Synthesis, characterisation and catalytic activity of non-crystalline organic-inorganic hybrid material comprising Keggin-type manganese(II)- substituted phosphotungstate and salen

Article abstract of DOI:10.1080/10610278.2011.636445

A new inorganic-organic hybrid material comprising Keggin-type mono manganese-substituted phosphotungstate and salen was synthesised. The synthesised hybrid material was systematically characterised by various physicochemical techniques such as elemental

Full text of DOI:10.1080/10610278.2011.636445

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Supramolecular Chemistry
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Synthesis, characterisation and catalytic activity of
non-crystalline organic–inorganic hybrid material
comprising Keggin-type manganese(II)-substituted
phosphotungstate and salen
Ketan Patel a , Pragati Shringarpure a & Anjali Patel a
a Chemistry Department , Faculty of Science, M.S. University of Baroda , Vadodara , 390
002 , India
Published online: 02 Dec 2011.
To cite this article: Ketan Patel , Pragati Shringarpure & Anjali Patel (2012) Synthesis, characterisation and catalytic activity
of non-crystalline organic–inorganic hybrid material comprising Keggin-type manganese(II)-substituted phosphotungstate and
salen, Supramolecular Chemistry, 24:3, 149-156, DOI: 10.1080/10610278.2011.636445
To link to this article: http://dx.doi.org/10.1080/10610278.2011.636445
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Supramolecular Chemistry
Vol. 24, No. 3, March 2012, 149–156
Synthesis, characterisation and catalytic activity of non-crystalline organic–inorganic hybrid
material comprising Keggin-type manganese(II)-substituted phosphotungstate and salen
Ketan Patel, Pragati Shringarpure and Anjali Patel*
Chemistry Department, Faculty of Science, M.S. University of Baroda, Vadodara 390 002, India
(Received 25 May 2011; final version received 24 October 2011)
A new inorganic–organic hybrid material comprising Keggin-type mono manganese-substituted phosphotungstate and
salen was synthesised. The synthesised hybrid material was systematically characterised by various physicochemical
techniques such as elemental analysis, thermal gravimetric analysis, FT-IR, UV–vis, electron spin resonane, 1H NMR, 13
C
MAS NMR and 31P NMR. The catalytic activity was evaluated for non-solvent liquid phase oxidation of styrene using O2.
The novelty of the present work lies in obtaining 42% conversion with 41% selectivity for styrene oxide with O2 under
solvent-free mild reaction conditions only in 4 h.
Keywords: Keggin; polyoxometalates; manganese-substituted phosphotungstate; hybrid material; oxidation; catalysis
1. Introduction
hybrids with POMs can lead to the development of more
efficient recyclable multifunctional catalysts.
Polyoxometalates (POMs) are inorganic and discrete
multimetal clusters that have found applications in
catalysis, material sciences and medicine (1–7). Their
properties may be modified depending on the elemental
composition, structure and counteractions (1). In addition,
the presence of organic substituents anchored on the POM
surface offers the possibility to tune the electronic features
of the resulting complexes, as well as their solubility,
reactivity and hydrolytic stability (4, 8, 9). The merging of
organic and inorganic domains is a developing field of
investigation focusing on the design of new hybrid
materials (10). In this respect, surface-appended organic
moieties carrying suitable functional groups are intro-
duced to foster the extended organisation of the POM
molecular units (7, 11, 12). This strategy was successfully
employed to obtain polymerisable (13), dendrimeric (14)
and supramolecular derivatives (15, 16).
Recently, Wang et al. reported organic–inorganic
hybrid aggregates based on Anderson-type POMs and
metal – schiff base (2c). To the best of our knowledge,
only one research article involving quaternary ammonium
salt of Keggin-type silicotungstate and metallosalen has
been reported (17). The catalytic activity has also been
evaluated for hybrid compounds involving metallosalen
and silicotungstates (18). Thus, most of the work has been
reported on silicotungstates, and studies on the phospho-
tungstates are very scarce. Moreover, it has also been
observed that no literature is available on the catalytic
aspects of hybrid compounds comprising phosphotung-
states and metallosalen.
Recently, we have reported one pot synthesis of
manganese(II)-substituted phosphotungstate and its cata-
lytic activity towards liquid phase aerobic oxidation of
styrene (19). It is known that the functionalisation of POM
can lead to the formation of new hybrid material with an
improved catalytic activity. It appears to be of interest to
functionalise the Mn(II)-substituted phosphotungstate
with an organic ligand, like salen, which would result in
interesting catalytic properties.
The development of organic–inorganic hybrid
materials remains a challenge and is an emerging field.
The organic–inorganic hybrid materials are categorised
into two classes. Those formed due to the non-covalent
interactions involving hydrogen bonds or van der Waals
interactions form the class I type; those involving strong
covalent or iono-covalent bonds form the class II type of
compounds (1d).
In the present paper, an attempt was made to synthesise
a new non-crystalline organic–inorganic hybrid material,
of class I type, comprising Keggin-type manganese-
substituted phosphotungstate and salen. The synthesised
hybrid compound was systematically characterised by
elemental analysis, thermal gravimetric analysis (TGA),
FT-IR, UV–vis, electron spin resonance (ESR), multi-
nuclear NMR (1H, 13C MAS and 31P NMR). The catalytic
The synthesis of hybrid materials can result in the
formation of non-crystalline or crystalline materials that
are isolated under mild conditions, following the room
temperature synthesis or under hydrothermal conditions.
In the field of catalysis, the study of organic–inorganic
*Corresponding author. Email: aupatel_chem@yahoo.com
ISSN 1061-0278 print/ISSN 1029-0478 online
q 2012 Taylor & Francis
http://dx.doi.org/10.1080/10610278.2011.636445
http://www.tandfonline.com

150
K. Patel et al.
CHO
N
N
OH
Reflux
2 h
H2N
HO
NH2
+
OH
Ethylene diamine
1 mole
Salicyaldehyde
2 moles
Salen
Scheme 1. Synthesis of salen.
activity of the synthesised complex was evaluated for
non-solvent liquid phase oxidation of styrene using
molecular oxygen.
was dissolved in a minimum amount of absolute alcohol.
The alcoholic solution of S was added dropwise to the
aqueous solution of PW11MnO39. The resultant mixture
was refluxed for 10 h with constant stirring, filtered to
obtain pale yellow powder. The isolated material was
washed with water as well as ethanol and dried at room
temperature (35–408C). The obtained hybrid material
(yield 0.88 g) was designated as PW11MnO39–S.
2. Experimental section
2.1 Materials
All chemicals used were of A.R. grade. Sodium tungstate
dihydrate, disodium hydrogen phosphate, sodium hydrox-
ide, MnCl2·4H2O, caesium chloride, acetone, ethylene
diamine, salicyaldehdyde and styrene were obtained from
Merck (Bombay, India).
The values of elemental analysis data for PW11Mn–S
are in good agreement with the theoretical values. Anal.
calcd: Cs, 14.32; W, 54.50; P, 0.84; Mn, 1.48; O, 21.98; C,
5.1; H, 0.43; N, 0.75. Found: Cs, 14.28; W, 54.45; P, 0.81;
Mn, 1.47; O, 21.49; C, 5.0; H, 0.43; N, 0.74.
2.2 Synthesis of salen
2.4 Synthesis of manganese(III) salen
The salen ligand (Schiff base) was synthesised followed by
the reported method (20), by refluxing the ethanolic
solution of the ethylenediamine and salicylaldehyde for 1 h.
The isolated yellow solid was designated as S (Scheme 1).
The Mn(III) salen was synthesised by the method reported
by Freire et al. (20). The observed values for S are in good
agreement with the theoretical values. Anal. calcd: C,
71.64; H, 5.97; N, 10.44. Found: C, 71.53; H, 5.94; N,
10.40.
2.3 Synthesis of inorganic–organic hybrid material
The synthesis of the hybrid material was carried out as
shown in Scheme 2. The PW11MnO39 was synthesised by
using the method as reported by us (19). The caesium salt
of PW11MnO39 (0.8985 g, 0.04 mmol) was dissolved in a
minimum amount of water, and S (0.067 g, 0.04 mmol)
2.5 Synthesis of physical mixture of PW11MnO39 and
salen
The physical mixture was synthesised by mixing
equimolar amount of PW11MnO39 and salen. Salen
N
N
OH
HO
N
N
OH HO
Water/Ethanol
reflux, 10 h
PW11MnO39
PW11MnO39–S
Scheme 2. Synthesis of PW11MnO39–S.

Supramolecular Chemistry
151
(0.067 g, 0.04 mmol) and the caesium salt of PW11MnO39
(0.8985 g, 0.04 mmol) were mixed and grinded properly in
a mortar and pestle till it becomes homogeneous in nature.
The obtained mixture was designated as PW11MnþS.
thermal techniques, the chemical formula of the isolated
hybrid material is proposed as Cs5[PW11O39Mn
(H2O)]·9H2O–(S) (PW11MnO39–S).
The FT-IR stretching vibration for S, PW11MnO39 and
PW11MnO39–S are presented in Table S1 (Supplementary
Information, available online). PW11MnO39 shows
stretching vibration at 1078, 1053, 956, 882 and
820 cm21 corresponding to PZO, WvO and WZOZW
2.6 Characterisation
Elemental analysis was carried out using JSM 5610 LV
combined with INCA instrument for EDX-SEM analyser
for the quantitative identification of metal ions. C, H and N
analyses were carried out using PerkinElmer 2400. The
total weight loss was calculated by the TGA method on the
Mettler Toledo Star SW 7.01 upto 6008C. FT-IR spectra of
the samples were recorded as the KBr pellet on the
PerkinElmer instrument. The UV–visible spectrum was
recorded at an ambient temperature on PerkinElmer 35
LAMDA instrument using the 1 cm quartz cell. The ESR
spectra was recorded on a Varian E-line Century series
X-band ESR spectrometer (liquid nitrogen temperature
and scanned from 2000 to 3200 Gauss). 1H solution NMR
was recorded in DMSO on Varian Mercury plus 300
instruments. 13C MAS NMR was recorded on Bruker DSX
300 MHz instrument. 31P solution NMR was recorded in
D2O on Bruker ACF 300 MHz instrument.
(21–23). The Dy difference of PZO stretching is 24 cm21
,
which is the typical Dy split value for Mn-substituted
phosphotungstate, indicating the presence of Mn in the
octahedral environment (19, 22, 23). The FT-IR spectrum
of PW11MnO39–S shows that all the stretching vibrations
correspond to PZO, WvO and WZOZW without any
significant shift except in the WZOZW bridges. These
was a slight shift from 820 to 813 cm21 observed in case of
PW11MnO39–S.
In addition to these vibrations, FT-IR spectrum can be
helpful in obtaining structural information concerning the
organic groups. A pair of the weak peaks at 1496 and
1460 cm21 was obtained for S and PW11MnO39–S, which
is attributed to both symmetric- and asymmetric-stretching
vibrations of aliphatic CZH bonds. Furthermore, the
stretching vibration of PW11MnO39–S for CZN, CvN
and aliphatic as well as for aromatic CH2 region can also
be observed. The results ensure a successful functionalisa-
tion of PW11MnO39 with salen. At the same time it has
also been observed that there is a shift in the peak position
from 1525 to 1545 cm21 corresponding to the CZO
stretching vibration. This interesting observation indicates
some interaction of salen with the PW11MnO39. It is well
known that the POMs is polydentate ligand with high
negative charge, which may be favoured by the formation
of hydrogen bond. Hence, in the present case, it may be
possible that the salen ligand gets bind to the large cluster
of POMs via hydrogen bond.
2.7 Catalytic activity
The catalytic activity was evaluated for the oxidation of
styrene using molecular oxygen as oxidant and tert-butyl
hydro peroxide (TBHP) as co-oxidant. Oxidation reaction
was carried out in a batch-type reactor operated under
atmospheric pressure. In a typical reaction, a measured
amount of catalyst was added to a three-necked flask
containing styrene and initiator TBHP (0.2 ml) at 808C.
The reaction was started by bubbling O2 into the liquid
with constant stirring on magnetic plate. The reaction was
carried out by varying different parameters such as the
amount of the catalyst and reaction time. After completion
of the reaction, the liquid product was extracted with
dichloromethane, dried with magnesium sulphate and
analysed on a gas chromatograph (Nucon 5700 model)
having a flame ionisation detector and an SE-30 packed
column (2 m length and 10% silica-based stationary
phase). Product identification was done by comparison
with authentic samples and finally by a combined gas
chromatography mass spectrometer (Hewlett Packard)
using an HP-1 capillary column (30 m, 0.5 mm ID) with EI
(70 eV).
The UV–vis spectra of Mn(III)-salen (MnS) and
PW11MnO39–S are shown in Figures 1 and 2. It is well
known that metalation of salen ligand with Mn(II) yields
the corresponding MnS complex distinctly identified by
UV–vis as well as by colour (brown) (18). As seen from
the UV–vis spectra for MnS (Figure 1), two absorption
bands were observed at 500 and 420 nm corresponding to
the typical d–d transition band for MnS complex,
indicating the presence of Mn(III) in the square pyramidal
environment/geometry. Whereas the UV–vis spectrum for
PW11MnO39–S (Figure 2) shows two absorption bands,
one strong at 291 nm corresponding to the [PW11O39]72
species. The other broad and weak absorption band at
around 393 nm, associated with d–d transition, which is a
typical value for Mn(II). The absence of any absorption
bands at 420 and 500 nm, which is the typical region for
MnS complex, indicates that salen moiety is not covalently
coordinated to the Mn centre. The presence of Mn(II)
species was further confirmed by ESR spectroscopy.
3. Results and discussion
The number of water molecules calculated from TGA
curve shows 4.6% weight loss corresponding to loss of 10
water molecules. From the elemental as well as the

152
K. Patel et al.
Table 1. 13C MAS NMR chemical shifts for PW11MnO39–S
and S.
0.32
0.28
0.24
0.20
0.16
0.12
Chemical shift (ppm)
PW11MnO39–S
Functional group
S
CH2 (bridge carbon)
CH2 (ring carbon)
54.81
121.53
124.4
135.08
171.52
166.51
57.82
116.22
119.20
130.84
161.91
165.91
(a) MnS
CZO
CvN
3.1 Magnetic spectroscopy
It is known that the presence of paramagnetic metal centre
generally results in a poorly defined NMR spectrum. As
mentioned earlier, for paramagnetic compounds electronic
relaxation is fast, good ESR spectrum cannot be obtained
at room temperature, while good NMR spectrum can be
obtained at room temperature.
400
450
500
550
600
650
λ (nm)
Figure 1. UV–vis spectra of MnS.
Paramagnetic compounds have different relaxation
times. If the electronic relaxation is slow, good ESR
spectra can be obtained at room temperature while, if the
electronic relaxation is fast, good ESR spectra can be
obtained only at low temperature. Mn(III)-salen com-
pounds are known to be ESR silent in the normal X-band
mode but absorb at g ¼ 8 in the high-field ESR
spectroscopy (18). If salen is covalently coordinated to
Mn, ESR is expected to be silent. However, a well resolved
six line hyperfine spectrum for PW11MnO39–S indicating
the presence of Mn (II) species.
In order to study the interaction between PW11MnO39
species and the salen moiety, solution (1H and 31P) and
1
solid (13C) NMR spectra were recorded. The H NMR
spectrum of PW11MnO39–S and S was recorded in D2O
and CDCl3 respectively. The chemical shifts for all
protons of the organic moiety (i.e. salen) in PW11MnO39–
S did not show any significant shift as compared to
S. However, broadening of the signal in case of
PW11MnO39–S is observed. If the Mn centre forms the
covalent bond with the salen moiety, significant chemical
shift is expected. The absence of any shift in the signals
indicate that S is not coordinate to Mn but is present in the
chemical environment of PW11MnO39 as an individual
moiety.
Thus UV–vis and ESR studies indicate the presence of
salen in the synthesised material and support the
observation drawn from FT-IR, i.e. the formation of the
hydrogen bond.
The chemical shift of the 13C MAS NMR values for S
and PW11MnO39–S is presented in Table 1. As shown in
Table 2, the ring C atoms (ZCH2Z) as well as CZOH
were significantly shielded and appeared upfield with
respect to the salen moiety. A very significant shifting
from þ171.5 to þ161.9 ppm can be attributed to the shift
2.0
0.32
0.28
1.6
1.2
(b) PW11MnO39–S
0.24
0.20
0.16
0.12
Table 2. Effect of amount of PW11MnO39–S on oxidation of
styrene.
Selectivity
(%)
0.8
400 450 500 550 600 650
λ (nm)
Amount of catalyst (mg)
Conversion (%)
BA
StyO
0.4
15
25
50
75
100
200
15
42
46
49
52
55
60
59
84
89
93
91
40
41
16
11
7
0.00
280
350
400
λ (nm)
450
500
550
9
Note: Styrene, 100 mmol; oxidant, O2 (1 atm); TBHP, 0.15 mmol; reaction time,
4 h; temperature, 808C.
Figure 2. UV–vis spectra of PW11MnO39–S.

Supramolecular Chemistry
153
of the electron density from OZH of salen to the bridging
O of PW11MnO39. It is known that for transition metal-
substituted POMs, the bridging O atom is expected to
generate basic sites (1) and as a consequence they attract
the protonated species towards them.
A neat reaction (without catalyst) was carried out and it
showed no conversion for the substrate, indicating that
there is no auto-oxidation taking place.
In the present case, OZH of the S is attracted towards
oxygen atoms of the PW11MnO39 and form hydrogen
bond. Hence upfield shifts for the C atom of the CZOH are
observed. In addition to this, NMR signal of the CvN
does not show any shift from the parent value. This
indicates that the N atom of the salen does not coordinate
with the Mn centre of PW11MnO39.
Upon comparison of the 1H and 13C NMR spectrum of
PW11MnO39–S, it was observed that the proton of the
CZOZH group is not deprotonated. This observation
indicates that the S moiety is not coordinated with the Mn
of the PW11MnO39, but is intactly attracted towards
PW11MnO39 via some chemical interaction.
3.3 Effect of amount of catalyst
The effect of the amount of catalyst on the conversion of
styrene is represented in Table 2. It is seen from the table
that the conversion increases up to 75 mg, after that it stays
almost constant.
The non-polar molecules such as hydrocarbons just
adsorb on the surface without entering the bulk. Thus, on
further increase in the amount, there may be blocking of
the active sites and thus increase in conversion is not
significant. This shows that it follows the adsorption
phenomenon rather than the typical pseudo-liquid
behaviour. In the present catalytic system, as the
concentration of the catalyst increases the number of
active sites available for the reaction to progress increases
which results in a difference in the selectivity of products.
On further increase in the concentration of the catalyst the
distribution of selectivity of products towards the more
stable product (i.e. benzaldehyde, BA). Due to the known
importance of epoxide, the amount of the catalyst was
optimised at 25 mg (24). Furthermore, the effect of
reaction time on oxidation of styrene was also studied.
From the above study, it may be proposed that salen is
present in the complex via H-bonding. The H of CZOZH
from salen forms hydrogen bond with the bridging O of the
Mn of the PW11MnO39 without disturbing the Mn-
substituted Keggin structure. The presence of intact
Keggin unit is further supported by 31P solution NMR
(Figure 3). A strong single-line 31P NMR spectrum
d ¼ 213.7 ppm was obtained. The value of 213.7 ppm is
the typical region for the mono metal-substituted Keggin-
type POM, indicating the presence of intact Keggin unit.
Thus spectral as well as the magnetic studies confirm
the presence of strong H-bonding between S and
PW11MnO39.
3.4 Effect of reaction time
It is seen from Table 3 that with the increase in reaction
time the conversion also increases. Initially, the increase in
the conversion is fast, and after 16 h a slow increase in the
conversion is observed. This may be due to the fact that
the alkene is consumed during the reaction, as a result the
amount of the reactant decreases, which then requires time
to reversibly bind with the oxidant. Furthermore, the rate
of desorption of the products formed from the catalyst
surface is faster, as a result the overall rate of the reaction
slows down resulting in a slow increase in the conversion
with time (24).
3.2 Catalytic activity
A detailed study on the oxidation of styrene with O2 and
TBHP (co-oxidant) was carried out to optimise conditions.
Table 3. Effect of reaction time on oxidation of styrene over
PW11MnO39–S.
Selectivity (%)
Reaction time (h)
Conversion (%)
BA
StyO
2
4
8
12
16
20
24
25
42
62
71
84
89
100
50
59
79
83
95
.99
.99
50
41
21
17
5
–
–
Note: Styrene, 100 mmol; oxidant, O2 (1 atm); TBHP, 0.15 mmol; reaction time,
4 h; temperature, 808C.
Figure 3. 31P solution NMR of PW11MnO39–S.

154
K. Patel et al.
As a result the overall rate of the reaction slows down
process. Hence, the overall reaction becomes a controlled
reaction, resulting in the formation of styrene oxide.
Furthermore, the almost same conversion as well as
selectivity towards BA for PW11MnO39þS and PW11Mn
indicates the presence of salen as a separate moiety. Thus,
for PW11MnO39þS, the salen moiety is free and not
attached to PW11MnO39 via chemical interaction, as a
result the Mn(II) active sites are easily available for
oxidation which results in a single selective product with
almost the same percentage conversion as that of
PW11MnO39.
and results in a slow increase in the percentage conversion
with time. The distribution of the product changes with
increase in the reaction time. Initially, after the completion
of 2 h, the equimolar of both products was observed. As
the reaction time increases the product selectivity shifts
towards BA. With an increase in the reaction time the
unstable intermediate, epoxide, is converted to the more
stable product BA. Due to the known industrial importance
of styrene oxide, the reaction time was optimised at 4 h.
The above data clearly indicates that the formed
hydrogen-bonded molecular assembly (formed by salen
and PW11MnO39) is the real active species responsible for
selectivity towards styrene oxide.
3.5 Effect of salen
Oxidation of styrene involves the formation of styrene
oxide and BA. It is known that if the oxidation reaction is
fast the product selectivity shifts towards the more stable
product (BA) rather than the less stable intermediate
(styrene oxide). While if the reaction is slow and
controlled, one styrene oxide is obtained as a major
product.
As seen from Table 4, PW11MnO39–S gives higher
conversion as compared to MnS, while better selectivity
towards epoxide than PW11MnO39. It is also observed
from Table 3 that the hybrid material gives 100%
conversion with single selective product in 24 h. So the
present catalyst is superior as compared to MnS and
PW11MnO39. Thus, there is a choice for the chemists to
select the reaction, with 100% conversion and single
selective product or with 42% conversion and 41%
selectivity towards styrene oxide.
In order to see the effect of salen and to confirm the
chemical interaction, the catalytic activity of PW11MnO39,
MnS and the physical mixture (PW11Mn þ S) was
evaluated for oxidation of styrene under the optimised
conditions of PW11Mn–S. The obtained results are
represented in Table 4.
As seen from Table 4, in case of MnS, only 24%
conversion with 71% selectivity for styrene oxide and 29%
selectivity for BA was obtained in 4 h (Table 4). However,
61% conversion with single selective product (i.e. BA)
was obtained when PW11MnO39 was used as a catalyst in
4 h. On the other hand, moderate conversion of 42% was
obtained for PW11MnO39–S with 41% selectivity towards
styrene oxide and 59% selectivity towards BA (Table 4).
The difference in the conversion as well as selectivity for
the hybrid material can be explained in the basis of
S. Mn(II) sites are the catalytic active sites which are
responsible for effective oxidation of styrene. In case of
PW11MnO39–S, these active sites Mn(II) are surrounded
by S moiety. As a result, the Mn(II) centre becomes
sterically hindered. This steric hindrance around the active
metal centre leads to the partial prevention of the oxidation
3.6 Test for leaching and heterogeneity
Any leaching of the active species from the support makes
the catalyst unattractive and hence it is necessary to study
the stability as well as leaching of PW11Mn–S. The
catalyst was filtered after the completion of the reaction
and the filtrate was characterised for UV–vis spec-
troscopy. For comparison, UV–vis spectra of
PW11MnO39–S in water were also recorded (Figure 4).
The absence of characteristic peaks in filtrate (Figure 5)
indicates that there is no leaching of PW11MnO39–S, and
the catalyst remains completely insoluble under reaction
condition and could be reused.
Furthermore, for the rigorous proof of heterogeneity, a
test was carried out by a filtering catalyst from the reaction
mixture at 808C after 2 h and the filtrate was allowed to
Table 4. Comparative data for oxidation of styrene using different catalysts.
Selectivity (%)
Amount of
catalyst (mg)
Amount of active species,
i.e. PW11Mn (mg)
Catalysta
Conversion (%)
BA
StyO
71
MnS
23
23
25
25
23
23
23
23
24
58
42
56
29
.99
59
PW11MnO39
PW11MnO39–S
PW11MnO39þS
41
.99
Notes: a Styrene, 100 mmol; oxidant, O2 (1 atm); TBHP, 0.2 ml; reaction time, 4 h; temperature, 808C; BA, benzaldehyde; StyO, styrene oxide.

Supramolecular Chemistry
155
very efficient. There is difficulty in the collection of
catalyst due to the loss during filtration. But if a large
amount of catalyst is used, the recycling of the catalyst was
significant. The recycle catalyst shows a decrease in the
conversion value upto 3%, whereas the distribution of the
products remains the same.
2.0
1.6
0.32
0.28
0.24
0.20
0.16
0.12
(b) PW11MnO39–S
1.2
4. Conclusions
The present paper reports synthesis of a new inorganic–
organic hybrid material. Spectral and magnetic studies
show that the Keggin unit retains its structure even after
the introduction of S. The heterogeneous nature of the
present hybrid material makes it attractive for the
oxidation of styrene. The superiority of the present
catalyst lies in obtaining 41% selectivity for styrene oxide
using molecular oxygen in a short period of time (4 h). The
catalyst was regenerated and reused up to three cycles after
simple workup. The present catalyst is multifunctional.
0.8
400 450 500 550 600 650
λ (nm)
0.4
0.00
280
350
400
λ (nm)
450
500
550
Figure 4. UV–vis spectra of PW11MnO39–S.
react up to 4 h. The reaction mixture was kept for 2 h and
filtrate was analysed on gas chromatogram. No change in
percentage conversion as well as percentage selectivity
was found. On the basis of the results, it can be concluded
that there is no leaching of the PW11MnO39–S, and the
present catalysts are truly heterogeneous in nature and fall
into the category C (25).
Acknowledgements
We are thankful to Indian Institute of Science (IISc), Bangalore,
for recording 13C MAS NMR spectra. Mr Ketan Patel and Ms
Pragati Shringarpure are thankful to Department of Science and
Technology (SR/S1/IC-13/2007), New Delhi, and Council of
Industrial and Scientific Research (CSIR, New Delhi) for
providing financial support.
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