Surfactant-assisted sol-gel synthesis of zirconia supported phosphotungstates or Ti-substituted phosphotungstates for catalytic oxidation of cyclohexene

Article abstract of DOI:10.1016/j.apcata.2014.05.025

The Keggin-type polyoxometalates (POMs) including phosphotungstate and Ti-substituted phosphotungstates were supported on zirconia (ZrO₂) matrix by a surfactant-assisted sol-gel copolymerization technique. The resultant ZrO₂ supported POMs composite materials (ZrO ₂-POMs) were characterized by X-ray power diffraction, transmission electron microscopy, Fourier transform infrared spectra, diffuse reflectance ultraviolet-visible spectra, thermogravimetry-differential scanning calorimetry analysis, and ³¹P magic angle spinning (MAS) nuclear magnetic resonance (NMR) analysis. The results showed that the ZrO₂-POMs materials have worm-like disordered mesoporous, and the primary structure of POMs is preserved after the formation of the composite materials. All the ZrO₂-POMs materials are catalytically active for the H ₂O₂-mediated oxidation of cyclohexene with relatively high selectivity to allylic oxidation products. Moreover, these materials exhibit high stability against leaching of active species, and can be easily recycled without obvious loss of activity and selectivity. The existence of relatively strong interaction between the POM units and the ZrO₂ framework should play important role in fabricating the stable heterogeneous ZrO ₂-POMs catalysts.

Full text of DOI:10.1016/j.apcata.2014.05.025

Accepted Manuscript
Title: Surfactant-assisted Sol-gel Synthesis of Zirconia
Supported Phosphotungstates or Ti-substituted
Phosphotungstates for Catalytic Oxidation of Cyclohexene
Author: Kaige Li Jing Wang Yongcun Zou Xiaojing Song
Hongcheng Gao Wanchun Zhu Wenxiang Zhang Jihong Yu
Mingjun Jia
PII:
S0926-860X(14)00359-7
DOI:
Reference:
http://dx.doi.org/doi:10.1016/j.apcata.2014.05.025
APCATA 14845
To appear in:
Applied Catalysis A: General
Received date:
Revised date:
Accepted date:
26-2-2014
30-4-2014
22-5-2014
Please cite this article as: K. Li, J. Wang, Y. Zou, X. Song, H. Gao,
W. Zhu, W. Zhang, J. Yu, M. Jia, Surfactant-assisted Sol-gel Synthesis of
Zirconia Supported Phosphotungstates or Ti-substituted Phosphotungstates for
Catalytic Oxidation of Cyclohexene, Applied Catalysis A, General (2014),
http://dx.doi.org/10.1016/j.apcata.2014.05.025
This is a PDF file of an unedited manuscript that has been accepted for publication.
As a service to our customers we are providing this early version of the manuscript.
The manuscript will undergo copyediting, typesetting, and review of the resulting proof
before it is published in its final form. Please note that during the production process
errors may be discovered which could affect the content, and all legal disclaimers that
apply to the journal pertain.

*Graphical Abstract (for review)
Page 1 of 35

*Highlights (for review)
Highlights
 Zirconia supported POM composites were synthesized by sol-gel technology.
 The POM clusters are highly dispersed in the composites.
 Strong interaction is built between POM units and ZrO2 framework.
 The composites show high catalytic activity in cyclohexene oxidation with H2O2.
 These catalysts are truly heterogeneous in nature, and easily recycled.
Page 2 of 35

*Manuscript
Surfactant-assisted Sol-gel Synthesis of Zirconia Supported
Phosphotungstates or Ti-substituted Phosphotungstates for Catalytic
Oxidation of Cyclohexene
Kaige Lia, Jing Wanga, Yongcun Zoub, Xiaojing Songa, Hongcheng Gaoa, Wanchun
Zhua,*, Wenxiang Zhanga, Jihong Yub, and Mingjun Jiaa,*
a
State Key Laboratory of Theoretical and Computational Chemistry, College of
Chemistry, Jilin University, 130021 Changchun, China
b
State Key Laboratory of Inorganic Synthesis and Preparative Chemistry, College of
Chemistry, Jilin University, Changchun 130012, China
* Corresponding authors: Tel: (+86)431-85155390; Fax: (+86)431-85168420.
Email addresses: jiamj@jlu.edu.cn (M. Jia)
wczhu@jlu.edu.cn (W. Zhu)
1
Page 3 of 35

Abstract
The Keggin-type polyoxometalates (POMs) including phosphotungstate and
Ti-substituted phosphotungstates were supported on zirconia (ZrO2) matrix by a
surfactant-assisted sol-gel copolymerization technique. The resultant ZrO2 supported
POMs composite materials (ZrO2-POMs) were characterized by X-ray power
diffraction, transmission electron microscopy, Fourier transform infrared spectra,
diffuse reflectance ultraviolet-visible spectra, thermogravimetry-differential scanning
calorimetry analysis, and 31P magic angle spinning (MAS) nuclear magnetic
resonance (NMR) analysis. The results showed that the ZrO2-POMs materials have
worm-like disordered mesoporous, and the primary structure of POMs is preserved
after the formation of the composite materials. All the ZrO2-POMs materials are
catalytically active for the H2O2-mediated oxidation of cyclohexene with relatively
high selectivity to allylic oxidation products. Moreover, these materials exhibit high
stability against leaching of active species, and can be easily recycled without obvious
loss of activity and selectivity. The existence of relatively strong interaction between
the POM units and the ZrO2 framework should play important role in fabricating the
stable heterogeneous ZrO2-POMs catalysts.
Keywords: Polyoxometalates, sol-gel copolymerization, zirconia, alkene, allylic
oxidation
2
Page 4 of 35

1. Introduction
Catalytic oxidation of alkenes with environmentally benign and inexpensive
oxidant, aqueous H2O2, is a highly desirable catalytic process for the synthesis of
various important chemical intermediates, like epoxides, α,β-unsaturated ketones and
alcohols [1,2]. Among various catalyst systems for alkene oxidation,
polyoxometalates (POMs), including metal-substituted POMs, have received much
attention for their hydrolytic and thermal stability, tunable acid and redox properties
[3-5]. Recently, numerous attempts on developing supported POM heterogeneous
catalysts have been made for the purpose to solve the separation and recycling
problems of homogeneous POM catalysts [6-11]. However, most of the supported
POM catalysts are not very stable during the liquid-phase oxidation course, and
leaching of active POM species easily occurs since it is quite difficult to establish a
stable linkage between the solid support and the POM clusters with relatively large
molecular size.
A few literature studies suggested that ZrO2 is a suitable support for immobilizing
POM clusters to obtain relatively active and stable heterogeneous ZrO2 supported
POMs (ZrO2-POMs) catalysts for the application in some liquid-phase catalytic
reactions and photo-catalysis processes [12-16]. For examples, Patel et al. reported
that ZrO2 supported lacunary undecatungtophosphate, prepared by impregnating
method, showed high activity and stability for the catalytic oxidation of styrene to
benzaldehyde with H2O2 [15]. Hu and co-workers reported that zirconia-supported
Ti-substituted Keggin-type POMs, prepared by hydrolysis of zirconium n-butoxide in
3
Page 5 of 35

the presence of Li5PW11TiO40 or K7PW10Ti2O40 cluster, exhibited excellent catalytic
activity and recyclability for the photo degradation of dye naphthol blue black [16]. It
was believed that the relatively high stability of ZrO2-POMs catalysts could be mainly
assigned to the existence of a strong interaction between the POMs clusters and ZrO2
support.
Recently, Armatas et al. developed a surfactant-assisted sol-gel copolymerization
route to prepare ZrO2-based 12-phosphomolybdic acid nanocomposite (ZrO2-PMA)
[17]. By using this method, ZrO2-PMA materials with ordered mesopore structure and
high loading of PMA could be obtained. It was found that the Keggin structure of
PMA clusters is preserved intact into the mesoporous frameworks, forming a solid
solution with the ZrO2 species. More interesting, the ZrO2-PMA materials show
exceptional catalytic activity and stability (against leaching) in oxidation of alkenes
using H2O2 as oxidant.
Here, we try to extend this surfactant-assisted sol-gel copolymerization strategy to
prepare ZrO2 supported phosphotungstate and Ti-substituted phosphotungstates, in
which the POM ions include [PW12O40]3-, [PW11TiO40]5-, and [α,α-P2W20Ti4O78]10-.
Previously, it was already known that these POMs are active homogeneous catalysts
for some selective oxidation reactions [18-20]. In the present work, we found that
mesoporous composites based on ZrO2 and the POMs could be obtained by sol-gel
copolymerization of the tetra-n-butyl-ammonium (TBA) salt of the POM, zirconium
oxide dichloride and Pluronic P123 (triblock copolymer EO20PO70EO20) surfactant.
The resultant materials were characterized by a variety of means, and their catalytic
4
Page 6 of 35

performance were investigated for the liquid-phase oxidation of cyclohexene using
H2O2 as oxidant.
2. Experimental
2.1. Materials
The following chemicals were purchased from commercial suppliers and were used
without further purification: 12-phosphotungstic acid (H3PW12O40), ZrOCl28H2O,
Ti(SO4)2, Na2WO42H2O, 85% H3PO4, glacial acetic acid, anhydrous ethyl alcohol,
P123 surfactant, tetra-n-butyl-ammonium bromide (TBABr), cyclohexene, 30 wt%
H2O2 aqueous solution, acetonitrile, and chlorobenzene.
The compounds of Na7[PW11O39] and Na9[A-PW9O34] were prepared following the
literature
procedures
[21,22].
TBA3[PW12O40],
TBA5[PW11TiO40]
and
TBA10[α,α-P2W20Ti4O78] were prepared in the light of the method reported in
31
literatures [18,23,24]. The FT-IR and P NMR spectra of these TBA salts of POMs
(TBA-POMs) were in good agreement with the references [18-24].
2.2. Preparation of ZrO2-POMs catalysts
ZrO2 supported POMs composites were prepared by a surfactant-assisted sol-gel
method similar to the literature procedure for preparing ZrO2-based
12-phosphomolybdic acid nanocomposites [17]. Typically, 1.6 g (5.0 mmol) of
zirconium oxide dichloride (ZrOCl28H2O) was added to the anhydrous ethanol
solution (10 mL) of P123 (0.6 g). The mixture was stirred vigorously at room
temperature for 2 h. After this, a clear solution of the TBA salts of phosphotungstate
or Ti-substituted phosphotungstates in 3 mL of warm CH3CN was added slowly into
5
Page 7 of 35

the above mixture solution under stirring. The obtained mixture was then heated to
70 °C, and homogeneous sol formed gradually under stirring. Transparent gel
appeared when all the solvents were vaporized in open air at 70 °C for 24 h. White
solid of the ZrO2-POM precursor were obtained after drying the gel at 100 °C under
vacuum for 10 h. The solid precursors were calcined at 250 °C for 4 h, and then at
350 °C for 6 h in air, to obtain three ZrO2 supported POMs composites, designated as
ZrO2-PW12O40, ZrO2-PW11TiO40 and ZrO2-P2W20Ti4O78 respectively. The loading of
POMs is around 0.05 mmol/g (based on P), which is determined by the feed ratio in
the starting solutions. For comparative studies, mesoporous zirconia (m-ZrO2) was
also prepared by the same method, just without addition of the POM compound.
2.3. Characterization methods
Powder X-ray diffraction (XRD) patterns were recorded on a Shimadzu XRD-6000
diffraction (40 kV, 30 mA) using Ni-filtered Cu Kα radiation. N2 adsorption/
desorption isotherms were measured at -196 °C using a Micromeritics ASAP 2010N
analyzer. The samples were evacuated at 150 °C for 24 h before measurements.
Specific surface areas were calculated using the BET model. The pore size
distributions were determined by the non-local density function theory (NLDFT)
method based on the adsorption data. Transmission electron microscopy (TEM)
images were taken with a H8100-IV electron microscope with an energy-dispersive
X-ray spectroscopy (EDX) operating at 200 kV. The samples were suspended in
ethanol by sonication and then picked up on a Cu grid covered with a carbon film.
Fourier transform infrared spectra (FT-IR) were recorded on a Nicolet Impact 410
6
Page 8 of 35

spectrometer. The diffuse reflectance ultraviolet-visible spectra (UV-vis) were
recorded on a Shimadzu 3600 instrument. The 31P MAS NMR spectra were recorded
on a 400MHz Bruker spectrometer. All 31P MAS NMR chemical shifts are referenced
to the resonances of monoammonium phosphate (NH4H2PO4) standard. A Netzsch
Thermoanalyser STA 449F3 was used for simultaneous thermal analysis combining
thermogravimetry and differential scanning calorimetry (TG-DSC) at a heating rate of
10 °C/min in air from 50-800 °C.
2.4. Reaction procedure for the catalytic oxidation of cyclohexene
The reaction was typically performed according to the following procedure: the
catalysts (contained 0.01 mmol POMs), 30% H2O2 (5 mmol) aqueous solution,
cyclohexene (2.5 mmol), chlorobenzene (2.5 mmol, as internal standard) and
acetonitrile (3 mL, as solvent) were added in a round-bottomed flask with the oil bath
equipment. (Catalyst : Oxidant : Cyclohexene = 1 : 500 : 250) The reaction mixture
was vigorously stirred and refluxed at 60 °C. The products of the reaction were
analyzed and quantified on a Shimadzu GC-8A gas chromatograph with HP-5
capillary column. Blank experiments showed that the oxidative reactions do not take
place without catalyst.
The hot catalyst filtration tests were performed to verify the heterogeneous nature
of catalytic systems. During the catalytic oxidation of cyclohexene, the solid catalyst
was separated from the reaction mixture by filtration after 1.5 h of the reaction and the
filtrate obtained was continuously stirred under the same reaction conditions. The
recyclability of the catalyst was also investigated by consecutive reusing the catalyst.
7
Page 9 of 35

In each catalytic cycle, the catalyst was recovered by filtration, washed with fresh
acetonitrile several times and dried at 100 °C under vacuum.
3. Results and discussion
3.1. Characterization of the materials
The low-angle XRD patterns of ZrO2-PW12O40, ZrO2-PW11TiO40,
ZrO2-P2W20Ti4O78 and m-ZrO2 are shown in Fig. 1A. A weak and broad reflection in
low-angle range could be observed for all tested materials, indicating that these
materials contain disordered mesopores [25, 26]. The wide-angle XRD patterns of the
ZrO2-POMs composites and m-ZrO2 are presented in Fig. 1B. The two broad peaks at
ca. 25 and 55o are attributed to the amorphous ZrO2 frameworks. The absence of
diffraction peaks associated with the crystalline phase of POMs suggests that the
POMs clusters should be highly dispersed in the ZrO2-POM composites.
The mesoporous characteristic of the composites could be further confirmed by
TEM observation. The TEM images of the two representative samples of
ZrO2-PW11TiO40 and ZrO2-P2W20Ti4O78 are displayed in Fig. 2. Worm-like mesopores
without long-distance order could be detected, with an average pore diameter of
around 2-3 nm. In addition, The EDX spectrum obtained on TEM for
ZrO2-P2W20Ti4O78 is shown in inset of Fig. 2B. The EDX spectra taken across a thin
area of mesoporous structures indicated a Zr/W atomic ratio of 13.5 for the
ZrO2-P2W20Ti4O78, which is quite consistent with the theory value of 14.1 (based on
the feed ratio in the starting sol-gel).
The nitrogen adsorption-desorption isotherms of m-ZrO2 and the three ZrO2-POM
8
Page 10 of 35

materials are shown in Fig. 3. All samples display a combination of type I and type IV
isotherms, which associated with a distinct condensation step at relative pressure (p/p0)
of 0.15-0.2 [27, 28], indicating that both micropores and mesopores are co-existed in
these composite materials. Compared with the support m-ZrO2, a pronounced increase
of BET surface areas could be observed after the introduction of POMs into the
matrix of ZrO2 (Table 1). This signifies the role of POMs in self-assembly process,
where it is believed that the POM anions are engaged specific interactions with the
surface micelles, promoting the formation of porous structure in the ZrO2-POM
materials [17]. The pore size distribution profiles, calculated from the absorption data
based on the NLDFT, are shown in inset of Fig. 3. Relatively broad pore size
distribution could be found for all the samples, confirming the co-existence of
disordered micropores and mesopores.
Notably, the disordered porous structure characteristic of the ZrO2-POM
composites is distinctly different from the literature reported ZrO2 supported
phosphormolybdic acid materials (ZrO2-PMA), which are prepared by a similar
surfactant-assisted sol-gel route but possess well-ordered mesoporous structure [17].
This could be mainly related to the use of different surfactant (P123 vs F127), as well
as the different solubility and sol-formation properties of the phosphormolybdic acid
(literature work) and the TBA salts of phosphotungstates (this work). In the present
work, a relative high preparation temperature of 70 oC (against 40 oC, in the literature
work) is used for the sol-gel process in order to avoid the appearance of undesirable
precipitate in the initial stage. Under this temperature, the rate of solvent evaporation
9
Page 11 of 35

is quickened, which might be another reason for the formation of non-uniformed
porosity of the ZrO2-POM materials.
The FT-IR measurements of the TBA-POMs compounds and the ZrO2-POMs
catalysts were also investigated (Fig. 4). As shown in Fig. 4A, the characteristic bands
of TBA3[PW12O40] include P-O bonds (～1081 cm-1), W=O bonds (～981 cm-1),
W-Ob-W (～895 cm-1), W-Oc-W (813 cm-1), which are agreed with the reference [24].
For
the
two
Ti-containing
compounds
TBA5[PW11TiO40]
and
TBA10[α,α-P2W20Ti4O78], the corresponding vibration bands shift slightly
accompanied with the appearance of shoulder peaks (e.g., 1062 and 1089 cm-1), which
is a indication of the distortion induced by the replacement of a W=O unit in the
parent Keggin structure with Ti=O [18]. For TBA5[PW11TiO40], the weak peak at 705
cm-1 should be related to the presence of Ti-OH species. As for
TBA10[α,α-P2W20Ti4O78], the appearance of weak peak at 721 cm-1 to should be
assigned to the presence of Ti-O-Ti species [23]. Besides, all the TBA-POMs samples
have the 1478 and 1380 cm-1, which are the characteristics bands of –CH3 in TBA+.
The FT-IR spectra of ZrO2-PW12O40, ZrO2-PW11TiO40, ZrO2-P2W20Ti4O78 are also
presented in Fig. 4B. The absorption bands at ～1090, ～937, ～833,～752 cm-1
could be attributed to the presence of P-O, W=O, W-Ob-W and W-Oc-W, respectively
[16,29], indicating that the primary Keggin units of POMs were maintained after the
formation of the ZrO2-POMs composites. Compared with the corresponded TBA salts,
all the absorption bands in this region turn to broader and red-shift, suggesting that
relative strong interaction should be present between the POMs units and the zirconia
10
Page 12 of 35

matrix. Especially, the bands at 937 cm-1 broadens and becomes less intense, which
might be assigned to the cleavage of some W=O bonds and possibly transformation
o
into W-O-Zr bonds during the preparation process (i.e., calcination at 300 C) [30].
The absence of absorption bands at 1478 and 1380 cm-1 suggest that the functional
groups of TBA have already be decomposed during the calcination process for
obtaining the ZrO2-POMs composites.
The UV-vis diffuse reflectance spectra of the representative sample of
TBA5[PW11TiO40], ZrO2-PW11TiO40 and m-ZrO2 support are shown in Fig. 5. Two
bands are observed at ～265 nm and ～325 nm in TBA5[PW11TiO40] could be
attributed to the oxygen-to-tungsten (O2-W6+) charge transfer at W=O and W-O-W
bonds of the POM cluster, respectively [16,29]. For ZrO2-PW11TiO40, a strong
blue–shift in O2-W6+ absorption peaks at ～208 nm and ～235 nm could be
observed, which might be also ascribed to the existence of strong chemical interaction
between the POM units and the ZrO2 matrix.
31P MAS-NMR spectra of TBA10[α,α-P2W20Ti4O78] and ZrO2-P2W20Ti4O78 are
shown in Fig. 6 TBA10[α,α-P2W20Ti4O78] revealed a major peak at -13.7 ppm and two
small peaks around the major peak. The small peaks could attribute to an impurity of
the starting material [31]. And the sample of ZrO2-P2W20Ti4O78 shows a signal at
−10.6 ppm, which is quite near from that of TBA10[α,α-P2W20Ti4O78]. This result
provides a direct evidence to confirm the preservation of heteropoly structure of the
POM units [30]. The slight shift of the signal indicates that the chemical environment
of P species varied somewhat, which can be attributed to the presence of relatively
11
Page 13 of 35

strong interaction between the POM units and ZrO2 support [30, 32]. On the basis of
related literatures [30-34], we may propose here that thermal-treatment of the ZrO2
supported POM catalysts could form at least two kinds of strong interaction between
ZrO2 and POM units. First, electrostatic interactions might be built between the POM
ions and Zr+ cations. Second, stable links like Zr-O-W bonds could be formed when
the surface Zr-OH groups strongly interact with the adjacent oxo W=O bonds (in the
POM units) at relatively high calcination temperature (e.g., 350 °C).
The TG-DSC profiles of the precursor of ZrO2-P2W20Ti4O78 are shown in Fig. 7.
The exothermic peak and weight loss in the range of 200-370 °C can be attributed to
the decomposition of [TBA]+ and the removal of P123 [35-37]. The second
exothermic peak along with the further weight loss at around 500 °C could be
assigned to the decomposition of POM and the transformation of amorphous ZrO2 to
the tetragonal ZrO2 [12,23,30]. These results suggest that calcining the precursor of
ZrO2-P2W20Ti4O78 at 350 °C (the temperature we adopted for preparing the catalyst)
could decompose the [TBA]+ and remove the surfactant but keep the structure of
POM preserved.
3.2. Catalytic properties
The catalytic performance of the ZrO2-POMs materials and the corresponding
homogeneous TBA salts were investigated for the H2O2-mediated liquid-phase
oxidation of cyclohexene. It is well known that cyclohexene is a suitable alkene to
study the issue of selectivity control for the POM-based catalysts, since it possesses
both highly reactive C=C double bonds and allylic hydrogen atoms [38]. Dependent
12
Page 14 of 35

on the nature of the catalysts and reaction conditions adopted, different product
distributions could be obtained, the main oxidative products include
2-cyclohexen-1-one (CyH-one), 2-cyclohexen-1-ol (CyH-ol), epoxycyclohexane
(epoxide), 1,2-cyclohexanediol (diol), and 7-oxa-bicyclo-[4.1.0]-heptan-2-one (keto
epoxide). In general, epoxide and diol could be produced via a heterolytic oxidation
process, while the allylic oxidation products (CyH-one and CyH-ol) are mainly
formed through a homolytic oxidation process [19]. Therefore, to obtain efficient
heterogeneous POM-based catalyst with high selectivity either to epoxide or to the
allylic oxidation products is a particularly interesting subject.
As shown in Table 1, the three TBA-POMs are active homogeneous catalysts for
the oxidation of cyclohexene. Their catalytic activity increase gradually in the order of
TBA3[PW12O40]
<
TBA5[PW11TiO40]
<
TBA10[P2W20Ti4O78]. Besides,
TBA3[PW12O40] shows relatively high selectivity to CyH-one (49%) and to epoxide
(25%). As for the two Ti-containing catalysts of TBA5[PW11TiO40] and
TBA10[P2W20Ti4O78], the selectivity to CyH-one decrease obviously with the increase
of Ti contents, while the selectivity to diol or to the keto epoxide increase
significantly. These results suggest that the introduced of Ti species may also
participate in the oxidative reaction directly by forming active intermediates like
Ti-OOH, which could oxidize olefin to epoxides and diol through a heterolytic
oxidation mechanism (heterolytic cleavage of the O-O bond) [39, 40]. Besides,
H3PW12O40 is also a very active homogeneous catalyst, it exhibits much higher
activity and diol selectivity than the corresponding TBA salt (TBA3[PW12O40]), which
13
Page 15 of 35

is an indicative that changing the types of countercation could also considerably
influence the catalytic performance of the POM-based catalysts.
All the three ZrO2 supported POMs catalysts are also catalytically active for the
oxidation of cyclohexene, with a same activity order as their corresponding TBA salts
(ZrO2-PW12O40 < ZrO2-PW11TiO40 < ZrO2-P2W20Ti4O78) (Table 1). Concerning the
fact that support m-ZrO2 just shows very low activity for the reaction, it can be
concluded that the main active sites of these ZrO2 supported POMs catalysts should
originated from the introduced POMs. Apparently, the difference in catalytic activity
of the three ZrO2-POMs catalysts could be mainly attributed to the different specific
surface areas of these catalysts, since their specific surface area increases with Ti
contents (as shown in Table 1), which is quite consistent with their activity order.
Notably, the three ZrO2-POMs catalysts exhibit relatively high selectivity to allylic
oxidation products, and the total selectivity to CyH-one and CyH-ol reaches above
70%, which is much higher than the corresponding TBA salts. The difference in
product selectivity between ZrO2-POMs and TBA-POMs suggests that the chemical
environment of the POM ions should be changed after the formation of ZrO2-POMs
composites.
In our case, it seems that the presence of Ti species in the two Ti-containing
ZrO2-POMs catalyst does not play significant role in influencing the catalytic activity
and selectivity, which is quite different from the corresponding TBA salts catalysts.
Previously, Kholdeeva et al. [19] found that homogeneous [PW11TiO40]5- catalyst is
more active and selective (to epoxide) than the Ti-free compound of [PW12O40]3- for
14
Page 16 of 35

the epoxidation of alkene. They suggest that the introduced Ti species may act main
active centers for the activation of H2O2 to form activated intermediate like Ti-OOH,
which is suitable for the formation of epoxide products through a heterolytic oxidation
mechanism. The formation of active Ti-containing hydroperoxides (like Ti-OOH)
could be confirmed by the color change of the homogeneous Ti-containing catalysts,
since the white color of the fresh Ti-substituted phosphotungstates turns to orange
after contacting with H2O2 [40, 41]. Similar phenomenon has been also observed for
our Ti-substituted phosphotungstates catalysts. However, such color change did not
observed for the ZrO2 supported Ti-substituted phosphotungstates catalysts, which
might be a hint that relatively strong interaction between Ti species and the ZrO2
support might be built (i.e., formation of Ti-O-Zr links) during the preparation process
(e.g., calcination) of ZrO2-POMs catalysts, leading to the catalytic role of Ti species is
restricted. This might be the main reason for why the product selectivity in
H2O2-based oxidation of cyclohexene turns out to be quite similar for the three
ZrO2-POMs catalysts.
Fig. 8A shows the kinetic profile of the oxidation of cyclohexene with H2O2 over
ZrO2-P2W20Ti4O78. In a duplicate reaction, the solid catalyst was separated by hot
filtration after 1.5 h reaction, and then the filtrate was further stirred for studying the
continuing activity at the reaction temperature. As shown in Fig. 8B, no further
conversion of cyclohexene could be detected in the filtrate, indicating that the
catalytic oxidation process is performed by truly heterogeneous ZrO2-POMs catalyst.
The recycling experiments of ZrO2-P2W20Ti4O78 for the oxidation of cyclohexene
15
Page 17 of 35

were also carried out. It was found that the catalyst maintains its activity and
selectivity even after four repeated catalytic cycles (Fig. 9), suggesting that the
ZrO2-POMs catalysts have very good recyclability.
According to the characterization results discussed above, it was known that the
POM clusters are highly dispersed in the ZrO2-POMs composites, and relatively
strong interaction are present between the POM units and ZrO2 support. Therefore,
the excellent stability and recyclability of the ZrO2-POMs catalysts should be mainly
assigned to the existence of stable links between the POM units and the ZrO2 matrix.
4. Conclusions
In summary, the present work demonstrates that mesoporous ZrO2-POMs
composites based phosphotungstate and Ti-substituted phosphotungstates could be
obtained through a surfactant-assisted sol-gel copolymerization route. These
composites exhibited high catalytic activity, selectivity and stability for the allylic
oxidation of cyclohexene with H2O2 as oxidant. The existence of relatively strong
interaction between the POM units and the ZrO2 framework plays important role in
fabricating active and stable heterogeneous ZrO2 supported POMs catalysts.
Acknowledgment
Financial support from the National Natural Science Foundation of China (No.
21173100 and 21320102001) is gratefully acknowledged.
References
[1] I.V. Kozhevnikov, Chem. Rev. 98 (1998) 171-198.
[2] O.A. Kholdeeva, R.I. Maksimovskaya, J. Mol. Catal. A: Chem. 262 (2007)
16
Page 18 of 35

7-24
[3] N. Mizuno, K. Yamaguchi, K. Kamata, Coord. Chem. Rev. 249 (2005)
1944-1956.
[4] N. Mizuno, K. Kamata, K. Yamaguchi, Top. Catal. 53 (2010) 876-893.
[5] C.L. Hill, J. Mol. Catal. A: Chem. 262 (2007) 2-6.
[6] J. Kasai, Y. Nakagawa, S. Uchida, N. Mizuno, Chem. Eur. J. 12 (2006)
4176-4184.
[7] K. Inumaru, T. Ishihara, Y. Kamiya, T. Okuhara, Angew. Chem. Int. Ed. 46 (2007)
7625-7628.
[8] N.V. Maksimchuk, M.S. Melgunov, J. Mrowiec-Bialon, O.A. Kholdeeva, J. Catal.
235 (2005) 175-183.
[9] S. Farhadi, M. Zaidi, Appl. Catal. A: Gen. 354 (2009) 119-126.
[10] N.V. Maksimchuk, M.N. Timofeeva, M.S. Melgunov, O.A. Kholdeeva, J. Catal.
257 (2008) 315-323.
[11] J. Juan-Alcañiz, E.V. Ramos-Fernandez, U. Lafont, J. Gascon, J. Catal. 269
(2010) 229-241.
[12] C. Trolliet, G. Coudurier, J.C. Védrine, Top. Catal. 15 (2001) 73-81.
[13] L. Pizzio, P. Vázquez, C. Cáceres, M. Blanco, Catal. Lett. 77 (2001) 233-239.
[14] P. Sharma, A. Patel, Appl. Surf. Sci. 255 (2009) 7635-7641.
[15] P. A. Shringarpure, A. Patel, Dalton Trans. 39 (2010) 2615-2621.
[16] C. Jiang, Y. Guo, C. Hu, C. Wang, Mater. Res. Bull. 39 (2004) 251-261.
[17] G.S. Armatas, G. Bilis, M. Louloudi, J. Mater. Chem. 21 (2011) 2997-3005.
[18] O.A. Kholdeeva, G.M. Maksimov, R.I. Maksimovskaya, C.L. Hill, Inorg. Chem.
17
Page 19 of 35

39 (2000) 3828-3837.
[19] O.A. Kholdeeva, Eur. J. Inorg. Chem. (2013) 1595-1605.
[20] C.N. Kato, S. Negishi, K. Yoshida, K. Nomiya, Appl. Catal. A: Gen. 292 (2005)
97-104.
[21] C. Brevard, R. Schimpf, G. Tourne, C. M. Tourne, J. Am. Chem. Soc. 105 (1983)
7059-7063.
[22] P. J. Domaille, Inorg. Synth. 27 (1990) 96-104.
[23] K. Nomiya, M. Takahashi, J.A. Widegren, T. Aizawa, Dalton. Trans. (2002)
3679-3685.
[24] S. Zhang, G. Zhao, S. Gao, Z. Xi, J. Mol. Catal. A: Chem. 289 (2008) 22-27.
[25] L. Xu, Y. Wang, Y. Guo, J. H. Clark, Green Chem. 10 (2008) 746-755.
[26] B. Lee, T. Yamashita, D. Lu, Junko N. Kondo, K. Domen, Chem. Mater. 14
(2002) 867-875.
[27] K.S.W. Sing, D.H. Everett, R.A.W. Haul, L. Moscou, R.A. Pierotti, J. Rouquerol,
T. Siemieniewska, Pure and Applied Chemistry 57(4) (1985) 603-619.
[28] E. Skliri, L.N. Lykakis, G.S. Armatas, RSC Adv. 4 (2014) 8402-8409.
[29] G. S. Armatas, A.P. Katsoulidis, D. E. Petrakis, P.J. Pomonis, J. Mater. Chem. 20
(2010) 8631-8638.
[30] E. López-Salinas, J.G. Hernández-Cortéz, I. Schifter, E. Torres-Garcia, Appl.
Catal. A: Gen. 193 (2000) 215-225.
[31] V. Kogan, Z. Aizenshtat, R. Neumann, New J. Chem. 26 (2002) 272–274.
[32] S. Thanasilp, J.W. Schwank, V. Meeyoo, S. Pengpanich, M. Hunsoma, J. Mol.
18
Page 20 of 35

Catal. A: Chem. 380 (2013) 49-56.
[33] F. Su, L. Ma, D. Song, X. Zhang, Y. Guo. Green Chem. 15 (2013) 885-890.
[34] J. Wang, Y. Zou, Y. Sun, M. Jia, W.R. Thiel, Chinese J. Catal. 35 (2014)
532–539.
[35] O.A. Kholdeeva, B.G. Donoeva, T.A. Trubitsina, G. Al-Kadamany, Eur. J. Inorg.
Chem. (2009) 5134-5141.
[36] C. N. Kato, K. Hayashi, S. Negishi, K. Nomiya, J. Mol. Catal. A: Chem. 262
(2007) 25-29.
[37] V. Dufaud, F. Lefebvre, G. P. Niccolai, M. Aouine, J. Mater. Chem. 19 (2009)
1142-1150.
[38] E.F. Murphy, T. Mallat, A. Baiker, Catal. Today, 57 (2000) 115-126.
[39] O.A. Kholdeeva, T.A. Trubitsina, M.N. Timofeeva, G.M. Maksimov, J. Mol.
Catal. A: Chem. 232 (2005) 173-178.
[40] L. Hua, Y. Qiao, Y. Yu, Z. Hou, New J. Chem. 35 (2011) 1836-1841.
[41] K. Hayashi, C.N. Kato, A. Shinohara, K. Nomiya, J. Mol. Catal. A: Chem. 262
(2007) 30-35.
19
Page 21 of 35

Figure Captions
Fig. 1. Low-angle (A) and wide-angle (B) XRD patterns of (a) m-ZrO2, (b)
ZrO2-PW12O40, (c) ZrO2-PW11TiO40, (d) ZrO2-P2W20Ti4O78.
Fig. 2. The typical TEM images of (A) ZrO2-PW11TiO40 and (B) ZrO2-P2W20Ti4O78,
and the TEM-EDX spectra of ZrO2-P2W20Ti4O78 (panel B, inset)
Fig. 3. Nitrogen absorption-desorption isotherms (A) m-ZrO2, (B) ZrO2-PW12O40, (C)
ZrO2-PW11TiO40, (D) ZrO2-P2W20Ti4O78. Insets: the corresponding NLDFT pore size
distributions.
Fig. 4. FT-IR spectra of (A): (a) TBA3[PW12O40], (b) TBA5[PW11TiO40], (c)
TBA10[α,α-P2W20Ti4O78]; (B): (a) ZrO2-PW12O40, (b) ZrO2-PW11TiO40 (c)
ZrO2-P2W20Ti4O78, (d) m-ZrO2.
Fig. 5. Diffuse reflectance UV–vis spectra of (a) m-ZrO2, (b) ZrO2-PW11TiO40, (c)
TBA5[PW11TiO40].
Fig. 6. The 31P MAS NMR spectrum of (A) TBA10[α,α-P2W20Ti4O78], (B)
ZrO2-P2W20Ti4O78.
Fig. 7. TG and DSC profiles of the precursor of ZrO2-P2W20Ti4O78.
Fig. 8. (A) Kinetic profile of oxidation of cyclohexene with H2O2 over
ZrO2-P2W20Ti4O78 catalyst (△); (B) Heterogeneous reaction check for
ZrO2-P2W20Ti4O78 catalyst (△), by continuing the reaction after removing the catalyst
(╳) by filtration at reaction temperature. Reaction conditions: 2.5 mmol of
cyclohexene, 0.01 mmol of catalyst, 5 mmol of H2O2 (30 wt% in water), 3 ml CH3CN,
60 °C reaction temperature.
20
Page 22 of 35

Fig. 9. ZrO2-P2W20Ti4O78 recycling in cyclohexene oxidation with H2O2; Reaction
conditions: 2.5 mmol of cyclohexene, 0.01 mmol of catalyst, 5 mmol of H2O2 (30
wt% in water), 3 ml CH3CN, 60 °C reaction temperature, every time reaction time: 9
h.
Table captions
Table 1 Cyclohexene oxidation with H2O2 is catalyzed by the series of ZrO2-POMs
and TBA-POMs catalysts.
21
Page 23 of 35

Table
Table 1 Cyclohexene oxidation with H2O2 is catalyzed by the series of ZrO2-POMs and
TBA-POMs catalysts.
Cyclohexene
Selectivity (%)
SBET
Catalyst
OH
/m2g-1
Conversion(%)
O
O
OH
OH
O
O
ZrO2-PW12O40
ZrO2-PW11TiO40
ZrO2-P2W20Ti4O78
m-ZrO2
44
119
126
31
-
28
43
45
4
43
43
42
28
17
49
35
16
27
30
33
32
9
-
17
17
16
29
51
14
23
40
13
-
10
9
-
4
7
H3PW12O40
57
33
41
59
9
15
5
TBA3[PW12O40]
TBA5[PW11TiO40]
TBA10[P2W20Ti4O78]
-
8
25
27
1
-
11
6
5
-
37
Reaction conditions: 2.5 mmol of cyclohexene, 0.01 mmol of catalyst, 5 mmol of H2O2 (30wt%
in water), 3 ml CH3CN as solvent, reaction temperature: 60 oC, reaction time: 9 h.
Page 24 of 35

Figure
Fig. 1. Low-angle (A) and wide-angle (B) XRD patterns of (a) m-ZrO2, (b) ZrO2-PW12O40, (c)
ZrO2-PW11TiO40, (d) ZrO2-P2W20Ti4O78.
Page 25 of 35

Figure
Fig. 2. The typical TEM images of (A) ZrO2-PW11TiO40 and (B) ZrO2-P2W20Ti4O78, and the
TEM-EDX spectra of ZrO2-P2W20Ti4O78 (panel B, inset).
Page 26 of 35

Figure
Page 27 of 35

Fig. 3. Nitrogen absorption-desorption isotherms (A) m-ZrO2, (B) ZrO2-PW12O40, (C)
ZrO2-PW11TiO40, (D) ZrO2-P2W20Ti4O78. Insets: the corresponding NLDFT pore size
distributions.
Page 28 of 35

Figure
Fig. 4. FT-IR spectra of (A): (a) TBA3[PW12O40], (b) TBA5[PW11TiO40], (c)
TBA10[α,α-P2W20Ti4O78]; (B): (a) ZrO2-PW12O40, (b) ZrO2-PW11TiO40 (c) ZrO2-P2W20Ti4O78, (d)
m-ZrO2.
Page 29 of 35