GeolCarp_Vol53_No2_99

GEOLOGICA CARPATHICA, 53, 2, BRATISLAVA, APRIL 2002

99 — 102

REDUCED CHARGE Al-PILLARED MONTMORILLONITES:

ON THE POSSIBILITY OF CONTROLLING THE PILLAR DENSITY

KRZYSZTOF BAHRANOWSKI

, ADAM GAWEŁ

, ANDRZEJ KIELSKI

ALICJA MICHALIK

, EWA M. SERWICKA

, EWA WISŁA-WALSH

KRYSTYNA WODNICKA

and CHRISTOPH ZWICKY

Faculty of Geology, Geophysics and Environmental Protection, Academy of Mining and Metallurgy,

al. Mickiewicza 30, 30-059 Kraków, Poland

Faculty of Ceramics and Material Sciences, Academy of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Kraków, Poland

Institute of Catalysis and Surface Chemistry, Polish Academy of Sciences, ul. Niezapominajek 8, 30-239 Kraków, Poland

Faculty of Mechanical Engineering and Robotics, Academy of Mining and Metallurgy, al. Mickiewicza 30, 30-059 Kraków, Poland

Eidgenössische Materialprüfungs- und Forschungsanstalt, Überlandstrasse 129, CH-8600 Dübendorf, Switzerland

(Manuscript received October 4, 2001; accepted in revised form December 13, 2001)

Abstract: The microporosity of Al-pillared Li-montmorillonites with different layer charges obtained by thermal treat-
ment at different temperatures was studied by nitrogen adsorption/desorption at —196 °C. A correlation was observed
between the decreasing sorption capacity of samples derived from lithiated precursors heated at ever increasing tempera-
ture and the increasing contribution of the collapsed pyrophyllite-like areas inactive in pillaring. The dominant micropore
radius remained below 3.5

—1

nm for all pillared samples, showing that, contrary to the common belief, the layer

charge reduction by Hofmann-Klemen effect fails to bring about substantial changes in the lateral interpillar distances.

Key words: porosity, nitrogen adsorption/desorption, pillar density, pillared clay, lithiation, reduced charge montmo-
rillonite.

Introduction

It has long been known that upon heating at about 200—300 °C
Li-exchanged montmorillonites gradually loose the cation ex-
change capacity (CEC) (Hofmann & Klemen 1950; Greene-
Kelly 1953; Calvet & Prost 1971; Alvero et al. 1994; Theng et
al. 1997; Karakassides et al. 1997). This effect is due to migra-
tion of the interlamellar Li

ions into the layer, causing the

neutralization of the layer charge. A similar phenomenon is
observed with other cations of small radius (< 0.7

—1

nm)

(Purnell et al. 1991). By applying different temperatures of the
heat  treatment,  montmorillonites  with  their  layer  charge  re-
duced to a different degree may be obtained (Calvet & Prost
1971;  Madejová  et  al.  1996;  Komadel  et  al.  1996).  This  ap-
proach has been used by several authors, who aimed at prepa-
ration of the pillared clays of different pillar density (Suzuki et
al. 1988, 1991; Horio et al. 1991; Purnell 1992; Jones & Pur-
nell 1993, 1994;  Sychev et al. 1997, 2000). Since the number
of pillars that may be incorporated between the clay layers de-
pends on the layer charge density of clay, it follows that upon
reduction of the layer charge fewer pillars with larger lateral
interpillar  distances  will  be  formed,  leading  to  the  develop-
ment of the sorption capacity of the micropore system as well
as  to  the  increase  of  the  micropore  radius.  Interestingly,  the
data presented in the quoted works usually show an opposite
effect, that is, the micropore specific surfaces and micropore
volumes actually decrease with lowering of the layer charge.
Such a phenomenon seems most puzzling and calls into ques-
tion the generally accepted views on the suitability of the lithi-
ating  treatment  as  a  means  for  generating  the  materials  with
controlled lateral spacing of pillars.

In the present study we decided to shed more light on this

problem by investigating the structure and texture of a series
of Al-pillared clays prepared from Li-montmorillonite, layer
charge of which was reduced prior to pillaring to a different
degree by applying thermal treatment at different tempera-
tures.

Experimental

Materials

The material used in this study was the less than 2 µm parti-

cle-size fraction extracted by sedimentation from Milowice
bentonite originating from the coal mine “Saturn” in Sosnow-
iec-Milowice, Poland. The structural formula of the montmo-
rillonite is Na

0.38

0.16

0.19

(Al

2.91

0.39

0.70

) [(Si

7.81

0.19

)

](OH)

and its cation exchange capacity 84 meq per 100 g.

The starting material for pillaring procedures was the lithium
form of this montmorillonite, obtained by treatment with 1 M
LiNO

solution, followed by multiple washing with distilled

water. The product, air dried at ambient temperature is referred
to as Li-mt. Li-mt was divided into six portions, and used for
preparation of LiX-mt series, by 24 h heating at temperature X
(50, 90, 120, 170, 200 and 250 °C). These materials were used
as matrices for preparation of Al-pillared samples, following
the procedure described by Vaughan & Lussier (1980). Chlo-
rhydrol (commercial aluminium hydroxychloride manufac-
tured by the Reheis Chemical Company) was added to the vig-
orously stirred water suspension of an appropriate lithiated
sample, in the amount corresponding to 6 mmol Al

per gram

ncserwic@cyf-kr.edu.pl

MECC ‘01

100 BAHRANOWSKI et al.

of clay. After 30 min of stirring the pH of the suspension (4.9)
was brought down to 2 with a diluted HCl solution and the
mixture heated at 70 °C for 30 min. After washing till the re-
moval of Cl

—

, and drying in air at 50 °C, a series of solids, de-

noted LiX/Al-mt, was obtained. After an additional 3 h of cal-
cination in air at 500 °C the samples were referred to as LiX/
Al-PILC.

Methods

XRD analyses were performed on oriented samples pre-

pared by spreading of the sample suspension on a glass slide
followed by drying at room temperature. The XRD patterns
were obtained with a DRON-3 diffractometer using Ni-filtered
Cu K

radiation. Nitrogen adsorption/desorption experiments

were performed at —196 °C using a static volumetric method
(Micromeritics ASAP 2000V2.03). Prior to adsorption the
samples were degassed down to 1.0

—3

Torr at 200 °C. Ad-

sorption/desorption of nitrogen was carried out in the relative
pressure range of 0.01 < p/p

< 1.00 using 0.1 g of sample. Spe-

cific surface areas were determined from adsorption isotherms
by applying BET and Langmuir equations (S

BET

and S

, re-

spectively) in the relative pressure range of 0.05 < p/p

< 0.25,

and the t method (S

), which consists in comparing the given

isotherm with the reference one obtained for a non-porous sol-
id (De Boer et al. 1966; Sing et al. 1985). In the present case
the  nitrogen  adsorption  isotherm  obtained  for  the  sodium-
montmorillonite heated for 20 h at 800 °C was used as a refer-
ence  absolute  isotherm.  Mesopore  surface  areas  were  deter-
mined  from  the  adsorption  branch  using  the  BJH  method
(S

BJH

) and, independently, from V

-t plots (S

) (Bahra-

nowski et al. 2000). Micropore surface areas (S

) were calcu-

lated by subtracting the mesopore surface areas S

from the

total specific surface areas S

. Mesopore volumes (V

) were

determined from the adsorption branch using the BJH method
(Barrett et al. 1973), while micropore volumes (V

) and the

micropore dominant radius (r

) were calculated with the MP

method (Mikhail et al. 1968; Mikhail & Brunauer 1975), using
0.25

—1

nm intervals for computations. For the sake of com-

parison the micropore volumes (W

) were also calculated us-

ing the Dubinin-Radushkevich method (Dubinin et al. 1975;
Dubinin 1981).

Results and discussion

The XRD patterns of LiX/Al-mt and LiX/Al-PILC samples

are presented in Figs. 1 and 2, respectively. The d

(001)

basal

spacing of all uncalcined samples is in the range 19.8—20.8

—1

nm (4.2°< 2

θ<4.5

°), and as the lithiating temperature

(X) increases the 001 peak becomes broader and less intense.
Simultaneously, the intensity ratio I

≅

—1

≅

—1

grows in the order 0.2 (50 °C), 0.2 (90 °C), 0.4 (120 °C), 1.3
(170 °C), 1.5 (200 °C), 2.0 (250 °C). Such a phenomenon can be
understood if one recalls that the reflection around 10

—1

θ ≅ 8.8

°), which is characteristic for the second order 002

peak of the

≅

—1

nm basal reflection of the pillared mont-

morillonite, may also be due to the montmorillonite with col-
lapsed pyrophyllite-like layers. Thus, the observed increase in

the intensity ratio I

≅

—1

≅

—1

may be interpreted as

due to the increasing contribution from the pyrophyllite-like
areas generated in lithiated samples by thermal treatment. It is
worthwhile to note that in the sample dried at 120 °C a weak
shoulder around 30

—1

nm (2

θ ≅ 3.0

°) may be seen, sug-

gesting a possible existence of interstratified areas with pyro-
phyllite-like/pillared smectite layer ordering. The XRD pat-
terns of calcined LiX/Al-PILC samples show a qualitatively
similar effect. The 001 reflections are broader and more dif-
fuse, with d

(001)

values between 17.3—18.3

—1

nm (4.8° < 2

< 5.1

°). Their intensity rapidly decreases with the increasing

temperature of preparation of the Li-mt precursor. In contrast,
the peak at ca. 10

—1

nm persists and its intensity relative to

the 001 reflection increases, so that it dominates the patterns of
Li200/Al-PILC and Li250/Al-PILC samples. Thus, it appears,
that upon increasing the temperature of lithiation, increasing
areas of montmorillonite become unable to reopen during

Fig. 2. XRD patterns of LiX/Al-PILC samples (X – temperature
of lithiation).

Fig. 1. XRD patterns of LiX/Al-mt samples (X – temperature of
lithiation).

Θ CuKα

250

200

170

120

Θ CuKα

ity

(

.u.)

250

200

170

120

REDUCED CHARGE Al-PILLARED MONTMORILLONITES 101

Parameter

Li-mt

Li50/Al-PILC

Li90/Al-PILC

Li120/Al-PILC Li170/Al-PILC Li200/Al-PILC

Li250/Al-PILC

BET

/g)

265

255

196

152

107

/g)

332

322

249

193

140

104

/g)

336

321

244

189

128

µp

/g)

306

288

214

171

104

/g)

BJH

/g)

µp

(cm

/g)

0.016

0.103

0.098

0.074

0.060

0.039

0.026

D-R

(cm

/g)

0.019

0.122

0.120

0.090

0.068

0.046

0.034

BJH

(cm

/g)

0.099

0.089

0.092

0.098

0.089

0.083

0.096

(nm

×10)

<3.5

Hysteresis loop

H-3

H-4

H-3

treatment with pillaring solution. This indicates that the layer
charge reduction is by no means homogeneous, and even at rel-
atively low temperatures collapsed areas exist in LiX-mt pre-
cursors. A similar effect has already been reported by other au-
thors (Madejová et al. 1996; Komadel et al. 1996). Komadel et
al. (1996), who investigated the Li-exchanged thermally treat-
ed Jelšový Potok montmorillonite. They found that on increas-
ing the temperature of heat treatment, ever larger non-swelling
areas appeared. They were identified as pyrophyllite-like lay-
ers, whose contribution reached 20 % already at a temperature
as low as 135 °C. In some cases the authors observed the for-
mation of interstratified pyrophyllite-like/smectite phases. The
latter phenomenon is also seen in the present study in the pil-
lared sample derived from Li120-mt. The existence of the su-
perstructure, indicated by the shoulder in the XRD diagram of
the uncalcined Li120/Al-mt sample, becomes more obvious af-
ter calcination. Here, the XRD pattern of Li120/Al-PILC
shows a resolved maximum at ca. 28

—1

nm (2

θ ≅ 3.2

°), con-

firming the occurrence at this temperature of pyrophyllite-like/
pillared smectite interstratification.

Fig. 3 shows the nitrogen adsorption/desorption isotherms

obtained for Li-mt and all investigated LiX/Al-PILC samples.
The highest sorption capacities are observed for the Li50/Al-
PILC and Li90/Al-PILC materials. Further increase of the pre-
treatment temperature results in a steady downward shift of the
corresponding  isotherms.  The  data  gathered  in  Table  1  show
that all samples have predominantly microporous character, as
the  specific  surface  area  associated  with  this  group  of  pores
corresponds  to  70—90  %  of  the  total  value.  The  existence  of
significant microporosity affects the applicability of the BET
formalism.  In  such  cases  the  Langmuir  equation  may  give  a
better fit. Indeed, when the specific surface areas are also cal-
culated using the t method, the S

values are very close to the

Langmuir specific surface areas. The most important message
conveyed by the data in Table 1 is that the main effect upon in-
creasing temperature of lithiation consists in reduction of the
total surface available for adsorption, which occurs on the ac-
count of decreasing microporosity. Starting from 120 °C pre-
treatment, all parameters describing microporosity become
steadily reduced. An important information is provided by the
MP method, which shows that in all samples the dominant mi-
cropore radius is below 3.5

—1

nm. Consequently, any possi-

ble change in the pillar density, in particular an increase of the
interpillar distance leading to the increase of the micropore

size, must be rather small and remains in the range not probed
by the MP method. Thus, the picture that emerges from the ni-
trogen sorption experiments confirms the conclusions drawn
on the basis of XRD analysis. The main effect occurring upon
increasing the temperature of thermal treatment of Li-mt pre-
cursor is the progressively increasing contribution of clay ar-
eas unable to open upon pillaring, rather than formation of
material of increasing micropore sizes.

The above described mechanism of pillaring appears to be

due to the inhomogeneous character of layer charge reduction
and might be specific for a given montmorillonite. However,
in view of a similar observation reported for Jelšový Potok
(Madejová et al. 1996; Komadel et al. 1996), and numerous

Fig. 3. N

adsorption/desorption isotherms at —196 °C for Li-mt

and LiX/Al-PILC samples (X – temperature of lithiation).

Table 1: Textural characteristics of LiX/Al-PILC (the meaning of abbreviations in the Methods).

Fig. 3

100

120

140

0,1

0,2

0,3 0,4

0,5 0,6

0,7 0,8

0,9

p/p

]

Li250/Al-PILC
Li200/Al-PILC
Li170/Al-PILC
Li120/Al-PILC
Li90/Al-PILC
Li50/Al-PILC
Li-mt

102 BAHRANOWSKI et al.

studies in which the main effect of lithiation at increasing tem-
peratures on the properties of pillared materials was the reduc-
tion of the sorption capacity (Purnell 1992; Jones & Purnell
1993, 1994; Suzuki et al. 1988, 1991; Horio et al. 1991; Sychev
et al. 1997, 2000), it seems that the phenomenon is of a general
nature. Thus, the common belief that the layer charge reduc-
tion by Hofmann-Klemen effect allows an easy control of lat-
eral interpillar distances and enhancement of the textural prop-
erties appears to be unsubstantiated.

Conclusions

Investigation of the reduced charge Li-montmorillonite ther-

mally treated at increasing temperatures and pillared with Al
oligomers shows that the layer charge reduction occurs in an
unhomogeneous manner, resulting in ever increasing areas in-
active in the pillaring process. In consequence, the sorption
capacity of the samples, associated mainly with the microporos-
ity, diminishes. No evidence of an increase of the dominant mi-
cropore radius can be detected by the MP calculations, indicat-
ing that, any effect, if present, remains below the 3.5

—1

radius probed by this method. In view of the similar sorption
characteristics of pillared reduced charge clays reported previ-
ously by other authors, the proposed explanation of the phe-
nomena accompanying lithiation appear to be of general sig-
nificance.

Acknowledgment: This work is dedicated to Professor
Leszek Stoch, on the occasion of his 70

birthday and 50 years

of scientific activity. The financial support by the Polish Com-
mittee for Scientific Research, KBN, Warsaw, within Re-
search Project 3 T09A 079 14 is gratefully acknowledged.

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