A Novel Synthesis Method of Al/Cr Pillared Clay and its Application in the Catalytic Wet Air Oxidation of Phenol

Article abstract of DOI:10.1007/s10562-018-2579-x

Abstract: Natural bentonite clay was pillared with Al and Cr polycations (Al-PILCs and Cr-PILCs) as well as with both Al and Cr mixed polycations (Al/Cr-PILCs) at different Al/Cr molar ratios. The structural and textural properties of the pillared clay catalysts were characterized by various techniques, such as high resolution scanning electron microscopy with energy dispersive system, powder X-ray diffraction, adsorption–desorption of N₂ at ? 196?°C, Fourier transform infrared spectroscopy, thermogravimetric analysis, cation exchange capacity and zeta potential. The obtained results showed that the intercalation of polycations in the interlamellar space of clay led to changes in the clay properties, basal spacing and specific surface area of a mixed PILCs were higher compared to single PILCs. Moreover, diffractograms of the Cr-PILCs confirmed the presence of a Cr₂O₃ phase. The catalytic wet air oxidation (CWAO) of the phenol was studied in a batchwise slurry reactor, at the experimental conditions of 100?°C temperature, 10?bar oxygen (O₂) partial pressure, at pH 3.0, 100?mg/L concentration and 0.4?g of catalyst. All catalysts were analysed to determine the conversion of phenol and removal of total organic carbon (TOC). The results showed that, Al-PILCs displayed higher removal of phenol and TOC conversion than Cr-PILCs. In the case of mixed Al/Cr-PILCs, maximum removal values of phenol and TOC conversion were achieved with Al/Cr molar ratio of 1:1. Moreover, aromatic intermediate products as catechol, hydroquinone and benzoquinones which are identified in the beginning of the phenol oxidation oxidized into short-chain carboxylic acids such as oxalic acid, maleic acid, formic acid, acetic acid and malonic acid. The obtained results demonstrate the potential of Al/Cr-PILCs catalyst as robust and cheap catalyst for CWAO of phenol under mild reaction conditions. Graphical Abstract: [Figure not available: see fulltext.].

Full text of DOI:10.1007/s10562-018-2579-x

Catalysis Letters (2018) 148:3655–3668
https://doi.org/10.1007/s10562-018-2579-x
A Novel Synthesis Method of Al/Cr Pillared Clay and its Application
in the Catalytic Wet Air Oxidation of Phenol
Jeffrey Baloyi1,2 · Thabang Ntho2 · John Moma1
Received: 15 August 2018 / Accepted: 26 September 2018 / Published online: 29 September 2018
© Springer Science+Business Media, LLC, part of Springer Nature 2018
Abstract
Natural bentonite clay was pillared with Al and Cr polycations (Al-PILCs and Cr-PILCs) as well as with both Al and Cr
mixed polycations (Al/Cr-PILCs) at different Al/Cr molar ratios. The structural and textural properties of the pillared clay
catalysts were characterized by various techniques, such as high resolution scanning electron microscopy with energy dis-
persive system, powder X-ray diffraction, adsorption–desorption of N2 at −196 °C, Fourier transform infrared spectroscopy,
thermogravimetric analysis, cation exchange capacity and zeta potential. The obtained results showed that the intercalation
of polycations in the interlamellar space of clay led to changes in the clay properties, basal spacing and specific surface area
of a mixed PILCs were higher compared to single PILCs. Moreover, diffractograms of the Cr-PILCs confirmed the pres-
ence of a Cr2O3 phase. The catalytic wet air oxidation (CWAO) of the phenol was studied in a batchwise slurry reactor, at
the experimental conditions of 100 °C temperature, 10 bar oxygen (O2) partial pressure, at pH 3.0, 100 mg/L concentration
and 0.4 g of catalyst. All catalysts were analysed to determine the conversion of phenol and removal of total organic carbon
(TOC). The results showed that, Al-PILCs displayed higher removal of phenol and TOC conversion than Cr-PILCs. In the
case of mixed Al/Cr-PILCs, maximum removal values of phenol and TOC conversion were achieved with Al/Cr molar ratio
of 1:1. Moreover, aromatic intermediate products as catechol, hydroquinone and benzoquinones which are identified in the
beginning of the phenol oxidation oxidized into short-chain carboxylic acids such as oxalic acid, maleic acid, formic acid,
acetic acid and malonic acid. The obtained results demonstrate the potential of Al/Cr-PILCs catalyst as robust and cheap
catalyst for CWAO of phenol under mild reaction conditions.
Graphical Abstract
Keywords Al/Cr-PILCs · Ultrasound treatment · Phenol · Catalytic wet air oxidation
1 Introduction
Electronic supplementary material The online version of this
In recent years, pillared clays (PILCs) have received consid-
article (https://doi.org/10.1007/s10562-018-2579-x) contains
erable attention for applications as catalysts and catalyst sup-
port due to their excellent catalytic properties such as high
specific surface area, porous structure and high thermal sta-
bility [1–3]. The PILCs solids are obtained by separating the
supplementary material, which is available to authorized users.
* John Moma
john.moma@wits.ac.za
Extended author information available on the last page of the article
Vol.:(0123456789)
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J. Baloyi et al.
clay layers through pillaring with an agent such as hydroxyl
polycations of polynuclear metals (Al, Fe, Ti, Cr and Zr).
These pillaring agents can be intercalated between adjacent
layers of clay, maintaining the spacing between adjacent
layers upon water removal and improving pore structure
[4–6]. The pillaring process involves exchanging the inter-
layer polycations with large inorganic hydroxyl polycations
of polynuclear cationic metals, which are converted into
oxides that are strongly bonded to clay the after calcina-
tion. The common traditional method of PILCs synthesis,
involves addition of metal ions, such as Al3+, Zr4+, Cr3+ and
Fe3+, into a basic solution (NaCO3 or NaOH) and stirred at a
certain temperature for hours (3–12 h), followed by 6–24 h
or 1–5 days of aging at 60–80 °C to obtain hydroxyl poly-
cations, which are pillaring agents solution. The precursor
pillaring agent solution is added into the diluted clay min-
eral suspension (typical 2% in weight), which is followed by
thermal treatment of intercalated clay mineral [7].
mixed chromium and aluminium pillars at different Al/Cr
ratios because of high thermal stability of Al-PILCs [20,
21]. In the literature, limited studies have been reported on
the synthesis of PILCs using methods such as ultrasound
and microwave irradiation with direct use of natural dry clay
without chemical treatment.
Phenol is an organic pollutant commonly found in indus-
trial wastewater emitted from refineries, paint, coking opera-
tions, petrochemicals, pulp and paper, pharmaceuticals, resin
manufacturing and plastics [1–4]. It was chosen as the model
compound in this study, because of its refractory nature.
Phenol is listed as priority water pollutants by the World
Health Organisation (WHO) and it is considered toxic above
2 mg/L [15]. Phenol and its derivatives are common pollut-
ants of ground water, because of its low biodegradability
and toxic for animals and human beings [22]. Therefore,
effective treatment of industrial wastewater contaminated
by refractory organic compounds such as phenol is crucial.
Catalytic wet air oxidation (CWAO) has attracted much
attention for treatment of refractory organic compounds
such as phenol that cannot be completely removed by con-
ventional treatment methods. In CWAO, refractory organic
compounds and oxidizable inorganic compounds can be oxi-
dized with air or oxygen (O2) at high operation temperature
(150–325 °C) and pressure (0.5–20 MPa) in the presence of
a robust catalyst.
However, the preparation of PILCs as catalysts has not
been extended to commercial scale, due to the difficulties
involved in performing the laboratory synthesis procedure
at industrial scale due to prolonged synthesis times and
large amount of water involved [8, 9]. Numerous studies
have made efforts to simplify the synthesis of PILCs, in
order to lower the synthesis times and the volumes of water
required. The use of concentrated clay suspensions (50 wt%)
and direct addition of natural clay without purification into
pillaring agent solution has been proposed as strategies
used by various studies to decrease the volume of water
required, while the synthesis time has been decreased by
the use of microwave and ultrasound treatment method [6,
9–15]. These studies found that the volume of water required
can be reduced by 70–95%, whereas the synthesis time can
be decreased to 70–93%. In addition, PILCs composed of
mixed polycations are reported to increase specific surface
area, thermal stability and catalytic properties of the PILCs
compared to a single polycations. Baloyi et al. [16] reported
that in the mixed polycations PILCs, one of the polycations
might be responsible for structural and thermal stability with
the required pore structure, whereas the catalytically active
sites might be enhanced by the correct choice of a second
polycation. Gil et al. [17] found that Al-PILCs structural
and acidic properties can be enhanced by the addition of a
second polycation pillaring agent such as Fe, Cr, Ti and Zr,
etc. The preparation of Cr-PILCs have attracted attention in
the catalytic applications due to the different oxidation states
of chromium [4]. However, numerous studies reported that
Cr-PILCs are not stable at high temperatures. Mata et al.
[18] reported a collapse of Cr-PILCs after calcination in
air above 200 °C while Hopkins et al. [19] observed the
collapse of Cr-PILCs basal spacing of 1.54 nm after calcina-
tion at 300 °C. Several reports have found that the thermal
stability of Cr-PILCs was improved by the preparation of
In the current study, ultrasound treatment method was
used both during the aging and the intercalation of pillaring
agents to synthesize PILCs containing chromium and alu-
minium pillars with different Al/Cr molar ratios. Moreover,
dry clay was directly added to the pillaring solutions without
purification. Structural and textural properties of PILCs were
characterized by PXRD, HRSEM-EDS, FTIR, TGA, Zeta-
sizer and BET surface area measurements. The efficiency
of these PILCs catalysts in the CWAO of phenol in aqueous
phase under mild reaction conditions was investigated. Phe-
nol oxidation intermediates generated by CWAO of phenol
were identified and discussed.
2 Experimental
2.1 Materials and Reagents
All reagents used in this study were analytical grade and
used without further purification. Natural bentonite (Bt) clay
supplied by ECCA Holdings (Pty) Ltd from Cape Town,
South Africa, with cation exchange capacity (CEC) of
78.6 meq/g, was used as starting material. Phenol (99.5%)
was purchased from Sigma Aldrich Chemical Co. Sodium
hydroxide (98.7%), hydrochloric acid (37%), aluminum
nitrate nonahydrate, chromium (III) nitrate nonahydrate, ace-
tic acid (99.0%) and HPLC-grade methanol were obtained
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A Novel Synthesis Method of Al/Cr Pillared Clay and its Application in the Catalytic Wet Air…
3657
from Merck. All solutions were prepared with high purity
water (resistivity 18.2 MΩcm) purified by a Milli-Q water
system (Millipore).
FTIR spectrometer with a Miracle single bounce dia-
mond attenuated total reflectance (ATR) cell from PIKE
Technologies. Thermogravimetric (TG) and differential
thermogravimetric (DTG) analyses were performed on
a Perkin–Elmer model 6000 thermal analyzer. Samples
were heated from 35 to 900 °C with nitrogen atmosphere,
at a heating rate of 10°C/min under nitrogen flow rate of
20 mL/min. The surface charge of the samples were deter-
mined using a Nano Zetasizer (Malvern Instruments, UK).
The cation exchange capacity (CEC) was determined by
the ammonium acetate method using Kjeldhal technique.
2.2 Pillared Clay Catalysts Preparation
As previously described in the literature by Tumol et al.
[15], the pillaring solutions were prepared by slowly add-
ing NaOH solution (0.4 M, 500 mL) into Al(NO3)3·9H2O
(0.4 M, 250 mL) or Cr(NO3)3·9H2O (0.4 M, 250 mL) solu-
tion under vigorous stirring at room temperature [23–26].
The NaOH was added at constant rate of approximately
5 mL/min to reach the final molar ratio of 2:1. The pillar-
ing solutions were ultrasonicated for 10 min at room tem-
perature prior to use. The mixed Al/Cr pillaring solutions
with different molar ratios (Al/Cr mole ratios of 1:3, 1:1,
3:1 and 6:1) of Al/Cr were prepared by using the above pro-
cedure at room temperature. The intercalation was carried
out by direct addition of dry clay into the single or mixed
aluminium and zirconium pillaring solutions under vigor-
ous stirring at room temperature for 30 min, using a ratio of
25 mL pillaring solution/1 g of clay. The ion exchange reac-
tion was sped up by using ultrasonic treatment at 25 °C for
10 min. Finally, the solid fractions were separated by cen-
trifugation for 8 min, washed several times with high purity
water, oven-dried at 120 °C for 16 h, and then calcined in a
muffle furnace at 500 °C for 2 h. The obtained pillared clay
catalysts were denoted as Al-PILC, Cr-PILC, Al/Cr-PILC
(1:3), Al/Cr-PILC (1:1), Al/Cr-PILC (3:1) and Al/Cr-PILC
(1:1), respectively.
2.4 Catalytic Wet Air Oxidation Experiments
The experimental studies were carried out in a 350 mL
semi batch Teflon-lined autoclave reactor (Parr Instru-
ments Co., model 4531M) equipped with a stirrer, an
electric heating jacket, gas inlet and liquid sampling
port. The temperature inside the reactor was measured
by a thermocouple and a proportional-integral-derivative
(PID) controller was used to keep the temperature constant
inside the reactor. (A schematic of the experimental setup
is presented in Figure S1). Prior to CWAO experiments,
operational parameters were optimized from preliminary
experiments (results are shown in Figs. S2–S7). The
stirring speed was maintained at 800 rpm. The reaction
temperature was set at 100 °C and the mixture inside the
reactor was pressurized with 10 bar oxygen. In a typical
run, the reactor was charged with aqueous suspension of
phenol (200 mL, 1000 mg/L) and 2 g/L of catalyst at ini-
tial pH of 3. After air was purged in the reactor with pure
nitrogen, the reactor system was pressurized with 5 bar
of pure nitrogen pressure for 5 min. Once the desired
temperature was reached, oxygen was introduced into the
reactor immediately to get a 10 bar of total pressure. This
point was set as the initial reaction time known as “zero
time”. Samples were withdrawn at regular intervals and
immediately filtered using 0.45 µm once-off syringe fil-
ters to analyze the phenol concentration. All experiments
were conducted in triplicates to check the reproducibility
of the results. The data reported in this study were the
average calculated from the repeat experiments. Experi-
mental errors are less than 5% between repeat experiments
throughout the study.
2.3 Characterization Techniques
The morphology and chemical composition of natural
and pillared bentonite samples were characterized by
high resolution scanning electron microscopy with an
energy dispersive X-ray spectrometer (HRSEM–EDS,
FEI Nova NanoSem). Powder X-ray diffraction (XRD)
analysis was performed in a high resolution Bruker AXS
D8 diffractometer with CoKα-radiation (40 kV, 40 mA,
1.78897 Å) in the 2θ range from 5° to 80° at scanning
speed of 0.02°/s. Nitrogen adsorption–desorption meas-
urements were performed at −196 °C using Micromeritics
ASAP TrisStar 3000 instrument. Prior to the measurement
all samples were degassed under vacuum at 150 °C for 4 h.
The specific surface area (SBET) was determined using the
Brunauer–Emmett–Teller (BET) method. The total pore
volumes (Vt) were calculated at a relative N2 pressure of P/
P0 ∼ 0.99. While the pore size distribution was calculated
using the Barrett–Joyner–Halenda (BJH) method based on
the desorption branch of the isotherms. Fourier transform
infrared (FTIR) spectrometer peaks were obtained from
4000 to 400 cm−1 over 200 scans using a Varian 640-IR
The stability and reusability of the best pillared clay cata-
lyst was evaluated by successive use of the same catalyst
sample in the phenol oxidation reaction. The experiments
carried out using the same conditions as the above CWAO
experiments. After each experimental run, the catalyst
was recovered by centrifugation for the next use, added to
100 mL CaCl2 solution (4%) and stirred for 120 min at room
temperature to remove all adsorbed organics.
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J. Baloyi et al.
pillared bentonite clay indicate the irreversibility of cationic
exchange of the intercalated polycations.
2.5 Analytical Methods
The zeta potential analysis results for natural and pillared
bentonite clay in the variation of initial pH in the range 1 to
10 is indicated in Fig. 2. The surface charge of the natural
bentonite clay exhibits highly negative zeta potential values
over the entire pH range studied due to isomorphous substi-
tution of structural Si4+ by Al3+ and pH dependent charge
from the hydroxyl groups at the edges of the bentonite clay
particles [31]. At pH 10, the zeta potentials of pillared clay
catalysts have more negative charges than that of natural
bentonite clay due to the high amount of hydroxyl (–OH)
groups on pillared bentonite clay surface. However, the
zeta potentials of the pillared clay catalysts decrease and
convert to positive values with the decrease in initial pH
values towards acidic conditions. This may be attributed to
the increase in H+ ions on the surface of pillared bentonite
clay catalysts, which can effectively increase the electropo-
sitivity of the catalysts, therefore aiding anionic contami-
nants removal in aqueous solution [32]. Single metal (Al
or Cr) polyoxycations pillared clays show a higher positive
charge compared to mixed Al/Cr polyoxycations pillared
clays, which may be attributed to the combination of more
hydroxide ions (–OH) in the mixed Al/Cr polyoxycations ({[
AlxCry(OH)z(H2O)n](3x+3y−z)+}).
A high performance liquid chromatography (HPLC) (Shi-
madzu LC-10AD), equipped with an ODS-3 column and
UV–Vis detector was used to determine the phenol concen-
tration in solution. The mobile phase was composed of high
purity water and methanol in a ratio of 60:40 (v/v) at a flow
rate of 1 mL/min. The evolution of phenol intermediates
during the reaction was analysed by HPLC following the
procedure described by Ma et al. [5]. The total organic car-
bon (TOC) content was determined in filtered aliquots of the
reaction mixture by Shimadzu TOC analyzer. The pH values
of solutions were determined using a Mettler–Toledo FE 20
pH meter. The concentration of leachate Cr3+ was measured
by inductively coupled plasma-atomic emission spectrom-
eter (ICP-AES) spectroscopy.
3 Results and Discussion
3.1 Characterization Studies
SEM micrographs of the raw natural bentonite and the pil-
lared bentonite clays synthesized using aluminium and chro-
mium metal oxides are shown in Fig. 1. The results clearly
indicate loosened and randomly distributed closely-packed
structure after pillaring. The structure of all the pillared clay
catalysts is similar to that of the natural bentonite clay indi-
cating that the structure of the natural clay was not destroyed
by pillaring with single or mixed metal oxide pillars. The
energy dispersive X-ray spectroscopy (EDS) analysis of
natural and pillared bentonites are presented in Table 1.
EDS analysis clearly shows the presence of elements like
Si, Al, Fe, Na, Mg, Ca, K and Cr in the samples. The Al
and/ or Cr oxides metal contents increase as result of cation
exchange with Na and Ca due to pillaring by Al, Cr and Al/
Cr pillars. Therefore, confirming that the intercalation of Al
or Cr is produced by a cation exchange reaction between the
exchangeable small interlayer polycations in the interlayer
space of bentonite clay and Al, Cr and Al/Cr pillars in solu-
tion. While the higher relative amount of Si/Al indicate the
presence of quartz and muscovite [27, 28].
The XRD patterns of the natural and pillared bentonite
clay samples in the 5°–80° 2θ angle range are compared
in Fig. 3. The corresponding basal spacing calculated by
Bragg’s law (2d sin θ=nλ, where d is the basal spacing, θ
is the angle of diffraction, λ is the wavelength, and n is the
path differences between the reflected waves, which equals
an integral number of wavelength) at d001 smectites reflec-
tion are shown in Table 1. The XRD patterns reveal that
natural bentonite clay is composed of Na-montmorillonite
as principal crystalline phase, accompanied by other impu-
rities such as quartz, muscovite and calcite. The detected
crystalline phases for natural bentonite clay are in accord-
ance with similar studies [30, 33]. After intercalation of Al,
Cr or Al/Cr metal oxides pillars into the natural bentonite
clay structure, the d001 peaks shifted slightly towards small
2θ values therefore increasing the basal spacing. Moreover,
the d001 peak intensity decreases with pillaring compared
to that of the natural bentonite clay. The XRD spectra of
natural bentonite and pillared clay samples are very similar,
suggesting that the structure of natural bentonite clay was
not destroyed after pillaring process. In the case of Cr-PILC
sample, the d001 diffraction peak collapses and the peaks
around 18°, 18.5° and 37° can be ascribed to the presence
of Cr2O3 in the diffractograms. This is in agreement with
previous studies by De León et al. [20] and Chen et al. [34]
who observed the d001 peak collapsing at temperatures above
300 °C. Therefore this can be attributed to the fact that the
d001 plane in the Cr-PILC has tendency of losing the order
The cation exchange capacity (CEC) of the natural
bentonite and pillared bentonite clay samples was meas-
ured by the ammonium acetate exchange according to the
Kjeldhal method. Normally the CEC of natural bentonite
clay is > 60 meq/100 g. The obtained CEC value of the
bentonite clay in Table 1 is within the reported values of
75–80 meq/100 g for South African bentonite clay [29, 30].
As shown in Table 1, the CEC of the clay decrease with pil-
laring from 78.6 to 21 meq/100 g; the decrease attributable
to the dissolution of octahedral cations of the 2:1 layers and
protonation of the SiO groups. The low CEC values of the
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A Novel Synthesis Method of Al/Cr Pillared Clay and its Application in the Catalytic Wet Air…
3659
Fig. 1 HRSEM microphoto-
graphs of a Bt, b Al-PILC, c
Cr-PILC, d Al/Cr-PILC (1:3), e
Al/Cr-PILC (1:1), f Al/Cr-PILC
(3:1) and g Al/Cr-PILC (6:1)
in the stacking of the pillared clay layers after calcination at
400–500 °C. The calcination temperature at 500 °C leads to
appearance of the Cr2O3 phase shown by the peaks identified
in Cr-PILCs, which are not detected in the corresponding
exchanged mixed pillared bentonite clay (Al/Cr-PILCs).
1630, 978, 906 and 561 cm−1 are typical of montmorillon-
ite, while the peaks at 771 and 661 cm−1 corresponds to the
presence of quartz [20]. The peak at 1487 cm−1 corresponds
to the occurrence of small amounts of calcite or dolomite
[35]. The peak at 3583 cm−1 is assigned to the OH-stretching
vibration (Al-Al-OH) structural groups of montmorillonite.
After pillaring of clay the Al-PILCs, Al-PILC (3:1) and
Al-PILC (6:1) peaks at approximately 3583 cm−1 increase
Figure 4 shows the FTIR spectra (400–4000 cm−1
)
recorded in transmission mode for natural bentonite clay and
pillared bentonite clay samples. The peaks at 3583, 3294,
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3660
J. Baloyi et al.
Table 1 Physicochemical characteristics of natural and pillared bentonite clay samples by EDX, CEC and XRD
Sample
Al/Cr ratio Na (%) Mg (%) Al (%) Si (%) K (%) Ca (%) Fe (%) Cr (%) Si/Al CEC
(meq/100 g)
d001 (nm)
Bentonite
Al-PILC
Cr-PILC
Al/Cr-PILC 1:3
Al/Cr-PILC 1:1
Al/Cr-PILC 3:1
Al/Cr-PILC 6:1
–
–
–
3.73
0.96
0.95
0.96
0.97
0.98
0.93
2.29
1.24
1.13
1.19
1.22
1.23
1.21
15.31
31.98
11.42
16.02
19.02
27.76
29.19
65.09
59.14
58.54
58.98
59.01
58.14
59.11
2.39
1.82
1.16
1.71
1.61
1.63
1.68
2.09
0.27
0.16
0.26
0.21
0.24
0.26
9.10
4.55
4.21
4.01
4.38
4.40
4.45
0.00
0.00
22.13
16.87
13.58
5.61
4.25
1.85
5.13
3.68
3.10
2.09
2.03
78.6
1.24a
1.52
0.90
1.36
1.50
1.41
1.38
40
30
27
21
24
33
3.17
aBasal spacing of non-calcined bentonite
3583
1630
611
771
3294
30
20
10
0
Na-Bt
Bt
978
906
Al-PILC
561
Cr-PILC
Al/Cr-PILC (1:3)
Al/Cr-PILC (1:1)
Al/Cr-PILC (6:1)
Al/Cr-PILC (3:1)
Al-PILC
pHzpc
7
Cr-PILC
0
1
2
3
4
5
6
8
9
10
11
Al/Cr-PILC (1:3)
Al/Cr-PILC (1:1)
pH
-10
-20
-30
-40
-50
Al/Cr-PILC (3:1)
Al/Cr-PILC (6:1)
4000
3500
3000
2500
2000
1500
1000
500
Fig. 2 Zeta potential (mV) of the natural and pillared bentonite clays
-1
Wavelength (cm )
as a function of pH
Fig. 4 FTIR spectra of natural and pillared bentonite clay samples
d(001)
in intensity which is indicative of an improvement in the
water retention capacity of the clay material due to pillar-
ing. The intensities of Cr-PILC, Al-PILC (1:3) and Al-PILC
(1:1) decrease and the peaks broaden. This decrease in the
intensity of the peaks is attributed to the dehydration and
dehydroxylation steps during pillaring [27]. The broadening
of the peaks could be due to the introduction of additional
–OH groups, as a result of the pillaring of the clay [35, 36].
The broad peak at 3294 cm−1 depicts the OH vibrational
stretching of intercalated water in montmorillonite. The
peak at 1630 cm−1 can be assigned to OH vibration bend-
ing of H–O–H. After the clay was pillared the intensities
of the Al-PILC, Al-PILC (3:1) and Al-PILC (6:1) peaks at
approximately 1630 cm−1 increased due to replacement of
hydrated larger interlayer cations which decreases the –OH
peaks. Moreover, the peaks at 1630 cm−1 shifted to higher
wavelength (approximately between 1639 and 1660 cm−1)
due to pillaring process. The Si-O-Si stretching vibration
in the tetrahedral structure of the bentonite clay peak is
Al/Cr-PILC (6:1)
Al/Cr-PILC (3:1)
Al/Cr-PILC (1:1)
Al/Cr-PILC (1:3)
Cr-PILC
Al-PILC
Bt
10
20
30
40
50
60
2theta ( )
Quartz Calcite
Cr2O3
Na-montmorillonite
Muscovite
Fig. 3 X-ray diffractograms of the natural and pillared bentonite clay
samples
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A Novel Synthesis Method of Al/Cr Pillared Clay and its Application in the Catalytic Wet Air…
3661
observed at 978 cm−1. The peaks at 906 and 771 cm−1 can be
assigned to Al–Al–OH and Mg–Al–OH groups, respectively.
After pillaring, intensities of these peaks at 978, 906 and
771 cm−1 decrease and the peaks shift to lower wavelengths.
This shift can be attributed to the sharing of hydroxyl (OH)
peaks between Al and Cr that are located in the octahedral
positions [20, 27]. The Si–O–Mg and Si–O–Al species are
donated by peaks at 561 and 661 cm−1 respectively. The
FTIR spectral data provide evidence of the successful pil-
laring of the bentonite clay.
to hinder the loss of specific surface area of Cr-PILC. Chen
et al. [34] obtain similar results in which the optimum Al/Cr
molar ratio in the pillaring process was determined to be 2:2.
The micropore volumes, total pore volume and pore diam-
eters of pillared bentonite clay materials increased with an
increase in the specific surface area of pillared bentonite clay
samples. The higher values of pore diameters and total pore
volumes indicate that most of the total pores were formed
from mesoporous structure. In conclusion it was observed
that textural analyses of pillared bentonite clay confirm that
the pillaring process led to the significant increase in the
porosity, which is one of the most significant characteristics
of the catalyst.
The textural characterization of natural and pillared ben-
tonite clay, including the specific surface areas (SBET), total
pore volumes (Vp), mcro volume (Vmic) and average diam-
eter of the pores (Dp) are presented in Table 2. The specific
surface area of Al, Cr and Al/Cr increase significantly after
pillaring process, which confirm that Al and Cr were effi-
ciently intercalated into the interlayer space of bentonite clay
sheets. Compared with the natural bentonite clay and single
metal oxides pillars (Al or Cr), the mixed metal oxides pil-
lars (Al/Cr) with different molar ratios exhibit higher specific
surface areas. This shows that the mixed metal oxides pillars
of Al/Cr polycations that formed were larger than the Al or
Cr polycations, which is an indicative that bentonite clay
layers were more separated and resulted in a more porous
structures being formed. The specific surface area of Cr-
PILCs increased slightly compared to starting natural clay,
while the Al-PILCs increased significantly. This indicates
a higher thermal stability of the Al-PILCs compared with
the Cr-PILC which collapse with the loss of their interlami-
nar spacing at the calcination temperature of 500 °C. These
results tally with the XRD analysis results in this study as
well as with other studies in the literature [20, 21, 34]. The
mixed metal oxides pillared clay of Al/Cr at the molar ratio
of 1:1, shows a high specific surface area of 180 m2 g−1. This
proves that the mixed metal oxides pillars (Al/Cr) was more
effective at increasing specific surface area than Al or Cr
metal oxides pillars alone and the significance of introduc-
ing Al metal oxides pillars during the intercalation process
The TGA and DTG curves of the natural and pillared
bentonite clays are shown in Fig. 5. The TGA curves of
the pillared clay samples are similar to that of natural ben-
tonite clay. The TGA and DTG curves recorded from 35
to 900 °C of the both natural and pillared bentonite clay
show three stages of mass loss. The first step in the range of
49–170 °C is attributed to the loss of physically adsorbed
water as well as the loosely bound water molecules present
in the interlayer space of the pillared bentonite clay. The
second step, at about 390–511 °C is related to the loss of
water molecules coordinated to the pillars. It is worth noting
that, at this stage, the removal of water is not associated with
destruction of bentonite clay mineral structure indicating
the high thermal stability of the clay which relates to their
crystalline structure. Whereas the third step in the range of
548–704 °C can be ascribed to the breakdown of the clay
crystal structure. At this temperature range, the loss in mass
of the pillared bentonite clays peaks are lower compared to
that of the natural clay, confirming the addition of Al and Cr
amount during the pillaring and less volatiles. The observed
differences in the TGA and DTG curves can be explained
by the differences in the dehydration and dehydroxylation
process, which can be ascribed to the presence of different
species of Al and Cr in the interlayer and external surface,
and also hydration energies differences [15].
Table 2 The textural
Sample
Al/Cr ratio
S1BET (m2/g)
V2pore (cm3/g)
V3mic (cm3/g)
d4pore (nm)
characterization of prepared
catalysts and raw bentonite clay
Bentonite
Cr-PILC
Al-PILC
Al/Cr-PILC
Al/Cr-PILC
Al/Cr-PILC
Al/Cr-PILC
–
–
–
1:3
1:1
3:1
6:1
61
67
147
150
180
168
159
0.11
0.13
0.32
0.29
0.36
0.33
0.31
0.089
0.091
0.099
0.1
0.116
0.111
0.109
10.1
9.6
8.5
8.1
7.2
7.8
8.4
1Specific surface area (SBET
)
2Total pore volume at p/p0 ~0.99
3Micropore volume
4Pore diameter
1 3

3662
J. Baloyi et al.
100
BET specific surface area and basal spacing, respectively,
were achieved, whereas in a conventional synthesis method,
low BET specific areas were reached in up to 7 days reac-
tion times. Ding et al. [9] synthesized Al/Cr-PILCs cata-
lysts using a one-step high-temperature and high-pressure
(HTHP) method. They have found that after 4 h of calcina-
tion at 550 °C, the specific surface area and maximum basal
spacing reached 266–362 m2/g and 2.06 nm, respectively.
Compared to conventional methods, the method significantly
reduced the steps and material consumption. However, high
temperatures, high pressures and long synthesis periods are
required compared to ultrasonic treatment method. As in
the case of the compared textural properties in Table 3, the
beneficial effect of ultrasonic method on the Al/Cr-PILCs
was observed. The obtained results can be explained by sta-
bility of Al/Cr-PILCs catalyst, which can be attributed to
the uniform pillaring obtained by the use of ultrasonic treat-
ment. Moreover, the use of ultrasonic treatment plays a role
in accelerating the [Al13]7+ diffusion within the clay layers
[14]. The use of ultrasonic treatment method present numer-
ous advantages such as reduction of the synthesis time from
days to less than 20 min. The method does not require any
heat for the process, therefore, saving in costs and less safety
risks. Finally, the addition of dry clay directly into pillar-
ing solution results in less consumption of water and space
required. Therefore, the textural properties of Al/Cr-PILCs
catalyst herein is comparable and even better compared to
other previous reported using conventional method.
(a)
95
90
85
80
75
70
Cr-PILC
Al/Cr-PILC (3:1)
Al/Cr-PILC (1:3)
Al/Cr-PILC (1:1)
Al/Cr-PILC (6:1)
Al-PILC
Bt
100
200
300
400
500
600
700
800
900
Temperature ( C)
0.0
-0.2
-0.4
-0.6
-0.8
-1.0
-1.2
-1.4
-1.6
(b)
Al/Cr-PILC (6:1)
Al/Cr-PILC (3:1)
Cr-PILC
Al/Cr-PILC (1:1)
Al/Cr-PILC (1:3)
Al-PILC
Bt
100
200
300
400
500
600
700
800
900
Temperature ( C)
3.2 Catalytic Wet Air Oxidation of Phenol
Fig. 5 TG (a) and DTG (b) curves of natural and pillared bentonite
clay samples
The catalytic activity of various pillared bentonite clay cata-
lysts were investigated using phenol as a model pollutant
and the results for phenol removal and total organic car-
bon (TOC) conversions are displayed in Fig. 6. It was found
in the WAO (in the absence of catalyst) of phenol that the
removal of phenol and TOC conversion were 9% and 2%,
respectively, indicating that the phenol was stable under our
The textural properties of Al/Cr-PILCs prepared in
this study was compared with previously reported similar
catalysts prepared by different methods in Table 1. From
Table 1, with ultrasonic treatment, synthesis at room tem-
perature and 20 min reaction time, 180 m2/g and 1.50 nm of
Table 3 Comparison of Al/Cr-PILCs catalyst textural properties with previously reported similar Al/Cr-PILCs catalysts prepared by different
methods
Catalysts
Al/Cr ratio
1:1
Method of sythensis
Ultrasonic
Reaction conditions
S1BET (m2/g)
180
d001 (nm)
1.50
CEC (meq/ Refs.
log)
Al/Cr-PILC
T=25 °C
21
This work
t
reaction =20 min
pH 5
Al/Cr-PILC
Al/Cr-PILC
Al/Cr-PILC
1:3
2:5
2:2
Convetional
HTHP
T=120 °C
96
362
125
1.72
2.06
1.57
–
–
–
[27]
[9]
t
reaction =7 days
T=140 °C
treaction =36 h
Conventional
T=25 °C
[34]
treaction =48 h
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A Novel Synthesis Method of Al/Cr Pillared Clay and its Application in the Catalytic Wet Air…
3663
100
90
80
70
60
50
40
30
20
10
0
100
80
60
40
20
0
Al/Cr-PILCs (1:1)
Al/Cr-PILCs (3:1)
Al/Cr-PILCs (1:3)
Al/Cr-PILCs (6:1)
Al-PILCs
Cr-PILCs
Bt
(a)
(b)
Without a catalyst
Al/Cr-PILC (1:1)
Al/Cr-PILC (3:1)
Al/Cr-PILC (1:3)
Al/Cr-PILC (6:1)
Al-PILC
Cr-PILC (1:1)
Bt
Without catalyst
0
30
60
90
120
150
180
0
20
40
60
80
100 120 140 160 180
Time (min)
Time (min)
Fig. 6 Phenol (a) and TOC (b) conversions in the CWAO reaction of phenol over the natural and pillared bentonite clay samples (200 mL of
reaction solution, 1000 mg/L phenol concentration, 0.4 g of catalyst, pH 3.0, 10 bar O2 pressure and 100°C)
operating conditions. The adsorption experiments was stud-
ied under the same operating conditions using nitrogen in
place of air. Under these conditions, 10% and 5% of phenol
removal and TOC conversion, respectively, were adsorbed
on the catalysts, which could be ascribed to the large specific
surface area of the catalysts which results in an increase
number of active sites in the catalysts. The phenol removal
and TOC conversion using natural bentonite clay were found
to be 8% and 2%, respectively, which was almost inactive
toward the CWAO of phenol. In contrast, the use of the vari-
ous pillared clays as catalysts yielded good catalytic activi-
ties towards phenol removal and TOC conversion in the
CWAO reaction with the mixed metal oxides being the most
active catalysts. After 120 min reaction, the phenol removal
reached 92%, 95%, 97% and 100% over the Al/Cr-PILC
(6:1), Al/Cr-PILC (1:3), Al/Cr-PILC (3:1) and Al/Cr-PILC
(1:1) catalysts, respectively. High mineralization of the phe-
nol was obtained and approximately 84% TOC conversion
was achieved within 180 min of the reaction in using Al/Cr-
PILC (1:1) catalyst. The TOC conversion decreased from 84
to 36%, with minimum in case of Cr-PILC. The TOC con-
version values decreased in the following order: Al/Cr-PILC
(1:1)>Al/Cr-PILC (3:1)>Al/Cr-PILC (1:3)>Al/Cr-PILC
(6:1)>Al/Cr-PILC>Cr-PILC. The performance of mixed
pillared clay catalysts specifically Al/Cr-PILC (1:1) cata-
lyst can be attributed to the highest surface area (180 m2/g),
which contribute to more active sites in the CWAO process.
The high thermal stability of the Al/Cr-PILCs prevented the
collapse of the bentonite clay interlayer structure during the
calcination step of the pillaring process, the increasing sta-
bility of the pillared clay towards Cr3+ leaching and possible
enhancement of surface acidity of the clay after pillaring,
that is significant for the oxidation of phenol as reported in
our previous studies [23]. Moreover, the increased positively
charged surface of the Al/Cr-PILCs favored electrostatic
attraction between the catalyst and negatively charged phe-
noxide ion in solution at acidic pH values, thus, enhancing
the adsorption of phenol on its surface.
The catalytic activities of the various catalysts in the
CWAO reaction of phenol are summarized in Table 4. In
the literature, there are no data available on the application
of Al/Cr-PILCs as catalysts in the CWAO processes. As
chromium being a good oxidant catalyst due to its different
oxidation states, in literature, there are limited studies where
Al/Cr-PILCs was used as a catalyst in oxidation reactions
[20, 21, 27, 34]. The use of noble metals (Pt, Ru…), tran-
sition metal oxides (TiO2, CeO2…) and carbon materials
(GO, r-GO…) for CWAO of phenol has been carried out
by numerous researches and their works show that these
catalysts give high activity [37–41]. Rocha et al. [37] inves-
tigated the use of Pt/TiO2-Ceria as catalysts for the CWAO
of phenol and found that Pt/TiO2-Ceria catalyst showed 96%
of phenol conversion and complete TOC conversion. It was
also reported that the presence of Ce in the catalysts pre-
vents deactivation of the catalysts due to carbon deposition
on the surface. Davies et al. [41] applied Pt/SiC catalyst for
CWAO of phenol in aqueous solutions and the catalyst dis-
played good performance and was very durable for CWAO
of phenol, achieving 99.95% phenol removal and 95% TOC
conversion at a reaction temperature of 160 °C and total
pressure of 7 bar. Yang et al. [37] reported on a CWAO of
phenol in a batch reactor using CeO2-TiO2 catalysts and the
1 3

3664
J. Baloyi et al.
Table 4 Summary of the
comparative study for phenol
oxidation with CWAO using
different catalysts
Catalyst
Operating conditions
Pollutant
Results
Refs.
[37]
Pt/TiO2/Ceria
T=160 °C
Phenol (1000 mg/L)
[Ph]=96%
P=10 bar
[TOC]=100%
Batch experiments
t
reaction =2 h
Pt/SiC
T=160 °C
Phenol (1000 mg/L)
Phenol (1000 mg/L)
Phenol (1000 mg/L)
Phenol (1000 mg/L)
Phenol (1000 mg/L)
Phenol (1000 mg/L)
[Ph]=99.95%
[TOC]=95%
[41]
P=7 bar
Batch experiments
t
reaction =5 h
CeO2-TiO2
MnOx-CeO2
GO
T=150 °C
[Ph]=100%
[TOC]=80%
[39]
P=30 bar
Batch experiments
t
reaction =150 min
T=100 °C
[Ph]=100%
[TOC]=77%
[38]
P=10 bar
Batch experiments
t
reaction =60 min
T=155 °C
[Ph]=100%
[TOC]=84%
[40]
P=25 bar
Batch experiments
t
reaction =40 min
r-GO
T=155 °C
[Ph]=100%
[TOC]=80%
[40]
P=25 bar
Batch experiments
t
reaction =120 min
Al/Cr-PILC(1:1)
T=100 °C
[Ph]=100%
[TOC]=80%
[This work]
P=10 bar
Batch experiments
t
reaction =120 min
CeO2-TiO2 mixed oxides catalysts display the higher activ-
ity. Phenol removal and TOC conversion were about 100%
and 77% after 120 min in the CWAO of phenol at reaction
temperature of 150 °C and the total pressure of 30 bar. Low
cost metal oxides, such as MnOx-CeO2 have been reported
by Arena et al. [38] as more active catalysts in the CWAO
of phenol at low temperature as 100 °C and total pressure
of 10 bar. It was found that the complete phenol removal
was achieved within 40 min reaction time, while 80% of
TOC conversion was achieved after 60 min. This high cata-
lytic activity of the MnOx-CeO2 catalysts was attributed to
the rapid adsorption of phenol and its intermediates. How-
ever, MnOx-CeO2 catalysts were reported to suffer from
deactivation because of deposition of carbon on the cata-
lyst surface. Yang et al. [40] used graphene oxide (GO) and
chemically reduced graphene oxides in the absence of any
metals, in the catalytic wet air oxidation (CWAO) of phenol
in a batch reactor. The GO was reported to have the highest
catalytic activity, with complete removal of phenol achieved
after 40 min, and 84% TOC conversion was obtained after
120 min at reaction temperature of 155 °C and total pressure
of 25 bar. While complete phenol oxidation and 80% TOC
conversion was achieved over r-GO.
terms of phenol removal and TOC conversion in the CWAO
of phenol. Whereas, the metal oxides such as CeO2 have
been reported to attract a lot attention, due to their good
chemical and thermal stability and resistance to both acidic
and basic media [37]. However, these best catalytic activities
are obtained at high operation conditions of CWAO reaction
and using expensive metals. Therefore, considering the low
cost of Al, Cr and natural clays, the use of Al/Cr-PILC (1:1)
catalyst synthesized in this study appeared to be the most
promising for CWAO of phenol.
Since Al/Cr-PILC (1:1) catalyst exhibited the highest
catalytic performance in the reaction, it was further con-
sidered for the study of the evolution of reaction intermedi-
ates. The yield of intermediates in the course of the phenol
CWAO reaction was monitored and the evolution profiles
of the identified intermediates are indicated in Fig. 7. The
identification of intermediates during phenol oxidation
was performed by HPLC analyses by direct comparison of
retention times with the corresponding standards. The main
intermediates identified were pyrocatechol, benzoquinone,
hydroquinone, oxalic acid, maleic acid, formic acid, ace-
tic acid and malonic acid. These intermediates have been
identified by several other studies for phenol oxidation
conducted using CWAO process [42, 43]. Refractory aro-
matic ring compounds such as hydroquinone are formed at
As shown in Table 3 noble metal catalysts containing Ru
and Pt catalysts, have shown the best catalytic activities in
1 3

A Novel Synthesis Method of Al/Cr Pillared Clay and its Application in the Catalytic Wet Air…
3665
1000
Table 5 Stability, leaching and basal spacing features of reused Al/
Cr-PILC catalyst in the removal of phenol and TOC over optimum
conditions
Phenol
Hydroquinone
Formic acid
Acetic acid
Oxalic acid
Malonic acid
Maleic acid
800
700
600
500
400
300
200
100
0
TOC measured
TOC calculated
800
600
400
200
0
No. of runs Phenol
removal in
TOC con-
version in
Leaching
of Cr3+
d001 (nm)
120 min (%) 180 min (%) (Mg L−1
)
First use
Re-use 1
Re-use 2
Re-use 3
Re-use 4
Re-use 5
Re-use 6
100
100
100
98
95
91
84
81
80
77
74
71
67
0.065
0.096
0.110
0.118
0.124
0.160
0.297
1.50
1.48
1.47
1.44
1.44
1.40
1.38
0
20 40 60 80 100 120 140 160 180
Time (min)
87
0
20
40
60
80
100
120
140
160
180
the ortho-oxidation pathway and para-oxidation pathway to
catechol and hydroquinone as the first step. Hydroquinone is
converted to p-benzoquinone, while the conversion of hyd-
roquinone to o-benzoquinone is considered negligible, since
o-benzoquinone was not detected by HPLC in our study.
The aromatic intermediates break down into open cleavage
products such as maleic acid, which oxidize into short chain
carboxylic acids such as formic, malonic, acetic and oxalic
acids. At the end of the reaction acetic, formic and oxalic
acids break down into and CO2 and H2O. Figure 7 (inset)
compares calculated and measured TOC for the measured
concentrations of phenol and the quantified intermediates
identified in the reaction solution with time. The difference
between the TOC calculated and TOC measured is consid-
ered to correspond to the unidentified intermediates, mostly
the solid polymer formed in the reaction [43]. Based on
the results obtained there are no differences between the
measured and calculated throughout the reaction, suggesting
that all the reaction intermediates were identified or possible
contribution of unidentified soluble compounds is consid-
ered negligible.
Time (min)
Fig. 7 Evolution of phenol and intermediates in the CWAO reaction
of phenol over the Al/Cr-PILC (1:1)
the beginning of the CWAO reaction. Then some aromatic
intermediates such as pyrocatechol and benzoquinone were
detected in the middle of the phenol oxidation reaction. The
aromatic intermediates need to be oxidized since they are
more toxic than phenol itself [44]. In this study the col-
our of the phenol solution was observed to change during
the reaction; the colourless phenol solution at the begin-
ning turned light brown to dark brown then light brown.
The dark brown colour can be attributed to the formation of
benzoquinone. The light brown solution turned colourless
due to oxidation of benzoquinone to short chain carboxylic
acids [45, 46]. With the increase in phenol oxidation reac-
tion time, short chain carboxylic acids such as oxalic acid,
maleic acid, formic acid, acetic acid and malonic acid are
formed in the reaction. Hydroquinone showed its highest
concentration after about 15 min of the reaction whilst all
the other intermediated were highest in concentration at
about 60 min all of which are further oxidized. Acetic and
oxalic acids concentrations decrease more slowly, suggesting
that these short chain acids are more difficult to oxidize at
these operation conditions. These results are in agreement
with the work of Guo [44]. and Ma et al. [18]. The forma-
tion of polymeric compounds during the CWAO of phenol
as reported Zapico et al. [43], was quantified by means of
the TOC measurements. The detailed radical nature of phe-
nol CWAO mechanism has been already well documented
in the literature [47–50] and the discussion of our reaction
mechanism experimental results were limited to our findings
in this study. The proposed reaction mechanism is based
on the work of Santos et al. [48] for phenol CWAO mecha-
nism. Through the identified intermediates in our study it
can be suggested that phenol oxidation proceeds through
3.3 Catalyst stability and reusability
The Al/Cr-PILC catalyst with ratio of 1:1 was used six con-
secutive times for the CWAO of phenol solution to evaluate
its stability and reusability, and the results are presented in
Table 5. After four runs the phenol removal efficiency was
still above 98%, suggesting that no significant loss of the
catalytic activity occurred. However, the TOC conversion
noticeably decreased from 84 to 67%, indicating that more
hydroxylation instead of phenol mineralization occurred
with the catalyst reuse. Therefore, it is recommended that
four runs for the catalyst was feasible in order to achieve
almost complete phenol removal and 74% TOC conversion
under studied conditions. There was a negligible decrease in
the catalyst BET surface area from 180 to 172 m2/g after the
successive reuse indicating negligible change in the surface
1 3

3666
J. Baloyi et al.
The results of the CWAO experiments showed that all
tested pillared clay catalysts were active in both removal of
phenol and TOC conversion. The highest conversion was
obtained with Al/Cr ratio of 1:1 Al/Cr, since 100% phe-
nol removal was achieved after 120 min, and 84% TOC
conversion was obtained after 180 min under the low tem-
perature and oxygen partial pressure conditions of 100 °C
and 10 bar in the oxidation of phenol by CWAO. Aromatic
intermediates such as catechol, hydroquinone and benzoqui-
nones were obtained and it was found that these intermedi-
ate compounds oxidized into short-chain carboxylic acids
such as oxalic acid, maleic acid, formic acid, acetic acid and
malonic acid. And these short-chain carboxylic acids were
then oxidized directly into carbon dioxide and water. The
reusability of the catalyst showed superior catalytic activity
and stability under our operating conditions. Moreover, in all
experimental runs, chromium leaching was found to be less
than 1 mg/L, which was lower than the maximum allowed
by legislation.
Run 6
Run 5
Run 4
Run 3
Run 2
Run 1
First use
50
10
20
30
40
60
2 theta ( )
Fig. 8 XRD patterns of fresh and reused Al/Cr-pillared clay catalyst
(Al/Cr ratio 1:1) under optimal experimental conditions
composition of the catalyst during use. This result suggests
that the noticeable decrease in Al/Cr-PILCs catalytic activ-
ity for the consecutive oxidation reaction runs was unlikely
caused by surface area pore site blockage. The Cr3+ leaching
concentration was less than 1 mg/L after six times catalyst
reuse, which exhibited high stability of the Al/Cr-PILCs
catalyst. The results of the catalytic activities and stability
of the Al/Cr- PILCs catalyst revealed that they are good cata-
lyst and have the sufficient stability in the CWAO of phenol
reaction. Therefore, making Al/Cr-PILCs catalyst promising
materials for treatment of refractory organic pollutants.
The structure of the catalyst after each catalytic run was
evaluated by XRD analysis and the results are indicated in
Table 5 and Fig. 8. No evident decline of basal spacing was
observed after the consecutive reaction runs. The excel-
lent stability of the basal spacing might be due to the stable
structure of the catalyst. The XRD patterns before and after
the consecutive use showed no obvious change in the posi-
tion of the d001 basal spacing of the Al/Cr-PILC catalyst.
These results showed that the Al/Cr-PILC catalyst had great
stability and good reusability.
Acknowledgements We kindly acknowledge the financial support of
the National Research Foundation (NRF)/ Department of Science and
Technology (DST) of South Africa under Professional Development
Programme (PDP) Project No. 96719 and Mineral Science Council of
South Africa (Mintek) under CWO of Wastewater Project No. ADE
164.
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Affiliations
Jeffrey Baloyi1,2 · Thabang Ntho2 · John Moma1
1
2
Molecular Science Institute, School of Chemistry, University
Advanced Materials Division, MINTEK, Private Bag X3015,
of the Witwatersrand, P/Bag 3, WITS 2050, Johannesburg,
South Africa
Randburg 2125, South Africa
1 3