Highly selective synthesis of cyclododecanone over mesostructured VSBA-15 catalysts

Article abstract of DOI:10.1016/j.apcata.2010.08.007

Highly ordered VSBA-15 mesoporous catalysts have been synthesized under pH-adjusting direct hydrothermal (pH-aDH) method using vanadyl sulphate hydrate and Pluronic P123 as the sources of vanadium and template, respectively. The mesoporous catalysts characterized by sophisticated instrumental techniques, viz. ICP-AES, XRD, N₂ adsorption, ESR, UV-vis DRS, ⁵¹V MAS NMR, ²⁹Si MAS NMR and TEM show their two-dimensional mesostructures with tetrahedral vanadium species on the silica surface. The well ordered VSBA-15 catalysts have been used in the oxidation of cyclododecane (CDD) with hydrogen peroxide (H₂O₂, 30%) to find their catalytic activities. The regenerated VSBA-15 catalysts have been examined to find their catalytic stabilities. VSBA-15(5) catalyst has been washed with ammonium acetate solution to investigate the leaching of vanadium species in the framework of silica network of SBA-15, and the catalytic activity of washed VSBA-15(5) has also been examined in the catalytic reactions. Moreover, the hydrothermally stable VSBA-15(5) catalyst has also been examined in the catalytic reaction to find the effect of its catalytic activity. Additionally, the influences of various reaction parameters such as temperature, time, ratios of reactant and solvents on the oxidation of CDD have been investigated. Based on the catalytic results, VSBA-15(5) catalyst is found to be a highly active, recyclable and promising heterogeneous catalyst for selective synthesis of cyclododecanone (CDDO).

Full text of DOI:10.1016/j.apcata.2010.08.007

Applied Catalysis A: General 388 (2010) 22–30
Contents lists available at ScienceDirect
Applied Catalysis A: General
journal homepage: www.elsevier.com/locate/apcata
Highly selective synthesis of cyclododecanone over mesostructured
VSBA-15 catalysts
M. Selvaraj∗, D.W. Park
School of Chemical and Biomolecular Engineering, Pusan National University, Busan 609-735, Republic of Korea
a r t i c l e i n f o
a b s t r a c t
Article history:
Received 12 April 2010
Received in revised form 28 July 2010
Accepted 5 August 2010
Available online 12 August 2010
Highly ordered VSBA-15 mesoporous catalysts have been synthesized under pH-adjusting direct
hydrothermal (pH-aDH) method using vanadyl sulphate hydrate and Pluronic P123 as the sources of
vanadium and template, respectively. The mesoporous catalysts characterized by sophisticated instru-
mental techniques, viz. ICP-AES, XRD, N2 adsorption, ESR, UV–vis DRS, 51V MAS NMR, 29Si MAS NMR and
TEM show their two-dimensional mesostructures with tetrahedral vanadium species on the silica sur-
face. The well ordered VSBA-15 catalysts have been used in the oxidation of cyclododecane (CDD) with
hydrogen peroxide (H2O2, 30%) to find their catalytic activities. The regenerated VSBA-15 catalysts have
been examined to find their catalytic stabilities. VSBA-15(5) catalyst has been washed with ammonium
acetate solution to investigate the leaching of vanadium species in the framework of silica network of
SBA-15, and the catalytic activity of washed VSBA-15(5) has also been examined in the catalytic reactions.
Moreover, the hydrothermally stable VSBA-15(5) catalyst has also been examined in the catalytic reac-
tion to find the effect of its catalytic activity. Additionally, the influences of various reaction parameters
such as temperature, time, ratios of reactant and solvents on the oxidation of CDD have been investigated.
Based on the catalytic results, VSBA-15(5) catalyst is found to be a highly active, recyclable and promising
heterogeneous catalyst for selective synthesis of cyclododecanone (CDD O).
Keywords:
VSBA-15
pH-adjusting direct hydrothermal method
Hydrothermal stability
Catalytic stability
Oxidation of CDD
© 2010 Elsevier B.V. All rights reserved.
1. Introduction
chemical supports for large organic molecules [5–8,16–18]. How-
ever, SBA-15 has better hydrothermal stability than MCM-41 due
Vanadium-containing microporous catalysts synthesized using
different hydrothermal conditions were used in several oxidation
reactions to produce a variety of selective products [1], however,
their catalytic activities strongly depend on the syntheses methods
[2,3]. The microporous catalysts restrict the accessibility of large
substrate molecules due to their smaller pore sizes. The described
limitations have been overcome by the discovery of mesoporous
M41s materials in 1992 [4]. Since Beck and co-workers reported
the mesoporous MCM-41 synthesized using direct hydrothermal
method [4], extensive research efforts have focused in the catalytic
applications of heteroatoms containing mesoporous silicas [5–8].
Highly ordered mesoporous VMCM-41 materials were successfully
synthesized and used in the oxidation of aromatics [6–14], and
they gave good catalytic activities with good selective products.
In 1998, Zhao et al. reported a new family of mesoporous SBA-15
synthesized under acidic hydrothermal condition [15]. Particularly,
all the mesoporous materials (MCM-41 and SBA-15) have received
much attention from researchers in the past decade because of their
potential applications such as catalysts, adsorbents and guest–host
to thicker pore walls [4,15], and it has also larger pore size than
MCM-41. But, the pure SBA-15 material is unable to be used directly
as a catalyst due to the lack of acidities. Hence, it has reported
that the guest species like heteroatoms or organic moieties can be
introduced into the framework of SBA-15 to increase the active
sites and thus improve the catalytic activity [16,17]. Therefore, it
is of great importance to introduce heteroatoms into mesoporous
silica materials prepared under strongly acidic conditions. Sev-
eral research reports are available, as the heteroatoms (M = Al, Fe,
V, Ti and Ga) were incorporated in the framework of SBA-15 by
direct synthesis or post-synthetic grafting method, for creating the
active sites on the silica surface, and the synthesized materials were
tested for their catalytic activities for a certain reaction [19–24].
As mentioned in a previous report [25], it is difficult to incorpo-
rate the high amounts of heteroatoms into the mesoporous silica
walls under the highly acidic conditions, and the introduced met-
als exist in the cationic form rather than as their corresponding
oxo species. Most heteroatoms introduced by post-synthesis proce-
dure may be located with octahedral coordination in the resultant
materials [21,24,26]. On the basis of such a serious limitation, the
mesoporous MSBA-15 (M = metal species) materials synthesized
by different hydrothermal conditions were reported [19,22,23,25].
It was also found that the MSBA-15 mesoporous materials have
∗
Corresponding author. Tel.: +82 51 510 2397; fax: +82 51 512 8563.
E-mail address: chems@pnu.edu (M. Selvaraj).
0926-860X/$ – see front matter © 2010 Elsevier B.V. All rights reserved.
doi:10.1016/j.apcata.2010.08.007

M. Selvaraj, D.W. Park / Applied Catalysis A: General 388 (2010) 22–30
23
highly active sites on the silica pore walls from the observation of
their characterization results. The MSBA-15 materials were widely
used as the catalysts in the several catalytic applications [19–24].
Particularly, Selvaraj and his research group successfully reported
the mesoporous MSBA-15 materials (M = Cr, Mn, Sn, Ti, Ga, Al and
Nb) synthesized using pH-adjusting direct hydrothermal (pH-aDH)
method [27–35]. From these characteristic results, one can con-
clude that MSBA-15 catalysts have higher hydrothermal stabilities
than MMCM-41 catalysts. The MSBA-15 catalysts were used in dif-
ferent catalytic reactions to produce the selective fine chemicals
[28,30–32,34,35].
In 2004, Hess et al. reported the highly dispersed vanadia on
SBA-15 synthesized using organic moiety functionalized vanadium
precursor [36]. Recently, Gao et al. reported the VSBA-15 synthe-
sized using ammonium metavanadate as a vanadium source under
different pH values [37]. Du et al. reported the VSBA-15 prepared
using oxovanadium (V) chloride as a vanadium precursor using
atomic layer deposition method [38], and the synthesized VSBA-15
were used in the oxidation of methanol to produce formaldehyde
with a good selectivity. Jurado et al. reported the selective epoxida-
tion of alkenes using highly active VSBA-15 catalysts, which were
synthesized using vanadium oxytriisopropoxide and ammonium
metavanadate as the sources of vanadium [39]. However, to the
best of our knowledge, the highly ordered two-dimensional VSBA-
15 catalysts synthesized by pH-adjusting direct hydrothermal
(pH-aDH) method using vanadyl sulphate hydrate as a vanadium
precursor have not been clearly reported with enhanced hydrother-
mal stabilities and catalytic activities, in the open literature, and the
removal of V2O5 crystallites on the surface of catalysts is not openly
reported.
again stirred for another 1 h. Then, 9 g of TEOS together with the
required amount of vanadyl sulphate solution (nSi/nV = 5, 10, 15,
20, 25 and 50) were added to the solution mixture, yielding a gel-
like solution with pH > 2. The resulting mixture was again stirred
for 24 h at 313 K before it was transferred into an autoclave to be
hydrothermally treated at 373 K for 24 h. After the hydrothermal
process, the solid products were recovered by filtration, washed
several times by water, and dried overnight at 373 K. The molar
composition of the gel was 1 TEOS/0.2–0.02 VOSO4·xH2O/0.016
P123/0.43 HCl/127 H2O. Finally, the samples were calcined at 813 K
in air for 6 h for complete removal of the template. The calcined
samples are denoted as VSBA-15(5), VSBA-15(10), VSBA-15(15),
VSBA-15(20), VSBA-15(25) and VSBA-15(50).
For comparison studies, the calcined SiSBA-15 was synthe-
sized according to the published procedure [27], and VMCM-
41(nSi/nV = 40) was also synthesized using a basic direct hydrother-
mal method [40]. The calcined VMCM-41 catalyst is denoted
as VMCM-41(40). The synthesized samples were characterized
according to our previous published procedure [27,40].
2.3. Preparation of hydrated VSBA-15
The hydrated VSBA-15 materials were obtained after the cal-
cined samples were exposed to moisture for 2 days under ambient
conditions.
2.4. Evaluation of hydrothermal stability
The hydrothermal stabilities of V-containing mesoporous silica
catalysts viz. VSBA-15(5) and VSBA-15(50) were investigated using
boiling water (373 K) at autogenous pressure for 168 h.
Herein we report the two-dimensional mesoporous VSBA-15
catalysts synthesized by pH-aDH method. To investigate the meso-
porous nature with the environments of vanadium species on
the silica network of pore walls, the synthesized VSBA-15 cata-
lysts have been characterized by ICP-AES, XRD, N2 adsorption, ESR,
UV–vis DRS, 51V MAS NMR, 29Si MAS NMR and TEM. Moreover,
the catalytic activities of VSBA-15 catalysts have been also inves-
tigated after chemical and hydrothermal treatments. The calcined
VSBA-15 catalysts have been used for the synthesis of cyclodode-
canone (CDD O) by the liquid-phase oxidation of cyclododecane
(CDD) with H2O2 under different optimal conditions.
2.5. Characterization
The elemental composition of the resultant solid catalysts was
analysed by ICP-AES (Perkin Elmer, Optima 3000) after the sam-
ples were dissolved in a HF solution. The small-angle XRD patterns
were recorded under ambient conditions on a Shimadzu XRD-6000
˚
with Cu K␣ radiation (ꢀ = 1.5406 A). The X-ray tube was operated
at 40 kV and 30 mA, while the diffractograms were recorded in the
2ꢁ range of 0.6–10◦ with a 2ꢁ step size of 0.01 and a step time of
10 s. The d-spacing and unit cell parameters were calculated using
√
the corresponding formulas, nꢀ = 2d sin ꢁ and a0 = 2d100
/
3. Nitro-
2. Experimental
gen adsorption/desorption measurements were conducted using
a Quantachrome Autosorb-1 by N2 physisorption at 77 K. All cal-
cined catalysts were outgassed for 3 h at 250 ◦C under vacuum
(p < 10−5 mbar) in the degas port of the sorption analyzer. The spe-
cific surface areas of the catalytic samples were calculated using
the BET method. The pore size distributions were calculated from
the adsorption branch of the isotherm using the thermodynamics-
based Barrett–Joyner–Halenda (BJH) method. The pore volume was
determined from the adsorption branch of the N2 isotherm. The
pore wall thickness (tw) was also calculated by unit cell parame-
ter (a0) and pore diameter (dp). ESR spectra of the as-synthesized
and calcined catalytic samples were recorded at the X-band at
293 K on a Bruker ESP 300 spectrometer. The magnetic field was
calibrated with a Varian E-500 gaussmeter and the microwave fre-
quency was measured by a HP 5342A frequency counter. 51V and
29Si NMR spectra were recorded at 11.75 T on a Varian INOVA 500
NMR spectrometer with a CPIMS probe. 29Si MAS spectra were
measured at 99.3 MHz using a SiN rotor at 3 kHz. 51V NMR spectra
were acquired at 131.275 MHz with 5.0 ␮s recycle delays using SiN
rotors 5 mm in diameter with spinning at 3 kHz. The chemical shifts
were estimated in ppm using tetramethylsilane (TMS) and VOCl3
as reference chemicals for 29Si and 51V, respectively. All spectra
were recorded at room temperature. UV–vis spectra were mea-
2.1. Chemicals
Triblock copolymer poly(ethylene glycol)-block-poly(propylene
glycol)-block-poly(ethylene glycol) (Pluronic P123, molecular
weight = 5800, EO20PO70EO20), tetraethylorthosilicate (TEOS),
hydrochloric acid (HCl), vanadyl sulphate hydrate (99.999%) were
all purchased from Aldrich Chemical Inc. All chemicals were used
as received without further purification. Millipore water was used
in all experiments. For the oxidation of cyclododecane (CDD), CDD,
hydrogen peroxide (30% H2O2), acetonitrile (MeCN), diethyl ether
and acetone, were all purchased from Aldrich Chemical Inc.
2.2. Synthesis
Mesoporous VSBA-15 catalysts with nSi/nV = 5, 10, 15, 20, 25
and 50 in gel, were synthesized using the pH-adjusting direct
hydrothermal (pH-aDH) method. In a typical synthesis of VSBA-15,
4 g of Pluronic P123 was stirred with 25 ml of water for 4 h to get a
clear solution. In order to adjust the pH of this solution above 1.8, we
added an aqueous HCl solution with nH O/nHCl ratio of 295 (75 ml of
0.25 M HCl solution) to the solution, and the mixture solution was
2

24
M. Selvaraj, D.W. Park / Applied Catalysis A: General 388 (2010) 22–30
sured using a Perkin-Elmer 330 spectrophotometer equipped with
a 60 mm Hitachi integrating sphere accessory. Powder samples
were loaded in a quartz cell with Suprasil windows, and spec-
tra were collected in the 250–800 nm wavelength range against a
quartz standard. Transmission electron microscopy (TEM) images
were collected on a JEOL 2010 electron microscope operated at an
acceleration voltage of 200 kV.
amounts of polymeric vanadium oxide formation (non-framework
V2O5 crystallites species) are observed with octahedral coordina-
tion in the resultant materials, and the uniform pore structures of
VSBA-15 materials are sometimes destroyed. The VSBA-15 cata-
lysts synthesized by an impregnation method, however, give good
catalytic activities [41–48], but a major decrease of catalytic activ-
ities may be observed in the recycle studies due to the leaching of
high amounts of non-framework V2O5 crystallites species. We thus
propose the pH-adjusting direct hydrothermal method to prepare
VSBA-15 materials.
2.6. Catalytic studies
Liquid-phase oxidation of CDD was performed in a 100 ml Parr
autoclave reactor at various reaction conditions. In a typical exper-
imental procedure, 1 g of CDD, 15 ml of MeCN, 2 mole ratio of
H2O2-to-CDD and 50 mg of VSBA-15(5) were taken for this catalytic
reaction, which was performed at 373 K for 24 h. After completion
of the reaction, the VSBA-15(5) catalyst was separated, and the
products were extracted with diethyl ether and analysed by gas
chromatography (GC, Nucon 5700) with a carbowax column using
reference samples. Additionally, the products were further con-
firmed by GC–MS (Hewlett Packard G1800A) equipped with a HP-5
capillary column. This catalytic reaction was also performed with
catalysts containing different V concentration. To find an optimal
condition in the presence of VSBA-15(5), we repeated the oxidation
of CDD with different reaction parameters such as time, tempera-
ture, mole ratios of reactants (CDD-to-H2O2).
Generally, the mesoporous VSBA-15 catalysts synthesized
under highly acidic conditions (pH < 1) have low amounts of vana-
dium on their silica surfaces (Eqs. (1) and (2)) because the V–O–Si
bonds are easily dissociated due to increasing hydrolysis rate of
vanadyl sulphate hydrate, as the vanadium precursor, with TEOS,
as the silicon precursor. However, lowering the acidity of the solu-
tion may decrease the hydrolysis rate of the vanadium precursor to
match that of the silicon precursor. This might enhance the inter-
action between the V–OH and Si–OH species in the synthesis gel.
However, one more reason may be that the divalent sulphate ion
from vanadyl sulphate hydrate creates HSO4−-ions in the synthe-
sis medium. The HSO4−-ions bind weakly with the surfactant. We
tried to incorporate high amounts of vanadium on the silica net-
work of SBA-15, as the pH of the synthesis gel was adjusted by a
molar ratio of nH O/nHCl = 295 to reach around 2.2, which is above
the zero net char2ge of silica. Since the pH of the synthesis medium
rises above the zero net charge of silica, the negative charge of the
silica species automatically forms in the synthesis gel, which might
enhance the tendency to interact with the [O V–OH]+ species (Eqs.
(3)–(5)). With decreasing H+ concentration in the synthesis gel, the
concentration of vanadyl hydroxyl species increases (Eqs. (3)–(5)).
Hence, under lesser concentration of H+ in the synthesis gel at pH
around 2.2, not only the partially condensed silica species are able
to form V–O–Si bond with [O V–OH]+ species but also the struc-
tural and textural order of the mesoporous framework are still
maintained. Based on the results, it is remarkably observed that the
high amounts of vanadium incorporated into SBA-15 depend upon
the concentration of H+ ions. The following mechanism is clearly
explained for the high amounts of vanadium loaded on the silica
surface.
2.7. Experimental procedures for stability of catalyst
The regenerated vanadium-containing mesoporous catalysts
viz. VSBA-15(5), VSBA-15(50) and VMCM-41(40) were prepared in
the following procedure, and they were reused in the oxidation of
CDD to determine their stabilities. In a typical experimental proce-
dure, the VSBA-15(5) catalyst used in a catalytic run was separated
from the reaction mixture, washed with acetone several time, and
dried at 393 K. Finally, the VSBA-15(5) catalyst was calcined at 773 K
for 6 h in air to remove the adsorbed species, and the regenerated
VSBA-15(5) catalyst was reused again in further catalytic runs. A
similar procedure was used for recycling studies of other catalysts
like VSBA-15(50) and VMCM-41(40). After completion of the reac-
tion, the catalyst was filtered and analyzed by ICP-AES to find the
percentage of V. The conversion of CDD and the selectivity of the
products were calculated using the standard formulas, followed by
analyzing results of GC and GC-MS.
≡ Si–OCH2CH3 (aq) + H2O(aq)(p−H→<2) ≡ Si–OH(aq) + HOCH2CH
3 (aq)
(1)
In another experiment for finding the catalytic stability of VSBA-
15(5), the original catalyst was treated with ammonium acetate
solution in order to remove the extra-framework vanadium species.
In a typical procedure, about 0.3 g of the calcined VSBA-15(5) cat-
alyst was washed with 1 M ammonium acetate (90 ml) solution
under constant stirring for 12 h at ambient temperature. Finally,
the VSBA-15(5) catalyst was filtered and calcined at 773 K for 6 h in
air to remove the adsorbed species. The treated catalyst is denoted
as washed VSBA-15(5) catalyst (W-VSBA-15). The washed catalyst
was analyzed by ICP-AES to find the percentage of V, and it was
used in the catalytic studies of CDD oxidation. Subsequently the
filtrate solution obtained from the treatment of the washing exper-
iment was also used for the oxidation of CDD under similar reaction
condition.
(pH<2)
2+
VOSO4 (aq) + 2H+
ꢀ
VO
+ H2SO4 (aq)
(2)
(aq)
(aq)
VO2+ + H2O(p−H→>2)[HO–V O]+ + H+
≡ Si–OH + H2O(p−H→>2) ≡ Si–O− + H3O+
[HO–V O]++ ≡ Si–O−(p−H→>2)O V–O–Si ≡ (–OH)
(3)
(4)
(5)
To prove that the mesoporous structure of the materials was
retained after vanadium incorporation, we characterized the cal-
cined VSBA-15 catalysts synthesized using pH-aDH method by
ICP-AES, XRD, N2 adsorption, ESR, UV–vis DRS, 51V MAS NMR, 29Si
MAS NMR and TEM.
The hydrothermally stable VSBA-15(5) catalyst was also used in
the catalytic oxidation of CDD.
3.1. ICP-AES
3. Results and discussion
The calcined VSBA-15 catalysts synthesized with different nSi/nV
ratios have been characterized by ICP-AES for the analysis of ele-
mental compositions, and the characteristic results are listed in
Table 1. In all cases, the nSi/nV ratios of the calcined catalysts are
higher than nSi/nV ratios in the synthesis gel because the vanadium
precursors are highly dissociated under acidic conditions. In this
The mesoporous VSBA-15 materials are synthesized by an
impregnation method/organic moiety functionalized method
[41–48]. Even though the syntheses methods are good for the
preparation of VSBA-15 materials, as already reported [25,36], high

M. Selvaraj, D.W. Park / Applied Catalysis A: General 388 (2010) 22–30
25
Table 1
Physicochemical properties of VSBA-15 catalysts synthesized using different conditions.
Catalysts
nH O/nHCl
nSi/nV ratio
a0 (Å)
ABET (m2/g)
dp (Å)
Vp (cm3/g)
tw = a0 − dp (Å)
2
Gel
Calcineda
VSBA-15(5)
295
295
295
295
295
295
40
5
10
15
20
25
50
–
14.8
18.2
31.5
42.3
53.4
87.5
–
122.9
122.6
122.2
121.3
119.7
118.8
118.2
45.4
709
742
773
820
863
890
908
869
710
83.4
83.6
84.1
84.7
85.1
86.3
87.4
27.7
83.8
1.03
1.08
1.09
1.10
1.11
1.17
1.07
0.72
1.07
39.5
39.0
38.1
36.6
34.6
32.5
30.8
17.7
38.6
VSBA-15(10)
VSBA-15(15)
VSBA-15(20)
VSBA-15(25)
VSBA-15(50)
SiSBA-15
VMCM-41(40)
–
–
40
–
42.2
19.5
W-VSBA-15(5)b
122.4
a0, unit cell parameter; ABET, specific surface area; dp, pore diameter; Vp, pore volume; pore wall thickness (tw) = unit cell parameter (a0) − pore diameter (dp).
a
The results of nSi/nV ratios in the products are determined by ICP-AES.
Washed catalyst.
b
synthesis method, the nSi/nV ratio decreases from 87.5 to 14.8 due
to the higher amount of vanadium incorporated on the pore walls of
SBA-15 by the formation of more vanadyl hydroxyl groups because
of adjusting the pH values from 1.6 to >2.
3.3. N2 adsorption
The N2 adsorption–desorption isotherms of calcined VSBA-15
catalysts synthesized with nSi/nV ratios are shown in Fig. 2. All
isotherms are replicated as IV type isotherms according to the
IUPAC classification and all exhibited a H1-type broad hysteresis
loop, which is typical of large-pore mesoporous solids [50]. As the
relative pressure increases (p/p0 > 0.6), all isotherms exhibit a sharp
step characteristic of capillary condensation of nitrogen within uni-
form mesopores, where the p/p0 position of the inflection point
is correlated to the diameter of the mesopore. Since SBA-15 has a
hexagonal arrangement of mesopores connected by smaller micro-
pores [51,52], the broad hysteresis loop observed in the isotherms
of VSBA-15 samples reflects the long mesopores, which limit the
emptying and filling of the accessible volume. The textural proper-
ties of VSBA-15 catalysts are given in Table 1. With the decrease of
nSi/nV ratio up to 14.8, textural properties such as specific surface
area, pore diameter and pore volume of VSBA-15 catalysts system-
atically decrease; as a result, the pore wall thickness increases with
the increase of vanadium content in the catalysts (Table 1). This
evidence strongly supports the conclusion that the high amounts
of vanadium are incorporated on the pore walls of SBA-15. Fur-
thermore, the structural and textural properties of W-VSBA-15(5)
are still maintained, as shown in Figs. 1 and 2, such characteristic
results are also shown in Table 1.
3.2. XRD
The as-synthesized VSBA-15 catalysts prepared using differ-
ent nSi/nV ratios show poorer quality of XRD peaks than SiSBA-15
(not shown). After removal of the polymeric surfactant by calcina-
tion process, the XRD patterns of VSBA-15 catalysts become better
resolved, as shown in Fig. 1. This may be due to some atomic rear-
rangement occurring inside the mesoporous catalysts during the
calcination. The XRD patterns of all VSBA-15 catalysts exhibit four
well-resolved peaks that are indexed to the (1 0 0), (1 1 0), (2 0 0),
and (2 1 0) reflections of the hexagonal space group p6mm. Due
to the higher V species incorporated into SBA-15, the XRD peaks
shift to lower angles, and VSBA-15(5) has lesser intensity of (2 1 0)
peak in the XRD pattern than other VSBA-15 catalysts. Based on
this observation, it is interesting to note that VSBA-15(5) has higher
thicker hexagonal walls than other VSBA-15, as reported in the lit-
erature [49]. Since the XRD peaks shift to lower angle by increasing
vanadium-content, an increase of unit cell parameters is observed
as compared to those of the purely siliceous SBA-15, as shown in
Table 1. This is because the V–O bond length is higher than the
Si–O bond length due to the larger ionic radius of V (49.5 pm) than
Si (40 pm).
Fig. 1. XRD powder patterns of calcined VSBA-15 catalysts.
Fig. 2. Nitrogen adsorption isotherms of VSBA-15 catalysts.

26
M. Selvaraj, D.W. Park / Applied Catalysis A: General 388 (2010) 22–30
3.4. ESR
ESR spectra of as-synthesized VSBA-15 catalysts (Fig. 3) exhibit
an axially symmetrical signal of tetravalent vanadium, which orig-
inates from the d1 electron interaction with nuclear spin (51V,
In = 7/2, natural abundance 99.8%). The ESR intensity of VO2+ in
the SBA-15 samples increases with the decrease of nSi/nV, as a
result, the line width systematically increases with increasing the
amount of VO2+ species. The spectra are characterized by an axially
symmetric set of eight lines originating from vanadyl VO2+ species
coupled to its own nuclear spin. The spin Hamiltonian parameters A
tensor and g tensor are g = 1.945, A = 191 G and g = 1.981, A = 65
ꢀ
ꢀ
⊥
⊥
G, where A is the hyperfine coupling constant. The g values and
hyperfine coupling constants are typical of vanadyl VO2+ species
with square pyramidal coordination [10,53]; the similar reports are
observed in several mesoporous molecular sieves [53]. Moreover,
the well-resolved ESR spectra show that the vanadyl VO2+ species
assigned in square pyramidal coordination is only observed for the
as-synthesized VSBA-15 samples.
ESR peaks of calcined VSBA-15 catalysts completely disappear
due to the absence of an unpaired electron on the 3dxy orbital of
the vanadium, as shown in Fig. 3. These results indicate that the
VO2+ species become into V5+ species after calcinations, because
no signal attributable to the V4+ species could be detected in the
calcined VSBA-15 sample. From the ESR results, one can observe
that the VO2+ species assigned in square pyramidal coordination
for as-synthesized VSBA-15 catalysts and the V5+ species in cal-
cined catalysts are incorporated on the silica surface of SBA-15.
The ESR results reveal that the “efficiency” of V species being incor-
porated into silica pore walls increases with decreasing the nSi/nV
ratios from 87.5 to 14.8. These results are in good agreement with
the literature [53], where V species are reportedly present, as the
Fig. 4. 51V MAS NMR spectra of VSBA-15 catalysts.
rated on the surface of silica pore walls. However, from the ESR
results, we could not find whether the V5+ species only is incor-
porated in the framework of SBA-15 or the non-framework V2O5
crystallites species are dispersed on the surface of VSBA-15 where
the high amounts of V species are loaded. Moreover, no informa-
tion is observed regarding the environmental nature of V5+ species
from the ESR results obtained at 293 K. Thus, the calcined VSBA-15
catalysts have been further characterized by 51V MAS NMR, 29Si
MAS NMR and UV–vis DRS.
V
5+species in calcined VSBA-15 catalysts is exclusively incorpo-
3.5. NMR
51V NMR spectra for calcined VSBA-15 catalysts give useful
information to differentiate between the various local coordina-
tion environments of V species. The as-synthesized VSBA-15 does
not give any NMR active signal, which could be due to the absence
of any diamagnetic V species. The 51V NMR spectra of the calcined
VSBA-15 catalysts exhibit a signal around −535 ppm as shown in
Fig. 4. The intensities of the spectra increases with the increase
of V species incorporated in the framework of silica when the
nSi/nV ratios are decreased. For instance, the calcined VSBA-15(5)
has a higher intensity than that of calcined VSBA-15(50). The NMR
peak around −535 ppm (relative to VOCl3) is a strong indication
of tetrahedral V5+ species incorporated on the silica surface [54].
However, in the similar spectrum, an addition of a weak peak
around −300 ppm shows that small amounts of non-framework
V2O5 crystallites species are assigned on the silica surface with
higher coordination [55]. From the 51V MAS NMR characteristic
results, it is clear that the non-framework V2O5 crystallites species
are accessible to water molecules. To prove this statement, we have
further characterized the W-VSBA-15(5) by 51V MAS NMR spec-
troscopy. The 51V MAS NMR spectrum of W-VSBA-15(5) shows a
strong peak around −535 ppm only, which can be assigned to the
tetrahedral V5+ species incorporated into silica surface, as shown in
Fig. 4. A weak shoulder around −300 ppm in W-VSBA-15(5) com-
pletely disappeared during the washing process, as shown in Fig. 4.
This evidence strongly supports the suggestion that a small amount
of non-framework V2O5 crystallites species are dispersed on the
surface of pore walls in rich vanadium-containing SBA-15. It is
also observed that the non-framework V2O5 crystallites species
are completely removed by the washing process (Fig. 4); such a
result confirms that the non-framework V2O5 crystallites species
Fig. 3. ESR spectra of VSBA-15 catalysts.

M. Selvaraj, D.W. Park / Applied Catalysis A: General 388 (2010) 22–30
27
Fig. 5. 29Si MAS NMR spectra of SiSBA-15 and VSBA-15(10).
Fig. 6. UV–vis DR spectra of VSBA-15 catalysts.
dispersed on the VSBA-15 interact with water molecule easily to
become five or six coordination [55]. Furthermore, the results of
29SiMAS NMR spectra show that the intensity of Q3 peak in VSBA-
15(10) is much lesser than that of siliceous SBA-15, as shown in
Fig. 5; this observation clearly supports the stabilization of vana-
dium species via silanol groups (defect sites). However, we were not
able to observe from the results of 51V MAS NMR spectra whether
the regular tetrahedral V5+ species or disordered tetrahedral V5+
species (central V5+ with V O double bond and V–O single bonds)
were incorporated in the framework of silica walls. Thus, the fresh
calcined and hydrated VSBA-15 catalysts have been further char-
acterized by UV–vis DRS.
absorption band around 455 nm shows the non-framework V2O5
crystallites species dispersed on the silica surface (Fig. 6). This result
is strongly confirmed by 51V MAS NMR spectra. But the major-
ity of V5+ species still remain in a tetrahedral coordination upon
hydration. We suggest that the isolated tetrahedral V5+ species are
incorporated on the silica surface of SBA-15 [56].
3.7. TEM
Fig. 7 shows the TEM images of the VSBA-15(10) catalyst. The
TEM images show well ordered hexagonal arrays of 1D mesoporous
channels, further confirming that VSBA-15 catalyst has a 2D p6mm
hexagonal structure. The distance between two consecutive centres
of hexagonal pores estimated from the TEM image is ca. ∼11 nm.
The average thickness of the wall is ca. ∼3.9 nm, which is much
larger than that for MCM-41. The pore diameter is around ∼8.4 nm,
which is in agreement with the N2 adsorption measurements. The
above results show that the VSBA-15 catalysts have well uniformly
pore diameters.
The above results of ICP-AES, XRD, N2 adsorption, ESR, UV–vis
DRS, 51V MAS NMR, 29Si MAS NMR and TEM suggest the successful
incorporation of a larger amount of vanadium species into SBA-15
by a pH-aDH method without affecting the structural and textural
order.
3.6. UV–vis DRS
UV–vis DRS spectra of freshly calcined, and hydrated VSBA-15
show the nature and the coordination of V species incorporated into
the mesoporous silica (Fig. 6). The strong UV–vis absorption spec-
tra of freshly calcined VSBA-15 appear between 270 and 285 nm
(Fig. 6). With the decrease of nSi/nV ratios from 50 to 5, the ꢀ values
(wavelength) of absorption peaks shift from 270 to 285 nm, and
their intensities increase with the increase of V species incorpo-
ration. The absorption band is assigned to the low-energy charge
transfer (CT) transition between tetrahedral oxygen ligands (O2−
)
and central V5+ species [10–13,56], and the CT band serves as a
good indicator for the tetrahedral V5+ species incorporated in the
framework of silica walls. However, we were not able to clearly
identify whether the regular tetrahedral V5+ or the disordered
tetrahedral V5+ species are incorporated into the silica pore walls.
Then, the hydrated VSBA-15 was analysed using UV–vis DRS. The
hydrated VSBA-15 catalysts show two UV absorption peaks at 275
and 375 nm toward lower energy, as shown in Fig. 6. The strong
absorption band around 275 nm is attributed to the regular tetra-
hedral V5+ species incorporated on the surface of pore walls of
SBA-15, and a broad absorption band around 375 nm shows the
partially disordered tetrahedral V5+ species incorporated on the
silica surface [10–13,56]. The absorption peak around 375 sug-
gests that the disordered tetrahedral V5+ species attracted by water
molecules transform into penta-or-octahedral V5+ species [56]. The
3.8. Hydrothermal stability
To investigate the hydrothermal stability, we have treated the
calcined V-containing mesoporous catalysts such as VSBA-15(5)
and VSBA-15(50) in boiling water at 373 K for 168 h. A study of
hydrothermal stability is an important factor to find the distinct
structural and textural parameters of VSBA-15, and it can be very
helpful to find the catalytic stability because VSBA-15 catalysts
can be used for the synthesis of bulk aromatics in chemical indus-
tries. After hydrothermal treatment of VSBA-15(50), its structural
and textural parameters decreases (Table 2) because it has high
numbers of free Si–O–Si bonds due to low amounts of V species
being incorporated into the silica pore walls. These results sug-
gest that the hydrothermal treatment for VSBA-15(50) may cause

28
M. Selvaraj, D.W. Park / Applied Catalysis A: General 388 (2010) 22–30
Table 3
Oxidation of CDD with different VSBA-15 catalystsa.
Catalysts
Conversion
of CDD (%)
Selectivity of
CDD O (%)
Selectivity of
CDD–OH (%)
VSBA-15(5)
71.4
63.7
55.7
47.6
40.4
27.3
37.3
89.5
82.4
75.8
70.1
58.1
35.3
63.8
10.5
15.1
18.4
21.3
23.4
15.3
16.3
VSBA-15(10)
VSBA-15(15)
VSBA-15(20)
VSBA-15(25)
VSBA-15(50)
VMCM-41(40)
a
Reaction conditions: 50 mg of catalyst; 1 g of CDD; 2 mole ratio of H2O2-to-CDD;
15 ml of MeCN; reaction temperature, 373 K; reaction time, 24 h.
3.9. Oxidation of CDD
The oxidation of CDD has been performed over VSBA-15 cata-
lysts, as shown in Scheme 1. The effects of reaction parameters such
as temperature, time and CDD-to-H2O2 ratios, have been studied to
find optimal values of CDD conversion and CDD O selectivity. The
performance of catalytic stability has been also found through the
studies of recycles. After chemical and hydrothermal treatments,
the vanadium concentrations and catalytic activities in the VSBA-
15(5) catalysts have been also investigated.
The oxidation of CDD was carried out with 2 moles of
H2O2-to-CDD and 15 ml of MeCN at 373 K for 24 h over
mesoporous V-containing mesoporous catalysts viz. VSBA-
15(5), VSBA-15(10), VSBA-15(15), VSBA-15(20), VSBA-15(25)
and VSBA-15(50), as shown in Table 3. The order of cat-
alytic activity found on the selectivity of CDD O is as follows:
VSBA-15(5) > VSBA-15(10) > VSBA-15(15) > VSBA-15(20) > VMCM-
41(40) > VSBA-15(25) > VSBA-15(50). VSBA-15(5) exhibits the best
performance with a conversion of CDD (71.4%) and a selectivity of
CDD O (89.5%). As shown in Table 3, the selectivity of cyclodo-
decanol (CDD–OH) increases from VSBA-15(5) to VSBA-15(25)
because the active sites on the silica surface are insufficient
to convert CDD O from CDD–OH due to the low amounts of
vanadium incorporated in the framework of silica walls. Thus,
VSBA-15(5) has higher selectivity of CDD O than other VSBA-15
catalysts (Table 3). The higher activity of VSBA-1(5) is presumably
ascribed to its two-dimensional space and to the high loadings
of tetrahedral V5+ species on the surface of SBA-15, resulting in a
higher number of accessible active sites because the tetrahedral
V5+ species incorporated in the framework of SBA-15 produces
high numbers of Lewis acid sites to enhance the catalytic activity
in the oxidation of CDD. Moreover, if we compare the catalytic
activity for the catalysts with similar vanadium-content as shown
in Table 1, the selectivity of CDD O in VSBA-15(20) is signifi-
cantly higher as compared to that of VMCM-41(40), as shown
in Table 3. It may be concluded from this evidence that the well
ordered mesoporosity material with high vanadium loadings plays
an important catalytic role in the production of CDD O with a
high selectivity. On the basis of catalytic activities of different
Fig. 7. TEM micrographs of calcined hexagonal mesoporous VSBA-15(10).
some shrinkage of mesopores and some structural disorder. But the
physicochemical properties (structural and textural parameters) of
VSBA-15(5) almost remain constant (Table 2) after hydrothermal
treatment because it has the high numbers of Si–O–V and V–O–V
bonds [14,56], which make thicker pore walls of VSBA-15(5) due
to the higher tetrahedral V5+ species incorporated on the surface
of pore walls. However, the VSBA-15(5) catalyst has physicochem-
ical properties lower than that of the freshly calcined VSBA-15(5)
(Tables 1 and 2) because the non-framework of V2O5 crystallites
species dispersed on the VSBA-15(5) are mostly removed during
the hydrothermal treatment. The physicochemical results of VSBA-
15(5) and W-VSBA-15(5) are almost identical (Tables 1 and 2). This
evidence strongly confirms that the non-framework V2O5 crystal-
lites species dispersed on the VSBA-15 are completely removed
during the hydrothermal treatment, even though the VSBA-15(5)
has better hydrothermal stability than VSBA-15(50) due to the
thicker pore walls, which are more stable against further attack by
boiling water (Table 2). Therefore, the treated VSBA-15(5) is called
“hydrothermally stable VSBA-15(5)”.
Table 2
Physicochemical properties of VSBA-15 catalysts treated by hydrothermal treatments, and VSBA-15 catalysts reused in three cycles.
Catalysts
Hydrothermal
nSi/nV ratio
a0 (Å)
ABET (m2/g)
dp (Å)
Vp (cm3/g)
tw = a0 − dp (Å)
Temperature (K)
Time (h)
Gel
Calcineda
VSBA-15(5)
VSBA-15(50)
3rd Runb
373
373
–
168
168
–
5
50
–
19.4
87.6
19.4
87.7
122.6
111.7
122.3
114.5
715
610
713
740
83.7
82.4
83.8
84.1
1.03
0.90
1.07
0.95
38.9
29.3
38.5
30.4
3rd Runc
–
–
–
a0, unit cell parameter; ABET, specific surface area; dp, pore diameter; Vp, pore volume; pore wall thickness (tw) = unit cell parameter (a0) − pore diameter (dp).
a
The results of nSi/nV ratios in the products are determined by ICP-AES.
The regenerated VSBA-15(5) catalyst was reused for recycling reactions.
The regenerated VSBA-15(50) was reused for recycling reactions.
b
c

M. Selvaraj, D.W. Park / Applied Catalysis A: General 388 (2010) 22–30
29
Table 4
Oxidation of CDD over the regenerated VSBA-15 catalysts and W-VSBA-15(5)a.
Table 5
Oxidation of CDD using different reaction conditions over VSBA-15(5)a.
Catalysts
Conversion
of CDD (%)
Selectivity of
CDD O (%)
Selectivity of
CDD–OH (%)
Catalysts
Conversion
of CDD (%)
Selectivity of
CDD O (%)
Selectivity of
CDD–OH (%)
VSBA-15 (5)b
VSBA-15 (50)b
VMCM-41(40)b
W-VSBA-15(5)
VSBA-15(5)c
63.2
27.2
32.4
62.1
63.2
5.2
76.3
35.1
58.4
76.4
82.4
10.3
8.3
18.1
12.1
14.3
18.0
17.6
3.2
VSBA-15(5)b
VSBA-15(5)c
VSBA-15(5)d
VSBA-15(5)e
VSBA-15(5)f
VSBA-15(5)g
75.3
60.1
55.3
76.3
55.2
73.2
84.3
71.2
62.3
83.2
59.3
82.3
10.4
15.3
10.3
11.1
6.3
SiSBA-15
Absence of catalyst
10.2
4.2
2.1
a
Reaction conditions: 50 mg of catalyst; 1 g of CDD; 2 mole ratio of H2O2-to-CDD;
a
Reaction conditions: 50 mg of catalyst; 1 g of CDD; 2 mole ratio of H2O2-to-CDD;
15 ml of MeCN; reaction temperature, 373 K; reaction time, 24 h.
b
15 ml of MeCN; reaction temperature, 373 K; reaction time, 24 h.
Reaction temperature: 393 K.
Reaction temperature: 353 K.
Reaction time: 12 h.
Reaction time: 48 h.
1 mole ratio of H2O2-to-CDD.
4 mole ratio of H2O2-to-CDD.
b
c
The results were obtained after three runs.
The catalyst was treated with boiling water for the investigation of its hydrother-
c
d
e
mal stability.
f
g
V-containing mesoporous catalysts, VSBA-15(5) is found to be a
promising heterogeneous catalyst in the liquid-phase oxidation of
CDD for the highly selective synthesis of CDD O, and its catalytic
activity is higher than VMCM-41 [9].
tivity of CDD O have been obtained (Table 4), thus indicating that
major activity is only due to vanadium species incorporated in the
framework of SBA-15.
Several vanadium-containing mesoporous catalysts viz. VSBA-
15(5) and VSBA-15(50) and VMCM-41(40), have been examined to
determine their catalytic stabilities. Initially, the mesoporous cat-
alysts used in the catalytic reaction usually suffer from the loss
of catalytic activities, and hence the catalysts need to be regen-
erated by calcination. The used catalysts were washed four times
with acetone and dried at 393 K overnight. Finally, the catalysts
were calcined at 773 K for 6 h in air for complete removal of the
organics and unreacted CDD molecules. The regenerated catalysts
have been reused for this reaction, which was carried out using the
similar reaction conditions, as noted in Table 4. The conversion of
CDD and selectivity of CDD O decrease in the first two runs (not
shown in Table 4). On the basis of first two runs, one can observe
that the non-framework V2O5 crystallites species leach on the cat-
alytic surface (the ranges of nSi/nV ratios obtained by ICP-AES are
from 19.4 to 87.7 (Table 2)). The conversion of CDD as well as the
selectivity of CDD O remains constant after three runs, indicating
that the vanadium ions cannot be further leached on the meso-
porous matrix, which is in good agreement with ICP-AES results of
filtrate solutions where no vanadium species is detected. Further-
more, the W-VSBA-15(5) has also been used in this reaction for the
investigation of catalytic stability. The effect of this study is nearly
identical to that of three runs of VSBA-15(5), as shown in Table 4.
However, it is interesting to note that after three runs the catalysts
show nearly the same conversion value, indicating that no more
leaching of vanadium from the VSBA-15 matrix will be observed
by ICP-AES analysis. This reaction was also carried out using SiSBA-
15 synthesized by pH-aDH method [27] as well as without catalyst.
In both cases, about ∼5–4% conversion of CDD and ∼8.3–10% selec-
The hydrothermally stable VSBA-15(5) was also used in the
liquid-phase oxidation of CDD; its catalytic activity is also similar
to that of W-VSBA-15, as shown in Table 4. Since the VSBA-15(5) is
a promising catalyst in this catalytic oxidation reaction, it has been
further used to find the best optimal conditions.
To obtain the highly selective synthesis of CDD O over VSBA-
15(5), the liquid-phase oxidation of CDD was carried out using the
different optimal conditions such as reaction temperature, reaction
time and ratios of reactant (CDD: TBHP).
The oxidation of CDD was conducted with different reaction
temperatures and times using the reaction conditions noted in
Table 5, for obtaining a highly selective synthesis of CDD O. When
the reaction temperatures and times are decreased from 373 to
353 K and 24 to 12 h, respectively, the rates of both CDD consump-
tion and CDD O formation decrease. The feasible CDD conversion
decreases with the decrease of temperature and time. This may
be due to the formation of fewer Lewis active sites on the surface
of the catalyst at low reaction temperature and time, which may
not be sufficiently supported to produce CDD O from CDD–OH.
Moreover, the conversion and selectivity have not significantly
increased when the reaction temperature and time are increased
from 373 to 383 K and 24 to 36 h, respectively. However, at above
390 K and 46 h, the conversion of CDD and the selectivity of CDD
O
decrease due to the formation of overoxidation products such as
1, 2-cyclododecandione and 1, 2-cyclododecandiol, as shown in
Scheme 1. This evidence strongly supports the conclusion that
a low reaction temperature/time (<353 K/12 h) or high reaction
temperature/time (>390 K/46 h) is unsuitable to highly produce
Scheme 1. Catalytic oxidation of CDD with H2O2 over vanadium-containing catalysts.

30
M. Selvaraj, D.W. Park / Applied Catalysis A: General 388 (2010) 22–30
CDD O with a good selectivity. One can conclude with confidence
from the catalytic results obtained with different reaction tem-
peratures/times that the reaction parameters, like temperature of
373 K and reaction time of 24 h, are favourable to produce a high
selectivity of CDD O.
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To find the best ratio of CDD-to-H2O2 for the highly selective
synthesis of CDD O, we carried out the oxidation of CDD with dif-
ferent ratios of H2O2-to-CDD using the reaction conditions noted
in Table 5. When this reaction is carried out with 2 mole ratios of
H2O2-to-CDD, the increase of conversion of CDD as well as selec-
tivity of CDD O is observed. The selectivity of CDD O decreases in
1 mole ratio of H2O2-to-CDD. We consider that the quantity of reac-
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4. Conclusions
The highly ordered two-dimensional VSBA-15 catalysts syn-
thesized with high tetrahedral vanadium loadings have been
successfully used in the oxidation of CDD to CDD O with a good
selectivity. ICP-AES results of VSBA-15 catalysts show that the
amounts of vanadium species are highly incorporated into SBA-
15. 29Si MAS NMR results of VSBA-15 catalysts also confirm that
high amounts vanadium species are incorporated on the silica sur-
face. The studies of XRD and N2 adsorption show that the structural
and textural properties of VSBA-15 catalysts cannot be affected by
the high amounts of vanadium species incorporated in the pore
walls of SBA-15. ESR spectra of as-synthesized VSBA-15 show the
amounts of square pyramidal V4+ species located on the silica sur-
face. The results of 51V MAS NMR spectra show that high numbers
of tetrahedral V5+ species are incorporated in the framework of sil-
ica surface and small amounts of non-framework V2O5 crystallites
species are dispersed on the silica surface. In the hydrated VSBA-15
catalysts, UV–vis DRS results clearlyshow thedistinctofregularand
disordered tetrahedral V5+ species incorporated into silica surface.
TEM results confirm the VSBA-15 catalysts synthesized with well
ordered hexagonal and uniform pore diameters. From the studies of
hydrothermal stability, VSBA-15(5) is found have better hydrother-
mal stability than VSBA-15(50) due to the thicker pore walls. On the
basis of all catalytic studies, it is clearly found that the VSBA-15(5)
catalyst is a highly active, recyclable and promising heterogeneous
catalyst for the selective synthesis of CDD O.
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