A cellulose-supported Mn(salen)Cl complex as an efficient heterogeneous catalyst for the selective oxidation of benzylic alcohols

Article abstract of DOI:10.1007/s13738-014-0457-z

[Mn(salen)Cl-cellulose] was synthesized by immobilization of homogeneous Mn(salen)Cl complex on cellulose and characterized by FT-IR, TGA and atomic absorption spectroscopy. The resulted catalyst exhibited moderate to high reactivity in the oxidation of benzylic alcohols into carbonyl compounds using oxone as oxidant in ambient conditions. The catalytic activity of Mn(salen)Cl and [Mn(salen)Cl-cellulose] in this reaction was investigated. The heterogeneous catalyst showed higher catalytic activity with respect to neat Mn(salen)Cl complex.

Full text of DOI:10.1007/s13738-014-0457-z

J IRAN CHEM SOC
DOI 10.1007/s13738-014-0457-z
ORIGINAL PAPER
A cellulose-supported Mn(salen)Cl complex as an efficient
heterogeneous catalyst for the selective oxidation of benzylic
alcohols
•
Mohammad Ali Nasseri Arezou Mohammadinezhad
•
Mehri Salimi
Received: 11 December 2013 / Accepted: 25 April 2014
Ó Iranian Chemical Society 2014
Abstract [Mn(salen)Cl-cellulose] was synthesized by
immobilization of homogeneous Mn(salen)Cl complex on
cellulose and characterized by FT-IR, TGA and atomic
absorption spectroscopy. The resulted catalyst exhibited
moderate to high reactivity in the oxidation of benzylic
alcohols into carbonyl compounds using oxone as oxidant
in ambient conditions. The catalytic activity of Mn(sa-
len)Cl and [Mn(salen)Cl-cellulose] in this reaction was
investigated. The heterogeneous catalyst showed higher
catalytic activity with respect to neat Mn(salen)Cl
complex.
The field of surface chemistry describes a variety of
natural or synthetic materials suitable for chemical surface
reactions, with the main objective to improve the overall
capacity for applications [2, 3]. A lot of these supports are
derived from inorganic compounds, such as silica and other
oxides; however, some important reports describe excellent
results in which are related to biopolymers [1, 4, 5]. In this
sense, the most abundant biopolymer is cellulose. It has
excellent thermal, mechanical properties and biocompati-
bility, also for economic and scientific reasons, is a
promising material for biochemical engineering [6, 7]. It
was widely applied as an efficient support and/or template
for the synthesis of inorganic materials. For example,
palladium nanoparticles were synthesized on the cellulose
matrix and employed as an active catalyst for the hydro-
genation reaction of phenol and the Heck coupling of
styrene and iodobenzene [8]. In recent years, cellulose has
gained renewed significance as a raw material and still
possesses a high potential for future applications [9–12].
Schiff bases are versatile ligands synthesized from the
condensation of an amino compound with carbonyl com-
pounds and these coordinate to metal ions.
Keywords [Mn(salen)Cl-cellulose] Á Cellulose Á Oxone Á
Oxidation
Introduction
Recently, science and technology have devoted to the
introduction and applications of environmentally friendly
and sustainable catalysts. One of the useful strategies for
the preparation of such catalysts is the attachment of
organic or inorganic materials to different solid supports.
The homogeneous catalysts suffer from the drawbacks of
poor catalyst recovery and product separation [1]. Low
toxicity, moisture resistance, air tolerance, great selectivity,
easy handling, and low cost are some of the beneficial
features of this method that make it an efficient alternative
to homogeneous catalyst.
Schiff’s base and their complexes possess remarkable
properties as catalysts in various biological systems,
polymers, dyes, antimicrobial activities, antifungal activi-
ties, antiviral activities insecticides, antitumor and cyto-
toxic activities, plant growth regulator, enzymatic activity
and pharmaceutical fields. A variety of schiff’s base and its
complexes have been studied extensively. Several model
systems, including those with bidentate, tridentate, tetra-
dentate, multidentate Schiff base ligands, and their coor-
dination chemistry of Mn attract much attention because of
its biological relevance and its own interesting coordina-
tion chemistry such as geometry, flexible redox property,
and oxidation state [13].
M. A. Nasseri (&) Á A. Mohammadinezhad Á M. Salimi
Department of Chemistry, College of Sciences,
University of Birjand, 97175-615 Birjand, Iran
e-mail: manaseri@birjand.ac.ir
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J IRAN CHEM SOC
In this work, [Mn(salen)Cl-cellulose] was synthesized
by immobilization of homogeneous Mn(salen)Cl complex
on cellulose. We reported comparison of the catalytic
activity of homogeneous Mn(salen)Cl complex and heter-
ogeneous Mn(salen)Cl complex in the oxidation reaction.
Oxidation reaction carried out using a variety of oxidants.
Among these oxygen donors, oxone is the best oxidant
[14–20].
complex. LiCl (29.15 mmol, 1.224 g) was then added.
After adding 3 gr cellulose and stirring the mixture over-
night at room temperature, product was isolated by
removing the solvent. The crude product was washed with
water, EtOH and ether, respectively. Further drying of the
solid product was carried out in an oven at 80 °C for 8 h
[21].
General procedure for catalytic oxidation of benzyl
alcohol with oxone catalyzed by Mn(salen)Cl-cellulose
Materials and methods
In a typical procedure, a suspension of supported Mn
catalyst (0.3 gr), oxone (1 mmol, 0.614 g), aromatic
alcohols (1 mmol) and ethanol (3 ml) was stirred at room
temperature for 30 min. After completion of the reaction
(TLC), the catalyst was filtered and the mixture was
extracted with diethyl ether (3 9 5 ml), the organic layer
was washed with brine (5 ml), dried over MgSO4, and
concentrated. Purity determinations of the products were
accomplished by TLC on silica-gel polygram SILG/UV
254 plates.
All chemicals and solvents were purchased from Merck or
Fluka and were used without purification.
The structure of Mn(salen)-cellulose was obtained by
interpretation of the spectra TGA50 (Shimadzu-Japan) on a
platinum cell, FT-IR 4300 (Shimadzu-Japan) on a range
from 400 to 4,000 cm-1 and atomic absorption spectros-
copy (AA-6300 Shimadzu-Japan).
Preparation of catalyst
To examine the effect of catalyst, we studied this
reaction in the absence of catalyst. Our results showed
the oxidation of benzyl alcohol with oxone even in the
absence of catalyst also gave benzaldehyde, but in
lower yield (8 %). Oxone is a versatile oxidizing agent
for the oxidation of alcohols but according to the liter-
ature, in the absence of catalyst, its reaction time is too
long [22–24].
3-Trimethoxysilylpropylamine (10 mmol, 1.76 ml) was
added to a suspension of salicylaldehyde (10 mmol,
1.044 ml) in absolute ethanol (10 ml) in ambient temper-
ature. The solution instantly became yellow, indicating the
formation of the schiff base. After 30 min, Mn(CH3COO)2
.4H2O (5 mmol, 1.225 g) was added to the solution, and
the mixture was further stirred for 2 h to form Mn(salen)
Scheme 1 Preparation of
cellulose-supported Mn(III)
catalyst
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J IRAN CHEM SOC
Fig. 1 Derivative TG (DTG)
curve Mn(salen)Cl-cellulose
Results and discussion
Characterization of catalyst
Scheme (1) shows synthesis procedure of [Mn(salen)Cl-
cellulose].
Spectroscopic techniques including FT-IR, TGA and
atomic absorption analysis were used to study the structure
of the manganese(III)-salen complex. The loading of Mn
on cellulose was 0.266 mmol/g estimated by AAS.
The derivative TG (DTG) curve of Mn(salen)Cl-cellu-
lose is shown in the Fig. 1. The recorded curve showed
weight loss at temperatures ranging from 265 to 575 °C,
which corresponds to the loss of salen complex and cel-
lulose [25]. The loading of Mn on cellulose was about
0.255 mmol/g, as determined from the 87.709 % loss in
mass between 265 and 575 °C, in accordance with the
result obtained from AAS. Covalent bands are responsible
for high thermal stability of catalyst and it caused the
catalyst stable up to 300 °C.
Figure 2 shows the FT-IR spectra of the cellulose,
Mn(salen)Cl and supported complexes of Mn. The
spectrum of Mn(salen)Cl shows a band at 1,632 cm-1
due to imine t(C = N) and another band at 1,596 cm-1
attributed to the aromatic ring vibrations (Fig. 2c). The
peak at 646 cm-1 is assigned to the stretching vibrations
of the Mn–O bond of the complex. In addition, the
observed peaks in the region of 1,300–1,600 cm-1 are
due to C–O, C–N and aromatic ring. In Fig. 2b, cellulose
exhibited bands at 3,200–3,500 cm-1 and 1,000–
1,300 cm-1 that related to OH, C–O and an anomeric
carbon. There were some differences between the FT-IR
spectra of the pure catalyst (Fig. 2c) and supported cat-
alyst (Fig. 2a). In the spectrum of Mn(salen)Cl-cellulose
(Fig. 2a), the vibration intensity of bonds became very
weak.
Fig. 2 FT-IR spectra of: a [Mn(salen)Cl–cellulose], b cellulose and
c [Mn(salen)Cl]
The effect of solvent on the oxidation of benzyl alcohol
with oxone catalyzed by Mn (salen)Cl-cellulose
To find the optimized conditions, benzyl alcohol was used
as a model substrate. The oxidation reactions were carried
out in various solvents at room temperature. According to
the Table 1, n-hexane, chloroform and acetone showed
These observations clearly showed the attachment of
salen complex onto cellulose.
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J IRAN CHEM SOC
Table 1 Effect of solvent on the oxidation of benzyl alcohol cata-
lyzed by [Mn(salen)Cl-cellulose]
Table 3 Optimization of oxidant amount in the oxidation of benzyl
alcohol catalyzed by [Mn(salen)Cl-cellulose]
Entry
Solvent
Benzaldehyde %
Time (min)
Entry
Oxidant (mmol)
Yield %
1
H2O
0
50
60
60
85
40
50
0
30
180
60
1
2
3
0.5
1
50
85
85
2
Toluene
3
CH3CN
1.5
4
CH3OH
40
Reaction conditions: benzyl alcohol (1 mmol), catalyst (0.3 gr),
CH3CH2OH (2 ml), T = 25 °C, t = 30 min
5
CH3CH2OH
CH3CH2OCOCH3
1,4-Dioxane
DMF
30
6
60
7
60
Table 4 The effect of catalyst amount in the oxidation of benzyl
alcohol with oxone catalyzed by [Mn(salen)Cl-cellulose]
8
120
90
9
Acetone
80
92
60
60
80
0
Entry
Catalyst (g)
Yield (%)
10
11
12
13
14
n-Hexane
Dichloromethane
THF
180
90
1
2
3
4
5
0.1
40
63
85
85
8
90
0.2
Chloroform
Solvent free
90
0.3
180
0.4
Reaction conditions: benzyl alcohol (1 mmol), catalyst (0.3 gr), ox-
one (1 mmol), Solvent (2 ml), T = 25 °C
None
Reaction conditions: benzyl alcohol (1 mmol), oxone (1 mmol),
CH3CH2OH (2 ml), T = 25 °C, t = 30 min
Table 2 Effect of different oxidants on the oxidation of benzyl
alcohol catalyzed by [Mn(salen)Cl-cellulose]
Optimization of the oxidant and catalyst amount
in the reaction of benzyl alcohol with oxone catalyzed
by Mn(salen)Cl-cellulose
Entry
Oxidant
Yield of benzaldehyde (%)
Time (min)
1
2
3
4
5
Oxone
UHP
85
30
15
50
0
30
600
600
600
600
To find the optimized conditions, we changed the amount of
oxidant and catalyst for the oxidation of benzyl alcohol using
oxone in the presence of ethanol at room temperature. Con-
sequently, the best conditions used for the oxidation of benzyl
alcohol by this catalytic system were the 0.3 g catalyst and
1 mmol oxidant (Tables 3 and 4). Control experiments indi-
cated that in the absence of the catalyst, the reaction at the same
condition gives benzaldehyde in a rather low yield of 8 %.
H2O2
KIO4
None
Reaction conditions: benzyl alcohol (1 mmol), catalyst (0.3 gr), sol-
vent (2 ml), oxone (1 mmol), T = 25 °C
high yield but the time for their reactions is too long.
Ethanol (entry 5) shows high yield in the shortest time so it
was chosen as reaction medium for the oxidation of benzyl
alcohols.
Oxidation of alcohols with oxone catalyzed
by Mn(salen)Cl-cellulose
Based on the selective conversion of benzyl alcohol to the
corresponding aldehyde during the experiments (Table 5,
entry 1–6), the oxidation of aliphatic alcohols was inves-
tigated at room temperature in the presence of oxone. The
oxidation of benzylic alcohols occurred selectively to give
the corresponding aldehydes in 30–120 min (Table 5,
entries 1–6), while aliphatic alcohol such as ethanol
remained unreactive. A possible reason for this is the
known reactivity of oxone toward benzylic C–H bonds
[22]. As shown in Table 5, the electron-rich groups
exhibited high yields and short times, possibly these groups
stabilize the intermediates produced along the reaction
pathway for the incorporation of oxygen into the molecule.
This hypothesis is reinforced by our results for the
The effect of different oxidants on the oxidation
of benzyl alcohol with oxone catalyzed
by Mn(salen)Cl-cellulose
The effect of different oxidants on the catalytic activity of
Mn(salen)Cl-cellulose in the oxidation of benzyl alcohol
was studied. We chose different oxidants such as Oxone,
KIO4, UHP and H2O2. The results are summarized in
Table 2. When KIO4, UHP and H2O2 were used as the
oxygen source in ethanol, Mn(salen)Cl-cellulose, pro-
duced low benzaldehyde. Oxone is a strong, cheap and
versatile oxidizing agent. The results showed that in the
presence of oxone as a oxygen source higher yield was
obtained.
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J IRAN CHEM SOC
Table 5 Oxidation of aromatic
alcohols with oxone catalyzed
by Mn (salen)Cl and
O
OH
Oxone/ Cat
EtOH
H
[Mn(salen)Cl-cellulose]
X
X
X= OH, Cl, OCH3
Entry R1
Time (min) Yield (%)
TOF(h-1
)
Mn(salen)Cl
Mn(salen)Cl-
cellulose
Mn(salen)Cl Mn(salen)Cl-
cellulose
Aldehyde
acid
Aldehyde acid
1
2
OH
30
20
30
0
85
82
0
3.748
1.875
20.5
5.125
OH
120
10
11
OH
3
4
5
OH
OH
120
120
120
54
40
25
8
21
17
100
80
0
10
5
3.375
2.5
6.25
5
HO
Cl
Cl
61
1.562
3.812
OH
Reaction conditions: alcohols
(1 mmol), oxone (1 mmol),
catalyst (0.3 gr), CH3CH2OH
(2 mL), T = 25 °C
Even after 10 h, the product
was not observed
OH
6
120
30
13
50
14
1.875
3.125
H3CO
a
7
8
CH3CH2OH
Cellulose
600
600
–
–
–
–
–
–
–
–
–
–
–
a
a
–
oxidation reactions of alcohols. For these compounds, the
electron-rich groups were directly oxidized, and thus, the
appropriate reaction occurs immediately with excellent
yield. In aromatic rings containing electron-attraction
groups, the yields were lower. The 4-hydroxy benzyl
alcohol can be converted to the corresponding aldehyde in
high yield, but the time of this reaction is longer than the
oxidation of benzyl alcohol (Table 5, entries 1 and 3). For
the 2-chloro benzyl alcohols, the yield of the product is
moderate (Table 5, entry 5). The best result in the short
time was obtained for the oxidation of benzyl alcohol in
85 % yield (Table 5, entry 1). In addition, we observed that
the mainly products are the benzaldehyde, not acid.
attach to cellulose, and therefore the catalytic activity of
Mn(salen)Cl does not reduce. Owing to the abilities of
support to highly disperse the active complexes, and to
control the local concentrations of the reactant and product
around active sites, the immobilization led to an increase in
catalytic performance in the oxidation reactions (calculated
in terms of TOF). One of the problems of homogeneous
catalysis using Mn(III)-salen complexes is the formation of
catalytically inactive oxomanganese species that limits
recycling of the catalyst. One of the potential advantages of
polymer-bound catalysts is the possibility of suppressing
this side reaction due to the spatial separation of individual
metal complexes on the solid support [26–34]. So the cat-
alytic activity of Mn(salen)Cl complex can be modified by
attaching to cellulose through covalent bonding.
Comparison of catalytic activity of Mn(salen)Cl
and Mn(salen)Cl-cellulose
Catalyst reusability and stability
To show the effect of immobilization on the catalytic
activity of Mn(salen)Cl in the alcohol oxidation with oxone,
we repeated all reactions under optimization conditions.
The results are summarized in Table 5. Our results showed
that the turnover frequency (h-1) (TOF) for the Mn(sa-
len)Cl-cellulose catalyst was higher than the Mn(salen)Cl
catalyst. One probable explanation is that the ligands of
Mn(salen)Cl complex are not oxidized when complex
It is important to verify the stability of the heterogeneous
catalyst since metal leaching from catalyst becomes
responsible for decreasing of the catalytic activity in suc-
cessive cycles. To assess long-term stability and reusability
of [Mn(salen)Cl-cellulose], oxidation of benzyl alcohol
was chosen as model reaction, and recycling experiments
were carried out with a single sample of the catalyst. After
123

J IRAN CHEM SOC
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each experiment, the catalyst was filtered, washed with
H2O, dried and reused. After reusing catalyst for four
consecutive times, the yield was 80 %. The results showed
that the catalytic activity of Mn(salen)Cl-cellulose is
appropriate for more than four-time reuse (Fig. 3).
Conclusions
In this study, we have developed a highly efficient oxida-
tion of benzylic alcohols to carbonyl compounds by oxone
in the presence of Mn(salen)Cl-cellulose. Mn(salen)Cl
complex was successfully immobilized on to cellulose and
this heterogeneous catalyst was used at neutral and mild
reaction conditions to convert benzyl alcohols to aldehyde
and acid. This catalyst also remarkably alters ratio of
aldehyde/acid, which may attributed to the immobilization
mode and support effect of heterogeneous catalyst.
Mn(salen)Cl-cellulose has received considerable attention
as an inexpensive, eco-friendly, highly reactive, easy to
handle and non-toxic catalyst for various organic trans-
formations, affording the corresponding products in
excellent yields. Moreover, this catalyst as a recoverable
catalyst can be easily separated from reaction mixture by a
simple filtration and reused repeatedly and can be stored
for a long time without any reducing of its reactivity.
Acknowledgments The authors are thankful to the University of
Birjand Research Council for the financial support of this work.
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