
cycles exacerbate this condition. The effect of freezing on clay mineral properties is
of consider able importance to the behaviour of soils in cold climates.
Freezing and thawing has long-lasting effects on the rheological properties of 2:1
type clay minerals. Schwinka and Mortel (1999), for example, have found that the
viscosity and plasticity of illite suspensions are increased by freezing and thawing.
The effects are enhanced by the addition of small amounts of Na
2
SiO
3
, but reduced
by addition of dival ent cations. Freezing and thawing also increase weight loss on
dehydration. These phenomena are attributed to the formation of water layers
bound to the clay mineral surface but are not observed with kaolins. The effects of
freezing–thawing cycles on the illite slurries are persistent. They extend up to the
sintering process and even influence the texture and mechanical strength of the
ceramic material obtained from the slurries.
7.2.2. DEHYDRATION OF CLAY MINERALS
On heating, all clay minerals pass through a temperature range in which they are
dehydrated to various degrees. In the upper region of this temperature range de-
hydration and dehydroxylation may overlap. Dehydration causes changes that can
be controlled and utilised. Loss of adsorbed water alters the macro- and micropo-
rosity of the clay mineral as well as its plasticity. Interlayer spaces collapse and CEC
dispersed
freeze drying air drying
Fig. 7.2.1. Effect of freeze-drying on the aggregation of clay mineral particles (Lagaly, 1987).
7.2.2. Dehydration of Clay Minerals 291