Polyoxometalate-based hybrid mesostructured catalysts for green epoxidation of olefins

Article abstract of DOI:10.1016/j.ica.2011.05.024

Novel hybrid polyoxometalates (POM) of α-H₃PW ₁₂O₄₀·nHMPA and α-H₃PMo ₁₂O₄₀·nHMPA composed of α-H ₃PW₁₂O₄₀ and H₃PMo ₁₂O₄₀ heteropoly acides (HPAs) and hexamethylphosphoramide (HMPA) organic substrate has been synthesized and purified. SBA-15 mesoporous silica is synthesized, using P123 surfactant via hydrothermal method, and functionalized with aminopropyl functional groups via grafting method. The synthesized mesostructured supports are used for intercalation of the hybrid POMs. The parent Keggin HPAs are also immobilized within the supports to perform closer and more efficient investigation. After characterization, effect of functional groups on immobilization pattern and quality is taken into consideration. The mesostructured organic-inorganic hybrid materials are characterized using X-ray diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic absorption, and FT-IR analysis. The newly designed hybrid catalysts are investigated for heterogeneous epoxidation of olefins. Effects of temperature, oxidant, and catalyst amount are studied and the reaction conditions are optimized. An interpretation of the differences in the catalytic activity of the precursors is put forward and their catalytic activity is compared with their HPA counterparts. Furthermore, effects of functionalization on catalyst activity, stability, and reusability are taken into consideration. Results reveal that the designed mesostructured POM based hybrid catalysts can selectively and efficiently epoxidize olefins in presence of hydrogen peroxide as oxidant. The catalysts are shown to be heterogeneous and reusable without significant loss of activity in the proceeding rounds.

Full text of DOI:10.1016/j.ica.2011.05.024

Inorganica Chimica Acta 376 (2011) 1–9
Contents lists available at ScienceDirect
Inorganica Chimica Acta
journal homepage: www.elsevier.com/locate/ica
Polyoxometalate-based hybrid mesostructured catalysts for green epoxidation
of olefins
⇑
Z. Karimi, A.R. Mahjoub , S.M. Harati
Department of Chemistry, Tarbiat Modares University, 14115-175 Tehran, Iran
a r t i c l e i n f o
a b s t r a c t
Article history:
Novel hybrid polyoxometalates (POM) of
a
-H PW12O40ꢀnHMPA and -H PMo12O40ꢀnHMPA composed of
a
3
3
Received 8 December 2010
Received in revised form 8 May 2011
Accepted 20 May 2011
3 3
a-H PW12O40 and H PMo12O40 heteropoly acides (HPAs) and hexamethylphosphoramide (HMPA) organic
substrate has been synthesized and purified. SBA-15 mesoporous silica is synthesized, using P123 surfac-
tant via hydrothermal method, and functionalized with aminopropyl functional groups via grafting
method. The synthesized mesostructured supports are used for intercalation of the hybrid POMs. The par-
ent Keggin HPAs are also immobilized within the supports to perform closer and more efficient investi-
gation. After characterization, effect of functional groups on immobilization pattern and quality is taken
into consideration. The mesostructured organic–inorganic hybrid materials are characterized using X-ray
diffraction (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM), atomic
absorption, and FT-IR analysis. The newly designed hybrid catalysts are investigated for heterogeneous
epoxidation of olefins. Effects of temperature, oxidant, and catalyst amount are studied and the reaction
conditions are optimized. An interpretation of the differences in the catalytic activity of the precursors is
put forward and their catalytic activity is compared with their HPA counterparts. Furthermore, effects of
functionalization on catalyst activity, stability, and reusability are taken into consideration. Results reveal
that the designed mesostructured POM based hybrid catalysts can selectively and efficiently epoxidize
olefins in presence of hydrogen peroxide as oxidant. The catalysts are shown to be heterogeneous and
reusable without significant loss of activity in the proceeding rounds.
Available online 6 June 2011
Keywords:
Hybrid polyoxometalate
Hexamethylphosphoramide
SBA-15
Amine-functionalized
Heterogeneous oxidation catalyst
Ó 2011 Elsevier B.V. All rights reserved.
1
. Introduction
which a central PO
4
tetrahedron is surrounded by a W12O36 or
Mo12
O36 shell [6,10]. They have attracted much attention as envi-
Production of epoxides is of growing interest in chemical and
ronmentally benign acids and efficient and highly selective electro-
philic and oxidation catalysts [10,11]. Besides, versatility and
accessibility of Keggin series have led to synthesis of various hy-
brid materials based on the Keggin structure [12,13]. In the last
few years there have been a rapidly growing number of reports
in the literature addressing the use of polyoxometalate as both
homogeneous and heterogeneous catalysts [6,14–16]. However,
to the best of our knowledge, only few reports are involved with
catalytic applications of organic–inorganic hybrid Keggin POMs
[13].
petrochemical industries due to their applications as organic inter-
mediates in pharmaceutical synthesis and as monomers in produc-
tion of various functional polymers [1–5]. However, epoxidation
reaction sometimes requires hazardous chemicals which leave acid
wastes. Therefore, designation of efficient heterogeneous catalysts
is an important step to going green, which involves both rigorous
control of the surface chemistry and geometry at micro, meso,
and macroscales. Besides, high active phase dispersion is inevitable
for fast mass transfer of reactants to the active catalytic sites. Thus,
much interest has been driven to production of unique mesostruc-
tured systems with uniform and tailored pore structure.
Since the polyoxometalates were found highly efficient for
epoxidation of olefins by Venturello et al. [17,18] and Ishii et al.
Polyoxometalates (POMs) are an immense class of oxygen
bridged metal cluster anions of mainly tungsten and molybdenum
2 2
[19,20] in 1980s, various H O -based epoxidation systems have
been efficiently catalyzed by POM based systems [21,23]. However,
there exist some drawbacks about POMs catalysts: their thermal
stability is relatively low and they are extremely soluble in polar
solvents, which thus results in separation problems. Moreover, sur-
[
6,7]. Some POMs, such as phosphotungstic acid (H
and phosphomolybdic acid (H PMo12 40, PMo), have a soccer ball-
shaped Keggin structure [8] with a molecular size of ca. 1 nm [9] in
3
PW12O40, PW)
3
O
2
ꢁ1
face area of unsupported POMs is usually low (1–10 m /g ) which
is an important restriction for catalytic systems. All these
limitations may be satisfied by grafting POMs within proper
⇑
Corresponding author. Tel.: +98 21 82883443; fax: +98 21 88009730.
E-mail address: mahjouba@modares.ac.ir (A.R. Mahjoub).
0
020-1693/$ - see front matter Ó 2011 Elsevier B.V. All rights reserved.
doi:10.1016/j.ica.2011.05.024

2
Z. Karimi et al. / Inorganica Chimica Acta 376 (2011) 1–9
heterogenizing supports, which is a great step to designation of
efficient heterogeneous POM based catalysts [22–25].
ethanol (1 g equal to 5.41 mmol) of is then added to the solution
and stirred for an hour. The resulting yellow precipitate is collected
and sucked as dry as possible on a fritted-glass funnel. The precip-
itate is recrystalized from an acetonitrile solution to yield orange
A new paradigm in heterogeneous catalysis is made possible by
application of MCM-type [25,26] and SBA-type [27] mesoporous
materials as appropriate host matrices for finely dispersion of POM
species within the uniform and tailored pores and channels [28,29]
and efficient improvement of their catalytic activity [19,20].
SBA-15 mesoporous silica (prepared using neutral block copoly-
mer), with well ordered hexagonal arrays of cylindrical channels,
narrow pore size distributions, remarkable thermal stability (up
crystals of H
3
PMo12
O
40ꢀn[C
6 18 3
H N OP] labeled as HMPAMo.
2.2.3. Synthesis of SBA-15
Synthesis of SBA-15 is conducted according to the literature
[32]. Pluronic P123 is used as a template, and the silica source is
TEOS, which is added drop wise to an acidic P123 aqueous solution
and then stirred at 40 °C for 24 h at 600 rpm. The reaction mixture
is acidified by adding HCl aqueous solution. The molar ratio of the
to 1000 K), large uniform pore size (5–30 nm), high surface area
2
(
600–1000 m /g), thick silica walls (3.1–6.4 nm) [23,30], and a
large number of silanol groups at the surface of its channels [31],
serves as an intriguing support for the purpose. Furthermore,
SBA-15 mesopores are interconnected by micropores [32,33], en-
abling the pore surfaces to be accessed in three dimensions in cat-
alytic reactions.
This efficient support may be modified with 3-aminopropyl-tri-
ethoxysilane (APTES) via grafting method, in which hydroxyl
2
groups of SBA-15 react with the organosilane to form a NH func-
reaction mixture is 1TEOS:0.0168 P123:5.8HCl:155H O. The pow-
der is recovered by filtration, dried overnight, and further calcined
at 600 °C for 6 h.
2
2
.2.4. Synthesis of SBA-15/NH
To prepare functionalized mesoporous supports via post-syn-
thesis method, 1.0 g of pure calcined SBA-15 is reacted with
0 mL of 0.01 M solution of APTES in dried toluene under reflux
for 8 h. The resultant white solid is filtered off, washed with dried
toluene, dried under vacuum, and labeled as SBA/NH
2
5
tional group [34–37]. This process is an important extension, since
it allows significant modification of physical and chemical proper-
ties. Furthermore, rational design and tailoring of hydrophobicity,
adsorption characteristics and reactivity is possible with the hybrid
organic/inorganic material. In addition to these merits, functional
groups enhance the interactions of POM with the support material
either by physical forces (i.e. hydrogen bonding and electrostatic
interactions) or chemical bonding and allow higher and more sta-
ble loading [29,38,39].
In this work, we report synthesis of novel organic–inorganic hy-
brid materials with hexamethylphosphoramide (HMPA), as the or-
ganic group, and tungsten and molybdenum Keggin POMs, as the
inorganic foundation. The polyoxometalate has the capacity to be
reduced and can form crystals, and HMPA, with a large dipole mo-
ment, has unusually high basicity [40,41] and can form complex
with polyoxometalate acids. The newly synthesized complexes
are then incorporated within pristine and functionalized SBA-15
mesostructured supports and studied as heterogenous catalytic
systems for oxidation of olefins. Oxidation conditions are opti-
mized and heterogeneous nature, efficiency, selectivity, and reus-
ability of the catalysts are truly assessed.
2
.
2
.2.5. Immobilization of PMo on mesoporous supports
Preparation of novel materials based on silica and polyoxometa-
lates is performed by the suspension of 1 g of mesoporous silica
SBA-15 or SBA/NH ) in 10 ml of POM solution (0.7 M) in 1:1 mix-
(
2
ture of acetonitrile and ethanol. The resulting mixture is gently
stirred in 70 °C until equilibrium is reached (typically 24 h). The so-
lid product is then centrifuged and washed several times with eth-
anol. The precipitate is then dried at room temperature and labeled
as SBA/HW, SBA/HMo, SBA/HMPAW, SBA/HMPAMo, [SBA/NH
2
]/
HMPAW, [SBA/NH ]/HMPAMo.
2
2.3. Characterization of the samples
Characterization of the synthesized organic–inorganic hybrid
mesoporous materials is performed by different conventional tech-
niques. X-ray powder diffraction (XRD) data are acquired on a
Philips X-Pert diffractometer using Cu K
a radiation. TEM images
are acquired using Loe 906 Microscope operating at 100 kV. Atomic
Absorption data are acquired by contrAA 700 using flam technique.
FT-IR spectra are recorded on a Nicolet, IR 100 FT-IR Spectrometer.
2
. Experimental
2.1. Chemicals
2.4. Catalytic experiments
All materials are of commercial reagent grade and are
Catalytic epoxidation reactions are performed in a round bottom
flask in an oil bath under magnetic stirring (400 rpm). Unless other-
wise specified, all oxidation reactions are carried out under reflux at
purchased from Merck Company. P123 (poly(ethylene oxide)
poly(propylene oxide)–poly(ethylene oxide) (EO20–PO70–
EO20)) is purchased from Aldrich Company. Tungstophosphoric
Acid -H [PW12 40] (labeled as HW) is produced via the procedure
reported earlier [42]. Molybdophosphoric Acid -H [PMo12
labeled as HMo) is synthesized according to literature [43].
–
2 2
atmospheric pressure with 1 mmol of olefin, 2 ml of 30% H O as oxi-
a
3
O
dant, 1 ml of dichloromethane as solvent, n-dodecane (0.1 mmol) as
internal standard for product quantification, and 0.025 g of hybrid
mesostructured catalysts. Catalytic reactions are monitored using
Gas chromatography, GC, HP5890 chromatogram.
a
3
40
O ]
(
2
2
.2. Catalyst preparation
.2.1. Synthesis of HMPAW (H
3
PW12
O
40ꢀn[C
6
H
18
N
3
OP])
3. Results and discussion
Phosphotungstic acid (2 g equal to 0.694 mmol) is dissolved in
:1 mixture of ethanol and water and HMPA (0.385 g equal to
.08 mmol) is then added to the solution. The resulting yellow
1
2
3.1. Immobilization of POMs within SBA-15 and SBA-15-NH
2
transparent solution is left to produce yellow crystals of
40ꢀn[C OP] labeled as HMPAW.
The immobilization pattern profits from an acid–base interac-
tion between the POMs (as Brønsted acid) and silanol groups pres-
ent at the surface and inside the channels of the mesoporous silica
(as the base) [35,36]. In fact, chemically active surface silanol
3
H PW12
O
6 18 3
H N
2
.2.2. Synthesis of HMPAMo (H
Phosphomolybdic acid (1.65 g equal to 0.902 mmol) is dissolved
in a 1:1 mixture of water and ethanol and HMPA solution in
3
PMo12
O
40ꢀn[C
6 18 3
H N OP])
groups (–Si–OH) of silica matrix are protonated in an acidic
+
medium (produced by POMs) to form –Si–OH
2
groups [44], which

Z. Karimi et al. / Inorganica Chimica Acta 376 (2011) 1–9
3
2
n
p
m
qꢁ
serves as a counter ion for the (–SiOH
+)(H
P
M
O
ꢀHMPA) poly-
and channels of the mosoporous supports. Actually, separate
detectable crystal phases of material are differentiated based on
XRD patterns. When the impregnated guest species are finely
dispersed within the support, they does not give rise to a separate
crystal phase and hence, no peaks corresponding to the bulk phase
of the guest molecules is detected in XRD [50]. The d100-spacing
anion [45–47] (Scheme 1). Furthermore, hydrogen bondings are
present between M@O , M–O , M–O , P@O and amine groups of
t
c
e
HMPA with HO–Si. These interactions ensure a stable and non-
physically adsorption pattern for POM intercalation within the
pristine SBA silica support.
As for SBA-15-NH
ical immobilization of POM by taking advantage of its overall neg-
ative charge, producing (–SiNH
Scheme 1). Besides, stronger hydrogen interactions are produced
with aminopropyl functionalized support, compared to the non-
functionalized one. Hence, a more stable heterogeneous catalyst
is produced with higher amount of guest molecules retained with-
in the support.
2
/POMs, the functional group promotes chem-
(calculated from Bragg equation) and unit cell parameters
p
(calculated using a
0
= 2d100
/
3) of the designed materials are
+
ꢁ
3
)(H
n
P M
p m
O
q
ꢀHMPA) polyanion
determined based on the low-angle XRD patterns (Table 1).
According to scanning electron microscopy (SEM) images
(Fig. 2a–c), morphology of SBA supports have remained intact both
after functionalization and POM immobilization. Transmission
electron microscopy (TEM) is also applied to observe the extent
of order in these materials and also the shape and regularity of
channels. The micrographs demonstrate that the hexagonal or-
dered channel structure of parent SBA-15 has been retained in
the designated hybrid materials (Fig. 2d and e).
(
3
.2. Characterization of the hybrid materials
Low angle powder X-ray diffraction (XRD) patterns of pure, func-
The amount of incorporated POMs is estimated based on atomic
absorption data (Table 1). As expected, aminopropyl functional
group improves immobilization process and higher content of
POM is immobilized within the functionalized support.
tionalized, and POM containing SBA-15 (Fig. 1A) confirm the struc-
tural maintenance upon both functionalization and incorporation of
guest molecule. The three distinct Bragg diffractions (1 0 0), (1 1 0),
and (2 0 0), are characteristics of a highly ordered two-dimensional
hexagonal mesoscopic structure of pure SBA-15 [48]. Presence of
the reflections in all diffractograms is indicative of the hexagonal
pore structure retainment after incorporation of the guest mole-
cules. The intensity and resolution of reflections is based solely on
the degree of ordering in the material’s pore array and arises from
the electron density gradient between the walls and pores. POM
immobilization within the mesoporous host reduces the order of
the mesostructured system and leads to increased phase cancella-
tion between X-rays scattered from the walls and pores regions.
Thus, a decrease in the intensity of the Bragg reflections in the low
angle region is evidenced [49]. Similar pattern is observed for the
functionalized hybrid materials (Fig. 1B). No peaks characteristic
of POM is detected in the wide angle region. This may be attributed
to the fine dispersion of the POM species within the copious pores
FT-IR analysis is applied to shed light on the structure of
HMPAW and HMPAMo species (electronic supplement and
Fig. 3A(c and e)). Comparing the IR spectra of the parent heteropoly
acid with the corresponding HMPA hybrid material, the vibrational
band corresponding to the P@O bond of HMPA exhibits a red-shift
ꢁ1
ꢁ1
from 1203 cm
in HMPA to 1172 cm
in the hybrid POMs
(Table 2). This suggests that the every HMPA has produced
hydrogen bonding with protons of the parent heteropoly acid
[13]. This combination weakens the P@O band and strengthens
the P–N band of the organic group.
FT-IR spectra of pristine, functionalized, and POM impregnated
SBA mesoporous supports are studied for structural characteriza-
tion purposes (Fig. 3A). Pristine SBA-15 gives a broad low-
ꢁ1
frequency band at 3427 cm
due to those silanol groups or
silanol nests with cross hydrogen bonding interactions. Bands
OH
R
OH
R
Intermolecular
R
Hydrogen bonding
HO
+H R
+H
OH
OH
Acid-base
interaction
Hydrogen bonding
HO
Support-HMPA
H+
R
R
OH
R
HO
Scheme 1. POM immobilization drill within SBA-type mesostructured supports. Some types of interactions are shown. For nonfunctionalized supports R = –OH and for
functionalized ones R =
NH2
.

4
Z. Karimi et al. / Inorganica Chimica Acta 376 (2011) 1–9
(A)
1
00
(a)
(
b)
c)
(
(
d)
e)
(
0
1
2
3
4
5
6
7
8
9
2
θ [°]
(B)
(
(
(
a)
b)
c)
0
1
2
3
4
5
6
7
8
9
10
2θ [°]
Fig. 1. Low-angle XRD patterns of: (A) (a) SBA-15, (b) SBA/HMPAW, (c) SBA/ HMPAMo, (d) SBA/HW, (e) SBA/HMo; (B) (a) SBA/NH
2 2 2
, (b) [SBA/NH ]/HMPAW; (c) [SBA/NH ]/
HMPAMo.
Table 1
Structural properties and chemical composition of mesostructured hybrid materials.
Sample
d
100 (nm)
Unit cell parameter (nm)
W or Mo content (wt.%)
SBA-15
7.95
7.3
7.6
7.5
7.3
7.4
7.2
7.1
9.2
8.4
8.8
8.7
8.4
8.6
8.3
8.2
–
–
SBA-15/NH
SBA/HW
SBA/HMo
SBA/HMPAW
SBA/HMPAMo
2
17.2
12.2
16
8.69
21.9
13.8
[
[
SBA/NH2]/HMPAW
SBA/NH2]/HMPAMo
ꢁ
1
ꢁ1
corresponding to Si–O–Si are detected at 1079 cm and 809 cm
As for the aminopropyl functionalized support, bands correspond-
ing to NH functional group are found around 2925, 1510 and
91 cm (Fig. 3B). The primary structure of unsupported POMs
.
catalysts and effects of solvent, catalyst amount, temperature,
and oxidant are taken into consideration.
2
Dichloromethane, acetonitrile, and methanol are studied as
models to study the solvent effect on the epoxide yield (Table 3).
All the reactions are carried out under identical conditions with
0.025 g catalyst, 1 mmol substrate (cyclooctene), 2 ml hydrogen
peroxide, and 1 ml solvent. It seems that dichloromethane is the
best solvent for the purpose.
ꢁ1
6
is also identified by distinct IR bands which are summarized in Ta-
ble 2. Actually, clear observation of POM is hindered as a result of
the strong background of silica support. Owing to the overlap of
ꢁ1
8
68 cm M–O–M and P–O bands with those of Si–O–Si stretching
vibrations of mesoporous supports, only few bands corresponding
to POM are discernable after immobilization within the supports.
Besides, the slight shift in the position of peaks serves as an evi-
dence for interaction between the mesostructured host and guest
molecules.
In order to optimize the catalyst amount, catalytic reactions are
performed using different amounts (0.01, 0.025, 0.05 g) of SBA/
HMPAW catalyst. Other parameters are kept constant (1 mmol
2 2
cyclooctene, 2 ml hydrogen peroxide, and 1 ml CH Cl ) and the
systems are refluxed for 12 h. Higher epoxides yield is achieved
by enhancing the catalyst amount from 0.01 g to 0.025 g (Table 3).
Further increase in the catalyst amount produces no significant
influence. Thus, the following experiments are carried out with
0.025 g of catalyst.
3
3
.3. Epoxidation reaction
.3.1. Optimizing the reaction conditions
Reaction conditions are optimized in presence of SBA/HMPAW
as a model for other synthesized POM containing heterogeneous
Temperature can also be important for achieving the optimum
result. To study the effect, reaction is performed at room

Z. Karimi et al. / Inorganica Chimica Acta 376 (2011) 1–9
5
2 2
Fig. 2. SEM images of: (a) SBA/HMPAW, (b) [SBA/NH ]/HMo, and (c) SBA/HW, and TEM images of (d) SBA/HMPAMo and (e) [SBA/NH ]/HMPAW.
temperature and under reflux conditions with 0.025 g of SBA/
HMPAW catalyst, 1 mmol of cyclooctene, 2 ml hydrogen peroxide,
and 1 ml CH Cl . The oxidation reaction proceeds much slower at
2 2
room temperature and reflux temperature is shown to be more
proper for the purpose (Table 3).
active and selective towards epoxide product. Blank reaction is car-
ried out to specify the role of catalyst. No conversion is achieved in
the absence of catalyst. Similar oxidation reactions are also carried
out with bare silica supports and only negligible catalytic activity is
evidenced (about 4% conversion for cyclooctene). These clearly im-
ply that the immobilized POM species are responsible for the high
oxidation activity of the hybrid catalysts. Besides, the large pores
and channels of the SBA support permit diffusion of relatively large
cyclic alkenes and epoxidation proceeds selectively inside copious
pores and channels of the mesostructured support.
Generally, the reaction is believed to proceed via the mecha-
nism postulated for Keggin type POMs. Monomeric, dimeric, and
tetrameric peroxo species are generated by the reaction of poly-
oxometalates with hydrogen peroxide, and the peroxo species
are postulated to be the catalytically active species for epoxidation
Either tertiary butyl hydrogen peroxide (TBHP) or H
applied as oxidant for epoxidation of olefins. Better conversion
rates are evidenced in presence of H (Table 3). Besides, hydro-
2 2
O can be
2 2
O
gen peroxide is much preferred as an eco-friendly green oxidant
which generates only water as a by-product and has a high content
of active oxygen species [52–55]. Hydrogen peroxide is also more
desirable because of safety and ease of handling. It’s noteworthy
that in absence of oxidant, no peroxo is produced and the oxidation
does not occur.
3.3.2. Effect of the nature of catalyst
2 2
of olefins with POM/H O system [51,56,57]. Closer monitoring of
Catalytic activity of the newly designed hybrid mesostructured
materials is studied for epoxidation of olefins. Results are summa-
rized in Table 4. All the catalytic systems are shown to be highly
the oxidation process indicates that the nature of addenda atoms
plays a vital role in activity of the hybrid catalysts. Tungsten-based
systems exhibit higher reactivity compared to molybdenum

6
Z. Karimi et al. / Inorganica Chimica Acta 376 (2011) 1–9
2
Fig. 3. FT-IR spectra of: (A) (a) SBA-15, (b) SBA/HMPAW, (c) HMPAW, (d) SBA/HMPAMo, (e) HMPAMo, (f) SBA/HW, (g) HW, and (B) (a) SBA/NH2, (b) [SBA/NH ]/HMPAW, (c)
HMPAW, (d) [SBA/NH ]/HMPAMo and (e) HMPAMo.
2
Table 2
Position of FT-IR characteristic bonds corresponding to unsupported POMs and HMPA.
Compound FT-IR characteristic bands
ꢁ
1
ꢁ1
ꢁ1
HMPA
HW
HMo
980 cm (P–N), 1203 cm (P@O), 1290 cm (C–N)
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
795 cm (W–O
c
–W), 890 cm (W–O
–Mo), 868 (Mo–O
b
–W), 982 cm (W@O
–Mo), 964 cm (Mo@O
d
), 1080 cm (P–O
a
)
ꢁ
1
ꢁ1
ꢁ1
789 cm (Mo–O
c
b
d
), 1064 cm (P–O
a
) [48]
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
HMPAW
609 cm (bending P–O), 807 cm (W–O
c
–W), 895 cm (W–O
b
–W), 987 cm (W@O), 1081 cm (P–O
a
), 1172 cm (P@O), 1298 cm (C–N), 1462,
ꢁ
1
2
3
913 & 3189 cm (–CH )
ꢁ
1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
ꢁ1
HMPAMo
c b a
609 cm (bending P–O), 804 cm (Mo–O –Mo), 878 cm (Mo–O –Mo), 972 cm (Mo@O), 1066 cm (P–O ), 1172 cm (P@O), 1298 cm (C–N),
ꢁ
1
1
3
462, 2920 & 3209 cm (–CH )
analogs. Actually, tungsten POMs are usually regarded as stronger
acids compared to their molybdenum counterpart [58]. It seems
that the epoxidation reaction rate and epoxides yield is controlled
and determined by catalyst’s acid strength [59]. Hence, tungsten
POMs show higher catalytic activity. Speaking of the acid strength
to justify our achievements, we decided to compare the catalytic
performance of phosphomolybdic [55] and phosphotungstic acids
immobilized within SBA-15 for oxidation of cyclooctene as a model
for other olefins (Table 4). As expected, SBA/W and SBA/Mo pro-
duce higher yield of epoxide, with SBA/W being more active. The
higher activity of the SBA/W catalyst is clearly justified regarding
its higher acidity. As mentioned before, in the hybrid POMs, the
acidic protons of the parent heteropoly acids are engaged in a
hydrogen bonding with the oxygen of the P@O group of HMPA.

Z. Karimi et al. / Inorganica Chimica Acta 376 (2011) 1–9
7
Table 3
mesoporous host. Indeed, higher amount of Guest molecules are
believed to be more finely dispersed via chemical immobilization
within the NH functionalized SBA-15, compared to the non-func-
2
Optimization of the cyclooctene oxidation reaction conditions using SBA/HMPAW
hybrid catalyst (1 mmol substrate, 0.1 mmol n-dodecane, 2 ml oxidant, and 1 ml
solvent). Results are reported after 12 h of reaction.
tionalized mesoporous. Thus, more catalytic active sites are avail-
able and oxidation reaction is more efficient. This is in
accordance with the epoxidation results, higher epoxides yield is
achieved with the aminopropyl functionalized systems (Table 4).
To study the oxidation procedure more closely, the reaction is
performed under the optimized conditions (i.e. 0.025 g catalyst,
Amount of SBA/HMPAW
catalyst (g)
Solvent Oxidant Temperature
Conversion
(%)
0
0
0
0
0
0
.025
.025
.025
.01
.05
.025
CH
CH
MeOH
2
Cl
CN
2
H
H
H
H
H
H
2
O
2
O
2
O
2
O
2
O
2
O
2
2
2
2
2
2
reflux
reflux
reflux
reflux
reflux
room
71.5
64.3
58.4
65.6
71.8
54.7
3
CH
2
CH
2
CH
2
Cl
Cl
Cl
2
2
2
2 2 2 2
1 mmol cyclooctene, 2 ml H O , 1 ml CH Cl , 0.1 mmol n-dodecane
under reflux) and epoxides yield is considered at different time
intervals (Fig. 4). Interestingly, the reaction proceeds more slowly
in presence of aminofunctionalized hybrid catalysts. This may be
attributed to the hindrance and restrictions for diffusion of the re-
agent inside the pores and channels of the mesostructured catalyst.
However, as mentioned above, higher yield is accomplished with
the functionalized catalysts.
temperature
reflux
0
.025
2
CH Cl
2
TBHP
63.5
Hence, their acidity is reduced and as the reaction procedure is af-
fected by this factor, lower catalytic activity is evidenced with the
hybrid POM based systems.
Catalytic performance of the heterogeneous catalysts is also re-
lated to fine dispersion of the active catalytic species within the
supports. Here, the immobilizing support and the nature of func-
tional groups are shown to have crucial effect on oxidation process.
Actually, as mentioned previously, functionalization improves
sorption properties and guest incorporation within SBA-15
3.3.3. Recyclability of catalysts
To assess stability and reusability, catalysts are separated from
the reaction mixture after each experiment, washed several times
and dried before being applied in the subsequent run. During the
second cycle of reaction with SBA/HMPAW and SBA/HMPAMo
Table 4
Catalytic performance of the designed mesoporous catalysts for epoxidation of olefins (0.025 g catalyst, 1 mmol substrate, 0.1 mmol n-dodecane, 2 ml H
data are reported after 12 h of reaction. = TON (turnover number) is defined as mmol of epoxide/mmol POM.
2
O
2
, and 1 ml CH
2
Cl
2
). The
⁄
⁄
Catalyst
Substrate
Product
Conversion (%)
Selectivity to epoxide (%)
TON
SBA/HMPAW
Cylopentene
Cyclohexene
Cyclooctene
Cyclododecene
Cyclooctene
Cylopentene
Cyclohexene
Cyclooctene
Cyclododecene
Cyclooctene
Cyclooctene
Cyclododecene
Cyclooctene
Cyclododecene
Cylopentene oxide
Cyclohexene oxide
Cyclooctene oxide
Cyclododecene oxide
Cyclooctene oxide
Cylopentene oxide
Cyclohexene oxide
Cyclooctene oxide
Cyclododecene oxide
Cyclooctene oxide
Cyclooctene oxide
Cyclododecene oxide
Cyclooctene oxide
Cyclododecene oxide
76
77
71
79
90
56
57
59
74
88
79
88
68
83
97
89
99
99
99
97
87
98
99
99
99
98
99
99
409
376
394
435
462
287
264
306
394
332
321
347
228
278
SBA/W
SBA/HMPAMo
SBA/HMo
[
SBA/NH
2
]/HMPAW
]/HMPAMo
[
SBA/NH
2
1
00
SBA/HW
SBA/Hmo
SBA/HMPAW
SBA-NH2]/HMPAW
SBA/HMPAMo
[
[SBA-NH2]/HMPAMo
0
0
4
8
12
Time (Hours)
Fig. 4. Comparison of catalytic activity of heterogeneous mesostructured catalysts for epoxidation of cyclooctene (0.025 g catalyst + 1 mmol cyclooctene + 2 ml H
2 2
O + 1 ml
CH Cl + 0.1 mmol n-dodecane under reflux.).
2
2

8
Z. Karimi et al. / Inorganica Chimica Acta 376 (2011) 1–9
Round 1
Round 2
Round 3
Round 4
1
00
0
SBA/HMPAW
SBA/HMPAMo
[SBA-NH2]/HMPAW
Catalyst
[SBA-NH2]/HMPAMo
Fig. 5. Catalytic activity of heterogeneous catalysts during the recycling process for cyclooctene epoxidation.
catalysts, a slight reduction is evidenced in the epoxide yield
Fig. 5) which may be attributed to leaching of unstable immobi-
References
(
[
[
1] M.K. Tse, C. Dobler, S. Bhor, M. Klawonn, W. Magerlein, H. Hugl, M. Beller,
Angew. Chem., Int. Ed. 43 (2004) 5255.
2] D.V. Deubel, G. Frenking, P.S. Gisdaki, W.A. Herrmann, N. Rosch, J.
Sundermeyer, J. Acc. Chem. Res. 37 (2004) 645.
lized POM. Nevertheless, the catalytic activity is nearly retained
during the later runs. However, this activity loss is not evidenced
with the functionalized hybrid catalysts. This is due to stronger
chemical interactions between the hybrid POM and the support
and hence, more efficient retainment of the catalytic active sites
within the pore and channels of the functionalized support.
[
[
3] Y. Shi, Acc. Chem. Res. 37 (2004) 488.
4] M. Hickey, D. Goeddel, Z. Crane, Y. Shi, Proc. Natl. Acad. Sci. USA 101 (2004)
5794.
[5] E.M. McGiarrigle, D.G. Gilheany, Chem. Rev. 105 (2005) 1563.
[
6] M.T. Pope, C.K. Jorgensen, et al. (Eds.), Heteropoly and IsopolyOxometalates,
Inorganic Chemistry Concepts, vol. 8, Springer Verlag, West Berlin, 1983.
4
. Conclusion
[7] T. Yamase, Catal. Surv. Asia 7 (2003) 226.
[
[
8] J.F. Keggin, Nature 131 (1933) 908.
9] I.K. Song, M.S. Kaba, M.A. Barteau, Langmuir 18 (2002) 2358.
This work provides an efficient and easy method for designation
[
10] T. Okuhara, N. Mizuno, M. Misono, Adv. Catal. 41 (1996) 113.
[11] N. Dubey, S.S. Rayalu, N.K. Labhsetwar, S. Devotta, Int. J. Hydrogen Energy 33
2008) 5958.
of a novel and selective POM-HMPA hybrid heterogeneous cata-
lysts for epoxidation of relatively large olefins. -H
40ꢀnHM-
PA and -H PMo12
40ꢀnHMPA organic–inorganic hybrid POMs are
synthesized. SBA-15 and SBA-15-NH mesoporous silicas are also
synthesized and characterized. The mesostructured porous materi-
als are shown to be effective supports for immobilization of the hy-
brid POMs and their HPA analogs. XRD, SEM, and TEM analyses
reveal that structure of mesoporous supports remains intact after
POM grafting. Atomic absorption and FT-IR analyses confirm
POM immobilization. Results reveal that the sorption property of
the mesoporous support is improved by functionalization with
(
a
3
PW12O
[
[
12] T. He, J. Yao, Prog. Mater. Sci. 51 (2006) 810.
13] J.Y. Niu, X.Z. You, C.Y. Duan, Inorg. Chem. 35 (1996) 4211.
[14] I.V. Kozhevnikov, K.I. Matveev, Appl. Catal. 5 (1983) 135.
15] M. Misono, Stud. Surf. Sci. Catal. 75 (1993) 69.
16] C.W. Hu, M. Hashimoto, T. Okuhara, M. Misono, J. Catal. 143 (1993) 437.
17] C. Venturello, E. Alneri, M. Ricci, J. Org. Chem. 48 (1983) 3831.
[18] C. Venturello, R. D’Aloisio, J.C. Bart, M. Riai, J. Mol. Catal. 32 (1985) 107.
[19] Y. Ishii, K. Yamawaki, T. Yoshida, T. Ura, H. Yamada, M. Ogawe, J. Org. Chem. 52
a
3
O
2
[
[
[
(
1987) 1868.
20] Y. Ishii, K. Yamawaki, T. Ura, H. Yamada, T. Yoshida, M. Ogawa, J. Org. Chem. 53
1988) 3587.
[21] R.I. Kureshy, I. Ahmad, N.H. Khan, S.H.R. Abdi, K. Pathak, R.V. Jasra, J. Catal. 238
2006) 134.
[
(
(
NH
2
group.
[
22] N.K.K. Raj, S.S. Deshpande, R.H. Ingle, T. Raja, P. Manikandan, Stud. Surf. Sci.
Catal. 156 (2005) 769.
The newly designed mesoporous hybrids are shown to combine
catalytic activity of POMs with high surface area of mesoporous
supports. Such kind of materials can be efficiently used as environ-
mentally benign, green, and selective heterogeneous catalysts for
epoxidation of olefins. The epoxidation reaction conditions are
optimized and effect of chemical composition and structure is con-
sidered for both POM’s and the immobilizing mesoporous support.
[23] Z. Karimi, A.R. Mahjoub, Catal. Commun. 12 (2011) 984.
[
[
[
24] P.J. Collman, L. Zeng, I.J. Brauman, Inorg. Chem. 43 (2004) 2672.
25] I.V. Kozhevnikov, Catal. Rev. Sci. Eng. 37 (1995) 311.
26] W. Chu, X. Yang, Y. Shan, X. Ye, Y. Wu, Catal. Lett. 42 (1996) 201.
[27] N.Y. He, C.S. Woo, H.G. Kim, H.I. Lee, Appl. Catal. A 281 (2005) 167.
[
28] D. Carriazo, C. Domingo, C. Martín, V. Rives, J. Solid State Chem. 181 (2008)
046.
29] A. Bordoloi, F. Lefebvre, S.B. Halligudi, J. Catal. 247 (2007) 166.
2
[
Best catalytic results are achieved with 0.025 g of [SBA/NH
2
]/
[30] D.Y. Zhao, Q.S. Huo, J.L. Feng, B.E. Chmelka, G.D. Stucky, J. Am. Soc. Chem. 120
1998) 6024.
(
HMPAW in presence of H oxidant CH Cl solvent under reflux
2
O
2
2
2
[
31] P. Karandikar, K.C. Dhanya, S. Deshpande, A.J. Chandwadkar, S. Sivasanker, M.
Agashe, Catal. Commun. 5 (2004) 69.
conditions. The hybrid catalysts are shown to be heterogeneous
and reusable and may be recovered at least for four runs without
any significant loss in activity.
[32] F. Ehrburger-Dolle, I. Morfin, E. Geissler, F. Bley, F. Livet, C. Vix-Guterl, S.
Saadallah, J. Parmentier, M. Reda, J. Patarin, M. Iliescu, J. Werckmann,
Langmuir 19 (2003) 4303.
[
33] R. Ryoo, C.H. Ko, M. Kruk, V. Antochshuk, M. Jaroniec, J. Phys. Chem. B 104
(2000) 11465.
Acknowledgment
[
[
[
34] X. Wang, K.S.K. Lin, J.C.C. Chan, S. Cheng, J. Phys. Chem. B 109 (2005) 1763.
35] A.S.M. Chong, X.S. Zhao, J. Phys. Chem. B 107 (2003) 12650.
36] T. Yokoi, H. Yoshitake, T. Tasumi, Chem. Mater. 15 (2003) 4536.
Financial support of this work by Tarbiat Modares University is
gratefully acknowledged.
[37] M.C. Burleigh, M.A. Markowitz, M.S. Spector, B.P. Gaber, Chem. Mater. 13
2001) 4760.
38] N.K. Kala Raj, S.S. Deshpande, R.H. Ingle, T. Raja, P. Manikandan, Catal. Lett. 98
2004) 217.
(
[
(
Appendix A. Supplementary material
[
[
39] A. Bordoloi, A. Vinu, S.B. Halligudi, Appl. Catal. A Gen. 333 (2007) 143.
40] R.D. Birkhoff, J.M. Heller, H.H. Hubbell, L.R. Painter, J. Chem. Phys. 74 (1981)
200.
Supplementary data associated with this article can be found, in
the online version, at doi:10.1016/j.ica.2011.05.024.
[
41] G. Kraml, G. Gritzner, J. Chem. Soc., Faraday Trans. I 81 (1985) 2875.

Z. Karimi et al. / Inorganica Chimica Acta 376 (2011) 1–9
9
[
42] R. Massart, R. Contant, J.M. Fruchart, J.P. Ciabrini, M. Fournier, Inorg. Chem. 16
1977) 2916.
43] Booth, H.C. (Ed.), Inorganic Syntheses, vol. 1, McGraw Hill, New York, 1939.
44] Y.H. Guo, C.W. Hu, J. Cluster Sci. 14 (2003) 505.
45] F. Lefebvre, J. Chem. Soc., Chem. Commun. (1992) 756.
46] V.M. Mastikhin, S.M. Kulikov, A.V. Nosov, I.V. Kozhevnikov, I.L. Mudrakovsky,
M.N. Timofeeva, J. Mol. Catal. 60 (1990) 65.
47] S. Damyanova, J.L.G. Fierro, I. Sobrados, J. Sanz, Langmuir 15 (1999) 469.
48] N.A. Stephenson, A.T. Bell, J. Mol. Catal. A 258 (2006) 231.
49] M.H. Lim, C.F. Blanford, A. Stein, Chem. Mater. 10 (1998) 467.
50] N.Y. Hea, C.S. Woo, H.G. Kim, H.I. Lee, Appl. Catal. A: General 281 (2005) 167.
[51] B.S. Lane, K. Burgess, Chem. Rev. 103 (2003) 2457.
(
[52] R. Noyori, M. Aoki, K. Sato, Chem. Commun. (2003) 1977.
[53] D.E. De Vos, B.F. Sels, P.A. Jacobs, Adv. Synth. Catal. 345 (2003) 457.
[54] J.M. Bregeault, Dalton Trans. (2003) 3289.
[55] Z. Karimi, A.R. Mahjoub, F. Davari Aghdam, Inorg. Chim. Acta 362 (2009)
3725.
[56] N. Mizuno, M. Misono, Chem. Rev. 98 (1998) 199.
[57] L. He, H. Li, W. Zhu, J. Guo, X. Jiang, J. Lu, Y. Yan, Ind. Eng. Chem. Res. 47 (2008)
6890.
[58] I.V. Kozhevnikov, Russ. Chem. Rev. 56 (1987) 811.
[59] Z. Karimi, A.R. Mahjoub, Appl. Surf. Sci. 256 (2010) 4473.
[
[
[
[
[
[
[
[