Mono lacunary phosphomolybdate supported on MCM-41: Synthesis, characterization and solvent free aerobic oxidation of alkenes and alcohols

Article abstract of DOI:10.1039/c3dt52395k

A new catalyst comprising monolacunary phosphomolybdate and MCM-41 was synthesized and characterized by different physicochemical techniques. The catalytic activity was evaluated by carrying out solvent free aerobic oxidation of alkenes and alcohols. The

Full text of DOI:10.1039/c3dt52395k

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Mono lacunary phosphomolybdate supported on
MCM-41: synthesis, characterization and solvent
free aerobic oxidation of alkenes and alcohols
Cite this: Dalton Trans., 2014, 43,
2512
Nilesh Narkhede, Anjali Patel* and Sukriti Singh
A new catalyst comprising monolacunary phosphomolybdate and MCM-41 was synthesized and charac-
terized by different physicochemical techniques. The catalytic activity was evaluated by carrying out
solvent free aerobic oxidation of alkenes and alcohols. The catalyst showed 60% conversion of styrene
and 28% conversion of benzyl alcohol. The superiority of the present catalytic system lies in obtaining
better conversion under solvent free and aerobic conditions.
Received 30th August 2013,
Accepted 14th November 2013
DOI: 10.1039/c3dt52395k
www.rsc.org/dalton
are more efficient catalysts among the Keggin series for oxi-
dation reactions. In the case of POMs, the dependence of
Introduction
Catalyst development is no longer looked upon simply in redox potentials on elemental composition is dictated primar-
terms of optimizing atom and energy efficiencies, but as a ily by the presence or absence of the most oxidizing skeletal or
clean technology. Different aspects of the overall process “addenda” atom or early transition metal ions, in the present
design, such as choice of solvent-free/green solvent operation case, molybdenum. Tuning of these properties of phos-
and methods of catalyst separation and waste disposal, must phomolybdates can be accomplished at atomic/molecular
be considered. Oxidation reactions of alkenes and alcohols are levels by formation of lacunary species.4 The lacunary phos-
some of the most important reactions in organic chemistry.1 phomolybdate can be formed by the removal of one or more
They give a variety of products such as carbonyl compounds, molybdenum–oxygen (Mo–O) octahedral moieties from the
epoxides, diols, and products (aldehydes). The commercial saturated Keggin anion framework which increases the anionic
synthesis of such compounds often proceeds via oxidation by charge of the complex.4
toxic or hazardous stoichiometric oxidants such as chromates,
In the literature there are a few reports focusing on the syn-
permanganates and peroxides.2 In addition to safety consider- thesis of mono lacunary phosphomolybdate; however, not
ations, such technologies are also atom inefficient due to poor much attention has been devoted to this field due to the
selectivity and/or additional separation and waste treatment known difficulty in isolation, and poor thermal as well as
steps to isolate the product, and therefore economically kinetic stability. It was hence a challenging task to stabilize
disadvantageous.
these species onto a suitable support. Our group reported the
Solid catalysts able to utilise O2 as the oxidant can circum- isolation of the sodium salt of lacunary phosphomolybdate for
vent these limitations to afford alternative, continuous aerobic the first time and successfully stabilized the species onto
processes, with associated safety, economic, and environ- hydrous zirconia5 and alumina6 and evaluated its catalytic
mental benefits, and represent an elegant class of atom- activity in solvent-free oxidation of styrene and benzyl alcohol.
efficient molecular transformations. Considering the increas-
Thus the prospect of supporting these species on metal
ing environmental problems, it is highly desirable to use mole- oxide supports emerges as an attractive pathway to exploit the
cular oxygen in place of traditional corrosive, toxic and catalytic activity and opened the opportunity to create more
expensive oxidants.
active and selective catalytic systems. Therefore, as an exten-
In this context, polyoxometalates (POMs), especially sion of the previous work, it was thought of interest to use a
phosphomolybdates, have been gaining importance due to their mesoporous support. In the present paper, for the first time,
excellent redox properties.3 It is known that phosphomolybdates we report the synthesis and characterization of mono lacunary
phosphomolybdate supported on MCM-41 as well as its use as
a catalyst for carrying out aerobic oxidation of alkenes and
alcohols.
Polyoxometalate and Catalysis Laboratory, Department of Chemistry, Faculty of
A series of catalysts containing 10–40% of mono lacunary
phosphomolybdate (PMo11) and MCM-41 were synthesized.
Science, The M. S. University of Baroda, Vadodara, 390002, India.
E-mail: aupatel_chem@yahoo.com; Tel: +91-265 2795552
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The 30% loaded catalyst was characterized by various thermal, designated as (PMo11)1/MCM-41, (PMo11)2/MCM-41, (PMo11)3/
spectral, and surface techniques. The catalytic activity was eval- MCM-41, and (PMo11)4/MCM-41, respectively.
uated for the aerobic oxidation of alkenes and alcohols.
Styrene and benzyl alcohol were taken as model substrates. Characterization
The conditions for maximum conversion as well as selectivity
for the desired product were optimized by varying different
Elemental analyses were carried out using a JSM 5910 LV com-
bined with an INCA instrument for EDX-SEM. TGA was carried
parameters such as the % loading of catalyst, amount of the
out on the Mettler Toledo Star SW 7.01 up to 600 °C. Adsorp-
catalyst, reaction time, and reaction temperature. A catalytic
tion–desorption isotherms of samples were recorded on a
Micromeritics ASAP 2010 surface area analyser at −196 °C.
property for the recycled catalyst was evaluated for the oxi-
dation of styrene and benzyl alcohol under optimized
Specific surface area was calculated using the BET method
conditions.
from the adsorption–desorption isotherms. FT-IR spectra were
obtained by using the KBr wafer on the Perkin Elmer instru-
ment. The Raman spectra were recorded on a FT-Raman Spec-
Experimental
Materials
trophotometer Model Bruker FRA 106. The 31P NMR spectra
were recorded at 121.49 MHz using a 7 mm rotor probe with
85% phosphoric acid as an external standard. The spinning
rate was 5–7 kHz. Catalyst samples after treatment were kept in
a desiccator over P2O5 until the NMR measurement. The XRD
pattern was obtained by using a PHILIPS PW-1830. The con-
ditions were Cu Kα radiation (1.54 Å) and scanning angle from
0° to 60°. SEM was carried out using a JSM 5610 LV combined
with an INCA instrument for EDX-SEM.
All chemicals used were of A.R. grade. Sodium molybdate,
anhydrous disodium hydrogen phosphate, acetone, nitric acid,
CTAB (cetyltrimethyl ammonium bromide), TEOS (tetraethyl
orthosilicate), and sodium hydroxide were obtained from
Merck and used as received.
Synthesis of the support
MCM-41 was synthesized by following a method reported by Catalytic reaction
us.7 Surfactant (CTAB) was added to a very dilute solution of
The oxidation reaction was carried out in a batch-type reactor
NaOH with stirring at 60 °C. When the solution became homo-
geneous, TEOS was added dropwise, and the obtained gel was
aged for 2 h. The resulting product was filtered, washed with
distilled water, and then dried at room temperature. The
obtained material was calcined at 555 °C in air for 5 h and
designated as MCM-41.
operated under atmospheric pressure. In a typical reaction, a
measured amount of catalyst (0.227 wt% for styrene and
0.231 wt% for benzyl alcohol) was added to a two-necked flask
containing substrates (100 mmol) at 80 °C for styrene and
90 °C for benzyl alcohol. The trace amount of (0.2 mol%) tert-
butyl hydrogen peroxide (TBHP) was added to the reaction in
order to initiate the reaction. The reaction was started by bub-
bling O2 into the liquid. The reaction was carried out by
Synthesis of mono lacunary phosphomolybdate (PMo11
)
The mono lacunary phosphomolybdate (Na7[PMo11O39]·16H2O) varying different parameters such as amount of the catalyst,
was synthesized using our previously reported method.5 reaction time and reaction temperature. After completion of
Sodium molybdate dihydrate (0.22 mol, 5.32 g) and anhydrous the reaction, the catalyst was removed and the products were
disodium hydrogen phosphate (0.02 mol, 0.28 g) were extracted with dichloromethane and analysed on a gas chro-
dissolved in 50–70 mL of conductivity water and heated to matograph (GC) using an Rtx-5 capillary column. The product
80–90 °C followed by the addition of concentrated nitric was identified by comparison with the authentic samples and
acid in order to adjust the pH to 4.3. The volume was then finally by gas chromatography-mass spectroscopy (GC-MS).
reduced to half by evaporation and the product was separated
by liquid–liquid extraction with 50–60 mL of acetone. The Leaching test
extraction was repeated until the acetone extract showed the
Heteropoly acids can be quantitatively characterized by the het-
−
absence of NO3 ions (ferrous sulfate test). The extracted
eropoly blue colour, which is observed when it is reacted with
sodium salt was dried in air. The obtained material was desig-
a mild reducing agent such as ascorbic acid. In the present
nated as PMo11. Analytical calculated (%): Na, 7.65; Mo, 50.12;
study, this method was used for determining the leaching of
P, 1.47; O, 39.52. Found (%): Na, 7.60; Mo, 49.99; P, 1.44;
O, 39.92.
PMo11 from the support. Standard samples containing 1–5%
of PMo11 in water were prepared. To 10 mL of the above
samples, 1 mL of 10% ascorbic acid was added. The mixture
was diluted to 25 mL. The resulting solution was scanned at a
Synthesis of the catalyst (anchoring of PMo11 to MCM-41)
A series of catalysts containing 10–40% of PMo11 anchored to λmax of 785 cm−1 for its absorbance values. A standard cali-
MCM-41 were synthesized by the impregnation method. One bration curve was obtained by plotting values of absor-
gram of MCM-41 was impregnated with an aqueous solution bance against % concentration. 1 g of supported catalyst
of PMo11 (0.1/10–0.4/40 g mL−1 of double distilled water) and with 10 mL water was refluxed for 18 h. Then 1 mL of the
dried at 100 °C for 10 h. The obtained materials were supernatant solution was treated with 10% ascorbic acid.
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Development of blue colour was not observed, indicating that
there was no leaching. The same procedure was repeated with
styrene and benzyl alcohol and the filtrate of the reaction
mixture after completion of the reaction in order to check the
presence of any leached PMo11.
Table 1 Infrared absorption bands for PMo12 and PMo11
POMs
PMo12
Bands
Wavenumber (cm−1
)
P–O
1070
965
870, 790
1048, 999
935
MovO
Mo–O–Mo
P–O
MovO
Mo–O–Mo
PMo11
906, 855
Results and discussion
Catalyst characterization
EDX elemental analysis performed on the catalyst was consist-
ent with theoretical expected values (theoretical: P = 0.33%,
Mo = 11.23%; observed: P = 0.37%, Mo = 12.4%).
The TGA of PMo11 (Fig. 1) shows the initial weight loss of
16% from 30 to 200 °C. This may be due to the removal of
adsorbed water molecules. The final weight loss of 1.8% at
around 275 °C indicates the loss of water of crystallisation. The
TGA of (PMo11)3/MCM-41 (Fig. 1) shows that initial weight loss
up to 150 °C may be due to the removal of adsorbed water
molecules. No significant loss occurs up to 350 °C, which indi-
cates the stability of the catalyst up to 350 °C.
The removal of the Mo–O moiety from the parent 12-molyb-
dophosphoric acid (PMo12) leads to the thermally less stable
material PMo11. The thermal stability values of PMo11 and sup-
ported PMo11 are 305 °C and 350 °C respectively, which are
comparatively lower than the parent PMo12 (430 °C) and the
supported PMo12 (475 °C).8
Fig. 2 FT-IR of (a) PMo11, (b) MCM-41 and (c) (PMo11)3/MCM-41.
The decrease in the specific surface area for (PMo11)3/
MCM-41 (485 m2 g−1) as compared to that of MCM-41 (659 m2 g−1
is as expected and is the first indication of chemical inter-
action between available surface oxygen of PMo11 and the
proton of the silanol group of MCM-41.
The comparison of FT-IR bands of PMo12 and PMo11 is
shown in Table 1. The P–O stretching band (1070 cm−1) for
PMo12 splits into two new bands 1048 and 999 cm−1 in PMo11.
)
presence of these bands confirms the formation of lacunary
PMo11.9
The FT-IR spectrum of MCM-41 (Fig. 2) shows a broad band
around 1300–1000 cm−1 corresponding to νas(Si–O–Si). The
bands at 801 and 458 cm−1 are due to symmetric stretching
and bending vibration of Si–O–Si, respectively. The band at
966 cm−1 corresponds to νs(Si–OH). The FT-IR spectra of
PMo11 and (PMo11)3/MCM-41 are presented in Fig. 2. The
bands at 1048 and 999 cm−1, 935 and 906 cm−1 and 855 cm−1
in the parent PMo11 are attributed to asymmetric stretches of
P–O, MovO and Mo–O–Mo, respectively, and are in good
agreement with reported values.9 The FT-IR of (PMo11)3/
MCM-41 (Fig. 2) shows bands at 965 cm−1 and 918 cm−1
assigned to P–O and MovO stretches, respectively. The shift
in the bands is an indication of strong chemical interaction
between PMo11 and the silanol groups of MCM-41. Further it
is confirmed by FT-Raman as well as 31P-NMR.
This is due to the lowering in the symmetry from Td (PMo12
)
to Cs (PMo11) around the central heteroatom, phosphorus. The
FT-Raman spectra of PMo11 and (PMo11)3/MCM-41 are dis-
played in Fig. 3. The FT-Raman spectrum of PMo11 shows
typical bands at 947 (νs(Mo–Od)), 932 (νas(Mo–Od)), 891 and
550 (νas(Mo–Ob–Mo)), 355 (νas(Oa–P–Oa)) and 217 (νs(Mo–Oa)),
where Oa, Ob, Oc, and Od are attributed to the oxygen atoms
connected to phosphorus, to oxygen atoms bridging two mol-
ybdenums (from two different triads for Ob and from the same
triad for Oc), and to the terminal oxygen MovO, respectively.9
The FT-Raman spectrum of (PMo11)3/MCM-41 shows the reten-
tion of all the characteristic bands at 916 (νs(Mo–Od)), 876
Fig. 1 TGA curves of PMo11 and (PMo11)3/MCM-41.
2514 | Dalton Trans., 2014, 43, 2512–2520
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Fig. 5 Powder XRD of (a) PMo11, (b) MCM-41 and (c) (PMo11)3/MCM-41.
Fig. 3 FT-Raman spectra of (a) PMo11 and (b) (PMo11)3/MCM-41.
of a chemical interaction between the hydrogen of the surface
silanol groups of MCM-41 and PMo11.
(νas(Mo–Od)), 793 and 548 (νas(Mo–Ob–Mo)), 323 (νas(Oa–P–Oa))
and 140 (νs(Mo–Oa)), which indicates that the structure of
PMo11 has been retained after anchoring to the support.
However, a large shift was observed for all characteristic bands
indicating the presence of very strong interaction between the
oxygen of PMo11 and the hydrogen of the silanol group of
MCM-41.
The low angle XRD of MCM-41 displays an intense diffrac-
tion peak at 2θ = 2.43° and two peaks between 2θ = 3.2–4.8°,
which are assigned to the lattice faces (100), (110), and (200),
respectively, suggesting a hexagonal symmetry of MCM-41
(Fig. 5). No separate characteristic peak of the crystalline phase
of PMo11 was observed in (PMo11)3/MCM-41 which indicates
that PMo11 is finely dispersed inside the hexagonal channels
of MCM-41.
The 31P MAS NMR spectrum of PMo11 (Fig. 4) shows a peak
at 1.64 ppm corresponding to PMo11. This is in good agree-
ment with the previously reported one.5 The catalyst shows a
single peak at −1.77 ppm which indicates the presence of
undegraded PMo11 in the catalyst. The shifting in the peak for
the catalyst is due to the interaction of PMo11 species with the
support. It is reported that the lower shift, i.e. deshielding,
increases as the degree of adsorption and degree of fragmenta-
tion increase.10 In the present case, the observed chemical
shift, shielding, indicates the presence of chemical interaction
between PMo11 and MCM-41, rather than simple adsorption.
Also, a single peak in (PMo11)3/MCM-41 indicates that no frag-
mentation of PMo11 species takes place after incorporation to
the support. Thus it can be concluded that PMo11 remains
intact inside the channels of MCM-41, as well as the presence
The SEM images of MCM-41 and the catalyst are shown in
Fig. 6. The surface morphology of the support is retained in
the catalyst. This indicates no aggregates of PMo11 on the
surface of the catalyst as well as uniform dispersion of PMo11
inside the channels of the support.
The FT-IR and 31P MAS NMR shows that PMo11 remains
intact even after anchoring to the support. BET, FT-IR,
FT-Raman and 31P MAS NMR shows that there exists a strong
Fig. 6 SEM micrograph of (a) MCM-41, (b) (PMo11)3/MCM-41 and
(c) PMo11.
Fig. 4 31P MAS NMR of (a) PMo11 and (b) (PMo11)3/MCM-41.
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chemical interaction of PMo11 with the support. SEM and XRD
results show fine dispersion of PMo11 inside the channels of
the support.
Catalytic reaction
A detailed catalytic study was carried out on the oxidation of
styrene by varying different parameters like % loading, catalyst
amount, reaction time and temperature to optimize the con-
ditions for the maximum conversion.
Further in order to check whether the reaction is just auto-
oxidation or not, we have carried out control experiments
without a catalyst and no conversion of substrates indicates
that the reaction does not proceed through auto-oxidation.11
Three different oxidation processes were studied (Table 2),
(i) TBHP as the sole oxidant, (ii) using O2 as the sole oxidant
and (iii) employing O2 as the oxidant with 0.2 mmol TBHP as
the initiator. In the case of (i), the reaction did initiate but
exclusively polymeric product was obtained for styrene oxi-
dation whereas no conversion was obtained for benzyl alcohol.
On comparing the activities of (ii) and (iii) the conversions
were almost doubled in the case of (iii) with respect to (ii).
This is due to the fact that O2 as the sole oxidant requires a
longer time for activation, i.e. a larger induction period,
whereas in the case of TBHP as an initiator and O2 as the main
oxidant, TBHP forms species BuO•/•OH and hence activates O2.
Fig. 7 Effect of % loading on oxidation of styrene. Reaction conditions:
0.2 mmol TBHP; time: 8 h; temperature: 80 °C; amount of catalyst:
0.227 wt%.
benzaldehyde may be because of direct oxidative cleavage of
CvC of styrene and fast conversion of styrene oxide to BA.
Fig. 7 shows the effect of % loading of PMo11 on the oxi-
dation of styrene using molecular oxygen. There is a drastic
increase in the conversion of styrene on increasing loading
from 20% to 30%. The conversion reaches 60% for 30%
loaded catalyst and further increase in the loading to 40%
does not affect the conversion significantly. This is because, at
higher % loading, the active species agglomerate on the
surface of the support resulting in low accessibility to the
active sites.
The amount of catalyst has a significant effect on the oxi-
dation of styrene. The oxidation of styrene was carried out by
taking catalyst amount in the range 15 mg to 100 mg (Fig. 8).
Initially, on increasing catalyst amount from 10 mg to 25 mg,
the conversion increases sharply. Further increase in the cata-
lyst amount does not increase the conversion very significantly.
Therefore, 25 mg amount of catalyst has been considered as
optimum for the maximum conversion.
Oxidation of styrene with molecular oxygen
Generally, styrene oxidation gives styrene oxide, benzaldehyde
(BA), and benzoic acid (Scheme 1). However, under the present
reaction conditions, the major oxidation product obtained was
Table 2 Effect of various oxidants on the epoxidation of styrene
Selectivity
Substrate
Styrenea
Oxidant
% Conversion
BA
Others
97c
39
30
1
1
10
The effect of reaction time on the selective oxidation of
styrene was carried out by monitoring % conversion at
TBHP
O2
O2/TBHP
TBHP
O2
44
39
60
<1
11
28
3
61
70
99
99
90
Benzyl alcoholb
O2/TBHP
Reaction conditions: reaction temp.: a 80 °C, b 90 °C, reaction time:
a 8 h, b 24 h, catalyst amount 25 mg, TBHP: 0.2 mmol. c Polymer as the
major product.
Fig. 8 Effect of catalyst quantity. Reaction conditions: 0.2 mmol TBHP;
Scheme 1 Aerobic oxidation of styrene.
2516 | Dalton Trans., 2014, 43, 2512–2520
time = 8 h; temperature = 80 °C.
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Scheme 2 Aerobic oxidation of benzyl alcohol.
(BA) was obtained as a major product along with some
amount of benzoic acid as an overoxidation by-product
(Scheme 2). The effects of various reaction parameters like %
loading, catalyst amount, reaction time and temperature were
studied and are shown in Fig. 11.
Fig. 9 Effect of reaction time. Reaction conditions: temperature =
80 °C; amount of catalyst: 0.227 wt%; 0.2 mmol TBHP.
The optimized conditions for oxidation of benzyl alcohol
(28% conversion, 90% selectivity of BA) are as follows: amount
of the catalyst – 25 mg (0.231 wt%); time – 24 h and tempera-
ture – 90 °C.
Control experiments
The control experiments were carried out using MCM-41 and
PMo11 for oxidation of styrene and benzyl alcohol under the
optimized conditions (Table 3). It is seen from Table 3 that
MCM-41 is not much active towards both the reactions, indi-
cating that the catalytic activity is due to PMo11 only. The cata-
lytic performance of the catalyst is also presented. The
obtained results are almost the same as that of the supported
catalyst indicating that PMo11 is the real active species.
Fig. 10 Effect of reaction temperature. Reaction conditions: 0.2 mmol
TBHP; time = 8 h; amount of catalyst: 0.227 wt%.
Heterogeneity and the recycling test
Rigorous evidence of heterogeneity can be gained only by fil-
tering the catalysts before completion of the reaction and ana-
lysing the filtrate for activity.12 A test was performed by
filtering the catalyst from the reaction mixture at 90 °C for
benzyl alcohol and 80 °C for styrene after a definite period of
time, and the filtrate was allowed to react further. The reaction
mixture and the filtrate were analysed by a gas chromatogram.
No change in the % conversion indicates that the reaction is
not just auto-oxidation and the catalyst plays an important role
for selective conversion of the substrates. The present catalyst
falls into the category C, i.e. the active species does not leach
and the observed catalysis is truly heterogeneous in nature.12
The catalyst was recycled in order to examine its activity as
well as stability. The recycling studies were carried out for both
the reactions. After the reaction, the catalyst was separated
from the reaction mixture by simple centrifugation; the first
washing was given with dichloromethane to remove the pro-
ducts, then the subsequent washings were done by double dis-
tilled water and then dried at 100 °C, and the recovered
catalyst was charged for the further run. No appreciable
decrease in the conversion was observed up to two cycles
(Table 4). The reused catalyst was further characterized by
different time intervals. It is seen from Fig. 9 that initially with
increase in reaction time up to 6 h, the % conversion increases
slowly. The catalyst initially takes an induction period to accel-
erate the reaction. The maximum conversion was achieved at
8 h of reaction time. On further increase in the reaction time,
polymerization starts.
The effect of temperature on the oxidation of styrene was
investigated by varying temperature in the range of 60–90 °C
(Fig. 10). An optimum of 60% conversion was achieved at
80 °C temperature. When temperature was increased further to
90 °C there is an increase in the conversion but a significant
decrease in the selectivity of BA. This is due to over-oxidation
of BA to benzoic acid at elevated temperature. Hence, 80 °C
was considered as optimum for the maximum conversion as
well as selectivity.
The optimum conditions for maximum % conversion (60%)
of styrene to benzaldehyde are: loading of PMo11: 30%; catalyst
amount: 25 mg (0.227 wt%); time: 8 h; temperature: 80 °C.
Oxidation of benzyl alcohol with molecular oxygen
The catalyst was also explored for the aerobic oxidation of FT-IR, surface area, FT-Raman and SEM analysis in order to
benzyl alcohol. In oxidation of benzyl alcohol, benzaldehyde see any structural changes.
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Fig. 11 Optimization of parameters for oxidation of benzyl alcohol: (a) effect of % loading: TBHP (0.2 mmol), amount of catalyst (0.231 wt%), temp-
erature (90 °C), time (24 h); (b) effect of catalyst amount: TBHP (0.2 mmol), temperature (90 °C), time (24 h); (c) TBHP (0.2 mmol), effect of time:
amount of catalyst (0.231 wt%), temp. (90 °C); (d) effect of temperature: TBHP (0.2 mmol), amount of catalyst (0.231 wt%), time (24 h).
Table 3 Control experiments over the catalyst and the support
Selectivity (%)
Conversion
(%)
Styrene
oxide
Materials
MCM-41
Substrate
BA
Styrenea
<1
<1
43
25.2
60.0
28.0
>99
>99
64
90.7
70.0
90.0
—
—
3.8
—
26.0
—
Benzyl alcoholb
Styrenea
c
PMo11
Benzyl alcoholb
Styrenea
(PMo11)3/MCM-41
Benzyl alcoholb
Reaction conditions: 0.2 mmol TBHP, temp.: a 80, b 90 °C, reaction
time: a 8 h, b 24 h, catalyst amount 25 mg, catalyst amount: c 5.7 mg.
Table 4 Recycling studies for both the reactions
Cycle
Conversion (%)
Selectivity (%) BA
TONa/b
Fig. 12 (a) FT-IR and (b) Raman spectra of fresh and regenerated
Fresh
1
2
60a/28b
59a/26b
55a/25b
70/90
71/89
68/89
22 472/10 487
22 097/9738
20 599/9363
catalysts.
band position of the regenerated catalyst compared to fresh
(PMo11)3/MCM-41 indicates the retention of structure of
PMo11 in the catalyst. This was further confirmed by
FT-Raman analysis of the reused catalyst. The Raman spec-
trum of the spent catalyst (Fig. 12b) shows the retention of
a Styrene, b benzyl alcohol, O2: 0.2 mmol TBHP, temp., 80 °C, time,
a 8 h, b 24 h, catalyst amount 25 mg.
Characterization of the recycled catalyst
The FT-IR data for the fresh as well as the regenerated catalyst the typical bands of PMo11. However, the spectrum is
are presented in Fig. 12a. No appreciable shift in the FT-IR slightly different from the fresh one in terms of intensity.
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Conclusions
We have synthesized and characterized successfully a new het-
erogeneous oxidation catalyst comprising PMo11 and MCM-41.
The BET, FT-IR, and 31P MAS NMR studies show that the struc-
ture of PMo11 remains intact even after supporting on MCM-41
and there exists a strong chemical interaction with the
support. SEM and XRD results show fine dispersion of PMo11
inside the channels of the support. We were successful in
developing an environmentally benign protocol for selective
oxidation of styrene and benzyl alcohol under aerobic
conditions using PMo11 supported on MCM-41. The applica-
bility of the method was extended to different alkenes and
alcohols. In all the cases we found a high TON of >20 500 for
alkenes and >5500 for alcohols. The catalyst separation is very
simple and can be recovered after simple filtration and is reu-
sable as it retains its catalytic performance. In addition, the
advantages of this catalytic system also include good substrate
conversion under solvent free reaction conditions and fore-
most the use of molecular oxygen which make it a greener
alternative.
Fig. 13 SEM images of (a) fresh and (b) regenerated catalysts.
Table 5 Oxidation of different substrates by (PMo11)3/MCM-41
Conversion
(%)
Selectivity
(%)
Substrate
Products
TON
Styrenea
60
55
BA
70
59
22 472
20 599
α-Methyl
Acetophenone
styrenea
Benzyl alcoholb
Cyclohexanolb
Cyclopentanolb
1-Hexanolb
28
22
20
15
BA
90
>99
10 487
8989
8240
5618
Cyclohexanone
Cyclopentanone >99
1-Hexanal >99
a Alkenes, b alcohols, O2: 0.2 mmol TBHP, temp., a 80 °C, b 90 °C time,
a 8 h, b 24 h , catalyst amount, 25 mg.
Acknowledgements
We are thankful to the Department of Science and Technology
(SR/S5/GC-01/2009) and the University Grants Commission
(39-837/2010 (SR)), New Delhi, for the financial support. Ms
Sukriti Singh and Mr Nilesh Narkhede are thankful to DST,
New Delhi and UGC, New Delhi, respectively, for the award of
a research fellowship.
This may be due to the sticking of the substrates on the
surface, although this might not be significant in the reutiliza-
tion of the catalyst.
The SEM images of the fresh and recycled catalyst are
shown in Fig. 13. The surface morphology of the catalyst is
retained after reusing for several cycles. The above study shows
that there is no deactivation of the catalyst after reuse. The
BET surface area and pore diameter of the reused catalyst were
found to be 470.2 m2 g−1 and 3.68 nm, respectively, which are
Notes and references
comparable with the fresh catalyst (surface area: 485 m2 g−1
pore diameter: 3.45 nm).
;
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