Effect of Al and Ce on Zr-pillared bentonite and their performance in catalytic oxidation of phenol

Article abstract of DOI:10.1134/S0036024416090272

Catalysts based on pillared clays with Zr and/or Al and Ce–Zr and/or Al polycations have been synthesized from a Tunisian bentonite and tested in catalytic oxidation of phenol at 298 K. The Zr-pillared clay showed higher activity than the Al-one in phenol oxidation. Mixed Zr–Al pillars lead to an enhancement of the catalytic activity due to the modification of the zirconium properties. The clays modified with Ce showed high conversions of phenol and TOC thus showing to be very selective towards the formation of CO₂ and H₂O.

Full text of DOI:10.1134/S0036024416090272

ISSN 0036-0244, Russian Journal of Physical Chemistry A, 2016, Vol. 90, No. 9, pp. 1766–1773. © Pleiades Publishing, Ltd., 2016.
CHEMICAL KINETICS
AND CATALYSIS
Effect of Al and Ce on Zr-Pillared Bentonite and Their Performance
in Catalytic Oxidation of Phenol1
Saida Mnasri-Ghnimia and Najoua Frini-Srasraa,b
aLaboratoire de Physico-chimie des Matériaux Minéraux et leurs Applications,
Centre National des Recherches en Sciences des Matériaux, Technopole Borj Cédria, BP 95-2050 Hammam lif, Tunisia
bDépartements de Chimie, Faculté des Sciences de Tunis, 1060 Tunis, Tunisia
e-mail: saidamnasri@gmail.com, nfrini@gmail.com
Received April 24, 2015
Abstract—Catalysts based on pillared clays with Zr and/or Al and Ce–Zr and/or Al polycations have been
synthesized from a Tunisian bentonite and tested in catalytic oxidation of phenol at 298 K. The Zr-pillared
clay showed higher activity than the Al-one in phenol oxidation. Mixed Zr–Al pillars lead to an enhancement
of the catalytic activity due to the modification of the zirconium properties. The clays modified with Ce
showed high conversions of phenol and TOC thus showing to be very selective towards the formation of CO2
and H2O.
Keywords: pillared clays, heterogeneous catalysis, catalytic oxidation in water, phenol, hydrogen peroxide
DOI: 10.1134/S0036024416090272
1. INTRODUCTION
lared clay is the lack of stability at high temperatures.
Thus many attempts have been made to improve the
thermal stability of PILC. One appropriate way to
overcome such a problem is the pillaring of clay min-
erals with solutions containing two different cations
[4–8]. Another way to improve the distribution of the
active phase and increase the catalytic properties of
the PILCs is addition of cerium to the pillaring solu-
tion [9–12].
The aim of the present work is to prepare heteroge-
neous catalysts using natural clay as support. Those
catalysts were tested in the phenol oxidation in aque-
ous medium, using H2O2 as an oxidant, in ambient
conditions (298 K and atmospheric pressure).
We report in this paper the results obtained in the
oxidation of an aqueous solution of phenol over simple
Zr, Al, and mixed Ce–Al and/or-Zr-pillared benton-
ites prepared from zirconium tetrachloride. To our
knowledge, the intercalation of clay minerals with zir-
conium oligomers using zirconyl chloride (ZrOCl2)
and zirconium acetate as the source of the polycations
has been widely studied. Nevertheless, only one paper
describing the pillaring of clay using zirconium tetra-
chloride (ZrCl4) as precursor has been published [13].
New environmental requirements provide signifi-
cant challenges. Large volumes of wastewater are pro-
duced in chemical, petrochemical and pharmaceutical
industries. Such wastewater contains organic contam-
inants which can be treated through homogeneous
catalysts. Homogeneous catalysts must be separated
from the aqueous solution after the reaction, which in
turn increases the costs of the process and brings about
further contamination through the dissolved metal
ions. The use of heterogeneous catalysts constitutes a
true alternative for the phenol oxidation in aqueous
medium. Different solids have been employed to cata-
lyze the oxidation of organic compounds in wastewa-
ters: Transition metal exchanged zeolites [1] and
Cu2+- or Fe3+-montmorillonites have been proposed
as active catalysts for organic compounds oxidation in
aqueous phase [2]. Pillared interlayered clays (PILCs)
have received increasing interest due to their textural
and catalytic properties in different reactions [3].
PILCs or cross-linked clays are obtained by exchang-
ing the interlayered cations of clay minerals with bulky
inorganic polyoxocations followed by calcination. The
intercalated polycations increase the basal spacing of
the clay minerals, and after calcination, they are con-
verted to metal oxide clusters by dehydration and
dehydroxylation processes. These metal oxide clus-
ters, named pillars, prevent the collapse of the clay
mineral sheet structure. But, the inconvenience of pil-
2. MATERIAL AND METHODS
2.1. Starting Material and Solid Synthesis
The original clay is an interstratified illite/smectite
obtained from Gafsa, Tunisia (noted G), homoion-
1
The article is published in the original.
1766

EFFECT OF Al AND Ce
1767
ized with 1 M sodium chloride solution. This clay has mined by Kjeldahl method. The CEC was expressed as
been described in previous works [14–17]. In order to mili-equivalent per gram of the calcined sample.
modify the clay mineral, different intercalated solu-
tions are prepared. Zirconium tetrachloride was used
as a source of zirconium polycation. Al(NO3)3 · 9H2O
was used in the synthesis of aluminum polycations.
Ce(NO3)3 · 6H2O was the source of cerium. The
cerium molar percentage in all solutions was 20%. All
solutions are prepared by slow addition of NaOH
(0.2 M) to the solutions containing Zr, Al, Al–Zr, or
Ce–Al–Zr-cations at room temperature until a spe-
cific pH. For Zr- and Ce–Zr-pillaring solutions this
value was 2.8. It is the most widely used pH value for
Zr-PILCs [18]. Al-and Ce–Al-pillaring solutions
were adjusted to OH-/Al ratio of 2 (pH 4.2) [6, 19, 20].
In the case of Zr–Al and Ce–Zr–Al solutions, pH of
the solution was about 3.8 (near the pH corresponding
to the simple Al-oligomer) and the solution remained
clear. The obtained solutions were aged for 24 h at
room temperature. The clay pillaring process is carried
out through the ionic exchange of the previously aged
solutions, keeping a ratio of 10 mmol of total metal per
gram of clay. The intercalating solution is slowly added
to a 2% clay aqueous suspension and then it was stirred
for 24 h at room temperature. The solids were sepa-
rated by centrifugation and washed by dialysis with
distilled water. The final material was dried at 350 K,
powdered gently in a mortar, and calcined for 2 h at
823 K at a heating rate of 275 K min-1.
Brønsted and Lewis acid centers were determined
by FT-IR spectroscopy on the basis of butylamine
adsorption. n-Butylamine solution in cyclohexane
(10 mL) was added to 0.1 g of the catalyst. The mixture
was shaken at room temperature. After drying, each
sample was calcined at a different temperature. FT-IR
spectra were recorded in the region 1800–400 cm−1 on
a Perkin-Elmer spectrometer using the KBr pellet
technique.
Thermogravimetric analysis (TGA) was done
between room temperature and 1100 K on a Perkin-
Elmer TGS-2 instrument in air (35 cm3/min) at a
heating rate of 10°C min–1. The sample weight was
0.050 g.
2.3. Catalytic Study
The catalytic oxidation of phenol in a diluted aque-
ous medium was carried out in a 250 mL batch glass
reactor open to atmosphere and thermostatted at
298 K with stirring and constant monitoring of pH.
The device featured an automatic reagent dosing unit
(Dosimat 725 Metrohm) for the continuous addition
of hydrogen peroxide solution. For each test the reac-
tor was loaded with 100 mL of a phenol solution
(2.00 g/L) and 0.5 g of catalyst. The high purity hydro-
gen peroxide solution (10.00 g/L) was added at a
2 mL/h rate. The pH was continuously adjusted to 3.7
and an air flow was introduced in the reactor at a vol-
The samples are labeled in accordance with the
nature of pillars (Zr, Ce–Zr, Al, Ce–Al, Alx–Zry, and
Ce–Alx–Zry-bent). The subindex values indicate the ume rate of 2 L/h. Samples from the reaction medium
were taken at initial time and after 15 and 30 min, and
then at each hour until completing 5 h experiment.
The catalyst was removed from the samples using a
0.2 μm nylon filter. The samples were analyzed imme-
diately after taken. The course of the phenol conver-
sion and the appearance of the intermediate com-
pounds were followed by high-performance liquid
chromatography (HPLC) by means of a Waters 600E
liquid chromatograph equipped with an Aminex HPX
87 H (Biorad) column. The evaluation of the total
organic carbon (TOC) was carried out with a Dohr-
man DC-80 device.
metal percentage in the initial pillaring solution.
2.2. Characterization Methods
The X-ray diffraction (XRD) study was done on a
Panalylitical X’Pert HighScore Plus device with CuKα
radiation.
N2 adsorption-desorption experiments were car-
ried out at 77 K on Quantachrome (USA) instrument.
The N2 isotherms were used to determine the specific
surface areas (SA) using the BET equation. The
micropore volume was determined using the t-plot
method and the total pore volume of the samples, Vt,
was calculated at P/P0 = 0.99. Before each measure-
ment the samples were outgassed for 2 h at 403 K.
3. RESULTS AND DISCUSSION
3.1. Characterization of the Pillared Clays
3.1.1. XRD. Table 1 lists the basal spacing of PILCs
with single and mixed oxide pillars after calcination at
823 K. As a reference, basal spacing of the purified
clay calcined at the same temperature was shown as
The chemical analysis of the starting material and
modified samples was performed using AAS Vario
atomic absorption spectrometer.
Cation exchange capacity (CEC) was determined well. As shown in Table 1, the basal spacing increases
by Kjeldahl method. Samples of 200 mg were from 10 to 17–20.5 Å. This fact indicates that the min-
exchanged with the ammonium acetate (1 M) three eral was successfully pillared in all cases. Depending
times and then washed with anhydrous methanol; a on the kind of hydroxycation used as a pillar, the pil-
final wash was performed with deionized water three laring process leads more or less to an increase in the
times. The amount of ammonium retained was deter- interlayer spacing. The PILCs showed different
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SAIDA MNASRI-GHNIMI, NAJOUA FRINI-SRASRA
Table 1. Basal spacings, (after calcination at 823 K), micro-
pores volume, and specific surface areas of the starting and
pillared clays
beneficial effect on the structural properties of Al–Zr-
PILCs [15], indicating that the addition of Ce is likely
to provide favorable conditions for the formation of
the Zr and/or Al pillar, providing greater thermal
resistance [8, 23–26].
3.1.2. SBET. The surface area and micropore vol-
ume values of the studied solids (Table 1) are in agree-
ment with the X-ray diffraction results, revealing an
increase in the surface area of the purified bentonite
after the pillaring process in all samples, which is cor-
responding to the successful pillaring of the clay and
the consequent formation of microporous structures.
BET surface area and micropore volume of the sin-
gle pillared clay Zr- and Al-pillared clays are higher
than that of the mixed Zr–Al pillared clay. The same
observation is noted in previous works dealing with the
mixed Al/Zr, Al/Fe, and Al/Cr [7] and mixed Zr–V
[27]. Incorporating cerium resulted in a decrease in
specific surface area and micropore volume. It has
been reported in the previous literature that the spe-
cific surface area and the pore volume of pillared clay
decrease after addition of cerium [23, 28–30].
SBET (m2 g–1)
3
–1
d001 (Å)
Vμp (cm g )
Na-Bent
Zr-Bent
Ce–Zr-Bent
Al-Bent
Ce–Al-Bent
Al50Zr50-Bent
Ce–Al50Zr50-Bent
Al10Zr90-Bent
Ce–Al10Zr90-Bent
10
107
200
180
191
123
162
121
177
123
0.021
0.063
0.025
0.062
0.024
0.046
0.023
0.054
0.021
18.2
19.3
17.4
18.7
19.2
19.1
20.5
20.5
Table 2. Chemical analysis and cationic exchange capacity
of the starting and the pillared clays
CEC
Al, % Zr, % Ce, %
(meq/100g)
Na-Bent
Zr-Bent
Ce–Zr-Bent
Al-Bent
Ce–Al-Bent
Al50Zr50-Bent
Ce–Al50Zr50-Bent
Al10Zr90-Bent
Ce–Al10Zr90-Bent
78
24
22
43
32
27
26
25
23
24.80
20.34 16.71
20.18 16.57
34.60
34.11
26.48 17.31
26.11 16.96
25.05 20.98
24.97 20.88
0
0
0
0.55
0
0.42
0
0.30
0
0.41
3.1.3. Chemical composition. The chemical com-
position shows that Al content of clays modified with
solution containing Al cation are higher than that of
purified bentonite (Table 2), indicating it effective
introduction in the material. The amount of zirco-
nium taken up by the clay in the presence of aluminum
was larger than the amount taken up by the clay in the
case of simple Zr-pillared clay. In fact, the ZrO2 per-
centages are 16.71, 17.31, and 20.98% for the Zr-bent,
Al50Zr50-bent and Al10Zr90-bent samples respectively.
This indicates a modification of the zirconium pillar
by hydrolysis and/or polymerization [15]. In the case
of samples modified with cerium, the introduction of
cerium in the samples is revealed, showing a practi-
cally greater value in solids modified with the simple
system Ce–Al (0.42%) or Ce–Zr (0.55%), while
materials modified with mixed system Ce–Zr–Al
show values of 0.41 and 0.30%, respectively for Ce–
Al10Zr90-bent and Ce–Al50Zr50-bent samples. The
0
0
increases of basal spacing: 18.2 Å for Zr-PILC, 17.4 Å
for Al-PILC, 19.2 for Al50Zr50-PILC, and 20.5 Å for
Al10Zr90-PILC. Notice that the interlayer spacings of
PILCs with mixed oxide pillars of Zr and Al are higher
than that of PILCs with single oxide pillars of either Zr
or Al. The incorporation of Al leads to an increase in
the basal spacing with greater value for Al10Zr90-bent. same behavior was observed in [31]. Much less Ce was
incorporated into the pillars than originally present in
the solution. [32, 33] observed the same behavior
when pillaring a bentonite with Ce–Al polycation and
suggested that this low amount of cerium could be
explained from the structure proposed for the Al–Ce
polycation, different from the simple structure for the
Keggin ion [8, 27] in which a Ce atom in tetrahedral
coordination is surrounded by four Al13 units, giving
the possibility of generating pillars with a low Ce con-
tent.
Added to the enhancement of the basal spacing, the
addition of Al was found to provide a useful way to
improve the structural properties of the Zr-PILC. In
fact, the structure of mixed Al–Zr-PILCs is well-
ordered and remains stable after calcination at 823 K,
and this is not evident for the Zr-PILC sample which
shows a wider peak [15]. The broadness of the 001 dif-
fraction peak in the case of Zr-PILCs is widely
reported in the literature [21, 22] The addition of
cerium to the aluminum and zirconium intercalated
solution produces an enhancement of the basal spac-
ing from 17.4 to 18.7 Å for Al-bent and from 18.2 to
19.3 Å for Zr-bent. However, no considerable differ-
ences were observed in the basal spacing value of the
Ce–Al–Zr-PILC and Al–Zr-PILC samples
3.1.4. Cation exchange capacity. The CEC of all
pillared samples decreases compared to the CEC of
parent clay mineral. The low values of CEC after pil-
laring (Table 2) suggest the irreversibility of cationic
exchange. The residual exchange capacity of Zr-bent
(Table 1). Besides, the introduction of cerium has a is lower than that of Al-bent. This shows that zirco-
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EFFECT OF Al AND Ce
1769
973 K
873 K
773 K
973 K
873 K
773 K
673 K
673 K
473 K
298 K
473 K
298 K
Ce−Zr-bent
1700 1800 1400
Wavenumber, cm−1
Ce−Al10Zr90-bent
1400
1500
1600
1500
1600
1700
1800
Wavenumber, cm−1
973 K
973 K
873 K
773 K
673 K
873 K
773 K
673 K
473 K
473 K
298 K
298 K
Ce−Al50Zr50-bent
1700 18001400
Ce−Al-bent
1400
1500
1600
1500
1600
1700
1800
Wavenumber, cm−1
Wavenumber, cm−1
Fig. 1. FT-IR spectra of n-butylamine adsorbed on Ce-modified PILCs.
nium polyhydroxycations are more efficient in block-
The IR spectra of n-butylamine treated Ce–Zr-
ing negative charge than Al ones. The incorporation of and/ or Al-PILC at the temperature ranging from 298
cerium leads to a slight decrease in the CEC value in
all pillared samples. This decrease is more pro-
nounced in the case of Al-PILC.
to 973 K are presented in Fig. 1. The addition of the
cerium leads to the appearance, in all samples, of a
new IR band at 1638 cm–1 corresponding to Brønsted
acid sites [34]. Then, the bands assigned to n-butyl-
amine coordinated onto Brønsted acid sites (1540 cm–1)
in the case of Ce–Al-bent remains after evacuation at
673 K. Thus suggests the stronger nature of the Brøn-
sted acid sites when compared to the Al-PILC. This
result indicates that cerium improves the Lewis and
Brønsted surface acidity. Similar behavior was
observed in [26]. Finally, the intensities of the bands
assigned to n-butylamine coordinated onto Lewis and
Brønsted acid sites are slightly reduced in all samples
after thermal treatment.
3.1.5. Surface acidity. The surface acidity of origi-
nal clay and Zr-and/or Al-PILCs evaluated from IR
study of n-butylamine adsorbed on synthesized solids,
has been described in previous works [16]. The results
of surface acidity measurements indicated that:
(i) the modified forms of clay exhibit better Brøn-
sted and Lewis acidity compared to their starting form
(the Na-bent sample) which presents only weak Lewis
acid sites at 298 K;
(ii) Zr pillars leads to stronger acidity than Al pil-
lars;
3.1.6. TGA analysis. TGA curves of the neat and
Al- and/or Zr-pillared clays are shown in Fig. 2. The
(iii) the Zr–Al-PILC samples show the presence of
both Brønsted and Lewis acidity in high concentra- TGA curves for cerium modified pillared clays were
tions.
also examined to identify the role of the cerium on the
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SAIDA MNASRI-GHNIMI, NAJOUA FRINI-SRASRA
0
0
−10
−10
Ce-Al-Bent
Al
Zr-Bent
Ce-Zr-Bent
200 400 600 800 1000 1200
200 400 600 800 1000 1200
0
0
Ce-Al10Zr90-Bent
Al10Zr90-Bent
−10
Ce-Al50Zr50-Bent
Al50Zr50-Bent
−10
200 400 600 800 1000 1200
0
200 400 600 800 1000 1200
Na-Bent
−10
200 400 600 800 1000
T, K
Fig. 2. TGA curves for initial and modified clays.
surface of the clays and are given in Fig. 2. For all sam- corresponds to the desorption of surface water physi-
ples, the weight loss occurs in two steps. In the case of cally adsorbed from 298 to 473 K, whereas the second
the natural clays, two temperature ranges can be dis- one occurs between 673 and 973 K assigned to either
tinguished. The first, up to about 473 K, can be the dehydroxylation of the clay structure and the pillar
ascribed to the elimination of hydration water. The species incorporated after pillaring. As shown in
second, about 700 K, can be associated to the dehy- Fig. 2, however, there is a slight but visible dissimilar-
droxylation of the clay sheets. For all modified clays, ity in the TGA peak of each material. For example, Al-
the weigh losses took place in two steps. The first step PILC gives the TGA peak at 853 K, but for Zr-PILC
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EFFECT OF Al AND Ce
1771
Zr-Bent
Ce-Zr-Bent
Al-Bent
Ce-Al-Bent
Al50Zr50-Bent
120
100
80
(a)
Ce-Al50Zr50-Bent
Al10Zr90-Bent
Ce-Al10Zr90-Bent
Na-Bent
60
40
20
0
60
120
160
240
300
360
Time, min
80
60
Zr-Bent
Ce-Zr-Bent
Al-Bent
Ce-Al-Bent
Al50Zr50-Bent
(b)
Ce-Al50Zr50-Bent
Al10Zr90-Bent
Ce-Al10Zr90-Bent
Na-Bent
40
20
0
60
120
160
240
300
360
Time, min
Fig. 3. Catalytic activity of different catalysts in the phenol oxidation (a) and TOC (b) conversion in diluted aqueous medium at
atmospheric pressure and at 298 K.
this peak is at 673 K. The TGA results of mixed Al–Zr the initial contents of iron oxides found in natural clay
samples show a weight loss at about 878 K. The differ- (8.03%) [33]. The materials modified with Zr species
ence observed in the dehydroxylation temperature can show better results than the solids modified with Al
be due to the difference in the thermal stability of the species. The conversion observed with the clay modi-
synthesized materials. However, the TGA peaks for fied only with aluminum is ascribed to the iron species
samples containing cerium are observed at around found in the octahedral layers of these materials, as
883 K for all samples. This suggests that the introduc- established in previous works [27–30, 32–35]. As
tion of the cerium to the pillared clay may lead to a already mentioned, it has been found that Al10Zr90-
shift of the dehydroxylation temperature to a higher
temperature region, which can be related to the
improvement of the thermal stability observed for the
cerium modified materials.
bent sample shows better phenol conversion than
Al50Zr50-bent. The highest activity was achieved with
the clays modified with higher amounts of Zr (Zr-bent
and Al10Zr90-bent).
3.2. Activity of the pillared clays in CWPO of phenol.
On the other hand, it is important to mention that
Solid material prepared via pillarization of the Tuni- the use of cerium allows to increase the activity of the
sian bentonite with Zr and/or Al, Ce–Zr and/or Al obtained catalysts in phenol oxidation up to 100%
polycations possess excellent catalytic properties in the conversion within the first 2 h of the reaction. In fact,
phenol oxidation reaction. Likewise, considerable after comparing the results with those obtained with-
activity is found in the TOC mineralization (Figs. 3a out Ce, the enormous catalytic potential of these
and 3b). In Fig. 3a, which shows the phenol conver- materials once they have been modified via pillaring
sion as a function of the reaction time for the six mod- with mixed Ce–Zr and/or Al systems is evident, since
ified solids and the natural clay. We can observe that for the clay modified with Zr and Zr–Al the conver-
the starting purified clay (Na-bent) used in this cata- sion was 88% after 2.5 h, for Al-modified clay it was
lytic reaction is quite active. Calcined Na-bent 40% after 3 h, as compared to 100% conversion for all
reached only 31% conversion of phenol after 4 h of samples with cerium. In all the experiments with
reaction. The non-negligible catalytic activity (Zr-and/or Al-PILCs, Ce–Zr-and/or Al-PILCs)
obtained for calcined purified clay can be explained by modified clays, leaching of cerium, zirconium or alu-
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SAIDA MNASRI-GHNIMI, NAJOUA FRINI-SRASRA
300
200
150
100
50
Formic acid
Maleic acid
Oxalic acid
(a)
0
60
120
180
240
300
360
Time (min)
200
(b)
Formic acid
150
100
50
Maleic acid
Oxalic acid
0
100
200
300
400
Time (min)
Fig. 4. Time-evolution curves of aromatic intermediates from phenol with pillared clays; (a) Zr-bent, (b) Al-bent.
minum species was very low (0.01 ppm for Ce, ior as Zr-PILCs. And, as in the case of sample modi-
0.04 ppm for Zr, and 0.05 ppm for Al).
fied without cerium, the time-evolution curves of car-
boxylic acids also suggest a refractory behavior in the
experiments carried out with the Ce–Zr and/or Al-
bent catalyst (not shown in Fig. 4). In fact, the main
carboxylic acid in the case of Ce–Al-PILCs is oxalic.
And in the case of Ce–Zr-and Ce–Zr–Al-PILCs,
formic acid is the main product of this group of inter-
mediates.
TOC conversion levels (Fig. 3b) follow the same
tendency as phenol conversion. Regarding the TOC
conversion reaction (Fig. 3b), the favorable effect of
the cerium addition can also be observed. Enhance-
ment of the catalytic activity in oxidative reactions is
due to the oxygen storage capacity of cerium oxides.
Similar results were observed in [20, 36, 37].
From the above results, the addition of Ce shows
excellent performance. In order to discriminate
between homogeneous and heterogeneous oxidation,
several experiments were done in similar conditions
with soluble cerium species. In this way, subsequent
investigations of the catalytic activity of the clay mod-
ified with only Ce without Al and Zr is assessed. To
investigate the performance of cerium, the starting
clay is exchanged 4 times with 0.4 M Ce(NO3)3 · 6H2O
aqueous solution at 25°C for 24 h. In the same condi-
tions, the Ce-modified bentonite used in this catalytic
reaction is quite inactive (6% conversion of phenol
and 0% conversion of TOC after 20 h). This correlate
well with the result obtained through chemical analy-
sis and XRD patterns. In fact XRD analysis show that
there is an increase in the basal spacing from 12 to
14.3 Å upon insertion of cerium ions in the parent clay
which is attributed to the larger hydration sphere sur-
rounding the Ce ions in the interlayer. When calcined
at 823 K, Ce-bentonite sample shows collapse of the
layered structure. Besides, the chemical analysis data
of this solid after dispersing in the NaCl solution (1 M)
shows that about total of cerium is detected in the
solution of NaCl. This amount is removed by
According to the reaction pathway described in
[38], phenol conversion at first step leads to aromatic
intermediates like hydroquinone and benzoquinone.
These intermediates are then converted to carboxylic
acids. The kinetic curves for carboxylic acids forma-
tion are presented in Fig. 4. Some important differ-
ences can be pointed out among the different pillared
clays. In the case of Zr-PILCs, formic acid is the main
product of this group of intermediates, showing a
remarkable maximum concentration value and then a
decrease due to mineralization. In the case of Zr-bent
sample, the TOC conversion reaches its maximum
level after 200 min (Fig. 3b), although the phenol con-
version is 85% (Fig. 3a) from lower time (150 min) and
at this time the formic acid concentration did not fin-
ish its mineralization clays (Fig. 4a), in agreement
with the result reported by other authors [39]. This
trend is not observed in the case of Al-PILCs, suggest-
ing that whether mineralization is not taking place or
it is balanced by formic acid formation from other car-
boxylic acids in the reaction pathway. The main car-
boxylic acid in the case of Al-PILCs is oxalic. Al50Zr50-
Bent and Al10Zr90-bent samples show the same behav-
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EFFECT OF Al AND Ce
1773
14. S. Mnasri, N. Besbes, N. Frini-Srasra, and E. Srasra,
exchanged reaction. So the phenol oxidation reaction
is catalyzed by heterogeneous catalyst and the homog-
enous part fails to oxidize the phenol.
C.R. Chim. 15, 437 (2012).
15. S. Mnasri and N. Frini-Srasra, Clay Miner. 47, 453
(2012).
16. S. Mnasri and N. Frini-Srasra, Infrared Phys. Technol.
4. CONCLUSIONS
58, 15 (2013a).
Several Zr and/or Al and Ce–Zr and/or Al pillared
Tunisian clays are prepared. Modification of benton-
ite with these polyhydroxycations is a convenient
method of preparation of pillared solids. The catalysts
obtained by Zr or Zr–Al polyhydroxycations pillaring
are highly efficient for the phenol oxidation reaction in
diluted aqueous medium at very mild conditions
(298 K and atmospheric pressure). The stability of the
active phase of these catalysts is evident, showing
extremely low metal leaching.
17. S. Mnasri and N. Frini-Srasra, Surf. Eng. Appl. Elec-
trochem. 49, 336 (2013b).
18. S. V. Awate, S. B. Waghmode, and M. S. Agashe, Catal.
Commun. 5, 407 (2004).
19. R. T. Yang, N. Tharappiwattananon, and R. Q. Long,
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20. M. A. Vicente, M. A. Banares-Munoz, L. M. Gandia,
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21. N. Maes, I. Heylen, P. Cool, and E. F. Vansant, Appl.
Clay Sci. 12, 43 (1997).
The addition of Ce during the synthesis of the cat-
alysts leads to an increase of its activity in oxidation of
phenol and of the formed intermediate compounds
and is beneficial for the total oxidation (mineraliza-
tion) reaction. Finally, the activity of the catalysts is
found to increase with its acidity, which is related to
the Zr content and to the presence of cerium.
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