Clay Minerals (1999) 34, 647-655
Effect of heating and acid pre-treatment
pore size distribution of sepiolite
on
S. B A L C I
Department of Chemical Engineering, Faculty of Engineering and Architecture, Gazi University,
06570 Maltepe - Ankara, Turkey
(Received 21 May 1998; revised 15 January 1999)
A B S T R A C T : Due to its channels of molecular dimensions and a high specific surface area,
sepiolite has many industrial applications which require high resistance to thermal effects in addition
to a large surface area. On heating, sorbed water molecules are removed causing changes in the pore
size distribution. In this study, the effects of thermal treatment on the pore structure of sepiolite and
the acid-treated sepiotite samples were investigated. The solid density of sepiolite, measured by a He
displacement technique, was 2.08 g cm 3 and total porosity was ~0.58. Both of these values showed
an increase at 100~ then decreased with further temperature increase due to crystal deformation and
channel plugging which occurred at elevated temperatures. The BET surface area of the original
sepiolite was 148 mz g 1, and increased to 263 m 2 g-1 at 100~ and then started to decrease.
Approximately 16% of the total volume was in the micropores at 100~ The acid pre-activation
caused restrictions in possible crystal deformation during thermal treatment. The micropore volume
increased to 20% and BET surface area reached values >500 m 2 g-1 for the acid-treated samples.
Sepiolite is a fibrous, hydrated Mg-A1 silicate clay
mineral. Tetrahedral A1 is present at between
0.4-0.48 atoms per twelve tetrahedral sites. The
discontinuous octahedral sheets provide for infinite
channels along the fibre axis with cross-section of
-10 x 4 ~2. During crystal growth, a second set of
irregularly shaped pores with a diameter o f - 1 0 0
is created within the fibre. The presence of channels
and pores in the structure accounts for both the high
surface area and the capacity to adsorb various
materials. Hence sepiolite finds application as an
adsorbent and catalyst support (Martin Vivaldi &
Fenoll Hach-Ali, 1969; Serratosa, 1978; de la
Caillerie & Fripiat, 1992).
The need for the purification of wastes and the
recovery of valuable chemicals has grown in
parallel with developing technology. Consequently,
the demand for porous adsorbents with characteristics appropriate for different applications has also
increased. The textural and structural of sepiolite
make it a useful adsorbent and catalyst. The
adsorptive properties of sepiolite toward many
gases, vapours and liquids have been studied by
several authors (Lopez-Gonzalez et al., 1978; Serna
& van Scoyoc, 1978; Sugiura et al., 1991; Bernal &
Lopez-Real, 1993; Rodriguez & Martinez, 1993;
Daza et al., 1993; Guijarro et al., 1994; Llnal &
Erdo~an, 1998). Chemical reactions, such as
synthesis, polymerization, thermal decomposition
and/or dehydration of organic molecules in the
channels of sepiolite or sepiolite-supported catalysts
may prove to be useful applications of the mineral
(Corma & Mocholi, 1992; Corma et al., 1993;
Rankel, 1994; Bautista et al., 1996; Bahamonde et
al., 1996).
Important characteristics relating to sepiolite
applications are particle size, particle shape,
surface area, surface chemistry, together with
other physical and chemical properties specific to
a particular application. All of these mineral
properties are affected by thermal and chemical
treatments. It is well known that water in sepiolite
consists of: (1) hydroscopic water; (2) zeolitic
water; (3) bound water; and (4) structural water, all
of which leave the mineral at different temperatures
upon thermal treatment. During dehydration, the
9 1999 The Mineralogical Society
648
S. Balct
pore structure and surface area of the mineral are
altered due to changes in the crystal structure.
Variations in pore structure cause changes in the
active surface area and may also increase or
decrease the transport limitations. Hydroscopic
water is adsorbed on the sepiolite surface, the
amount depending on the relative humidity and it is
desorbed at lower temperatures. Zeolitic water fills
the narrow, regular channels in the octahedral
layers, is eliminated by outgassing a little above
100~ and its removal increases the surface area.
The water molecules sorbed on the edges of the
octahedral sheets constitute the bound water. All the
zeolitic water and approximately half of the bound
water are lost by 300~ At this stage of hydration
the crystal structure undergoes drastic change,
which is commonly referred to as 'crystal
folding'. The rest of the bound water is lost
between 300-500~ without any other significant
structural changes taking place. During the last
stage of dehydration, the structural water molecules
are removed at temperatures >700~ The formation
of enstatite, which causes a drastic decrease in
surface area starts above 800~ (Martin Vivaldi &
Fenoll Hach-Ali, 1969; Serratosa, 1978; Serna &
van Scoyoc, 1978; Kiyohiro & Otsuka, 1989; Balcl,
1996). Furthermore, purification of the mineral
using acid and base treatments causes changes in
the pore and crystal structure as well the
physicochemical properties of the mineral. Acid
pre-treatment causes the removal of Mg during
which some of the bound water and structural water
associated with the Mg coordinate may be taken
away before thermal dehydration (Jimenez Lopez et
al., 1978; Campelo et al., 1989; ~eti~li &
Gedikbey, 1990; Balcl, 1996; Myriam et al., 1998).
Since the diffusion mechanism in solids depends
on the pore size distribution, a knowledge of that
distribution will help to understand physical
adsorption and adsorption accompanied by surface
reaction in porous solids. Indeed, a particular
distribution of the pore sizes may enhance or
restrict the given application. The presence of
macro and mesopores (pores with larger dimensions) play a significant role in the mass transfer to
the micropores, where adsorption and/or chemical
reactions take place.
The changes occurring in the pore and crystal
structure of sepiolite minerals of different origin
with thermal and chemical treatments have been
investigated by several authors. Inagaki et al.
(1990) studied the micropore distribution of the
sepiolite from Turkey, treating the samples with
water vapour at various pressures. De la Caillerie &
Fripiat (1992) observed an increase in surface area
at 100~ and a rapid decrease with further
temperature increase. Sarkaya et al. (1993)
proposed a technique called thermoporometry, for
the evaluation of macro and mesopore size
distribution. This technique is based on the
evaluation of melting and vapourization of water
in the pores. Ruiz et al. (1996) studied the textural
changes of the mineral with thermal effects.
Nitrogen adsorption experiments showed that
crystal folding starts at 390~ and the microporosity first decreases, then stays constant up to
570~
Literature studies have shown that acid treatment
causes dissolution of the octahedral sheet, an
increase in porosity and the eventual formation of
amorphous silica. Moreover, a decrease in micro
and mesoporosities is observed for harsher acid
treatments (Jimenez Lopez et al., 1978; Lopez
Gonzalez et al., 1978, 1981; Rodriguez Reinoso et
al., 1981; Corma et al., 1984; Vicente et al., 1994;
Myriam et ai., 1998). Jimenez Lopez et al. (1978)
observed surface area development with HNO3
treatment at concentrations <1 M. Activation of
sepiolite with dilute HNO3 and subsequent heat
treatment were investigated by Lopez Gonzalez et
al. (1981). The changes in pore structure during
thermal decomposition of acid pre-activated
samples were investigated by Campelo et al.
(1989). Rodriguez et al. (1995) studied the
characterization of sepiolite treated with aqueous
HC1 solutions. The free silica formed during acid
activation, particularly using harsh treatments, has
little effect on the properties of the solid. Myriam et
al. (1998) observed that the BET surface area of
untreated sepiolite increases from 213 m2g 1 to
340 m2 g-1 when treated with 1 N HC1 solution.
There is an increasing demand for porous
materials as adsorbents and catalyst supports. The
abundance and availability of sepiolite reserves
together with its relatively low cost guarantee its
continued utilization. Most of the world sepiolite
reserves are found in Turkey. Thus it is important
to characterize those minerals and evaluate how
important physicochemical properties are altered
during chemical and thermal treatment. The
objective of this study was to investigate the pore
structure and sorption characteristics of sepiolite
and their variation with thermal dehydration and
acid treatment using H2SO4.
649
Pore size distribution in acid-treated sepiolite
TABLE 1. Chemical analysis of sepiolite and acid-treated sepiolite samples.
Sepiolite
Acid treated
MATERIALS
SiO2
MgO
A1203
62.70
92.54
24.45
1.95
2.96
1.61
AND METHODS
The sepiolite samples were from large deposits in
the Sivrihisar region, Central Anatolia. Acidactivation was carried out using 2 ~ H2SO4 at
room temperature for 24 h. The mineral particles
were filtered and washed with distilled water
several times, then dried at 40~
Chemical
analyses of the samples were obtained using a
Philips X'Cem spectrophometer (Table 1). Thermal
decomposition of sepiolite samples and acidactivated samples was carried out with an air flow
rate of 200 cm 3 min -1 in a Linsesis L81,
Thermogravimetric Analyzer, at a heating rate of
20~ min-1. 200 mg of particles near 0.91 mm in
equivalent diameter were used. Samples were
maintained at the final temperature for 30 min
prior to cooling (Balcl, 1996).
For the characterization studies, samples
(150-200 mg) were heated to the seleted dehydration temperatures at a rate of 20~ min -1. The
samples were kept at the selected final temperatures for 30 min, to allow the dehydration reactions
to be completed. The N2 adsorption-desorption
experiments of the samples were carried out at
77 K using a Quantaehrome Monosorb Surface
Analyzer. In Balcl's previous work (1996), the N2
data were analysed using the Brunaner-EmmetTeller method (Gregg & Sing, 1982). The mesoo
*~\
5
-....
\~
Sepiolite
Acid treated
8 Io
g
20
2 .=
0
I
100
I
200
I
300
P
400
I
500
I
600
I
700
I
800
I
900
11300
Temperature ( ~
FIG. 1. Thermal activation behaviour of sepiolite and
acid-treated sepiolite samples.
CaO
6.11
3.42
Na20
Fe203
2.61
0.05
0.89
0.22
K20
0.28
0.21
and macro-pore size distribution of the samples
was obtained using a 30,000 psi Mercury Intrusion
Porosimeter which enables pores having diameters
>6.7 nm to be measured. Solid densities of samples
were measured using a Micromerities 1302
Helium-Air Pycnometer. Knowing the apparent
and solid densities, total porosities were calculated.
Porosity is defined as the void fraction of the
samples (Gregg & Sing, 1982). X-ray diffraction
traces were obtained using a Philips PW3710 X-ray
diffractometer.
RESULTS
AND DISCUSSION
Hydroscopic water and zeolitic water were removed
from the sepiolite channels up to 220~ resulting in
9% weight loss (Fig. 1). The removal of bound
water occurred in two steps. The removal of some
bound water caused erystal deformation and mass
transfer limitations, caused by channel plugging,
decreased the dehydration rate in the final part of
the bound water removal. All the bound water was
desorbed by 750~ resulting in a -14% weight loss.
The loss of structural water occurred at higher
temperature and a further -1.5% extra weight loss
occurred up to 900~ No more weight loss was
observed during the isothermal period of 30 rain at
900~ In contrast, the acid treatment caused the
removal of octahedral Mg ions resulting in some of
the bound and structural water associated with Mg
coordination being removed along with a rearrangement of the crystal structure leading an increase in
pore volume. Hence, acid-treated samples showed
different thermal behaviour due to their rearranged
crystal structure and chemical composition. Acid
activation and the pre-drying of samples at 40~
caused the removal of some parts of hydroscopic
and zeolitic water before heat treatment.
Consequently, the observed weight loss up to
250~ was -7.5%. The weight loss observed
between 250-900~ was approximately half of
the value observed for the untreated mineral over
the same temperature range (Lopez Gonzalez et al.,
650
S. Balcl
TABLE 2. Pore size distribution data for sepiolite samples dehydrated at different temperatures.
Dehydration temperature, ~
100
200
300
500
Room
temperature
Characteristics
Total porosity (= pore vol./sample vol.)
Microporosity
Total pore volume, cm3/g
Pore volume corresponding
to pores d>6.7 nm, cm3/g
BET surface area, m2/g
Surface area corresponding
to pores d>6.7 nm, m2/g
Solid density from He-Air
displacement technique, g/cm3
Apparent density from mercury
intrusion technique, g/cm3
0.580
0.066
0.656
0.581
148
79
0.700
0.116
0.951
0.794
263
65
0.670
0.091
0.846
0.760
0.612
0.078
0.745
0.667
240
48
0.600
0.070
0.748
0.661
219
43
148
68
900
0.551
0.040
0.678
0.629
64
54
2.078
2.429
2.304
2.058
2.00
1.810
0.879
0.736
0.762
0.797
0.802
0.811
1981; Corma et al., 1984; Campelo et al., 1989;
Balcl, 1996).
The XRD patterns of sepiolite, acid-activated
sepiolite and sepiolite dehydrated at 900~ are
given in Fig. 2. There is evidence of a CaCO3
impurity in the sepiolite (Fig. 2a). Acid treatment
caused 92% removal of Mg resulting in a change in
the crystal structure of the mineral (Table 1, Fig. 2).
It is apparent that CaSO4 was formed during acid
treatment (Fig. 2b). Dehydration of sepiolite at
900~
caused a considerable change in the
mineral structure due to the formation of enstatite.
The results of N2 adsorption experiments which
were carried out between the relative equilibrium
pressure range of O.05<P/Po <0.30, were interpreted
using BET methods (Balcl, 1996) and the BET
surface area values of the samples, dehydrated at
different temperatures, are summarized in Tables 2
and 3. The variation of cumulative pore volume for
untreated mineral and acid-activated mineral corresponding to pores having dimensions >6.7 nm are
given in Figs. 3 and 4, respectively. The differential
pore volume distribution of the untreated mineral is
given in Fig. 5 and that of acid-activated mineral in
a
0
5
10
15
20
25
30
35
40
45
50
55
60
65
70
75
~
FIG. 2. XRD patterns Of (a) sepiolite; (b) sepiolite treated with 2 M H2804; (c) sepiolite dehydrated at 900~
651
Pore size distribution in acid-treated sepiolite
1.00
0.90
Sepiolite
0.80
.... t
0.70
i
0.60
~-~
-oA
0.50
ioo~
2oo~
/ ~ -
_
oooc
500~
~
" ~ ~
........
;::~ 0.40
0.30
0.20
0.10
0.00
0
0
0
0
0
0
0
0
0
0
0
0
0
0
0
Pore diameter, nm
FIG. 3. Variation of cumulative pore volumes with pore diameter for sepiolite samples dehydrated at different
temperatures.
2.00
1.80
1.60
1,40
--4,-- Acid treat.
t lO0~
~'~ L20
- - e - - 250oc
~?
I.oo
_.<>_ 300~
~
3500(2
0.80
~
A
./....u
500~
900oC
0.60
0.20
0.00 I
Pore diameter, nm
FIG. 4. Variation of cumulative pore volumes with pore diameter for acid-treated sepiolite samples dehydrated at
different temperatures.
652
S. Balcz
TABLE 3. Pore size distribution data for acid-treated sepiolite samples dehydrated at different temperatures.
Characteristics
Total porosity (= pore vol./sample vol.)
Microporosity
Total pore volume, cm3/g
Pore volume corresponding to pores
d>6.7 nm, cm3/g
BET surface area, mZ/g
Surface area corresponding to pores
d>6.7 nm, mZ/g
Solid density from He-Air
displacement technique, g/cm 3
Apparent density from mercury
intrusion technique, g/cm 3
Room
100
temperature
0.708
0.108
1.430
1.212
375
109
0.835
0.169
2.080
1.666
521
230
Dehydration temperature, ~
250
300
350
0.767
0.149
1.549
1.247
503
157
0.734
0.138
1.425
1.157
496
81
0.646
0.130
1.077
0.860
484
69
500
900
0.586
0.117
0.909
0.728
0.562
0.092
0.878
0.734
456
32
360
30
1.698
2.489
2.124
1.939
1.696
1.560
1.462
0.495
0.410
0.495
0.515
0.600
0.645
0.640
Fig. 6. Detailed information relating to surface area
and distribution of pore volumes for the untreated
and acid-activated samples dehydrated at different
temperatures, are reported in Tables 2 and 3.
The BET surface area of the original mineral was
~150 m 2 g-1 whilst the surface area of the sepiolite
corresponding to pores having a diameter >6.7 n m
was - 8 0 m 2 g-1. The total pore volume and total
porosity of the mineral were -0.66 cm 3 g 1 and
0.58 respectively. The pore volume corresponding
to those pores having a diameter >6.7 nm, was
- 0 . 5 8 cm 3 g-1. R e m o v a l o f z e o l i t i c w a t e r
0.60
0,50
0.40
r
Sepiolite
I
100~
,L
200~
--0-- 300~
p-
500~
0.30
900~
Q
0.20
0.10
0.00
~
Average pore diameter, nm
FIG. 5. Variation of differential pore volumes with pore diameter for sepiolite samples dehydrated at different
temperatures.
653
Pore size distribution in acid-treated sepiolite
0.60
0.50
r
Acid treat.
!
100~
2500(2
0.40
---0-- 3OO~
>
350~
0.30
500~
0.20
0.10
0.00
0
0
0
0
0
o
0
0
g
0
0
w
A v e r a g e pore diameter, n m
Fie. 6. Variation of differential pore volumes with pore diameter for acid-treated sepiolite samples dehydrated at
different temperatures.
molecules from the voids and channels of the
mineral increased the BET surface area at 100~
almost two-fold. Porosity values also showed an
increase. The percentage of the micropore volume
over the total volume was increased to 16 from 11
by dehydrating the mineral at 100~ Recalling that
the removal of bound water is achieved by the
direct removal of the water molecules coordinated
to Mg 2+ cations and that this breakage might cause
rearrangements within the crystal structure of the
mineral, then it is likely that crystal folding and, as
a result, micropore plugging occurred. Hence, the
BET surface area and micropore volume percentage
d e c r e a s e d as the p r e - t r e a t m e n t t e m p e r a t u r e
increased. Above 800~ the formation of enstatite
began, and therefore a considerable decrease both in
porosity and surface area was observed as expected.
Only 7% of the total pore volume corresponds to
micropore volume and the BET surface area was
also lowered to 64 m 2 g - l at 900~ (Table 2).
Moreover, the breaking of water molecules from the
octahedral sheets might decrease the strength of the
pore walls, which may result in pore wall opening
at elevated temperatures, thus causing an increase
in the amount of meso and macropore volumes.
Due to the pore wall deformation, pore volume
values in the macro and mesopore ranges showed a
slight increase with increasing temperature. The
macro and mesopore volume at 300~
was
0.67 cm 3 g 1 and stayed approximately constant
up to 500~
The surface area of the mineral
corresponding to the pores having diameter
>6.7 nm and BET surface area values were
~68 m 2 g 1 and 148 m 2 g 1, respectively, at that
dehydration temperature. The differential pore
volume of the samples showed an increase for the
pores having diameter >20 nm (Fig. 5). It was
apparent from these data that the average pore
diameter for the pores having diameters >6.7 nm
was - 5 0 nm. First a slight increase was observed in
the average pore diameter with increasing temperature, then it decreased (Fig. 4).
The acid pre-treatment and pre-drying at 40~
caused the removal of some of the zeolitic and
structural water associated with the octahedral
sheet, other than thermal dehydration (Fig. 1).
Acid treatment also caused the generation of
amorphous porous silica (Fig. 2). Together, these
654
S. Balct
might cause a development in the initial pore
structure as concluded from the literature studies.
The development of this pore size distribution with
acid activation caused an increase in BET surface
area of ~2.5 times the original value for the
mineral, accompanied by twice the microporosity
(Table 3; Figs. 4, 6). The percentage of the
micropore volume over the total was >15%. The
pre-removal of some parts of the water molecules
also lowered the percent weight loss during heat
effects (Fig. 1). The initial pore size development
and reduced weight loss may restrict the micropore
plugging brought on by thermal dehydration at
elevated temperatures. The BET surface area
reached 521 m 2 g-I with a microporosity of 0.17
at 100~ both of which remained nearly constant
up to 250~
The variations in both BET surface
area and microporosity were very small up to
350~ with a micropore volume around 20% in this
temperature range. The crystal folding and as a
result micropore plugging were at their smallest
value up to 700~
The BET surface area was
-425 m 2 g-1 at that temperature and it reduced to
360 m 2 g-1 at 900~ (Balcl, 1996). From the
chemical analysis and XRD patterns it is seen that
acid treatment caused the removal of some Mg
from octahedral coordination (Table 1, Fig. 2). The
Mg removal might limit the formation of simple
silica enstatite which means that micropore
plugging at elevated temperatures may be restricted.
However, samples with high surface area were
obtained at high temperatures.
On the other hand, the acid treatment caused the
leaching of some inorganic parts of the mineral other
than the removal of water. This leaching might
decrease the strength of the pore walls which results
in the pore wall opening. Hence, acid pre-treatment
might increase the amount of meso and macropore
volumes. The pore volume for the pores having the
dimensions >6.7 nm was 1.21 cm 3 g 1 for acidtreated sample while that of the untreated mineral
was 0.58 cm 3 g-1. These values showed an increase
at 100~
then started to decrease with further
temperature increase. Whereas the presence of large
pores may restrict the transport resistance within the
particle during selected applications, the presence of
a large amount of large pores may not be an
advantage in others. In such a case, the number of
large pores might be restricted by the controlled
acid-treatment.
The solid densities in the acid-treated series are
lower than those of untreated samples due to the
dissolution of Mg. Solid density values showed a
decrease above 100~ for both untreated sepiolite
and the acid-treated samples. This behaviour is
consistent with the decreasing trend of the surface
area, the total porosity and the microporosities
(Tables 2, 3).
CONCLUSIONS
The removal of zeolitic water causes an increase
both in surface area and microporosity. Acid
treatment resulted in improvements in the pore
size distribution. A solid with a surface area of
521 m 2 g i and a total porosity of 0.84 is obtained
for the sepiolite treated with 2 M H2SO4, then
dehydrated at 100~ For the acid-treated sample, a
solid with microporous structure and high surface
area was still obtained at elevated temperatures.
ACKNOWLEDGMENTS
This work was partially financed by the Research Fund
AFP/MMF 06/98-13 of Gazi University and by
Government Project DPT/96K 120780.
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