Cobalt-chitosan: Magnetic and biodegradable heterogeneous catalyst for selective aerobic oxidation of alkyl arenes and alcohols

Article abstract of DOI:10.1007/s12039-015-0959-y

A novel and biodegradable cobalt-chitosan as a magnetic heterogeneous catalyst was synthesized and characterized by XPS, FT-IR, EDX and TEM. Catalytic performance of cobalt- chitosan was tested by aerobic oxidation of alkyl arenes and alcohols. The results show that the catalyst exhibits excellent conversion for selective aerobic oxidation of various alkyl arenes, primary and secondary alcohols with air as the only oxidant. The catalyst can be easily separated by magnetic devices and reused for 5 runs without appreciable loss of activity. [Figure not available: see fulltext.]

Full text of DOI:10.1007/s12039-015-0959-y

c
J. Chem. Sci. Vol. 127, No. 11, November 2015, pp. 1927–1935. ꢀ Indian Academy of Sciences.
DOI 10.1007/s12039-015-0959-y
Cobalt-chitosan: Magnetic and biodegradable heterogeneous catalyst
for selective aerobic oxidation of alkyl arenes and alcohols
AHMAD SHAABANI∗, MAHMOUD BORJIAN BOROUJENI and
MONA HAMIDZAD SANGACHIN
Faculty of Chemistry, Shahid Beheshti University, G. C., P. O. Box 19396-4716, Tehran, Iran
e-mail: a-shaabani@sbu.ac.ir
MS received 12 May 2015; revised 7 August 2015; accepted 8 August 2015
Abstract. A novel and biodegradable cobalt-chitosan as a magnetic heterogeneous catalyst was synthesized
and characterized by XPS, FT-IR, EDX and TEM. Catalytic performance of cobalt- chitosan was tested by
aerobic oxidation of alkyl arenes and alcohols. The results show that the catalyst exhibits excellent conversion
for selective aerobic oxidation of various alkyl arenes, primary and secondary alcohols with air as the only
oxidant. The catalyst can be easily separated by magnetic devices and reused for 5 runs without appreciable
loss of activity.
Keywords. Cobalt; chitosan; heterogeneous catalyst; magnetism; aerobic oxidation; alkyl arene.
cobalt onto inorganic solid supports such as TiO2,31
silica,36 ZnO,37 zeolites,38,39 Montmorillonite,40 and
commercial carbon.41
1. Introduction
The selective oxidation of alkyl arenes and alcohols into
the corresponding carbonyl compounds is an impor-
tant goal in synthesis of fine chemicals.1 From green
chemistry point of view, improving methods for oxida-
tion which use clean oxidants and reduce the amount
of waste are important.2 Numerous homogeneous cat-
alysts are widely used for aerobic oxidation, however
they have drawbacks such as tedious catalyst separation
and pollution of the product with catalyst.3–9 Immobi-
lization of homogeneous catalyst on the solid supports
simplifies the reaction procedure and increases the sta-
bility and recyclability of the catalyst.10 In a few cata-
lysts that were developed for aerobic oxidation, noble
metal-based catalysts, such as Au, Pt, Pd, Ru, etc., are
widely used. However, these catalysts are expensive and
the reaction conditions such as strongly basic medium
often reduce the selectivity of the reaction.11–20
Using low cost and nontoxic base metal such as
cobalt as catalyst is worthwhile. Cobalt, due to strong
capability for dioxygen activation, is attractive in aer-
obic oxidation.21 Cobalt-based catalyst systems are
widely used for oxidation of alcohols,22–31 sulfides,32
oximes33 and alkylbenzenes.34,35 Recently, to overcome
the drawbacks of homogeneous cobalt-based catalysts,
much attention has been focused on immobilization of
Natural organic polymers such as cellulose and chi-
tosan are interesting materials as suitable supports
for catalytic applications due to low cost and high
surface area. These polymers have interesting prop-
erties such as biodegradability, biocompatibility, and
non-toxicity.42,43 Chitosan is a natural polysaccharide
prepared by deacetylation of chitin. Chitin is a natu-
ral polymer obtained from crabs, insects and shrimps.
Amine and hydroxyl groups in chitosan chain can
chelate with metal ion. Using chitosan as a support
was reported due to its advantage properties such as
low toxicity and high affinity for transition metal ions
in solution.44–50 It is important to note that not only
cobalt was easily separated from chitosan by chemi-
cal treatment or carbonization but also exhibited good
reusability by virtue of isolation using an external
magnet.
In continuation of our efforts to synthesize of chemi-
cally supported heterogeneous cellulose- and graphene-
based heterogeneous catalysts for organic reactions,51,52
herein we report the synthesis and characterization of
a new biodegradable and magnetic cobalt-based hetero-
geneous catalyst supported on chitosan, which is highly
effective for selective aerobic oxidation of alkyl arenes,
primary and secondary aromatic alcohols. The catalyst
can be easily separated by magnets and reused up to five
times without loss of activity.
∗For correspondence
1927

1928
Ahmad Shaabani et al.
chemicals were purchased from Merck and used with-
2. Experimental
2.1 Materials and equipment
out further purification.
X-ray powder diffraction (XRD) data were col-
lected on a STOE STADI P with scintillation detec-
Chitosan (degree of deacetylation =88%, Mv= tor, secondary monochromator using Cu Kα radiation.
320000) was purchased from Sigma-Aldrich. Other Cobalt determination was carried out on Flame atomic
Figure 1. HR-TEM images of cobalt-chitosan. Cobalt particles are shown by arrows.
Figure 2. EDX results of cobalt –chitosan.
Figure 3. XPS spectrum of cobalt-chitosan.

Cobalt-chitosan: Magnetic and biodegradable catalyst
1929
Figure 4. Infrared spectra of chitosan (A) and cobalt-chitosan (B).
absorption spectroscopy (FAAS, Shimadzu model AA- (TGA) was done using STA 1500 instrument heating at
680 atomic absorption spectrometer) with a Co hollow the rate of 10◦C min−1 in air.
cathode lamp at 240.7 nm, using an air-acetylene flame.
Transmission electron microscopy (TEM) analysis was
performed by LEO 912AB electron microscope in an
ultra-high vacuum. For TEM imaging, a small amount
of the catalyst was dispersed in ethanol and sonicated
for 15 min and then a drop of solution was deposited
on carbon- supported Cu grid. Products were ana-
lyzed using Agilent 7890A Gas Chromatograph. Inter-
nal standard method was used to measure the amount of
the obtained products. 1-Bromodecane was used as an
internal standard. XPS analysis was performed using a
VG multilab 2000 spectrometer (Thermo VG scientific)
in an ultra-high vacuum. Thermogravimetric analysis
2.2 Catalyst preparation
Chitosan (0.5 g) was suspended in 50 mL of water. To
this suspension, 15 mL of 0.1 M solution of cobalt (II)
chloride hexahydrate was added and stirring continued
for 4 h at room temperature. Then, 5 mL of solution
of NaBH4 (2.2 mmol, 0.08 g) was added dropwise to
control the reduction process and prevent agglomera-
tion of cobalt particles under vigorous stirring to the
suspension during 4 h. The reaction solution stirred at
ambient temperature for 2 h to complete reaction under
an inert atmosphere. Finally, the black suspension was
Table 1. Effects of the solvent, temperature, reaction time, amount of catalyst and initiator on oxidation of alkyl arenes
using cobalt-chitosan.
Entry
Solvent
Initiator
Amount of catalyst (mol %)
Time (h)
Temperature (◦C)
Yield (%)a
1
2
3
4
5
6
7
8
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
1,2-dichlorobenzene
n- heptan
NHPI
NHPI
NHPI
NHPI
NHPI
–
NHPI
NHPI
NHPI
NHPI
NHPI
1
2
3
4
5
5
5
5
5
5
5
8
7
5
3
2
8
2
2
2
8
8
80
80
80
80
80
80
70
80
60
70
100
50
60
70
85
95
Trace
60
60
10
0
9
10
11
n- hexan
EtOH
H2O
0
Reaction conditions: diphenylmethane (1 mmol), NHPI (1 mol %), 10 mL solvent, air
a GC Yield

1930
Ahmad Shaabani et al.
separated by an external magnet, and the catalyst (5 mol %, 0.023 g) and NHPI (1 mol %, 0.016 g)
washed with deionized water and dried under vacuum in 10 mL 1,2-dichlorobenzene. The flask was fitted
at 60◦C. The catalyst was characterized by TGA, EDX,
XPS and TEM.
with a reflux condenser and mixture was heated at 80◦C
with continuous bubbling of air with flow rate of 15 mL
min−1. After completion of the reaction (progress of the
reaction followed by TLC), catalyst was separated by an
external magnet and the reaction mixture was analyzed
by gas chromatography.
2.3 General procedure for oxidation of alkyl arenes
Alkyl arenes (1 mmol) were added in a two round-
bottomed glass flask containing a mixture of cobalt-chitosan
Table 2. Aerobic oxidation of alkyl arenes with cobalt-chitosan.
Entry
Alkyl arene
Product
Time(h)
Yield a (%)
1
2
3
4
5
4
4
6
6
4
95
95
90
90
95
6
7
4
4
90
90
8
4
95
9
4
4
95
90
10
Reaction conditions: alkyl arene 1 mmol, 5 mol % catalyst, 0.023 g), and NHPI (1 mol
%, 0.016 g) in 10 mL 1,2-dichlorobenzene
a GC Yield

Cobalt-chitosan: Magnetic and biodegradable catalyst
1931
Table 3. Effects of the solvent, temperature and base on
oxidation of phenylethyl alcohol using cobalt-chitosan.a
3. Results and Discussion
At first, cobalt (II) on chitosan was immobilized.
The dropwise addition of solution of cobalt chloride
(CoCl2.6H2O) to chitosan suspension resulted in a light
pink coloration, which indicated formation of cobalt
(II)-chitosan complex. Then cobalt-chitosan was pre-
pared by chemical reduction of this suspension. The
morphology and size of cobalt-chitosan have been
investigated by TEM. Figure 1 shows TEM images
of catalyst that clearly illustrate the rod-like morphol-
ogy of cobalt particles. Cobalt particles were dispersed
on chitosan and they are very active in the aerobic
oxidation.
The chemical composition of cobalt-chitosan was
investigated using energy dispersive spectroscopy
(EDS) analysis. The EDS analysis clearly illustrated the
presence of cobalt in the catalyst (figure 2).
The XPS analysis of cobalt -chitosan was done to
obtain information about oxidation state of cobalt par-
ticles and their interaction with chitosan (figure 3). The
main peaks appeared in survey scan are Co 2p, C 1s, O
1s and N 1s. In XPS analysis of catalyst, we observed
two peaks for cobalt that correspond to Co (0) and Co
(II). Co 2p 3/2 peak of Co 2+ species appeared in 781.66
eV and cobalt metal phase peak appeared in 777.56 eV.
The ratio of cobalt metallic to cobalt ion was about 60
Entry
Solvent
Temperature
Base
Yield (%) b
1
2
3
4
5
6
7
8
DMF
DMF
CH3CN
H2O
EtOH
EtOH
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
p-xylene
100
100
80
reflux
70
70
80
80
80
80
80
K2CO3
KOH
K2CO3
KOH
K2CO3
KOH
KOH
K2CO3
K2CO3
K2CO3
Et3N
Pyridine
–
60
60
65
10
30
30
70
20 C
35 d
95
85
70
10
9
10
11
12
13
80
80
aReaction conditions: phenylethyl alcohol (1.00 mmol), base
(0.50 mmol) , catalyst (5 mol%, 0.23 g), solvent 10 mL for
4 h, air.
bYield determined by GC analysis. cin the presence of
CoCl2.6H2O as a catalyst; dusing Co(II)-chitosan coordina-
tion polymer as a catalyst (without reduction of Co(II)).
2.4 General procedure for the oxidation of aromatic
alcohols
Alcohols (1 mmol) were added in a two round-
bottomed glass flask containing a mixture of cobalt-
chitosan (5 mol %, 0.023 g) and K2CO3 (0.5 mmol,
0.069 g) in 10 mL p-xylene. The flask was fitted with a
reflux condenser and mixture was heated at 80◦C with
continuous bubbling of air with flow rate of 15 mL
min−1. After completion of the reaction (progress of the
reaction followed by TLC), catalyst was separated by
an external magnet and the reaction mixture was ana-
lyzed by gas chromatography using a CP-Sil-8 fused
silica capillary column.
to 40. The Co 2p
binding energy of metallic cobalt
3/2
in 781.1 eV was shifted to 777.56 eV. N 1s and O 1s
binding energies of cobalt-chitosan were appeared in
401 and 533.23 eV, respectively. According to the liter-
ature, N 1s and O 1s binding energies of chitosan were
appeared in 399.4 and 532.2 eV, respectively.53 Binding
energies of N 1s and O 1s were found to be higher than
same peaks in chitosan.53 These upward shifts indicated
decreasing electron density around O and N due to elec-
tron draining to cobalt. The binding energies of Co 2p
Scheme 1. Oxidation of secondary benzylic alcohols.
Scheme 2. Oxidation of benzylic alcohol with cobalt-chitosan.

1932
Ahmad Shaabani et al.
Table 4. Oxidation of alcohols with cobalt-chitosan.
Table 4.
(continued)
substrate Time (h)
Entry
substrate
Time (h)
Product
Yielda (%)
Entry
Product
Yielda (%)
75
17
7:30
1
2
1
90
85
Reaction conditions: alcohol (1.00 mmol), K2CO3 (0.50
mmol), cobalt-chitosan (5 mol%, 0.23 g), p-xylene 5 mL at
80◦C, air. aYield determined by GC analysis.
1:20
in cobalt-chitosan shifted to the lower energy region.
These downwards shifts indicated that electron density
around cobalt increased since electron receiving from
the O and N atoms in the chitosan units. So XPS results
showed that the Co-N and Co-O coordinate bands were
formed and caused to stabilize of Co particles.53,54
Figure 4 shows the FT-IR spectra of the cobalt-
chitosan and chitosan. In chitosan spectrum, N-H and
O-H stretching vibration appeared in 3448 cm−1. Band
at 1653 and 1602 cm−1 displayed the presence of amide
and -NH2 group, respectively. Stretching vibration of
aliphatic C-H groups appeared at 2869 cm−1. Bands in
1422 and 1381 cm−1 region correspond to CH2 and CH3
bending vibration respectively C-N stretching vibration
appeared in the 920-1200 cm−1 region and overlapped
with characteristic absorption bands of saccharide
structure.53 C-O, O-H and C-N stretching vibration
bands of chitosan in catalyst spectrum showed red shift
obviously due to interaction of cobalt and chitosan.
Vacuum-dried cobalt-chitosan was employed for the
FAAS. The Co concentration obtained using calibra-
tion curve was prepared with cobalt solution standards.
The cobalt loading in catalyst was determined to be
13 wt %.
3
4
5
1
92
96
80
0.25
1:30
6
7
15
3
85
90
8
1
1
95
85
87
9
10
1:30
The catalytic activity of the cobalt-chitosan was
tested in the aerobic oxidation of alkyl arenes. The oxi-
dation of diphenylmethane was performed as a model
reaction and the effects of solvent, temperature, time
and amount of catalyst were investigated. The results
were summarized in table 1. The best conditions for aer-
obic oxidation of diphenylmethane were 5 mol % cata-
lyst, 1 mol % NHPI and in 1, 2-dichlorobenzene at 80◦C
using ambient air as oxidant (table 1, entry 5). Using
NHPI as an initiator was necessary to achieve the high
conversion in the oxidation of alkyl arenes.
11
8
86
12
13
4
1
91
95
To determine efficiency of catalyst for aerobic oxi-
dation, a large variety of alkyl arenes were subjected
to optimized conditions. As shown in table 2, the
results showed that the oxidation of alkyl arenes
using cobalt-chitosan were produced the correspond-
ing carbonyl compounds selectively in high yields.
Oxidation of toluene was produced benzaldehyde. Aer-
obic oxidation of the reactants with two benzylic sites
14
15
2
85
55
20
16
4:30
80

Cobalt-chitosan: Magnetic and biodegradable catalyst
1933
Figure 5. TGA of chitosan catalyst before and after reaction.
Figure 7. Reusability of the catalyst for oxidation of
phenylethyl alcohol by cobalt-chitosan.
could not be achieved with these catalysts (table 3,
entries 8 and 9). The optimum conditions are shown in
scheme 1.
Figure 6. XPS spectrum of cobalt-chitosan after reaction.
As shown in scheme 2, these conditions for oxidation
was carried out selectively in one position (table 2, of primary benzylic alcohols afforded benzaldehyde
entries 6,7). derivatives and no over oxidation product was detected.
In order to further assess the ability of the catalyst, The results are summarized in table 4. The different
the scope of the aerobic oxidation was further expanded rates of oxidation in benzylic alcohols indicate that the
to aromatic alcohols. The oxidation of phenylethyl alco- substitutions have important role in the reaction. The
hol was performed as a model reaction. We investigated oxidation reactions were faster with electron-donating
the effect of solvent on oxidation process. After testing group than with electron-withdrawing group. We found
a variety of solvents, p−xylene was determined to be that cinnamyl alcohol was oxidized to cinnamaldehyde
the best solvent for this reaction (table 3, entry 10). after 20 h in 55% yield (table 4, entry 15). Cyclohexanol
Oxidation of phenylethyl alcohol with catalyst under was not converted to cyclohexanone under our reaction
air atmosphere did not produce high yield of prod- conditions and the starting alcohol was recovered. 2-
uct at room temperature. The oxidation was progressed Phenylethane-1-ol and 3-phenylpropane-1-ol were con-
by rising temperature and it smoothly was done at verted to corresponding aldehydes in relatively good
80◦C under air atmosphere. Interestingly, CoCl2.6H2O yields. (table 4, entries 16 and 17).
and cobalt (II)-chitosan coordination polymer showed
The thermogravimetric analysis (TGA) curves are
little activity in oxidation and further improvement shown in figure 5. Different decomposition patterns

1934
Ahmad Shaabani et al.
Table 5. Comparison of results in literature and this work for oxidation of benzyl alcohol.
Entry
Catalyst
Oxidant
Temperature ◦C
Solvent
Time (h) Yield (%) Reference
37
1
2
cobalt(II)/ZnO
mono transition metal (Co,
Mn, Ni)-substituted Keggin-
phosphomolybdates
Co/NSC
1.5 MPa O2
H2O2
70
90
PEG-600
–
13
24
95
56.5
26
41
55
24
3
4
5
6
O2
O2
O2
110
110
80
–
16
4
3
98
89
100
95
cobalt-doped birnessite
toluene
toluene
p−xylene
Co3O4/AC
cobalt-chitosan
Air, K2CO3
80
1
This work
for chitosan and cobalt-chitosan were clearly observed. 4. Conclusion
These results confirmed formation of the new coordi-
In this work, the formation of cobalt-chitosan using a
simple method is described and its catalytic activity was
investigated for aerobic oxidation of alkyl arenes and
aromatic alcohols. The high activity of catalyst was due
cobalt distributed on the surface of chitosan. The cat-
alyst was inexpensive, stable, biodegradable and non-
toxic due to the use of chitosan as the support and was
readily reused and recovered due to the magnetic prop-
erty. Elongated Cobalt particles dispersed in the chi-
tosan matrix could be observed in TEM images. The
Catalyst showed high activity in the aerobic oxidation
of alkyl arenes and aromatic alcohol with negligible
amount of leaching of Cobalt into the solution. The
activity of the catalyst was affected by factors such as
solvent, temperature and base. Generally, the process
was eco-friendly, efficient and cost-effective.
nation polymer. TGA curves of the catalyst before and
after reaction showed that the catalyst was stable under
aerobic oxidation reaction conditions (table 4).
XPS spectrum of the catalyst after reaction is shown
in figure 6. Two species of cobalt that correspond
to Co(0) and Co(II) were observed in the spectrum
that confirmed that the catalyst was stable in reaction
conditions.
The stability and reusability of the catalyst were
examined by five sequential oxidation experiments for
the model reaction (figure 7). In fact, the cobalt-
chitosan was successfully recycled without any consid-
erable loss of activity and selectivity.
To rule out the possibility of cobalt leaching dur-
ing the reaction, a hot filtration test consisting of
the removal of the catalyst (after 5 min) was per-
formed. Upon catalyst removal, no further conversion
was observed even after extended time. Moreover, the
leaching of cobalt into the aqueous phase has also
been checked by the FAAS analysis. The FAAS results
showed that cobalt species do not exist in the solu-
tion. The absence of leached cobalt is an outstanding
result and indicates a strong chelation of the chi-
tosan amino and hydroxyl groups with cobalt species
and the cobalt catalyst worked in a heterogeneous
manner.
Acknowledgments
We gratefully acknowledge financial support from
the Catalyst Center of Excellence (CCE) of Shahid
Beheshti University and the Iran National Elites Foun-
dation (INEF).
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time and with milder conditions than the other works
in some cases. Our catalyst had advantages such as
high reusability, easy recoverability by magnet, hetero-
geneous nature and good selectivity and conversion in
the reaction.
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