Cerium-incorporated cage-type mesoporous KIT-6 materials: Synthesis, characterization and catalytic applications

Article abstract of DOI:10.1016/j.apcata.2009.11.016

Ce-KIT-6 with Si/Ce = 5, 10, 20, 50, 100 and 150 mesoporous molecular sieves were successfully synthesized by a hydrothermal method. The synthesized materials were characterized extensively to understand the physico-chemical properties of the materials. The low-angle powder XRD patterns of calcined Ce-KIT-6 material showed a phase that can be indexed to cubic Ia3d. The UV-vis spectra of Ce-KIT-6 samples exhibited two distinct bands with maxima at 265 and 300 nm due to charge transfer to Ce³⁺ and Ce⁴⁺ respectively. Their intensity increased with increase in the Ce content of the samples. The BET surface area was found to be in the range of 668.4-836.9 m²/g. HR-TEM images revealed well-ordered cubic 3-D mesoporous channels. The catalytic activity was tested in the liquid phase oxidation of cyclohexane with hydrogen peroxide. Cyclohexanol was found to be the major product. Ce-KIT-6 (5) gave 74% cyclohexane conversion, which is higher than other catalysts. Ce⁴⁺ is suggested to activate hydrogen peroxide by coordination to oxidise cyclohexane.

Full text of DOI:10.1016/j.apcata.2009.11.016

Applied Catalysis A: General 374 (2010) 11–17
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Cerium-incorporated cage-type mesoporous KIT-6 materials: Synthesis,
characterization and catalytic applications
*
Azhagapillai Prabhu, Loganathan Kumaresan, Muthaiahpillai Palanichamy, Velayutham Murugesan
Department of Chemistry, Anna University, Chennai 600025, India
A R T I C L E I N F O
A B S T R A C T
Article history:
Ce-KIT-6 with Si/Ce = 5, 10, 20, 50, 100 and 150 mesoporous molecular sieves were successfully
synthesized by a hydrothermal method. The synthesized materials were characterized extensively to
understand the physico-chemical properties of the materials. The low-angle powder XRD patterns of
calcined Ce-KIT-6 material showed a phase that can be indexed to cubic Ia3d. The UV–vis spectra of Ce-
KIT-6 samples exhibited two distinct bands with maxima at 265 and 300 nm due to charge transfer to
Ce3+ and Ce4+ respectively. Their intensity increased with increase in the Ce content of the samples. The
BET surface area was found to be in the range of 668.4–836.9 m2/g. HR-TEM images revealed well-
ordered cubic 3-D mesoporous channels. The catalytic activity was tested in the liquid phase oxidation of
cyclohexane with hydrogen peroxide. Cyclohexanol was found to be the major product. Ce-KIT-6 (5)
gave 74% cyclohexane conversion, which is higher than other catalysts. Ce4+ is suggested to activate
hydrogen peroxide by coordination to oxidise cyclohexane.
Received 26 July 2009
Received in revised form 9 November 2009
Accepted 12 November 2009
Available online 18 December 2009
Keywords:
Cage-type materials
Ce-KIT-6
Oxidation
Cyclohexane
Cyclohexanol
Cyclohexanone
Hydrogen peroxide
ß 2009 Elsevier B.V. All rights reserved.
1. Introduction
nanotube-type carbon such as CMK-9 as well as rod-type CMK-8.
The large pore mesoporous silica KIT-6 was prepared by Kim et al.
The discovery of silica-based and metal incorporated mesopor-
ous M41S, SBA-n and KIT-n families has attracted much attention
due to their high surface area, high pore volume, large pore
diameter and uniform pore size distribution [1–3]. These materials
find applications in the areas of catalysis, separation and
adsorption [4]. The large pore mesoporous silicas with cubic
Ia3d structure were synthesized by using triblock copolymers as
structure-directing agents under different synthetic conditions [5].
The incorporation of metal ions in the silica network of MCM-41,
MCM-48 and SBA-15 materials has been reported already [6–10]. A
brief report on cerium silicate analogues of MFI, BEA and MTW has
already been published [11]. In recent years, cerium containing
materials have been used as catalysts for selective oxidation of
organic compounds [12]. Cerium has proven to be a powerful
modifying agent for catalysts such as zeolites used in the cracking
of heavy oil [13]. However, the incorporation of Ce in zeolite
framework is rather difficult. The usefulness of mesoporous
materials can be improved by incorporation of heteroatoms such
as titanium [14–17], vanadium [18–20], aluminium [21,22] and
recently cerium [23]. Although pure KIT-6 has no catalytic sites, it
can be used as a template to fabricate new bicontinuous arrays of
[24]. KIT-6 exhibited cubic Ia3d symmetry; its structural consists
of interpenetrating bicontinuous networks of channels such as
those in MCM-48. Recently researchers have focused on the
substitution of catalytically active heteroatoms which could
exhibit considerable reactivity due to easily accessible active sites
within the mesoporous network [25]. The incorporation of Ce into
mesoporous materials like M41S, hexagonal SBA-15 and cubic KIT-
6 is expected to be rather easy because of the greater flexibility of
the network. Cerium-containing materials are not only important
but also an interesting class of materials [26]. The coordination of
cerium ions in the mesoporous materials can affect the catalytic
properties because the incorporation of cerium ions into the walls
of mesoporous silica material allows creation of Lewis and
Bronsted acid sites. In particular, cerium containing MCM-41
materials have exhibited many catalytic applications, including
vapor-phase dehydration of cyclohexanol, hydroxylation of 1-
naphthol with aqueous H2O2 and tert-butyl hydroperoxide,
selective acylation, alkylation, oxidation of cyclohexane to
cyclohexanol and n-heptane oxidation [27]. Ce-MCM-48 and Ce-
SBA-15 exhibited good catalytic activity and selectivity in the
acylation of alcohols, thiols, phenols and amines [28,29].
As mesoporous KIT-6 is a very recent material as regards
research interest, its application to catalysis is yet to be exploited
fully. Dubey et al. [30] reported the synthesis of polymer-silica
(KIT-6 with cubic symmetry) composite materials through in situ
* Corresponding author. Tel.: +91 44 22203144; fax: +91 44 22350397.
E-mail address: v_murugu@hotmail.com (V. Murugesan).
0926-860X/$ – see front matter ß 2009 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2009.11.016

12
A. Prabhu et al. / Applied Catalysis A: General 374 (2010) 11–17
radical controlled polymerization vinylmonomers inside the silica
mesopores for hydroxylation of phenol. There is no report on the
use of Ce-KIT-6 in the oxidation of cyclohexane with H2O2. In the
present paper, we first report on the synthesis and characterization
of cerium containing KIT-6 mesoporous materials. The environ-
ment of the cerium cation in Ce-KIT-6 molecular sieve was
determined by different methods, and the materials were also
tested for their catalytic activity.
II volumetric adsorption analyzer. Before adsorption measure-
ments all samples were outgassed at 200 8C in the degas port of the
adsorption analyzer.
X-ray photoelectron spectral analysis was carried out using a
Thermo Electron Corporation Instrument equipped with ultra-high
vacuum chambers. Mg K-alpha X-ray (100 W) was used as the
source at a take off angle of 5–758; a vacuum pressure of 10ꢁ6 to
10ꢁ7 Torr was maintained. The powder sample was fixed on a steel
holder with double-sided adhesive tape and analyzed as received.
An electron flood gun was used to reduce charge effects. High-
resolution spectra were obtained with an analyzer pass energy of
100 eV. The binding energies were referenced to an adventitious
carbon 1s line set at 284.8 eV. Gaussian line shapes were used to fit
the curves for C 1s, O 1s and N 1s, and a mixed Gaussian/Lorentzian
function was employed for Cu 2p, Ti 2p and Ce 3d. The elemental
compositions were estimated from the relative area intensities of N
(1s), Ti (2p) and O (1s) peaks. DR–UV–vis spectra of the materials
were recorded using a UV–vis spectrophotometer (Shimadzu
model 2450). BaSO4 was used as the standard for measurement in
the scan range 200–800 nm.
2. Experimental
2.1. Materials
Tetraethylorthosilicate (Merck) and cerium nitrate (Merck)
were used as the sources for silicon and cerium respectively.
Triblock copolymer, poly (ethylene glycol)-block–poly (propylene
glycol)-block–poly (ethylene glycol) (Pluronic P123, molecular
weight = 5800, EO20PO70EO20; Aldrich) was used as the structure-
directing template. Hydrochloric acid (30%) and n-butanol, as co-
solvent, were used in the synthesis, AR grade samples. H2O2
(28 wt.% in water) was titrated iodometrically prior to use.
Cyclohexane was purchased from Merck (India).
3. Results and discussion
2.2. Synthesis
3.1. XRD
Ce-containing KIT-6 (Ce-KIT-6) mesoporous materials were
prepared by the following procedure: triblock copolymer (P123)
(4 g) was added to distilled water (120 ml). HCl (30%) (7.2 ml) was
added to this solution. The mixture was then stirred for 4 h to make
the template dissolve in water and HCl. To this combination, n-
butanol (6 g) was added as co-solvent under stirring at 35 8C. After
one hour stirring, TEOS (8.6 g) and cerium nitrate (0.35 g) (Si/
Ce = 50) were added under the same condition. The mixture was
then stirred for 24 h at 35 8C, followed by 24 h heating at 100 8C in
an oven. The white solid product obtained after hydrothermal
treatment was filtered and dried at 100 8C without washing. The
solid material was calcined at 540 8C to remove the template.
The small angle powder XRD patterns of calcined Ce-KIT-6
materials are shown in Fig. 1. All of them showed characteristic
peaks indexed to cubic Ia3d. The high intensity peak observed in
the 2u range around 0.98 is indexed to (2 1 1). The intensity of this
(2 1 1) diffraction peak decreased with the increase in the Ce
content. Wide angle XRD patterns of Ce-KIT-6 showed that the
walls are amorphous, as there is no pattern in the 2u range 20–308.
The unit cell constants (a0) and the d-spacing values for all Ce-KIT-
6 materials are presented in Table 1. From the values in the table it
is evident that there is no corresponding variation of a0 and d-
spacing with the addition of cerium content. Hence, the wall
thickness and pore diameter might not be influenced by the
addition of cerium in KIT-6.
2.3. Catalytic studies
3.2. ICP-AES
The reaction was carried out in the liquid phase. The batch
reactor consists of a double-necked round bottomed flask fitted
with a condenser. The reaction mixture viz., cyclohexane, H2O2 and
solvent (5 ml), was taken in the reactor vessel and heated in an oil
bath to the requisite temperature with simultaneous stirring with
a magnetic stirrer. The liquid products were analyzed using a gas
chromatograph (Shimadzu GC-17) with a DB-5 capillary column
The cerium content in Ce-KIT-6 for different Si/Ce ratios was
determined using ICP-AES and the results are given in Table 1.
Under the synthesis conditions employed, the crystallization
(30 m ꢀ 0.25 mm ꢀ 0.25
mm) equipped with a flame ionization
detector. The products were also identified using a gas chromato-
graph coupled with a mass spectrometer (PerkinElmer Auto
System XL Gas Chromotograph with Turbo Mass Spectrometer) (EI,
70 eV); helium was the carrier gas.
2.4. Material characterization
Small angle powder X-ray diffraction (SAXRD) patterns were
recorded on a Rigaku diffractometer using Cu K
a radiation
(
l
= 0.154 nm) in the 2 range 0.88–108, typically running at a
u
voltage of 40 kV and a current of 100 mA. High-resolution
transmission electron microscopic (HRTEM) images were recorded
from the thin edges of particles supported on a porous carbon grid
using JEOL JEM-4000EX equipment operated at 400 kV with a field
emission gun. The cerium content in Ce-KIT-6 was determined
using inductively coupled plasma atomic emission spectroscopy
(ICP-AES) with a Labtum Plasma 8440 instrument. Nitrogen
adsorption isotherm was recorded at ꢁ196 8C on a Belsorb mini
Fig. 1. Powder XRD patterns of calcined Ce-KIT-6 with Si/Ce ratio of (a) 5, (b) 10, (c)
20, (d) 50, (e) 100 and (f) 150.

A. Prabhu et al. / Applied Catalysis A: General 374 (2010) 11–17
13
Table 1
Structural and textural parameters of Ce-KIT-6 of different Si/Ce ratios.
Si/Ceb (gel)
SBET (m2 gꢁ1
)
Vp (cm3 gꢁ1
d
)
dp (nm)
ds (nm)
a
c
e
f
Catalyst
a0 (nm)
Ce-KIT-6 (5)
20.86
21.63
21.18
22.07
21.18
20.38
12
16
836.9
753.9
743.6
668.4
776.0
786.4
1.24
1.24
1.17
1.05
0.75
1.08
6.19
6.86
6.42
6.63
6.47
5.84
4.24
3.95
4.39
4.41
4.12
4.35
Ce-KIT-6 (10)
Ce-KIT-6 (20)
Ce-KIT-6 (50)
Ce-KIT-6 (100)
Ce-KIT-6 (150)
25
54
103
152
pffiffiffi
a
Unit cell parameter calculated from the formula a0 ¼ d2 1 1
ꢀ
6.
b
c
d
e
f
From ICP-AES.
Specific surface area.
Pore volume.
Pore diameter.
Pore wall thickness.
from Ce-KIT-6 (5) to Ce-KIT-6 (100). It is attributed to enhanced
irregularity of mesopores with increase in Ce content.
3.4. HR-TEM
The highly ordered mesoporous structure of Ce-KIT-6 (50) is
also confirmed by HR-TEM images as shown in Fig. 3. The high-
resolution TEM image showed clear lattice fringes with a spacing of
about 20 nm. Such results again prove that the catalyst possesses
typical 3-dimensional cubic structure for KIT-6. It is further
confirmed that no agglomerated active species could be observed
on the surface, which means that the active phases are highly
dispersed or embedded in the silica matrix. The distance between
two consecutive centres of cubic pores estimated from the HR-TEM
image is ca. 20 nm. The average thickness of the wall is ca. 4–5 nm
and the pore diameter is around 6 nm. This is in agreement with
the results of the N2 adsorption study.
Fig. 2. Nitrogen adsorption–desorption isotherms of calcined Ce-KIT-6 with Si/Ce
3.5. XPS
ratio of (a) 5, (b) 10, (c) 20, (d) 50, (e) 100 and (f) 150.
reaction was non-stoichiometric and hence a higher Si/Ce ratio was
observed.
The XPS spectra of Ce-KIT-6 (5, 10, 20, 50, 100 and 150) are
shown in Fig. 4. Ce3+ compound gave 2 peaks corresponding to 3d5/
2 and 3d3/2 [31]. On the other hand Ce4+ compound was shown to
contain three characteristic regions, viz., from 880 to 890 eV BE due
to Ce 3d5/2, from 890 to 910 eV BE due to Ce3d5/2 and Ce3d3/2 and at
917 eV BE due to 3d3/2 of Ce4+. In the present study we also
observed similar peaks for Ce3+ and Ce4+ as shown Fig. 6. Hence, the
materials are established to possess both Ce3+ and Ce4+, as
discussed in the DR–UV–vis spectral analysis.
3.3. BET surface area analysis
Nitrogen adsorption-desorption isotherms of Ce-KIT-6 are
shown in Fig. 2 and the corresponding textural parameters are
also presented in Table 1. The N2 sorption isotherms are typically
type IV isotherm with hysteresis, which is characteristic of the
mesoporous nature of the materials. The BET surface area is in
the range 668.4–836.9 m2/g. A well-defined steep appearance in
the adsorption–desorption curves is observed around a relative
pressure (p/po) of 0.7. The BJH pore volume obtained from the
adsorption branch is about 1.24 cm3 gꢁ1, which is larger than the
thickness of the silica wall template. The pore volume decreased
3.6. DRS–UV–vis
The DRS–UV–vis spectra of Ce-KIT-6 (5, 10, 20, 50, 100 and 150)
are shown in Fig. 5. The materials were transparent from 400 to
800 nm, covering the entire visible region. There are absorption
Fig. 3. HR-TEM image of template-free mesoporous (a) Ce-KIT-6 (10) and (b) Ce-KIT-6 (50) in the direction of the pore axis.

14
A. Prabhu et al. / Applied Catalysis A: General 374 (2010) 11–17
Fig. 5. DR–UV spectra of calcined Ce-KIT-6 with Si/Ce ratio of (a) 5, (b) 10, (c) 20, (d)
50, (e) 100 and (f) 150.
H2O2 as a sacrificial oxidant in acetic acid at 70, 80 and 90 8C with a
feed ratio of (cyclohexane: H2O2) 1:1. The products were found to
be cyclohexanol and cyclohexanone. In addition, there are also
other products with cyclohexyl acetate as the major one. The
results of cyclohexane conversion and product selectivity are
presented in Table 2. The cyclohexane conversion over Ce-KIT-6
(Si/Ce = 5) rapidly increased from 70 to 80 8C but above 80 8C the
conversion decreased. The increase in conversion from 70 to 80 8C
is due to enhanced activation of H2O2. But above 80 8C, H2O2
started to decompose and hence cyclohexane conversion de-
creased. Although the catalyst contains both Ce3+ and Ce4+, as
evidenced from DRS–UV analysis, the main active sites that
activate H2O2 are considered to be Ce4+ [33]. However, Ce3+ can
generate Bronsted acidity, which can catalyze esterification of
cyclohexanol to cyclohexyl acetate by activating acetic acid. This
was also verified by heating acetic acid and cyclohexanol over Ce-
KIT-6 (5) which gave 40% conversion in 12 h reaction time. Though
Ce4+ is present in the framework with tetrahedral oxidic
coordination, it is capable of chemisorbing and activating H2O2,
as enhancing of coordination number more than six is quite
common for rare earth elements. When H2O2 is chemisorbed, only
Fig. 4. XPS spectra of Ce-KIT-6 with Si/Ce ratio of (a) 5, (b) 10, (c) 20, (d) 50, (e) 100
and (f) 150.
bands with lmax at 265 and 300 nm in the near ultraviolet region
from 200 to 400 nm. Bensalem et al. [32] have reported that the
charge transfer transition of O2ꢁ ! Ce3+ is occurring at 265 nm and
that of O2ꢁ ! Ce4+ is occurring at 300 nm. These transitions are
nearly coincident with the present samples. Hence, the samples
might have both Ce3+ and Ce4+ sites in the framework. The
intensities of these peaks are also in accordance with Si/Ce ratio,
that is, Ce-KIT-6 (5) shows more intense bands for Ce3+ and Ce4+
and the intensities decrease with increase in the Si/Ce ratios.
3.7. Catalytic studies
3.7.1. Effect of temperature
The liquid phase oxidation of cyclohexane was carried out over
Ce-KIT-6 (Si/Ce = 5, 10, 20, 50, 100 and 150) molecular sieves using
Table 2
Effect of temperature on the conversion of cyclohexane and product selectivity.
Catalyst
Temperature (8C)
Conversion (%)
Selectivity (%)
Cyclohexanol
Cyclohexanone
Others
Ce-KIT-6 (5)
70
80
90
58.7
74.1
68.1
84.1
80.6
81.1
9.9
10.2
8.6
6.0
9.2
10.3
Ce-KIT-6 (10)
Ce-KIT-6 (20)
Ce-KIT-6 (50)
Ce-KIT-6 (100)
Ce-KIT-6 (150)
70
80
90
55.1
68.1
66.1
84.8
80.1
81.4
8.8
11.3
7.2
6.4
8.6
11.4
70
80
90
49.1
61.8
58.1
84.6
77
8.3
14.1
8.2
7.4
8.9
81.2
10.6
70
80
90
44.3
55.1
51.4
84.8
79.2
80.8
7.1
10.1
6.4
8.1
10.7
12.8
70
80
90
40.6
52.1
47.3
85.5
79.2
81.1
6.2
9.1
8.2
9.3
11.7
10.7
70
80
90
31.2
40.1
38.6
84.1
79.4
78.3
5.8
8.4
8.6
10.1
12.2
13.1
Catalyst: 0.1 g; feed ratio 1:2; solvent: acetic acid; time: 12 h.

A. Prabhu et al. / Applied Catalysis A: General 374 (2010) 11–17
15
Scheme 1. Possible mechanistic pathway for the formation of cyclohexanol.
the distant oxygen is activated, by which cyclohexane could be
oxidized by insertion of oxygen across the C–H bond as shown in
the reaction Scheme 1. Hence, cyclohexanol is the principal
product in this oxidation.
Cyclohexane conversion and selectivity of products over Ce-
KIT-6 (10) followed nearly the same trend as that of Ce-KIT-6 (5).
The other catalysts with high Si/Ce ratios also exhibited similar
trends towards conversion and product selectivity to that of Ce-
KIT-6 (5). But the conversion over Ce-KIT-6 (5) was higher than
other conversions. The selectivity of cyclohexyl acetate over all Ce-
KIT-6 with high Si/Ce ratios was higher than Ce-KIT-6 (5), as the
former catalysts possess higher hydrophobicity than the latter.
Due to the enhanced hydrophobicity, they can rapidly expel water
as and when they are formed during esterification, thus shifting the
equilibrium to the ester side. This study revealed that Ce-KIT-6 (5)
is better than all other catalysts for cyclohexane conversion and for
cyclohexanol selectivity. Again, from this discussion it is clear that
both Ce3+ and Ce4+ are mainly planted on the surface of the channel
rather than being buried inside the wall.
The selectivity of cyclohexanol decreased at 80 8C due to its
oxidation to cyclohexanone. The selectivity of cyclohexanol
increased slightly at 90 8C, as its conversion to cyclohexanone
decreased. But the cyclohexanone selectivity at 90 8C is less than at
80 8C, which may be due to decomposition of H2O2. Based on the
above discussion, we conclude that the selectivity of cyclohexanol
can be increased by conducting oxidation in the temperature range
70–80 8C with high content of cyclohexane in the feed, so that
conversion of cyclohexanol to cyclohexyl acetate could be
suppressed. The last column in Table 2 indicates the selectivity
of other products, among which cyclohexyl acetate is the major
one. Cyclohexyl acetate selectivity increased with increase in
temperature and as mentioned above, the reaction is catalyzed by
Bronsted acid sites generated by the framework Ce3+ sites. Based
on the formation of cyclohexyl acetate and the oxidation of
cyclohexanol, it is inferred that the respective active sites are Ce3+
3.7.2. Effect of feed ratio
The effects of feed ratio on cyclohexane conversion and
cyclohexanol selectivity were studied with 1:1, 1:2, 1:3 and 1:4
at 80 8C over Ce-KIT-6 (5) for 12 h and the results are presented in
Table 3. The conversion increased rapidly from 1:1 to 1:2 feed ratio,
but the increase in conversion was not so rapid at 1:3 feed ratio.
There was only a slight increase in conversion with 1:4 feed ratio
compared to that with 1:2. Hence, there might be less competition
for cyclohexane than for cyclohexanol at 1:2 for oxidation.
However, it appears that cyclohexanol competition for oxidation
becomes more significant at 1:3 and 1:4. The selectivity to
cyclohexanol decreased and cyclohexanone increased with in-
crease in the H2O2 content in the feed. The selectivity to cyclohexyl
acetate at 1:3 and 1:4 was slightly higher than at 1:1. As the
reaction requires chemisorption of acetic acid and nucleophilic
and Ce4+
.
The formation of cyclohexanone is due to the oxidation of
cyclohexanol by the activated H2O2. The distant oxygen of
activated H2O2 is suggested to abstract two hydrogens from
cyclohexanol, as shown in the reaction Scheme 1. Yao et al. have
reported that the formation of peroxyacetic acid is the cause of the
conversion of cyclohexanol to cyclohexanone. Laha et al. reported
dehydration of cyclohexanol to cyclohexene over Ce-MCM-41 at
80 8C. But in the present study, though the reaction was carried out
at 70, 80 and 90 8C, the product analysis by GC-MS did not provide
any evidence in support of dehydration of cyclohexanol. In order to
verify the formation of peroxyacetic acid, we performed a control
reaction with H2O2 and cylcohexane over Ce-KIT-6 (5) without
using acetic acid as solvent. In this reaction also, cyclohexanone
was observed as the product. Hence, peroxyacetic acid may not be
Table 3
Effect of feed ratio on the conversion of cyclohexane and product selectivity.
Feed ratio
Conversion (%)
Selectivity (%)
Cyclohexanol
a
required species for the conversion of cyclohexanol to
Cyclohexanone
Others
cyclohexanone. But the conversion of cyclohexane was less for
the same feed ratios; hence the main role of acetic acid is only as a
1:1
1:2
1:3
1:4
60.1
74.1
81.8
83.2
84.1
80.6
73.6
69.1
7.9
10.2
13.9
17.8
8.2
9.2
solvent rather than
a mediator to oxidize cyclohexanol to
12.5
13.1
cyclohexanone. Although the reaction was feasible in the absence
of solvent, the latter is a necessity in order to enhance the
conversion.
Catalyst: Ce-KIT-6 (5); catalyst amount: 0.1 g; temperature: 80 8C; solvent: acetic
acid; time: 12 h.

16
A. Prabhu et al. / Applied Catalysis A: General 374 (2010) 11–17
Table 4
Table 6
Effect of reaction time on the conversion of cyclohexane and product selectivity.
Effect of solvent on the conversion of cyclohexane and product selectivity.
Reaction time (h) Conversion (%) Selectivity (%)
Cyclohexanol Cyclohexanone Others
Solvent
Conversion (%)
Selectivity (%)
Cyclohexanol
Cyclohexanone
Others
6
12
24
49.5
74.1
82.9
88.5
80.6
67.2
3.9
10.2
16.9
7.6
9.2
Acetone
30.7
74.1
10.3
82.2
80.6
93.0
17.8
10.2
2.8
0.0
9.2
4.2
Acetic acid
Methanol
15.9
Catalyst: Ce-KIT-6 (5); catalyst amount: 0.1 g; feed ratio 1:2; solvent: acetic acid;
Catalyst: Ce-KIT-6 (5); catalyst amount: 0.1 g; feed ratio 1:2; temperature: 80 8C;
temperature: 80 8C.
time: 12 h.
Table 5
conversion of cyclohexane in acetic acid than in methyl ethyl
ketone with H2O2 over TS-1 catalyst. Yao et al. reported partial
decomposition of H2O2 in methanol and acetone. The high
conversion of cyclohexane in acetic acid is attributed to the
formation of peracetic acid in acetic acid medium and hence
insertion of oxygen across the –OH bond of acetic acid by the
activated H2O2 might occur in the present reaction. In addition, one
can speculate that interaction of acetic acid with the oxidic sites of
framework through hydrogen bonding could partly enhance the
coordination capability of cerium. Hence, it can better adsorb and
activate H2O2 and subsequently oxidize cyclohexane. But such a
strong hydrogen bonding property is not possible with either
acetone or methanol. Acetone and methanol gave high selectivity
to cyclohexanol. The side product was not obtained in acetone, but
it was observed in methanol. Hence, acetone is better than
methanol, but acetic acid is better than all other solvents because it
gave higher conversion than others.
Effect of catalyst amount on the conversion of cyclohexane and product selectivity.
Catalyst weight (g) Conversion (%) Selectivity (%)
Cyclohexanol Cyclohexanone Others
0.05
0.10
0.15
0.20
29.4
74.1
78.9
82.8
87.8
80.6
76.6
66.8
6.0
10.2
14.6
17.4
6.2
9.2
12.8
15.8
Catalyst: Ce-KIT-6 (5); feed ratio 1:2; solvent: acetic acid; temperature: 80 8C; time:
12 h.
attack of cyclohexanol on acetic acid, a 1:1 feed ratio gives less
cyclohexyl acetate. Hence, based on the high conversion and the
lower selectivity to cyclohexyl acetate, 1:2 was chosen as the
optimum ratio.
3.7.3. Effect of reaction time
The effects of reaction time on conversion and product
selectivity were studied for 6, 12 and 24 h with the feed ratio
1:2 at 80 8C. The results are presented in Table 4. For a given feed
ratio, longer reaction time may not be good as it could promote
cyclohexanol esterfication to cyclohexyl acetate. This view is
clearly evident from the data in Table 4. The selectivity to
cyclohexanol decreased and cyclohexanone increased with the
increase in reaction time. Similarly the selectivity to cyclohexyl
acetate also increased. Hence, 12 h reaction time was chosen as the
optimum one based on the high selectivity of cyclohexanol and the
lower selectivity to cyclohexyl acetate. The recyclability of the
catalyst was also tested three times under similar conditions; no
significant change in conversion and product selectivity was
observed. Hence, Ce is suggested to be so firmly planted in the
framework that it cannot easily be leached away in acetic acid or
other solvents. Similar results were also reported by Yao et al.
4. Conclusions
The preparation of Ce-KIT-6 with different ratios of Si/Ce was
successfully accomplished. The incorporation of cerium allowed
mesoporous materials to retain the cubic array of pores, which is
confirmed by HR-TEM and low-angle XRD. The catalytic study also
proved that cerium planted KIT-6 molecular sieves are effective
catalysts for the selective formation of cyclohexanol from the
oxidation of cyclohexane. Although Ce4+ are shown to be the main
active sites that activate H2O2 in the reaction, their planting largely
in the channel surface is an important observation in this study.
Ce3+ is not required for cyclohexane oxidation but their presence
appear to be unavoidable in the synthesis of Ce-KIT-6. Recyclability
of the catalyst without leaching of cerium in the presence of
solvent is also an important characteristic of the catalysts. The use
of acetic acid as a solvent is better than that of either acetone or
methanol, since acetic acid gave high conversion and high
selectivity to cyclohexanol. Based on this study it is tentatively
concluded that any quadrivalent metal ions in the framework of
mesoporous material with high coordination property are capable
of oxidizing cyclohexane to cyclohexanol by activating H2O2. This
study should be extended to incorporate other quadrivalent rare
earth metal ions to prove this conclusion.
3.7.4. Effect of catalyst amount
The effects of catalyst loading on cyclohexane conversion and
product selectivity were studied with 0.05, 0.1, 0.15 and 0.20 g of
the catalyst with a feed ratio 1:2 at 80 8C. The results are presented
in Table 5. The conversion increased rapidly from 0.05 to 0.10 g
catalyst but the increase was not so much above 0.10 g. There is an
approximately 50% decrease in acid sites in 0.05 g catalyst and
hence less conversion was observed. The increase in conversion
was not very rapid above 0.1 g catalyst and hence the content of
free H2O2 may not be high. The selectivity of cyclohexanol
decreased with increase in catalyst amount as the selectivity of its
dependent products, namely, cyclohexyl acetate increased gradu-
ally. This study therefore illustrates that a catalyst weight of 0.10 g
is the optimum one for high cyclohexane conversion and high
selectivity to cyclohexanol.
Acknowledgements
The authors acknowledge University Grants Commission (UGC),
New Delhi for providing funds under special assistance-DRS
programme and Department of Science and Technology (DST),
New Delhi, under FIST programme for equipment facilities in the
Department. One of the authors, L. Kumaresan is thankful to CSIR,
New Delhi, for the award of a senior research fellow grant (SRF).
3.7.5. Effect of solvents
The effects of solvents on conversion and product selectivity
were studied at 80 8C with the feed ratio 1:2 and 0.1 g catalyst. The
results are presented in Table 6. Acetic acid showed higher
conversion than other solvents. Sooknoi et al. [34] reported higher
References
[1] C.T. Kresge, M.E. Leonowicz, W.J. Roth, J.C. Vartuli, J.S. Beck, Nature 359 (1992)
710–712.

A. Prabhu et al. / Applied Catalysis A: General 374 (2010) 11–17
17
[2] J.S. Beck, J.C. Vartuli, G.J. Kennedy, C.T. Kresge, W.J. Roth, S.E. Schramm, Chem.
Mater. 6 (1994) 1816–1821.
[3] J.S. Beck, J.C. Vartuli, W.J. Roth, M.E. Leonowicz, C.T. Kresge, K.D. Schmitt, C.T.W.
Chu, D.H. Olson, E.W. Sheppard, S.B. McCullen, J.B. Higgins, J.L. Schlenker, J. Am.
Chem. Soc. 1141 (1992) 10834–10843.
[18] W. Zhang, J. Wang, P.T. Tanev, T.J. Pinnavaia, Chem. Commun. (1996) 979–980.
[19] Z. Zhang, J. Suo, X. Zhang, S. Li, Chem. Commun. (1998) 241–242.
[20] R. Neumann, A.M. Khenkin, Chem. Commun. (1996) 2643–2644.
[21] A. Prabhu, L. Kumaresan, M. Palanichamy, V. Murugesan, Appl. Catal. A: Gen. 360
(2009) 59–65.
[4] G. Oye, W.R. Glomm, T. Vralstad, S. Volden, H. Magnusson, M. Stocker, J. Sjoblom,
Adv. Colloid Interf. Sci. 123–126 (2006) 17–32.
[22] Y. Li, W. Zhang, L. Zhang, Q. Yang, Z. Wei, Z. Feng, C. Li, J. Phys. Chem. B 108 (2004)
9739–9744.
[5] F. Kleitz, S.H. Choi, R. Ryoo, Chem. Commun. (2003) 2136–2137.
[6] J.M. Kim, S.K. Kim, R. Ryoo, Chem. Commun. (1998) 259–260.
[7] A. Sayari, J. Am. Chem. Soc. 122 (2000) 6504–6505.
[8] Y. Liu, A. Karkamkar, T.J. Pinnavaia, Chem. Commun. (2001) 1822–1823.
[9] D. Zhao, J. Feng, Q. Huo, N. Melosh, G.H. Frederickson, B.F. Chmelka, G.D. Stucky,
Science 279 (1998) 548–552.
[10] D. Zhao, Q. Huo, J. Feng, J. Kim, Y. Han, G.D. Stucky, Chem. Mater. 11 (1999) 2668–
2672.
[11] S.C. Laha, P. Mukherjee, S.R. Sainkar, R. Kumar, J. Catal. 207 (2002) 213–223.
[12] G.A. Molander, Chem. Rev. 92 (1992) 29–68.
[13] T.M. Tri, J. Massardier, P. Gallezot, B. Imelik, in: Tokyo, T. Seiyama, K. Tanabe (Eds.),
Proceeding of the Seventh International Congress on Catalysis, Vol. A, Elsevier,
Amsterdam, Netherlands, 1980, p. 266.
[14] W.H.Zhang,J.Lu,B.Han,M.Li,J. Xiu,P.Ying,C.Li, Chem. Mater. 14(2002)3413–3421.
[15] F. Chiker, J.P. Nogier, F. Launay, J.L. Bonardet, Appl. Catal. A:Gen.259 (2004) 153–162.
[16] G. Lapisardi, F. Chiker, F. Launay, J.P. Nogier, J.L. Bonardet, Catal. Commun. 5
(2004) 277–281.
[23] A.S. Araujo, J.M.F.B. Aquino, M.J.B. Souza, A.O.S. Silvab, J. Solid-State Chem. 171
(2003) 371–374.
[24] T.W. Kim, F. Kleitz, B. Paul, R. Ryoo, J. Am. Chem. Soc. 127 (2005) 7601–7610.
[25] Y. Shao, L. Wang, J. Zhang, M. Anpo, J. Phys. Chem. B 109 (2005) 20835–20841.
[26] K.M.S. Khalil, J. Coll. Interf. Sci. 315 (2007) 562–568.
[27] M.D. Kadgaonkar, S.C. Laha, R.K. Pandey, P. Kumar, S.P. Mirajkar, R. Kumar, Catal.
Today 97 (2004) 225–231.
[28] Z. Wangcheng, L. Guanzhong, G. Yanglong, G. Yun, W. Yanqin, W. Yunsong, Z.
Zhigang, L. Xiaohui, J. Rare Earths 26 (2008) 515–522.
[29] W. Yao, Y. Chen, L. Min, H. Fang, Z. Yan, H. Wang, J. Wang, J. Mol. Catal. A: Chem.
246 (2006) 162–166.
[30] A. Dubey, M. Choi, R. Ryoo, Green Chem. 8 (2006) 144–146.
[31] E. Cano, M.A. Garcia, M.A. Villegas, G. Battaglin, J. Llopis, J.M. Bastidas, J. Sol–Gel
Sci. Technol. 27 (2003) 293–299.
[32] A. Bensalem, J.C. Muller, F.B. Verduraz, J. Chem. Soc., Faraday Trans. 88 (1992)
153–154.
[33] M.N. Timofeeva, S.H. Jhung, Y.K. Hwang, D.K. Kim, V.N. Panchenko, M.S. Melgunov,
Yu.A. Chesalov, J.S. Chang, Appl. Catal. A: Gen. 317 (2007) 1–10.
[34] T. Sooknoi, J. Limtrakul, Appl. Catal. A: Gen. 233 (2002) 227–237.
[17] G. Lapisardi, F. Chiker, F. Launay, J.P. Nogier, J.L. Bonardet, Micropor. Mesopor.
Mater. 78 (2005) 289–295.