Aerobic oxidation of alcohols and alkenes over a novel lacunary phosphomolybdate anchored to zeolite Hβ

Article abstract of DOI:10.1039/c5ra03550c

Lacunary phosphomolybdate anchored to zeolite Hβ was synthesized and characterised by various physicochemical techniques. The catalytic activity was evaluated for aerobic oxidation of benzyl alcohol and styrene. The catalyst was found to be efficient, especially in terms of selectivity for the desired benzaldehyde product (for benzyl alcohol-90%, styrene-72%), very high turnover number (alcohols > 3500, alkenes > 18000) as well as recyclability. The viability of the catalyst was also extended to various alkenes and alcohols.

Full text of DOI:10.1039/c5ra03550c

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PAPER
Aerobic oxidation of alcohols and alkenes over a
novel lacunary phosphomolybdate anchored to
zeolite Hb†
Cite this: RSC Adv., 2015, 5, 36270
*
Sukriti Singh, Nilesh Narkhede and Anjali Patel
Lacunary phosphomolybdate anchored to zeolite Hb was synthesized and characterised by various
physicochemical techniques. The catalytic activity was evaluated for aerobic oxidation of benzyl alcohol
and styrene. The catalyst was found to be efficient, especially in terms of selectivity for the desired
benzaldehyde product (for benzyl alcohol – 90%, styrene – 72%), very high turnover number (alcohols >
3500, alkenes > 18 000) as well as recyclability. The viability of the catalyst was also extended to various
alkenes and alcohols.
Received 27th February 2015
Accepted 13th April 2015
DOI: 10.1039/c5ra03550c
www.rsc.org/advances
in different type of species6 in the acidied aqueous solution,
1. Introduction
mainly due to hydrolytic decomposition because of kinetic
instability and equilibrium rate.6,7 It was hence a challenging
task to stabilize these species onto a suitable support.
One of the most signicant reactions in organic chemistry1 is
oxidation reaction of alkenes and alcohols, which provide a
variety of products; among them carbonyl compounds, epox-
ides, diols, and aldehydes are of much importance.2 The
oxygenated derivatives, especially carbonyl compounds, have
widespread applications in dyestuff, perfumery and agro
chemical industries.3 From the view point of growing environ-
mental worries, catalytic methods utilizing green oxidant such
as O2 are highly desirable for sustainable chemistry.
One of the option is to use heterogeneous catalyst. Hence,
the prospect of developing new catalytic methodology by sup-
porting various species on supports, emerges as an attractive
pathway and has opened the opportunity to create more active
as well as selective catalytic systems.
Polyoxometalates (POMs) are very good alternative in terms
of economic viability and easy to manufacture, where the
homogenous form can be easily heterogenized. They are excel-
lent candidates due to plethora of structural and compositional
variability4 and the modication of properties can be basically
accomplished by tuning the structural properties at the atomic
or molecular level of parent POMs. Among various POMs,
phosphomolybdates, have gained much importance due to
their excellent redox properties. The lacunary phosphomo-
lybdates can be synthesised by removing one or more molyb-
denum–oxygen (Mo–O) octahedral from the saturated Keggin
anion framework leading to an increase in the overall anionic
charge of the complex.5 The phosphomolybdates can be present
Our group has successfully stabilized the mono-lacunary
phosphomolybdate (PMo11) species onto hydrous zirconia,8
alumina,9 MCM-4110 and evaluated the catalytic activity for
solvent-free oxidation of styrene and benzyl alcohol. It is known
that the ‘support’ not only provides a mechanical means but
also modies the overall catalytic properties of the material.
Therefore, as an extension of our previous work, it was thought
of interest to use zeolites as support and to explore the redox
properties of the catalyst for oxidation reaction. Among zeolites,
zeolite beta (Hb) was selected due to following reasons: (1)
zeolite beta shows typical characteristics of ideal support, (2) it
is a high silica zeolite having large pores, (3) it has surface
silanol groups as well as hydrophobic character and (4) it is a
good candidate for catalysis due to inherent property of high
thermal and chemical stability.
A series of catalysts containing 10–40% of PMo11 and zeolite
were synthesized. The 30% loaded catalyst was characterized by
various physicochemical characterization techniques. The
catalytic activity was evaluated for the aerobic oxidation of
alcohols and alkenes, where benzyl alcohol and styrene were
taken as test substrates for reaction optimization (Scheme 1).
The conditions for maximum conversions were optimized by
varying different reaction parameters such as % loading,
amount of the catalyst, reaction time, and reaction temperature.
Some important factors for the formation of a successful
anchoring platform and better catalytic activity are the nature of
support, structural porosity and high specic surface area.
Polyoxometalates and Catalysis Laboratory, Department of Chemistry, Faculty of
Science, The M. S. University of Baroda, Vadodara-390002, Gujarat, India. E-mail:
aupatel_chem@yahoo.com
Hence,
a comparative study was carried out between
PMo11/MCM-41 and the present catalyst to investigate the effect
of support. The catalyst was regenerated and recycled up-to two
† Electronic supplementary information (ESI) available: SM 1 DTA curves of
support and catalyst can be found here. See DOI: 10.1039/c5ra03550c
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containing substrates (100 mmol) and 0.2 mmol tertbutyl
hydrogen peroxide (TBHP) in order to initiate the reaction. The
reaction mixture was maintained at 80 ꢀC for styrene and 90 ꢀC
for benzyl alcohol on REMI magnetic stirrer equipped with
preheated oil bath at 600 rpm. The reaction was started by
bubbling O2 (analytical grade) into the liquid. Different reaction
parameters were varied such as catalyst amount, reaction time
and reaction temperature. Aer completion of reaction, the
catalyst was removed by simple centrifugation. The products
were analysed on a gas chromatograph (GC) using Rtx-5 capil-
lary column aer extraction with dichloromethane and drying
with magnesium sulphate. Identication of the product was
carried out by comparing it with authentic samples and then by
gas chromatography-mass spectroscopy (GC-MS).
Scheme 1 Reaction scheme for oxidation of benzyl alcohol and
styrene.
2.6. Characterization
cycles without signicant decrease in the catalytic activity. The
scope of reaction was extended to variety of substrates.
TGA-DTA was obtained from Mettler Toledo Star SW 7.01
instrument up to 600 ꢀC (scan rate-10 ꢀC min; type of gas-
nitrogen and ow used-10 mL minꢁ1). FT-IR spectra were
obtained from Perkin Elmer instrument (number of scans-4 and
resolution-2 cmꢁ1) using the KBr wafer. The Raman spectra
were recorded on a Bruker RFS 27, with resolution-2 cmꢁ1 and
number of scans-4. Adsorption–desorption isotherms were
recorded on a Micromeritics ASAP 2010 surface area analyser at
ꢁ196 ꢀC using BET method. The 31P MAS NMR spectra were
recorded at 121.49 MHz by BRUKER Avance DSX-300 using a
7 mm rotor probe, spinning rate 5–7 kHz, Number of scans-
6400 and Pulse width-4. 85% H3PO4 was used as an external
standard and samples were kept in a desiccator over P2O5 until
the NMR measurement. The XRD pattern was obtained on
2. Experimental
2.1. Materials
All the chemicals used were of A. R. grade. Benzyl alcohol,
cyclopentanol, cyclohexanol, 1-hexanol, 1-octanol, styrene,
4-methyl styrene, a-methyl styrene, tert butylhydroperoxide
(70% aq. TBHP) and dichloromethane were obtained from
Merck and used as received. The sodium form of zeolite b (Si/Al
ratio 10) was used as received and was purchased from zeolites
and Allied Products, Bombay, India.
2.2. Treatment of the support (zeolite-Hb)
˚
PHILIPS PW-1830 using Cu Ka radiation (1.54056 A) and scan-
+
ning angle from 0ꢀ to 60ꢀ. Elemental analyses (EDX) and SEM
were carried out on JSM 5910 LV combined with an INCA
instrument for EDX-SEM.
In order to convert sodium salt of zeolite (Nab) to NH4 form
conventional ion exchange method11 using a 10 wt%, 1 M NH4Cl
aqueous solution was used. The resulting NH4+ form zeolite was
then converted to acidic form (H+ type) by calcination in air for 6
ꢀ
h at 550 C. The obtained material was designated as Hb.
3. Result and discussion
3.1. Catalyst characterization
2.3. Synthesis of mono lacunary phosphomolybdate (PMo11
)
30% loaded PMo11 on Hb (30% PMo11/Hb) showed highest
catalytic activity, as a result we have selected 30% PMo11/Hb for
further detailed characterizations with various techniques.
TGA curves of zeolite support and catalysts are shown in
Fig. 1. The initial weight loss of 16% from 30 to 200 ꢀC for PMo11
(Fig. 1a) may be due to the removal of adsorbed molecules of
water. The nal weight loss of 1.8% indicates the loss of water of
crystallisation at around 275 ꢀC. The zeolite support (Fig. 1b)
The mono lacunary phosphomolybdate (Na7[PMo11O39]$16H2O)
was synthesized by following method reported by us earlier.10
2.4. Synthesis of catalyst (anchoring of PMo11 to Hb)
Impregnation method was used to synthesise a series of cata-
lysts containing 10–40% of PMo11 anchored to Hb. One gram of
support was impregnated with aq. solution of 0.1–0.4 g of PMo11
(10–40 mL distilled water, respectively). The resultant material
ꢀ
showed a unique weight loss of 13–15% up to 250 C, which is
ꢀ
was dried at 100 C for 10 h. The as synthesised catalysts were
due to removal of physically adsorbed water molecules. Absence
of further weight loss beyond 250 ꢀC and up to 600 ꢀC indicates
retainment of zeolite Hb framework. The TGA of 30% PMo11/Hb
(Fig. 1c) shows initial weight loss of 11.2% up to 150 ꢀC due to
removal of physically adsorbed molecules of water. No
designated as 10% PMo11/Hb, 20% PMo11/Hb, 30% PMo11/Hb,
and 40% PMo11/Hb, respectively.
2.5. Oxidation of benzyl alcohol and styrene with molecular
oxygen
substantial weight loss was observed up to 350 ꢀC (2.6%),
ꢀ
The oxidation reaction was performed in a batch-type reactor indicating catalyst stability up to 350 C.
under atmospheric pressure. In a typical reaction appropriate
The DTA of PMo11 shows two endothermic peaks at 80 ꢀC
amount of catalyst was added in a two-necked ask (50 mL) and 140 ꢀC which may be due to loss of adsorbed water and
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Fig. 1 TGA curves of (a) PMo11, (b) Hb and (c) 30% PMo11/Hb.
water of crystallization (SM 1), respectively. In addition, a broad
exothermic peak in the region 270–305 ꢀC was also observed
which may be due to the decomposition of PMo11. DTA curve of
30% PMo11/Hb (SM 1) shows a very broad endothermic range
from 50 ꢀC to 150 ꢀC due to continuous loss of adsorbed water. A
ꢀ
broad exothermic peak at 350 C, was observed which may be
due to decomposition of PMo11. It can be concluded that
thermal stability of PMo11 increases on anchoring PMo11 to the
support. The TGA and DTA analyses are in agreement with one
another.
Fig. 2 Nitrogen sorption and pore size distribution of (a) Hb and (b)
30% PMo11/Hb.
The decrease in the specic surface area for 30% PMo11/Hb
(362 m2 gꢁ1) as compared to that of Hb (587.2 m2 gꢁ1) is as
expected and is the rst indication of chemical interaction
between available surface oxygen of PMo11 and Hb. The
nitrogen adsorption isotherms of support and catalyst are dis-
played in Fig. 2. The catalyst showed Type (IV) pattern with three
different stages: monolayer formation by adsorption of nitrogen
(P/P0 < 0.4) on the mesopore walls, a steep increase in adsorp-
tion (P/P0 ¼ 0.4–0.8) due to capillary condensation with
hysteresis loop, and multilayer adsorption on the outer surface
of the particles. The pore size of Hb zeolite in the present case is
around 2.5 nm. It is known that the size of lacunary PMo11 is
<1.0 nm, hence it can easily enter in the channels of Hb. A
decrease in the pore diameter was observed aer anchoring
PMo11 on to the support due to the presence of PMo11 inside the
zeolite framework by pore lling as well as surface coverage.
The FT-IR spectrum of PMo11 (Fig. 3a) shows asymmetric
stretching of P–O (1048 and 999 cmꢁ1), Mo]O (935 cmꢁ1) and
Mo–O–Mo (906 and 855 cmꢁ1) and the reported values are in
good agreement with reported litterarure.12 The FT-IR spectra
for Hb and 30% PMo11/Hb (Fig. 3c) shows broad band in the
range 1060–1090 cmꢁ1 due to asymmetric stretching vibration
O–T–O (nasym), which is subject to change due to its sensitive to
the silicon and aluminum ratio in the framework of zeolite. A
band (935 cmꢁ1) may be due to strong chemical interaction of
terminal oxygen of PMo11 with Hb, which was further conrmed
by FT-Raman analysis.
FT-Raman spectra of PMo11 and 30% PMo11/Hb are dis-
played in Fig. 4. The FT-Raman spectrum of PMo11 (Fig. 4a)
shows typical bands at 947 ns(Mo–Od), 932 nas(Mo–Od), 891 and
550 nas(Mo–Ob–Mo), 355 nas(Oa–P–Oa) and 217 ns(Mo–Oa), where
characteristic of the pore opening is observed due to band at
455 cmꢁ1
.
The bands of PMo11 at 1048 and 999 cmꢁ1 are not
11
visible due to merging with the broad band of Hb in the range
1060–1090 cmꢁ1. The shiing and broadening of Mo–O–Mo
band from 905 to 920 (shi of 15 cmꢁ1) and from 855 to
890 cmꢁ1 (shi of 35 cmꢁ1) as well as disappearance of Mo]O
Fig. 3 FT-IR of (a) PMo11, (b) Hb, and (c) 30% PMo11/Hb.
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intensity peaks at 2.62 and ꢁ4.53 ppm are also observed,
indicating high degree of adsorption as well as small
fragmentation.5,13
Fig. 6 shows XRD patterns of support Hb and 30% PMo11/Hb.
The characteristic crystalline peaks for PMo11 is generally
observed in the 2q range of 25 to 35. The XRD of catalyst is
almost similar to that of the support Hb with no separate
crystalline peak of PMo11. This shows a well dispersion of PMo11
inside the framework of the zeolite support. This is in good
agreement with the reported one.11
SEM images of Hb and 30% PMo11/Hb are shown in Fig. 7.
The semi-crystalline nature of PMo11 is distinctly visible in
Fig. 7a, at 1000ꢂ magnication and there was no change in the
surface morphology of the catalyst as compared to the support.
The catalysts shows a uniform dispersion of PMo11 in a non-
crystalline form on the surface of the support. The framework
of zeolite support is retained aer incorporation of PMo11
which is in good agreement with that of XRD spectra.
,
Fig. 4 FT-Raman spectra of (a) PMo11 and (b) 30% PMo11/Hb.
Thus, XRD and SEM conrm the uniform dispersion of
PMo11 and retainment of surface morphology of the support.
Oa, Ob, Oc, and Od are attributed to the oxygen atoms connected
to phosphorus, oxygen atoms bridging two molybdenum (from
two different triads for Ob and from the same triad for Oc), and
to the terminal oxygen Mo]O, respectively.10 The FT-Raman
spectrum of 30% PMo11/Hb (Fig. 4b) shows the retention of
all the characteristic bands, however, slight decrease in inten-
sity and shi in bands of ns(Mo–Od) – from 947 to 962 (shi of
15 cmꢁ1); nas(Mo–Od) – from 932 to 898 (shi of 34 cmꢁ1);
nas(Mo–Ob–Mo) – from 891 to 836 (shi of 55 cmꢁ1) and
nas(Oa–P–Oa) – from 355 to 323 (shi of 32 cmꢁ1), may be due to
a strong hydrogen bonded interaction between the terminal
oxygen of PMo11 and hydroxyl group of Hb this is further
conrmed by solid state 31P-NMR.
3.2. Catalytic oxidation reaction of benzyl alcohol and
styrene
In order to ensure the catalytic activity for the oxidation of
benzyl alcohol and styrene two reactions were carried out: (1) in
presence of support Hb as catalyst no conversion was found, (2)
reactions were also carried out in the absence of catalyst which
resulted in no oxidation reaction. In the present work, a detailed
catalytic study was carried out for the oxidation of benzyl
alcohol as well as styrene (Scheme 1) by varying different
parameters like % loading, catalyst amount, reaction time and
temperature to optimize the conditions for the maximum
conversion.
31P MAS NMR is the most important method to study the
structure, composition and chemical environment around
phosphorus in POM compounds. The 31P MAS NMR spectra of
PMo11 (Fig. 5a) shows a very intense peak at 1.64 ppm which is
in good agreement with the reported one.8 For catalyst, an
intense de-shielded peak at 0.145 ppm (Fig. 5b) indicates the
chemical interaction between PMo11 and Hb. However, two low
The effect of % loading of PMo11 on the oxidation of benzyl
alcohol using molecular oxygen under similar reaction condi-
tions (benzyl alcohol – 100 mmol, TBHP – 0.2 mmol, amount of
Fig. 5 31P MAS NMR spectra of (a) PMo11 and (b) 30% PMo11/Hb.
Fig. 6 Power XRD pattern of (a) PMo11, (b) Hb and (c) 30% PMo11/Hb.
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The amount of catalyst has a signicant effect on the
oxidation of benzyl alcohol, hence the catalyst amount was
increased from 10 mg (0.092 wt%) to 30 mg (0.2772 wt%)
(Fig. 8b). Initially a sharp increase in conversion was observed
on increasing the catalyst amount from 10 mg to 25 mg. On
further, increasing the catalyst amount the conversion does not
increase signicantly. Therefore, 25 mg amount of catalyst
(0.231 wt%), was found optimum for obtaining maximum
conversion.
The effect of reaction time was carried out by observing
change in % conversion at different time intervals. It was
observed on increasing the reaction time, the % conversion also
increases signicantly (Fig. 8c). It is known that more reaction
time is necessary to form a reactive intermediate (substrate +
catalyst) which subsequently leads to the products. Maximum
25.5% conversion was observed at 24 h. However, when the
reaction was allowed to continue till 25 h, no signicant
conversion was observed, but selectivity of benzaldehyde
decreases. The maximum conversion was achieved at 24 h of
reaction time.
Fig. 7 SEM images of (a) PMo11 (1000ꢂ magnification), (b) Hb and (c
and d) 30% PMo11/Hb.
ꢀ
catalyst – 25 mg (0.231 wt%), time – 24 h and temp – 90 C) is
shown in Fig. 8a. The conversion increases with increase in the
% loading of PMo11 from 10% to 20%. A signicant increase in
the conversion was observed on increasing the loading from
20% to 30%. The 30% loaded catalyst showed highest conver-
sion 25.5%, however there was no signicant change in the
conversion on further increasing the loading to 40%. No change
in conversion, at higher % loading might be possible due to
agglomeration of the active species on the surface of support
leading to low accessibility towards the active sites. It is also
seen that at higher loading the selectivity for benzaldehyde
decreases. Hence, 30% loading was selected for further study.
The effect of temperature was investigated in the range
ꢀ
ꢀ
70–100 C (Fig. 8d). At 90 C an optimum of 25.5% conversion
ꢀ
was achieved. On increasing the temperature further to 100 C
there was no signicant increase in the conversion however,
decrease in the selectivity of benzaldehyde was observed due to
over-oxidation of benzaldehyde to benzoic acid at elevated
temperature. Thus, 90 ꢀC was considered optimised tempera-
ture for obtaining maximum conversion as well as selectivity for
benzaldehyde.
Fig. 8 Reaction optimization: effect of (a) % loading, (b) catalyst amount, (c) reaction time and (d) reaction temperature. % Conversion is based
on benzyl alcohol.
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Following are the optimised conditions for obtaining secondary alcohol was much easier than primary alcohols
maximum % conversion (25.5%) of benzyl alcohol to benzal- (Table 1). The obtained trend was in agreement with the
dehyde (90% selectivity): loading of PMo11 – 30%, amount of reported ones. In all cases we were able to obtain outstanding
catalyst – 25 mg (0.231 wt%), time – 24 h, temperature – 90 ꢀC. TONs. However, the present catalytic system is not applicable to
A detail study, similar to benzyl alcohol oxidation, was less reactive long chain primary alcohols, like 1-octanol.
carried out for aerobic oxidation of styrene using 30%
In the case of alkenes, styrene, 4-methyl styrene produced
PMo11/Hb by changing various reaction parameters such as % 72% benzaldehyde and 67% acetophenone, respectively.
loading of PMo11, catalyst amount, time and temperature to a-Methyl styrene exhibited good conversion (59%) and 70%
optimize the conditions using oxidant O2 (Fig. 9). It is well selectivity for acetophenone. In all cases, excellent selectivity
known that oxidation of styrene gives styrene oxide as an (60–70%) of desired product with very high TON was obtained.
intermediate product. However, in most of the cases, benzal-
dehyde was obtained as major product via. (i) direct oxidative
cleavage of C]C bond of styrene and (ii) fast conversion of
styrene oxide to benzaldehyde. Similarly, in the present case,
benzaldehyde was characterized as the major (>72%) oxidation
product along with styrene oxide, acetophenone, benzoic acid
as minor products. All the reaction parameters play signicant
role in optimizing the best conversion of styrene and highest
selectivity for benzaldehyde.
The optimum conditions for maximum % conversion (57%)
of styrene to benzaldehyde (72% selectivity) are: loading of
PMo11 – 30%, amount of catalyst – 25 mg (0.227 wt%), time – 8 h,
temperature – 90 ꢀC.
3.4. Control experiments and heterogeneity test
The control experiments with Hb and PMo11 were performed
under optimized conditions (Table 2). It is seen from Table 2
that the support is not active towards the oxidation reaction
suggesting that the activity is mainly due to PMo11. The same
reaction was performed by taking the active amount of PMo11. It
was observed that the active species produced 25.2% conversion
of benzyl alcohol with 91% selectivity of benzaldehyde. Similar
obtained activity for supported catalysts indicate that PMo11 is
the real active species.
Heterogeneity test was carried out for the oxidation of benzyl
alcohol.14 A test was performed by ltering the catalyst at 90 C
aer 18 h and the ltrate was allowed to react up to 24 h. The
ꢀ
3.3. Oxidation of different alcohols and alkenes
In order to explore the applicability of the method for a selective reaction mixture of 18 h as well as the ltrate of 24 h were ana-
aerobic oxidation different alcohol and alkene substrates were lysed by GC. No change in the conversion and selectivity was
studied (Table 1). The most common benzylic alcohols, and observed indicating that the catalyst falls into category C. Similar
primary and secondary alcohols were used as a substrates reactions were carried out for styrene oxidation. It also conrms
oxidized to corresponding aldehydes or ketones with moderate that the reactions are not just auto-oxidation but the catalyst
to good activity and exceptional selectivity. The oxidation of plays an important role for selective conversion of the substrates.
Fig. 9 Effect of (a) % PMo11 loading, (b) catalyst amount, (c) reaction time and (d) temperature on styrene oxidation under aerobic conditions.
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Table 1 Oxidation of various alcohols and alkenes over supported Table 2 Control experiments over the catalyst and the supporta
catalysts, under optimized conditionsa
Catalyst
Hb
Substrate
Conversion %
Selectivityc %
Substrate
% Con. Products
% Sele. TON
Benzyl alcohola
Styreneb
<1
<1
25.2
43
25.5
57
100
100
91
64
90
PMo11
Benzyl alcohola
Styreneb
25.5a
90
9551
30% PMo11/Hb
Benzyl alcohola
Styreneb
72
a
% Conversion is based on substrate a alcohol, b alkene – O2: 0.2 mmol
a
b
a
b
c
TBHP, temp. – 90 ꢀC, 80 ꢀC; time – 24 h, 8 h; selectivity for
benzaldehyde.
21a
20a
>99
>99
8580
8171
was done by dichloromethane. Further successive washings
were done by distilled water and dried at 100 ꢀC, and the
recovered catalyst was used for the next run. No considerable
loss in the activity was detected up to two cycles (Table 3).
In order to see the need for reactivating the catalyst by
9a
>99
3745
NCa
—
—
ꢀ
washing and heating at 100 C, we have carried out two exper-
iments as follows: (1) separating the catalyst and adding a fresh
one to the same solution and the reaction is further carried out
for 10 h and it was observed that the conversion increases by
10%, with similar selectivity for benzaldehyde. This indicates
that the catalyst may be poisoned by the product molecules
blocking the active sites. (2) On adding fresh benzyl alcohol in
the reaction mixture aer 24 h without further catalyst washing,
no increase in conversion was observed. This truly suggests that
washing procedure reactivates the catalyst.
57b
72
70
20 975
22 099
59b
The reused catalyst was further characterized by EDX, FT-IR
and SEM analysis. EDX elemental analysis performed on the
fresh (fresh: P ¼ 0.37%, Mo ¼ 12.0%) and recycled catalyst
(recycled: P ¼ 0.33%, Mo ¼ 11.3%) shows that all the elements
in the recycled catalysts are retained. The FT-IR data for the
fresh as well as the recycled catalysts are presented in Fig. 10.
The FT-IR of fresh catalyst shows bands at 890 cmꢁ1 and 918
cmꢁ1 assigned to Mo–O–Mo stretches. Similar bands were
observed for recycled 30% PMo11/Hb. No appreciable shi in
the FT-IR band position of the regenerated catalyst compared to
fresh 30% PMo11/Hb indicates the retention of structure of
PMo11 in the catalyst. Fig. 11 shows SEM images of the recycled
as well as fresh catalyst. Even aer three cycles the surface
morphology of the catalyst is retained. The above studies show
that there is no deactivation of the catalyst aer reuse.
50b
67
18 728
a
a
b
% Conversion is based on substrate: alcohols, alkenes – O2:
a
b
a
b
0.2 mmol TBHP, temp. – 90 ꢀC, 80 ꢀC; time – 24 h, 8 h; catalyst
a
b
amount, 0.231 wt%, 0.227 wt%. TON was calculated from the
formula, TON ¼ moles of product/moles of catalyst.
A leaching test was carried out by taking 1 g of catalyst with 10
mL of distilled water and was reuxed for 24 h. Then, 1 mL of the
supernatant liquid was reacted with 10% ascorbic acid solution.
No blue colour was observed, indicating that there was no
leaching. The same test was repeated by taking alcohols and the
ltrate of the reaction in order to see the presence of any leached
PMo11. The absence of blue colour indicates no leaching.
Table 3 Recycling studies under the optimised conditionsa
Cycle
Conversiona,b (%)
Selectivitya,b,c (%)
TONa,b
Fresh
1
2
25.5/57
25.0/55
24.0/54
90/72
89/68
89/66
9551/20975
9364/20306
8989/19937
3.5. Recycling study and characterization of regenerated
catalyst
a
% Conversion is based on substrate a benzyl alcohol, b styrene, O2: 0.2
mmol TBHP, catalyst quantity a 0.231 wt%, b 0.227 wt%. c Selectivity for
benzaldehyde.
The oxidation of benzyl alcohol and styrene were performed
over recycled catalysts (Table 3), under optimized conditions.
The catalyst was removed by centrifugation; the rst washing
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showed higher selectivity for benzaldehyde 72% as compared to
30% PMo11/MCM-41 (70%). The observed difference in
conversion as well as selectivity might due to difference in
surface area, pore geometry of zeolite Hb supercages.
3.7. Comparative study with reported catalysts
Table 5 comprises the catalytic activity of reported catalyst for
oxidation of benzyl alcohol and styrene.8,16–19 The catalyst 2.5%
Au–2.5% Pd/TiO2 showed excellent conversion (90%) in pres-
ence of 0.2 MPa O2 pressure in reactor at 100 ꢀC and high turn-
over frequency (TOF).18 It is important to note that costly metals
such as gold, Pd, and TiO2 are used as a catalyst under 0.2 MPa
pressure of O2. Superiority of the present catalytic system lies in
obtaining better conversion as well as selectivity under mild
reaction conditions with very high TON (9551).
Fig. 10 FT-IR spectra of fresh and recycled catalyst.
3.8. Reaction mechanism study
Three different oxidation processes were studied in order to
study the reaction mechanism (Table 4), (1) TBHP as the only
oxidant, (2) O2 as the only oxidant and (3) using 0.2 mmol TBHP
as initiator along with O2 as the oxidant. Control experiments
without a catalyst suggested that there was no conversion of
substrates and that the reaction does not proceed through auto-
oxidation.20
Fig. 11 SEM images of (a) fresh catalyst and (b) recycled catalyst.
3.6. Comparison of catalysts and effect of supports
In the rst case (1) entirely polymeric product was obtained
for styrene oxidation, however benzyl alcohol showed no
conversion. On comparing the activities of (2) and (3) the
conversions were almost doubled in the case of (3) with respect
to (2). This is because O2 is the only oxidant and it requires
extended activation time, i.e. a larger induction period.
Whereas, in the case of O2 as the main oxidant and TBHP as an
initiator, it forms species BuOc/cOH which activates O2. From
these observations, it was proposed that the reaction appears to
be a radical mechanism and activation of catalyst was necessary
for initiating the reaction, where TBHP acts as a radical initiator
only (Scheme 2). Our group had earlier proposed a mechanism
for aerobic oxidation of alkenes and alcohols,21 similarly in the
present case also we expect the same mechanism.
It is recognised that the support type not only shows a
mechanical role but also signicantly modies the catalytic
activity of the catalyst. The difference in catalytic activity and
product selectivity in oxidation reaction of benzyl alcohol may
be due to the nature of supports such as structural meso-
porosity and high specic surface area. Hence, the obtained
results of 30% PMo11/Hb were compared with 30%
PMo11/MCM-41.10 It is seen that benzyl alcohol conversion is
higher in the case of 30% PMo11/MCM-41 (28%). This might be
due to difference in the surface area of the catalysts where
surface area of 30% PMo11/MCM-41 (485 m2 gꢁ1) is higher
compared to 30% PMo11/Hb (362 m2 gꢁ1). Hence, the order of
catalytic activity observed was 30% PMo11/MCM-41 > 30%
PMo11/Hb. It is well known that the products distribution is
affected by acidity of the catalyst.
Table 4 Effect of various oxidants on both the reactionsa
Selectivity
Generally, oxidation of benzyl alcohol gives benzaldehyde
(major), with over oxidation product benzoic acid (minor).
These types of over oxidation reactions are directly promoted by
acidity of the catalyst. So, observed result i.e. lower selectivity
(90%) of benzaldehyde is attributed to acidity of the support. It
is well known that MCM-41 and zeolite Hb both exhibit acidic
character, however Hb (1.14 mmol gꢁ1) is much more acidic
than MCM-41 (0.82 mmol gꢁ1).15 Similar trend was observed for
oxidation of styrene (30% PMo11/MCM-41 ¼ 60% conversion,
30% PMo11/Hb ¼ 57% conversion), however 30% PMo11/Hb
Substrate
Oxidant
Conversion
Benzaldehyde
Others
Benzyl alcohola
TBHP
O2
O2/TBHP
TBHP
O2
<1
10
25.5
39
30
99
98
90
3
58
72
1
2
10
97c
42
28
Styreneb
O2/TBHP
57
a
Reaction conditions: reaction temp.: a 90 ꢀC, b 80 ꢀC, reaction time: a
b
c
24 h, 8 h, catalyst amount 25 mg, TBHP: 0.2 mmol. Polymer as the
major product.
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Table 5 Comparison of catalytic activity with the reported catalysts for oxidation of benzyl alcohol and styrene
Catalyst
Reaction conditionsa
Solvent (mL)
% Conv.
Products (% select.)
TON
Ref.
TBA–SiW11Ruc
1 : 2 : 120 : 110b
0.7 : 30 : 24 : 80b
— : — : 8 : 100
100 : 25 : 24 : 90b
3 : 300 : 5 : 90g
100 : 25 : 8 : 80g
100 : 25 : 8 : 80g
3
1.6
—
—
10
—
—
36
22
65
340
—
—
9551
—
16
17
18
Mo–V–O oxided
>99
91.6
90
34.8
93.9
72
e
2.5% Au–2.5% Pd/TiO2
74.5
25.5
15.7
21.7
57
30% PMo11/Hbf
This work
19
8
Mo-exchanged zeolite
f
20% PMo11/ZrO2
11 329
20 975
30% PMo11/Hbf
This work
a
c
% Conversion is based on substrate; benzyl alcohol: amount of catalysts (mg): time (h): temperature (ꢀC). b Benzyl alcohol; solvent ¼ isobutyl
acetate/ d toluene; e 0.2 MPa pO2, f TBHP was used as an initiator, g styrene, product (benzaldehyde).
Notes and references
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4. Conclusion
In conclusion, a novel mono lacunary phosphomolybdate
anchored to zeolite-Hb was synthesized and efficiently charac-
terized. Catalytic study was carried out for solvent free aerobic
oxidation of benzyl alcohol and styrene. The applicability of the
method was extended to different alkenes and alcohols. In all
the cases a high TON >18 000 for alkenes and >3500 for alcohols
was found. A basic comparative study on effect of support was
carried out by comparing zeolite-Hb based catalyst with meso-
porous MCM-41 and it was found that acidity of supports play a
role in selectivity of the desired product. The catalyst can be
recovered aer simple ltration and can be recycled without any
signicant change in the conversion.
Acknowledgements
AP and SS are thankful to the Department of Science and
Technology for the nancial support (Project number-SR/S5/
GC-01/2009). Mr Nilesh Narkhede is thankful to CSIR, New
Delhi, for the award of Senior Research Fellowship (SRF).
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36278 | RSC Adv., 2015, 5, 36270–36278
This journal is © The Royal Society of Chemistry 2015