Synthesis, characterization, and application of a manganese Schiff base complex containing salicylaldehyde-poly(vinylamine)/SBA-15 as a novel heterogeneous hybrid catalyst

Article abstract of DOI:10.1039/c3ra22579h

A manganese Schiff base complex containing salicylaldehyde-poly(vinylamine) /SBA-15 (Mn-SA-PVAm/SBA-15) composite was prepared as a novel heterogeneous catalyst through in situ polymerization of vinylamine within the nano-channels of mesoporous silica SBA-15 without using any organo silica precursors. The catalyst was characterized well by elemental analysis, FT-IR, AAS, DRS UV-Vis, XRD, BET, SEM and TGA techniques. The catalytic activity of the supported metal complex was studied in the selective oxidation of benzyl alcohols using t-butyl hydroperoxide as an oxidant. The effects of reaction temperature, solvent and the amount of catalyst as well as the recyclability of the catalyst were investigated. The Royal Society of Chemistry 2013.

Full text of DOI:10.1039/c3ra22579h

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Synthesis, characterization, and application of a
manganese Schiff base complex containing
salicylaldehyde–poly(vinylamine)/SBA-15 as a novel
heterogeneous hybrid catalyst
Cite this: DOI: 10.1039/c3ra22579h
ab
ab
a
Ahmad Reza Massah,* Roozbeh Javad Kalbasi* and Somayeh Kaviyani
A manganese Schiff base complex containing salicylaldehyde–poly(vinylamine)/SBA-15 (Mn–SA–PVAm/
SBA-15) composite was prepared as a novel heterogeneous catalyst through in situ polymerization of
vinylamine within the nano-channels of mesoporous silica SBA-15 without using any organo silica
precursors. The catalyst was characterized well by elemental analysis, FT-IR, AAS, DRS UV-Vis, XRD, BET,
SEM and TGA techniques. The catalytic activity of the supported metal complex was studied in the selective
oxidation of benzyl alcohols using t-butyl hydroperoxide as an oxidant. The effects of reaction
temperature, solvent and the amount of catalyst as well as the recyclability of the catalyst were
investigated.
Received 19th October 2012,
Accepted 2nd May 2013
DOI: 10.1039/c3ra22579h
www.rsc.org/advances
inorganic moieties with organic polymers has a high potential
1
. Introduction
1
6–19
for future applications.
Resultant materials are supposed to
The oxidation of alcohols to aldehydes and ketones is a widely
used chemical transformation in organic synthesis. Transition
metal complexes are effective and selective catalysts in the
oxidation of alcohols.
synthesize and, as well as porphyrins and phthalocyanines,
mimic the enzymatic catalysts present in biological systems.
The high electron-donating ability of the Schiff base ligand,
which facilitates the electron transfer rate, is expected to improve
the overall catalytic activity of an imine-functionalized compo-
20
have better mechanical and/or thermal properties. In these
cases the inorganic moiety is a mesoporous molecular sieve
crystalline framework material. These porous inorganic solids
1
2
–4
Schiff base complexes are easy to
with empty pores and channels are excellent hosts for intercala-
21,22
tion reactions.
Silica SBA-15 is a mesoporous molecular sieve,
5,6
which possesses uniform hexagonal channels ranging from 5 to
0 nm and a very narrow pore-size distribution. Its large internal
3
2
21
surface area (.800 m g ) allows for the dispersion of a large
number of catalytically active centers, whereas its thick frame-
7
–9
site.
Although transition metal complexes catalysts offer
23–25
work walls (3.1–6.4 nm) provide high hydrothermal stability.
interesting results in organic synthesis, these complexes are still
associated with problems of corrosion, contamination of
In continuing our previous work to develop new polymer–
inorganic hybrid materials as heterogeneous catalysts,
herein, we have focused our attention on the synthesis and
characterization of a manganese Schiff base complex contain-
ing salicylaldehyde–poly(vinylamine) supported on SBA-15
26–28
10–12
reaction products and difficulties of separation.
To overcome
these drawbacks, the heterogenization of homogeneous catalysts
is expected. Extensive investigations have been focused on the
incorporation of metal complex catalysts onto or into inert
supports such as molecular sieves, amorphous silica, and
(
Scheme 1) as a heterogeneous hybrid catalyst. The catalyst
was prepared via a simple method without using any organo
silica precursors. The catalytic activity of this novel hybrid
catalyst was tested for the selective oxidation of alcohols. This
catalyst could be reused several times (at least 5) without
significant loss of activity/selectivity.
1
3–15
polymers.
Organic–inorganic hybrid materials are other
supports for the heterogenization of homogeneous metal
complex catalysts. Organic–inorganic composite materials are
increasingly important for a variety of applications due to their
extraordinary properties which arise from the combination of the
different building blocks. The combination of nanoscale
2. Experimental
a
Department of Chemistry, Shahreza Branch, Islamic Azad University, 311-86145
Shahreza, Isfahan, Iran. E-mail: massah@iaush.ac.ir; rkalbasi@iaush.ac.ir;
Fax: +98-321-3213103; Tel: +98-321-3213211
2
.1. Catalyst characterization
b
The materials were characterized by atomic absorption
spectroscopy (AAS) (Perkin-Elmer, Analyst 300), X-ray diffrac-
Razi Chemistry Research Center, Shahreza Branch, Islamic Azad University,
Shahreza, Isfahan, Iran
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Scheme 1 Preparation of Mn–SA–PVAm/SBA-15.
tion (Bruker D8ADVANCE, Cu-Ka radiation), FT-IR spectro-
hyde (1.3 mL, 12.4 mmol) and dry methanol (7 mL) were
refluxed for 24 h at 70 uC under a nitrogen atmosphere. The
yellow precipitate obtained was washed with dry methanol (2
6 5 mL) to remove excess salicylaldehyde to give a Schiff base
ligand containing salicylaldehyde–poly(vinylamine)/SBA-15
(SA–PVAm/SBA-15).
scopy (Nicolet 400D in KBr matrix in the range 4000–400
2
1
cm ), BET specific surface area and BJH pore size distribu-
tion (Series BEL SORP 18, at 77 K), diffuse reflectance (DR) UV-
Vis spectroscopy (Shimadzu UV-265), thermo gravimetric
analysis (TGA, Setaram Labsys TG (STA) in a temperature
2
1
range 30–700 uC and with a heating rate of 10 uC min in an
atmosphere) and SEM (Philips, XL30, SE detector). The
2
.2.3. Preparation of manganese Schiff base complex
N
2
containing salicylaldehyde–poly(vinylamine)/SBA-15. A flask
containing a stirred suspension of manganese(II) acetate
tetrahydrate (2.45 g, 10 mmol) in 10 mL of dry methanol was
purged with nitrogen and then warmed to 40 uC under a
nitrogen atmosphere. SA–PVAm/SBA-15 (0.5 g) was added in
one portion and the resulting suspension was then refluxed
under a nitrogen atmosphere for 8 h at 70 uC to give a
manganese Schiff base complex containing salicylaldehyde–
poly(vinylamine)/SBA-15 (Mn–SA–PVAm/SBA-15). The collected
solid was washed with dry methanol (2 6 5 mL) and dried in
air.
The Mn content of the catalyst was estimated by decompos-
ing a known amount of the catalyst using perchloric acid,
nitric acid, fluoric acid, and hydrochloric acid, and the
transition metal content was estimated using an atomic
absorption spectrometer (Perkin-Elmer, Analyst 300).
1
13
products were identified by H NMR and C NMR spectro-
scopy (Bruker DRX-400 Avance spectrometer at 400 and 100
MHz, respectively). The melting points were measured on an
electro-thermal 9100 apparatus and reported without correc-
tion. The melting points compared satisfactorily with those
reported in the literature.
2.2. Catalyst preparation
2.2.1. Preparation of poly(vinylamine)/SBA-15. Mesoporous
silica SBA-15 was prepared through the addition of H PO as
3
4
2
9
described in the literature recently. The poly(vinylamine)/
SBA-15 composite was synthesized as follows:
In the first step, into a 50 mL round-bottomed flask, acryl
amide (0.25 g, 3.5 mmol), benzoylperoxide (0.025 g, 0.1 mmol),
SBA-15 (1 g) and THF (10 mL) were added. The solution was
stirred with a magnetic stirrer and refluxed for 4.5 h at 70 uC.
Then the solution was filtered and washed with THF (2 6 5
mL). The white precipitate was dried in air overnight. This
composite was named poly(acrylamide)/SBA-15 (PAA/SBA-15).
In the second step, into a 100 mL round-bottomed flask PAA/
2.3. General procedure for the oxidation of alcohols
0.1 g of catalyst and 2 mmol of benzyl alcohol were mixed in 5
mL of CH CN and the mixture was refluxed with continuous
3
stirring in an oil bath. 3 mmol of t-butyl hydroperoxide (TBHP)
was added to the refluxing mixture and the progress of the
reaction was monitored by thin layer chromatography (TLC)
(eluent: n-hexane : ethyl acetate = 16 : 4). After completion of
the reaction, the mixture was cooled to room temperature and
the catalyst was removed by filtration. Diethyl ether (15 mL)
was added to the filtrate and washed with brine (10 mL) and
SBA-15 composite (0.5 g), calcium hypochlorite (Ca(OCl) ) (2 g)
2
and distilled water (75 mL) were added. The solution was
stirred with a magnetic stirrer and refluxed for 4.5 h at 90 uC.
Then the solution was filtered and washed with hot distilled
water (2 6 200 mL). The white precipitate was dried in air
overnight. Finally, through the Hoffman reaction, the poly
(vinylamine)/SBA-15 (PVAm/SBA-15) composite was obtained.
2 4
H O (10 mL). The organic layer was dried over MgSO and the
2
.2.2. Preparation of Schiff base ligand containing salicy-
solvent was evaporated to offer the crude product. The pure
product was obtained by column chromatography over silica
gel using n-hexane/ethyl acetate mixed solvent as the eluent.
laldehyde–poly(vinylamine)/SBA-15. Into a 25 mL round-
bottomed flask, PVAm/SBA-15 composite (0.5 g), salicylalde-
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2
1
Moreover, the presence of peaks at 2800–3000 cm
corre-
sponds to the aliphatic C–H stretching in SA–PVAm/SBA-15. In
addition, in the FT-IR spectrum of SA–PVAm/SBA-15 (Fig. 1e),
2
1
the sharp band appearing at 1640 cm is due to the vibration
of CLN. The appearance of the above band shows that
salicylaldehyde is attached to the surface of PVAm/SBA-15
and the SA–PVAm/SBA-15 composite is obtained. As shown in
the Mn–SA–PVAm/SBA-15 spectrum (Fig. 1f), the band at
2
1
around 1640 cm , which corresponds to CLN, is shifted to
2
1
lower wave numbers (1570 cm ) (red shift). This may be due
to the interaction between the Mn cations and the CLN groups
and the formation of a ligand–metal complex. This means that
Mn–SA–PVAm/SBA-15 was successfully prepared.
The powder XRD patterns of the mesoporous silica SBA-15,
PVAm/SBA-15, SA–PVAm/SBA-15 and Mn–SA–PVAm/SBA-15 are
shown in Fig. 2. The low angle diffraction pattern of SBA-15
shows evidence of three reflections at 2h values of 0.5–3u,
including one strong peak, (100), and two weak peaks, (110)
and (200), corresponding to a highly ordered hexagonal
3
0
mesoporous silica framework.
The PVAm/SBA-15, SA–
PVAm/SBA-15 and Mn–SA–PVAm/SBA-15 samples show the
same pattern, indicating that the long-range order of the SBA-
1
1
5 (100) framework is retained after the hybridization of SBA-
5. However, it is observed that the intensity of these
characteristic diffraction peaks is decreased in these samples
compared with pure mesoporous silica SBA-15, which is
probably due to the presence of guest moieties on the
mesoporous framework of SBA-15, resulting in a decrease in
Fig. 1 FT-IR spectra of (a) mesoporous silica SBA-15, (b) PVAm, (c) PVAm/SBA-
1
5, (d) salicylaldehyde, (e) SA–PVAm/SBA-15 and (f) Mn–SA–PVAm/SBA-15.
3. Results and discussion
3.1. Characterization of catalyst
Fig. 1 shows the FT-IR spectra of mesoporous silica SBA-15,
PVAm, PVAm/SBA-15, salicylaldehyde, SA–PVAm/SBA-15 and
Mn–SA–PVAm/SBA-15. The characteristic band around 1080–
2
1
1090 cm
is due to the Si–O stretching in the Si–O–Si
structure, which is seen in Fig. 1a, c, e and f. In the FT-IR
spectrum of PVAm/SBA-15 (Fig. 1c), the new band at 1436
2
1
cm is the bending vibration absorption of the N–H bond.
2
1
Also, the presence of peaks at around 2800–3000 cm
corresponds to the aliphatic C–H stretching in the PVAm/
2
1
SBA-15 composite. Moreover, the peak around 3430 cm is
stronger than in the FT-IR spectrum of SBA-15, which is due to
the stretching vibration of the N–H groups. This is in
accordance with the spectrum of PVAm (Fig. 1b). In the FT-
IR spectra of SA–PVAm/SBA-15 (Fig. 1e) and Mn–SA–PVAm/
2
1
SBA-15 (Fig. 1f), the peak at around 3430 cm is the same as
in the FT-IR spectrum of SBA-15 (Fig. 1a), and is due to the
stretching vibration of the O–H groups, which shows that
amine functional groups are condensed by salicylaldehyde.
Fig. 2 Powder XRD patterns of (a) mesoporous silica SBA-15, (b) PVAm/SBA-15,
(c) SA–PVAm/SBA-15 and (d) Mn–SA–PVAm/SBA-15.
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2
Fig. 3 N adsorption–desorption isotherm of (a) pure SBA-15 and (b) PVAm/
SBA-15.
2
Fig. 4 N adsorption–desorption isotherm and BJH pore size distribution
obtained for SA–PVAm/SBA-15.
the crystallinity. In addition, the composites contain much
less SBA-15 due to dilution of the silicious material by PVAm,
salicylaldehyde and Mn; therefore, this dilution can also
account for a decrease in the peak intensity.
these textural properties are smaller than those found for
mesoporous silica SBA-15, PVAm/SBA-15 still has a mesopor-
ous form and does not block the pores of the SBA-15, hence it
is suitable to be developed and grafted with other organic
The N adsorption–desorption isotherms of the pure SBA-15,
2
PVAm/SBA-15, SA–PVAm/SBA-15 and Mn–SA–PVAm/SBA-15
samples are shown in Fig. 3, 4 and 5. The isotherms are
similar to the type IV isotherm with H1-type hysteresis loops at
high relative pressure according to the IUPAC classification,
characteristic of mesoporous materials with highly uniform
size distributions. From the two branches of the adsorption–
desorption isotherms, the presence of a sharp adsorption step
in the P/P
relative pressure P/P
0
region from 0.6 to 0.8 and a hysteresis loop at the
. 0.7 show that the materials possess a
0
well defined array of regular mesoporous (Fig. 3 and 4).
However, there are differences between the Mn–SA–PVAm/
SBA-15 sample and the others; the amount of adsorbed
nitrogen is lower at the same relative pressure and the step
and its sharpness are smaller. This phenomenon implies that
the Mn cations are incorporated into the pores of the SBA-15.
The specific surface areas and the pore sizes have been
calculated using the Brunauer–Emmett–Teller (BET) and
Barrett–Joyner–Halenda (BJH) methods, respectively. The
structure data of all the mesoporous materials (BET surface
area, total pore volume, and pore size) are summarized in
Table 1. It is clear that calcined SBA-15 has a high BET surface
2
21
3
21
area (1430 m g ), and a large pore volume (1.9 cm g ) and
pore size (9.9 nm), indicative of its potential application as a
host inorganic material. After hybridization with PVAm
through in situ polymerization, PVAm/SBA-15 exhibits a
smaller specific area, pore size and pore volume in comparison
with those of pure SBA-15, which might be due to the presence
of polymer on the internal surface of the SBA-15. Although
Fig. 5 N2 adsorption-desorption isotherm and BJH pore size distribution
obtained for Mn–SA–PVAm/SBA-15.
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Table 1 Surface data of the samples
2
21
3
21
Sample
BET surface area (m g
)
Pore volume (cm g
)
Pore diameter (nm)
Mesoporous silica SBA-15
PVAm/SBA-15
SA–PVAm/SBA-15
1430
856
442
104
1.90
1.08
0.39
0.25
9.90
8.81
4.89
2.25
Mn–SA–PVAm/SBA-15
Fig. 6 Scanning electron microscopy (SEM) photographs of (a) SBA-15 and (b) Mn–SA–PVAm/SBA-15.
compounds for the formation of novel ligands. The data for
the Mn–SA–PVAm/SBA-15 sample indicates a reduction in
surface area, pore size and pore volume compared to SA–
PVAm/SBA-15. This might be due to the incorporation of Mn
into the channels of SA–PVAm/SBA-15 and the pore size
distribution curve for this catalyst proves this idea (Fig. 5,
inset). These results are likely to be due to the presence of Mn
on the surface of the catalyst. Although these textural proper-
ties are smaller than those found for mesoporous silica SBA-
Fig. 6 shows scanning electron microscopy (SEM) photo-
graphs of SBA-15, and the Mn–SA–PVAm/SBA-15 composite.
We can see that there is negligible difference in the particle
surface morphology between SBA-15 and Mn–SA–PVAm/SBA-
15, which indicates that the polymerization, salicylaldehyde
grafting and coordination of Mn on the ligand, take place
more in the channels and less on the outer surfaces of the
catalyst.
The electronic spectral profiles of SA–PVAm/SBA-15 and
Mn–SA–PVAm/SBA-15 are reproduced in Fig. 7. The electronic
spectrum of the SA–PVAm/SBA-15 exhibits three bands at 370,
15, Mn–SA–PVAm/SBA-15 is still suitable to function as a
heterogeneous catalyst.
Fig. 7 UV-Vis spectra of (a) SA–PVAm/SBA-15 and (b) Mn–SA–PVAm/SBA-15.
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loss steps can be seen (Fig. 8b). The first step (around 7%, w/w)
appearing at a temperature of ,150 uC corresponds to the loss
of water (i.e., adsorbed water on the inner and outer surfaces
of SBA-15). The second weight loss (about 475–600 uC)
amounts to around 13% (w/w) which is related to the
degradation of the polymer. Compared to the PVAm curve,
the weight loss of PVAm/SBA-15 is mild, and indicates that
PVAm/SBA-15 is more thermo-stable than PVAm. Therefore,
after hybridization, the thermal stability is enhanced greatly
and this is very important for the catalyst application. For the
SA–PVAm/SBA-15 sample, three separate weight losses can be
seen (Fig. 8c). The first weight loss (around 4%, w/w)
appearing at a temperature of ,100 uC, corresponds to loss
of water. The second weight loss (around 30%, w/w) at 250–375
uC is related to the degradation of salicylaldehyde which is
attached to the surface of the PVAm/SBA-15. The third weight
loss of around 6% (w/w) at about 610–680 uC can be related to
the degradation of the PVAm chains. However, following
coordination to Mn cations, Mn–SA–PVAm/SBA-15 shows
higher thermo-stability compared to SA–PVAm/SBA-15. This
higher stability indicates that the complex with the metal is
formed.
Fig. 8 TGA curves of (a) PVAm, (b) PVAm/SBA-15, (c) SA–PVAm/SBA-15 and (d)
Mn–SA–PVAm/SBA-15.
2
64 and 234 nm and these are assigned to the n A p*, p A p*
3.2. Catalytic activity
3
1
and s A s* transitions, respectively. These bands shift to
higher wavelengths and the intensity of these bands is
decreased for Mn–SA–PVAm/SBA-15. However, the s A s*
transition is removed indicating the restructuring of the ligand
after coordination to the metal ion. The appearance of a weak
band due to a ligand to metal transition underneath the n A
p* transition makes this a band broad. No band could be
located in the higher wavelength region due to the expected d–
d transition.
Initially, benzyl alcohol was chosen as a model to test the
catalytic activity of the Mn–SA–PVAm/SBA-15 for the oxidation
of alcohols under different conditions (Table 2). In order to
investigate the effect of solvent, the oxidation of benzyl alcohol
(2 mmol) was carried out in several solvents using 0.1 g of the
catalyst at 65 uC (Table 2, entries 1–5). As can be seen, normal
hexane was not a suitable solvent for this catalytic system,
because its polarity was not high enough. The yield of the
reaction in water was very low, maybe because water can
hydrolysis the CLN bond of the catalyst, and also the mass
transfer is probably retarded because water may form a film
layer on the catalyst surface, and thus the possibility of
Fig. 8 shows the TGA curves of PVAm, PVAm/SBA-15, SA–
PVAm/SBA-15 and Mn–SA–PVAm/SBA-15 under an N
sphere. The weight loss (around 34%, w/w) of PVAm begins at
70 uC because of the thermo-degradation of the PVAm
2
atmo-
1
32
diffusing to active sites was reduced. Reaction in methanol
polymer chains and the degradation ends at 550 uC (Fig. 8a);
whereas for the PVAm/SBA-15 sample two separate weights
led to a good yield (80%) and selectivity (90%) during the
oxidative reaction, and acetonitrile was the best solvent with
Table 2 Oxidation of benzyl alcohol (2 mmol) with TBHP over Mn–SA–PVAm/SBA-15 under different conditions
a
Entry
Solvent
T (uC)
Catalyst (g)
TBHP (mmol)
Yield (%)
Selectivity (%)
1
2
3
4
5
6
7
8
9
1
1
1
1
1
1
n-hexane
Methanol
Water
65
65
65
65
65
reflux
25
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.1
0.08
0.12
0.14
0.1
0.1
0.1
—
3
3
3
3
3
3
3
3
3
3
3
2
4
5
3
70
80
30
60
80
90
48
65
80
90
90
65
75
65
5
.99
90
80
Solvent-free
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
Acetonitrile
90
.99
.99
.99
.99
.99
.99
.99
.99
90
40
reflux
reflux
reflux
reflux
reflux
reflux
reflux
0
1
2
3
4
5
80
.99
a
Isolated yield after 20 min.
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0% yield and selectivity of more than 99%, and was chosen as
the solvent for further investigation. Reaction in solvent-free
conditions gave 60% yield of benzaldehyde.
Paper
8
the amount of coordinated manganese, the yield remained
constant. In order to study the effect of the amount of SBA-15
on the oxidation reaction, several catalysts with various ratios
of SBA-15 to PVAm were prepared (Table 3, entries 9–11). The
results showed that the yield of benzaldehyde was increased
upon increasing the amount of SBA-15 in the catalyst. These
results are due to the large surface area of the Mn–SA–PVAm/
SBA-15 in comparison with Mn–SA–PVAm. When the amount
of SBA-15 is increased, the polymeric material is ordered in the
surface of catalyst. So the functional groups of the polymer are
free and more effective for salicylaldehyde grafting. The best
ratio of SBA-15 to acryl amide was found to be 4 (w/w).
In order to show the generality and scope of the new
protocol, several benzylic alcohols were oxidized in the
presence of Mn–SA–PVAm/SBA-15 (Scheme 2 and Table 4). In
all cases, the oxidation reaction proceeded smoothly to give
the corresponding benzaldehyde in moderate to good yields.
The reaction with benzylic alcohols carrying electron-donating
or electron-withdrawing groups gave the corresponding
product in good yields and high purity.
The recycling performance of the Mn–SA–PVAm/SBA-15 in
the oxidation reaction was also investigated. After each
reaction, the catalyst was filtered, washed with dry methanol
(2 6 5 mL), dried at room temperature and reused in the next
run without further purification. The catalyst could be reused
at least 5 times without appreciable decrease in yield and
reaction rate.
In order to prove the heterogeneous nature of the catalyst
and the amount of Mn leaching, a heterogeneity test was
performed, in which the catalyst (Mn–SA–PVAm/SBA-15) was
separated from the reaction mixture at approximately 50%
conversion of the starting material through centrifugation.
The reaction progress in the filtrate was monitored (Fig. 9). No
further oxidation reaction occurred even at extended times,
indicating that the nature of the reaction process was
heterogeneous and there was no progress for the reaction in
the homogeneous phase.
The effect of temperature was studied in the range 25–81 uC
in acetonitrile (Table 2, entries 5–8). The yield increased from
48% to 90% upon increasing the temperature from 25 uC to 81
uC, without any substantial loss of selectivity (.99%).
The effect of the amount of catalyst on the reaction was also
investigated. Generally, there was 5% yield in the absence of
catalyst (Table 2, entry 15), indicating that this was indeed an
Mn–SA–PVAm/SBA-15 catalyzed reaction and the yield was
increased by the catalyst. The optimum amount of catalyst (0.1
g) was determined from experiments corresponding to entries
6
and 9–11 in Table 2. No improvement in the yield was found
with further increase in the amount of catalyst. Hence, the
optimal amount of catalyst was chosen as 0.1 g based on 2.0
mmol of benzyl alcohol. Then, the best amount of the TBHP
was determined for the oxidation of benzyl alcohol (Table 2,
entries 6 and 12–14). The yield increased from 65 to 90% upon
increasing the amount of TBHP from 2 to 3 mmol. Upon
further increasing the amount of TBHP, the yield was
decreased. This may be due to the blocking of active sites of
3
3
the catalyst by water molecules from the TBHP solution.
The variation of the catalytic activity with the amount of
salicylaldehyde, Mn and SBA-15 in the catalyst on the
oxidation reaction was investigated. The results are summar-
ized in Table 3. The best yield of benzaldehyde was obtained
by using catalyst containing 25 mmol of salicylaldehyde per
gram of PVAm/SBA-15 (Table 3, entries 1–6). Higher amounts
of salicylaldehyde did not improve the yield to a great extent.
On the other hand, it was shown that the yield of the reaction
in the presence of Mn–SA–PVAm/SBA-15 as a catalyst was
higher than that with Mn–PVAm/SBA-15 (Table 3, entry 1). The
higher activity of Mn–SA–PVAm/SBA-15 is due to the existence
of the salicylaldehyde Schiff base ligand as an electron
donating ligand which facilitates the electron transfer. Also,
there was an upward trend of the benzaldehyde yield upon
increasing the amount of Mn from 5 to 20 mmol per g of SA–
PVAm/SBA-15 (Table 3, entries 6–9). Upon further increasing
In addition, to investigate the effect of SBA-15 on the
heterogeneity of the catalyst and prevention of Mn leaching,
another experiment was performed, in which the catalyst was
prepared without SBA-15, with the same amounts of the other
materials (Mn–SA–PVAm). Then, the reaction progress was
monitored over this catalyst and the amount of Mn leaching
was measured in the solution. The same reaction was carried
out over the Mn–SA–PVAm/SBA-15 and the amount of Mn
leaching was calculated under the same conditions. The
results are shown in Table 5. According to the results, Mn–
SA–PVAm showed less catalytic activity than Mn–SA–PVAm/
SBA-15 which can be related to a greater amount of Mn
leaching in the Mn–SA–PVAm system. Therefore, it can be
Table 3 Oxidation of benzyl alcohol over different Mn–SA–PVAm/SBA-15
a
catalysts
Salicylaldehyde
mmol per g of
PVAm/SBA-15)
Mn (mmol
per g of
SA–PVAm/SBA-15)
SBA-15
(g per g
of AA)
b
(
Yield
(%)
Entry
1
2
3
4
5
6
7
8
9
—
5
10
10
10
10
10
10
5
15
20
20
20
2
2
2
2
2
2
2
2
2
1
4
50
60
65
70
75
80
70
82
85
75
90
10
15
20
25
25
25
25
25
25
1
1
0
1
a
Reaction conditions: benzyl alcohol (2 mmol), catalyst (0.1 g),
b
TBHP (3 mmol), acetonitrile (5 mL), reflux, 20 min. Isolated yield
with more than 99% selectivity.
Scheme 2 Oxidation of benzylic alcohols catalyzed by Mn–SA–PVAm/SBA-15.
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Table 4 Oxidation of benzylic alcohols over Mn–SA–PVAm/SBA-15
b
Entry
1
Substrate
Product
Time (min)
20
Yield (%)
90
85
2
3
20
30
83
73
4
5
6
30
20
35
70
70
7
8
35
30
30
75
30
40
9
1
0
1
40
25
50
50
1
a
b
3
Reaction conditions: alcohol (2 mmol), TBHP (3 mmol), catalyst (0.1 g), CH CN (5 mL), reflux. Isolated yield with more than 99%
selectivity.
concluded that the mesoporous structure of SBA-15 prevents
Mn leaching and this mesoporous structure acts as a proper
cage (micro reactor) for the active species of the catalyst.
According to the above results, we can say that SBA-15 plays
two important roles in this composite. One of the greatest
advantages of Mn–SA–PVAm/SBA-15 is its ease of use due to its
powdery structure. Mn–SA–PVAm is adhesive and this char-
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both. The results clearly show that Mn–SA–PVAm/SBA-15
promotes the reaction more effectively than the other catalysts.
4. Conclusion
In summary, we have found that Mn–SA–PVAm/SBA-15 is a
new, efficient and environmentally benign catalyst for the
oxidation of alcohols in high yield, selectivity and purity. This
composite was prepared by a very simple method without
using any organosilica precursors (organosilica precursors are
expensive and using them involves complicated synthesis and
purification methods). Also, heterogenization increased the
activity of the homogeneous catalyst due to site-isolation and
cooperative effects between the SBA-15 support and the metal
complexes. Additionally, the ordered mesostructure of the
material also plays an important role in obtaining high
reactivity and selectivity towards the oxidation reaction.
Therefore, easy preparation without using expensive organo-
silica precursors, high thermal stability, high catalyst activity,
good reusability, and environmental friendliness are some of
the catalyst advantages. Further applications of this catalyst to
other transformations are currently under investigation.
Fig. 9 Heterogeneity test for the oxidation reaction.
Table 5 Effect of SBA-15 on catalytic activity
ab
Catalyst
Yield (%)
Mn leaching (ppm)
Mn–SA–PVAm/SBA-15
Mn–SA–PVAm
90
50
0.4
2.1
a
Reaction conditions: benzyl alcohol (2 mmol), catalyst (0.1 g),
b
TBHP (3 mmol), acetonitrile (5 mL), reflux, 20 min. Isolated yield
with more than 99% selectivity.
Acknowledgements
The support by Islamic Azad University, Shahreza Branch
(
IAUSH) Research Council and Center of Excellence in
acter makes it hard to separate it from the vessel, but after
mixing with SBA-15 and making the Mn–SA–PVAm/SBA-15
composite, it becomes a powder, which is easy to be used and
recycled. Additionally, a comparative reaction using Mn–SA–
PVAm/SBA-15 and Mn–SA–PVAm showed that Mn–SA–PVAm/
SBA-15 is more efficient, due to completion of the reaction in a
short time, which is related to the lower amount of Mn
leaching. Actually, in this composite, SBA-15 is a micro reactor
for the active species of the catalyst and the reaction occurs in
it.
Chemistry is gratefully acknowledged.
References
1
2
3
4
R. A. Sheldon and J. K. Kochi, Metal-Catalyzed Oxidation of
Organic Compounds, Academic Press, New York, 1981.
N. E. Leadbeater and M. Marco, Chem. Rev., 2002, 102,
3217.
K. C. Gupta and H. K. Abdulkadir, J. Macromol. Sci., Part A:
Pure Appl. Chem., 2008, 45, 53.
A. A. Costa, G. F. Ghesti, J. L. de Macedo, V. S. Braga, M.
M. Santos, J. A. Dias and S. C. L. Dias, J. Mol. Catal. A:
Chem., 2008, 282, 149.
A comparative study was conducted for the use of Mn–SA–
PVAm/SBA-15 with some of the reported catalysts for the
oxidation of benzyl alcohol with respect to the catalyst,
oxidant, reaction time, yield and selectivity of the products
(Table 6). It is noteworthy that the oxidation reactions with
5
N. S. Venkataramanan, S. Premsingh, S. Rajagopal and
K. Pitchumani, J. Org. Chem., 2003, 68, 7460.
reported catalysts were carried out with longer reaction times
and the benzaldehyde was obtained in low yield, selectivity or
Table 6 Comparison of Mn–SA–PVAm/SBA-15 with some other catalysts for the oxidation of benzyl alcohol
Entry
Catalyst
Time
Solvent
Oxidant
Yield (%)
Selectivity (%)
Ref.
1
2
3
4
5
6
7
8
TEMPO, Cu(II) catalyst
2 h
9 h
4 h
4 h
3 h
5 h
6 h
20 min
H
2
O
O
TBHP
H
H
H
O
O
2
25
55
52
53
28
90
60
90
86
—
34
35
15
36
37
38
39
CoO–CeO
Cr(salen)–NH
Alkali-treated ZSM-5
Zn [Fe(CN) ]?3H
TiO -supported nano Au
Mg2.5Ni0.5Al HT
Mn–SA–PVAm/SBA-15
2
n-hexane
Solvent free
2
-MCM-41
2
2
2
O
O
O
2
2
2
99
86
99
93
97
.99
H
2
O
2
6
2
O
Solvent free
Solvent free
Toluene
2
2
in ScCO
2
2
CH
3
CN
TBHP
This work
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6
I. F. J. Vankelecom and P. A. Jacobs, Catalyst immobilisa-
24 Y. Jung, S. Kim, S. J. Park and J. M. Kim, Colloids Surf., A,
2008, 313, 292.
25 C. Hess, U. Wild and R. Schl o¨ gl, Microporous Mesoporous
Mater., 2006, 95, 339.
tion on inorganic supports, in Chiral Catalyst
Immobilisation and Recycling, ed. P. A. Jacobs, I. F. J.
Vankelecom, D. De Vos, Wiley-VCH, Weinheim, Germany,
2
000.
26 R. J. Kalbasi, M. Kolahdoozan and M. Rezaei, Mater. Chem.
Phys., 2011, 125, 784.
7
P. Oliveira, A. Machado, A. M. Ramos, I. Fonseca, F.
M. Braz Fernandes, A. M. Botelho do Rego and J. Vital,
Microporous Mesoporous Mater., 2009, 120, 432.
Y. Guo, C. Hu, C. Jiang, Y. Yang, S. Jiang, X. Li and E. Wang,
J. Catal., 2003, 217, 141.
2
2
2
3
7 R. J. Kalbasi, M. Kolahdoozan, A. R. Massah and
K. Shahabian, Bull. Korean Chem. Soc., 2010, 31, 2618.
8 R. J. Kalbasi, M. Kolahdoozan, K. Shahabian and
F. Zamani, Catal. Commun., 2010, 11, 1109.
9 M. Colilla, F. Balas, M. Manzano and M. V. Regi, Chem.
Mater., 2007, 19, 3099.
8
9
J. Hu, K. Li, W. Li, F. Ma and Y. Guo, Appl. Catal., A, 2009,
364, 211.
1
1
1
1
0 S. A. Patel, S. Sinha, A. N. Mishra, B. V. Kamath and R.
N. Ram, J. Mol. Catal. A: Chem., 2003, 192, 53.
1 E. D. P. Carreiro and A. J. Burke, J. Mol. Catal. A: Chem.,
2
2 G. Grivani, S. Tangestaninejad and A. Halili, Inorg. Chem.
Commun., 2007, 10, 914.
3 J. Zhao, J. Han and Y. Zhang, J. Mol. Catal. A: Chem., 2005,
0 D. Y. Zhao, J. L. Feng, Q. S. Huo, N. Melosh, G.
H. Fredrickson, B. F. Chmelka and G. D. Stucky, Science,
1998, 279, 548.
006, 249, 123.
31 M. Salavati-Niasari, S. N. Mirsattari and M. Bazarganipour,
Polyhedron, 2008, 27, 3653.
32 Y. Chujo and T. Saegusa, Adv. Polym. Sci., 1992, 100, 11.
33 K. M. Parida and D. Rath, Appl. Catal., A, 2007, 321, 101.
34 P. J. Figiel, A. M. Kirillov, Y. Y. Karabach, M. N. Kopylovich
and A. J. L. Pombeiro, J. Mol. Catal. A: Chem., 2009, 305,
2
31, 129.
1
1
4 B. Z. Zhan and A. Thomposon, Tetrahedron, 2004, 60, 2917.
5 X. Wang, G. Wu, J. Li, N. Zhao, W. Wei and Y. Sun, J. Mol.
Catal. A: Chem., 2007, 276, 86.
6 M. Zanetti, S. Lomakin and G. Camino, Macromol. Mater.
Eng., 2000, 279, 1.
7 C. Yang, P. H. Liu, Y. F. Ho, C. Y. Chiu and K. J. Chao,
Chem. Mater., 2003, 15, 275.
8 M. T. Run, S. Z. Wu, D. Y. Zhang and G. Wu, Mater. Chem.
Phys., 2007, 105, 341.
1
78.
5 S. S. Deshpande and R. V. Jayaram, Catal. Commun., 2008,
, 186.
6 A. Jia, L. L. Lou, C. Zhang, Y. Zhang and S. Liu, J. Mol.
Catal. A: Chem., 2009, 306, 123.
7 S. R. Ali, V. K. Bansal, A. A. Khan, S. K. Jain and M.
A. Ansari, J. Mol. Catal. A: Chem., 2009, 303, 60.
8 X. Wang, H. Kawanami, S. E. Dapurkar, N.
S. Venkataramanan, M. Chatterjee, T. Yokoyama and
Y. Ikushima, Appl. Catal., A, 2008, 349, 86.
3
3
3
3
1
1
1
9
1
2
2
9 G. Kickelbick, Prog. Polym. Sci., 2003, 28, 83.
0 S. S. Ray and O. Masami, Prog. Polym. Sci., 2003, 28, 1539.
1 D. A. Ruddy, N. L. Ohler, A. T. Bell and T. D. Tilley, J. Catal.,
3
9 T. Kawabata, Y. Shinozuka, Y. Ohishi, T. Shishido,
K. Takaki and K. Takehira, J. Mol. Catal. A: Chem., 2005,
236, 206.
2
006, 238, 277.
2
2
2 L. Jiao and J. R. Regalbuto, J. Catal., 2008, 260, 342.
3 A. H. Lu and F. Schuth, Adv. Mater., 2006, 18, 1793.
RSC Adv.
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